rr-^rf^ 3 HYDROMETEOROLOGICAL REPORT N Seasonal Variation of 10-Square-Mile Probable Maximum Precipitation Estimates, United States East of the 105th Meridian U.S. DEPARTMENT OF COMMERCE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION U.S. NUCLEAR REGULATORY COMMISSION Sflver Spring, Md. April 1980 U.S. Department of Commerce U.S. Nuclear Regulatory National Oceanic and Atmospheric Commission Administration NUREG/CR-1486 Hydrometeorological Report No. 53 SEASONAL VARIATION OF 10-SQUARE-MILE PROBABLE MAXIMUM PRECIPITATION ESTIMATES, UNITED STATES EAST OF THE 105TH MERIDIAN Prepared by Francis P. Ho and John T. Riedel Hydrometeorological Branch Office of Hydrology National Weather Service Washington, D.C. April 1980 I o Digitized by the Internet Archive in 2013 http://archive.org/details/seasonalvariatioOOhofr TABLE OF CONTENTS Page Abstract 1 1. Introduction 1 1.1 Authorization 1 1.2 Purpose 1 1.3 Scope 1 1.4 Definitions 2 1.5 Previous study 2 2. Basic data 2 2.1 Background 2 2.2 Available station rainfall data 3 2.2.1 Storm rainfall 3 2.2.2 Maximum 1-day or 24-hour values, each month 3 2.2.3 Maximum 6-, 12- and 24-hr values, each month 3 2.2.4 Maximum recorded rainfall at first-order stations. ... 3 2.2.5 Data tapes, selected stations 3 2.2.6 Data tapes, 1948-73 5 2.2.7 Canadian data 5 3. Approach to PMP 5 3.1 Summary 5 3.2 Selected major storm values 5 3.3 Moisture maximization 6 3.3.1 The concept 6 3.3.2 Atmospheric moisture 6 3.3.3 Representative storm dew point 15 3.3.4 Maximum dev; point 15 3.3.5 Moisture adjustment 15 3.3.6 Elevation and barrier considerations 15 3.4 Transposition 16 3.4.1 Definition 16 3.4.2 Transposition limits 16 3.4.3 Transposition adjustment 17 3.4.4 Distance-from- coast adjustment for tropical storm rainfall 17 3.5 Total storm adjustment 17 3.6 Envelopment 20 3.6.1 Durationally 20 3.6.2 Seasonally 20 3.6.3 Regionally 20 4. Analysis 20 4.1 Introduction 20 4.2 Minimum PMP at 20 grid points 21 4.3 Statistical computations of taped rainfall data (1948-73). 21 4.4 Maximum observed rainfall values ..... 28 4.5 Maximum atmospheric moisture 28 iii Page 4.5.1 Precipitable water in soundings 28 4.5.2 Surface dew points 29 4.5.3 Seasonal variation of maximum moisture 29 4.6 Seasonal variation of rainfall at selected long record stations (1912-61) 29 4.7 Rainfall depth-duration relations 29 4.7.1 Within storms 29 4.7.2 Among storms 38 4.7.3 Analyses 38 4.8 Regional PMP gradients 46 4.9 Some observations on PMP patterns 46 5. Resulting PMP 48 6. Example of Use of PMP maps 48 7. Special problems 48 7.1 Stippled regions on PMP maps 48 7.2 Extreme precipitation at Mt. Washington, N.H 80 7.3 Point rainfall vs. lO-mi^ average rainfall 80 7.4 Storm adjustments greater than 150 percent 81 8. Observed storms within 50 percent of PMP . . 81 Acknowledgements 86 References 87 FIGURES la Storms controlling PMP (September through June) for 6 hours. . 11 lb Storms controlling PMP (September through June) for 24 hours . 12 lc Storms controlling PMP (September through June) for 72 hours . 13 2 Grid points used for transposition 18 3 Distance-f rom-coast adjustment for transposing tropical storm rainfall (from HMR No. 51) 19 4 6-, 24-, and 72-hr PMP for grid point 11, (37°N, 89 °W) (Data are moisture maximized and transposed storm rainfall depths 22 5a 4% probability values for 6-hr duration for November 24 5b Ratios of 4% probability values for 6-hr duration for November or December to the highest of the 12 monthly values 25 iv Page 5c Seasonal variation of ratios of monthly maxima to the highest monthly 4% probability values for 6-hr dura- tion (grid points 3, 6, 12, 13, 17, 20) 26 5d Month of maximum 4% probability values for 6 hours. . . 27 6a Example of analyzed maps of greatest observed 6-hr rainfall (November) 30 6b Example of analyzed maps of greatest observed 24-hr rainfall (November) 31 6c Example of analyzed maps of greatest observed 72-hr rainfall (November) 32 6d Seasonal variation of ratios of monthly maxima to the highest monthly maxima for maximum observed 6-hr rainfall (for grid points 3, 6, 12, 13, 17, 20) ... 33 7a Contiguous United States, maximum w (mm), surface to 500-mb, by half months, January through June (from Ho and Riedel, 1979) 34 7b Maximum 12-hr persisting 1000-mb dew points (°C) for October (Environmental Data Service, 1968) 35 7c Seasonal variation of ratios of atmospheric moisture for each month to that for the highest month (grid points 3, 6, 12, 13, 17, 20) 36 8 Example of seasonal variations of 1% and 4% proba- bility values, the maximum of record, and the ratio of each month's 4% probability value to that for the highest month. Three day rainfall for Duluth, Minnesota and Kansas City, Missouri (1912-61). Vertical scale gives both percentages, and depths in tenths of an inch 37 9 Seasonal variation of within storm 6/24-hr rain ratios for major storms in the study region 39 10a Within storm 6/24-hr rain ratios, January 40 10b Within storm 6/24-hr rain ratios, July 41 11 Within storm 6/24-hr rain ratios vs. magnitude of 24-hr depths 42 12 Seasonal variation of within storm 72/24-hr rain ratios for major storms in the study region 43 v Page 13 Within storm 72/24-hr rain ratios vs. magnitude of 72-hr depths 44 14 Depth-duration plots for November (grid points 3, 6, 12, 13, 17, 20). Upper curves are the ratios of rainfall for various durations to the 24-hr value (+ from PMP; X from 4% probability values). Lower curves show PMP values. Maximized rainfall values transposed to the grid point are shown with storm number (table 2) 45 15 Latitudinal variation by month of 6-hr PMP along longitude 91°W 47 2 16-25 6-hr 10-mi PMP, January through December, (in.) 49-58 2 26-35 24-hr 10-mi PMP, January through December, (in.) 59-68 2 36-45 72-hr 10-mi PMP, January through December , (in. ) 69-78 46 Example of variation of PMP depths with duration for mid- month of March and April for 40.5°N,87.5°W (see sec. 6). . . 79 47 Number of separate storms with rainfall 2l 50% of PMP for 6, 24, and 72 hours (number of storms _> 50% of PMP for July and August can be obtained from Riedel and Schreiner 1980) 82 TABLES 1 Data sources for determining maximum station precipitation of record, 4 2 Major storms selected for moisture maximization and transposition 7-10 3 Important storms centered in Canada near the U.S. border . . 14 4 Statistical analyses for 1948-73 station precipitation on magnetic tapes 23 5 Extreme precipitation amounts observed at Mt. Washington, N.H. (44°16N; 71°18W) during the winter season 81 6 Known storm rainfalls for 6, 24, and 72 hours that are within 50 percent of mid-month PMP for the month in which the storm occurred (July and August storms not included) . 84-85 VI SEASONAL VARIATION OF 10-SQUARE-MILE PROBABLE MAXIMUM PRECIPITATION ESTIMATES UNITED STATES EAST OF THE 105th MERIDIAN Francis P. Ho and John T. Riedel Hydrometeorological Branch Water Management Information Division Office of Hydrology National Weather Service, NOAA Silver Spring, Maryland ABSTRACT. Estimates of the upper limit to rainfall that the atmosphere can produce (probable maximum precipitation) are given in this study for durations from 6 to 72 hours for each month of the year for 10 mi 2 areas. The results are in a generalized form, that is, on maps allowing use for planning and design of any present or proposed hydrologic structure for the United States east of the 105th meridian. The probable maximum precipitation estimates show a smooth variation with duration, season, and location. 1. INTRODUCTION 1.1 Authorization This study was authorized and funded through Interagency Agreement No. NRC-01- 77-113 between the Nuclear Regulatory Commission (NRC) and the National Oceanic and Atmospheric Administration (NOAA) dated June 2, 1977. The Agreement was extended to October 1, 1979, by an amendment dated May 20, 1979. 1 . 2 Purpose The purpose of the study is to give seasonal variation of probable maximum precipitation (PMP) estimates for 10 mi 2 areas for the United States east of the 105th meridian. PMP estimates for durations of 6 to 72 hours, by 6-hr increments are required. 1 . 3 Scope PMP estimates for 6, 24, and 72 hours are given on generalized maps for each midmonth for 10 mi 2 areas. While smaller sized areas have greater PMP values, especially for the warm season, they will not be defined in this study. For the winter season, PMP for smaller areas are not appreciably different from the 10-mi 2 values in this study. All-season estimates of PMP, Hydrometeorological Report (HMR) No. 51, Probable Maximum Precipitation Estimates 3 United States East of the 105th Meridian, (Schreiner and Riedel 1978) set the greatest values that can be reached at some time during the year. They are accepted as upper bounds for the present study. 1 A Definitions Probable maximum precipitation (PI-IP) means the theoretically greatest depth of precipitation for a given duration that is physically possible over a particular drainage basin at a certain time of year. (American Meteorologi- cal Society 1959) . Realizing there are yet unknowns in the physical processes responsible for extreme rainfall, we refer to PMP values as esti- mates. Generalized PMP estimates are estimates determined for large regions that are now required or one would expect will be needed in the future. These are frequently presented as a series of isolines on a map for a given area size and duration. All-season PMP is the greatest PMP regardless of season. For the region of this study one can generally say that for all durations, the all-season PMP will fall sometime between June and September for every point. Our problem is to determine the variation from this all-season estimate for each month of the year. 1.5 Previous Study The only other study covering seasional variation of PMP for the entire region is HMR No. 33 (Riedel et al. 1956). Because the all-season values of HMR No. 51 differ from those of HMR No. 33, it follows that the estimates for each month in HMR No. 33 also need revision. All facets of PMP for the region were restudied and therefore the seasonal values differ from those in HMR No. 33. 2. BASIC DATA 2 . 1 Background As in all PMP studies, basic data are the extreme record storm rainfalls. Storm Rainfall in -the United States^ Depth- Area-Duration Data (Corps of Engineers 1945- ) is a kept-up-to-date catalog of many of the most extreme areal rainfalls. The data are maximum areal depths for standard area sizes and durations. Data for more than 600 storms have been published in this catalog. Additional data come from unofficial sources developed by the Hydrometeorological Branch (Shipe and Riedel 1976). Most of the storms include data from surveys after the storm or resulting flood, sometimes called bucket surveys 3 in which additional rainfall measure- ments are found. Some of these values are measured in regular rain gages, privately owned or owned by local agencies or companies but not included in usual published records. Other values are measured in small test tube type gages, oil cans, or buckets. Such unofficial catches are accepted after checks against other observations and weather patterns, discussions with observers, and more recently, against radar echoes and satellite pictures. It turns out that the most extreme point rainfalls of record are almost entirely from unofficial sources. This should be expected since there is practically no chance that the most extreme rainfall of a storm would occur over a preselected gage site. A shortcoming is that only very limited sur- veys and studies have been made for "out of season" (that is out of the season giving most intense rainfalls of the year) storms. Thus, we must augment our sample with extremes from regularly reporting precipitation stations and recognize that these values may not include the most extreme falls . 2.2 Available Station Rainfall Data Table 1 lists several data sets that were surveyed for obtaining the greatest rainfalls for each month. 2.2.1 Storm Rainfall This means Storm Rainfall in the United States, Depth- Area-Duration Data (Corps of Engineers 1945- ) and the augmented computer file of storm data (Shipe and Riedel 1976). The greatest value for 6, 12, 24, 48, and 72 hours for each month were used from these sources. 2.2.2 Maximum 1-Day or 24-Hr Values, Each Month These values (Jennings 1952) are for regularly reporting stations for the period of record through 1949. Because nonrecording stations are more numerous than recording stations and have longer records, the maximum values generally are from nonrecording stations . 2.2.3 Maximum 6-, 12- and 24-Hr Values, Each Month For 28 of the 37 states involved in this study, there are published (U.S. Weather Bureau 1951-61) maximum observed depths at recording stations for these durations for the period beginning, in most cases, in the 1940 's and ending in 1950. Exceptions are the few recorders going back many years at "first-order" Weather Bureau (now National Weather Service) stations. 2.2.4 Maximum Recorded Rainfall at First-Order Stations A paper (Jennings 1963) published the greatest depths at first-order sta- tions for durations from 5 minutes to 24 hours for about 200 stations in the study region. This is for the period of record through 1961. 2.2.5 Data Tapes, Selected Stations For about 50 stations in our study region, daily rainfall records have been put on magnetic tapes for the period 1912-61. From these tapes we can extract 1-, 2-, and 3-day maxima for each of the 12 months. Table 1. — Data sources for determining maximum station precipitation of record Item Type of Data Period of record Remarks United States Data Storm Rainfall Technical Paper No. 16 Technical Paper No. 15 Primarily maxi- For greatest known mum known areal storms - kept up to depths to date. Maximum 1-day Through 1949 or 24-hr values, each month for regular report- ing stations Maximum l-,2-,3-,6-, Through 1950 12-& 24-hr values for each month Mainly for warm season Available for 28 of the 37 states in study region Technical Paper No. 2 Maximum record- Through 1961 ed rainfall 5 min to 24 hr for 296 1st order stations Data tapes Data tapes Daily precipi- tation a. daily & b. hourly pre- cipitation for regu- ular report- ing stations 1912-1961 1948-73 Available for stations in study region More than 6500 stations Canadian Data Storm Rainfall Similar to U.S. storm rainfall Station Maxima Daily Observa- tion 1941-1970 See section 2.2.7 2.2.6 Data Tapes, 1948-73 These tapes include all observed hourly and daily rainfalls for the period measured both at recording and nonrecording stations, updating to 1973 the published data of pars. 2.2.2 to 2.2.5 for the durations we are interested in. 2.2.7 Canadian Data The Canadians have summarized maximum rainfall depths in much the same way we have in the United States. They maintain a catalog Storm Rainfall in Canada of greatest areal rainfall depths (Atmospheric Environment Service 1961- ). Another publication (Atmospheric Environment Canada 1973) gives the greatest single observed value for one day at each observing station for the period 1941-70. Yet another source (Department of Transport) lists the greatest single observed value for a day in the period 1931-58. A list of station locations (Department of Transport, Meteorological Branch 1970) was helpful in locating extremes near the northern bound of our study region. 3. APPROACH TO PMP 3.1 Summary Central to this study, as already mentioned, is HMR No. 51. For at least one midmonth the values of the present study reach the all-season PMP of HMR No. 51 for every geographical point. The basic approach used in HMR No. 51 is the approach adopted here. We will not repeat the various techniques, steps, and tests fully detailed in that report. More generally, a manual of PMP (World Meteorological Organization 1973) which summarizes PMP procedures that have been used in the United States is recommended for any reader who wishes to pursue the topic further. Development of PMP for each month consisted of the following operations on selected major record storm rainfalls. a. Moisture maximization b. Transposition c . Envelopment Brief discussions of these items follow. At times we extract liberally from HMR No. 51. 3.2 Selected Major Storm Values From the data sources listed we extracted those values that could signifi- cantly influence the level of PMP after they are moisture maximized (par. 3.3) and transposed (par. 3. A), for September through June and any portion of the study region. This was done by first plotting the most extreme depths for a given duration from Storm Rainfall in the United States on 12 maps, one for each month. We then extracted the greatest station values from the other data sources and added them to the plotted maps if they were of the same gen- eral level or greater than those already on the maps. Such maps were plotted for extreme values for durations of 6, 12, 24, 48 and 72 hours. On these 6 maps we also plotted the most extreme rainfall values for the region adjoin- ing the United States from the Canadian data sources. Table 2 lists the storms selected chronologically by month. We show the storm location by latitude, longitude, town, and State. The. storm number we used is given, as well as the Corps of Engineers' assignment number (if there is one) from Storm Rainfall in the United States (Corps of Engineers 1945- ) and the source of the data. The observed depth for the most criti- cal duration is given. Other information in table 2 is explained in par. 3.5. Figures la, lb, and lc show the locations of these most important storms together with the observed rainfall depths for 6-, 24-, and 72-hr durations respectively. The month of occurrence is also given. Table 3 lists the more important storms selected from the Canadian data. These have a bearing on the magnitude of PMP near the U.S. border. 3. 3 Moisture Maximization 3. 3.1 The Concept Moisture maximization is increasing storm rainfall depths for the storm location and season, for higher atmospheric moisture than was available in the actual storm. Signficant precipitation results from lifting moist air. Processes caus- ing this lifting, associated with horizontal convergence, have been de- scribed in numerous texts. Various attempts at developing a model that will reproduce extreme rainfalls are hampered by the lack of sufficient data within storms to adequately check the magnitudes of horizontal convergence, vertical motion, and other parameters. Since measurements of these para- meters during severe storms are not readily obtainable, the solution has been to use extreme record storm rainfalls as an indirect measure of para- meters, other than moisture, that are important to such events. We thus adjust storm rainfalls of record to the equivalent that would have occurred with maximum moisture and make the following assumption: The sample of extreme storms is sufficiently large so that near optimum "mechan- ism" (or efficiency) has occurred. By "mechanism" is meant a combined measure of all the important parameters to rainfall production, except moisture. The assumption thus circumvents a quantitative evaluation of "mechanism" and results in increasing the observed storm rainfall, assumed to occur with near optimum "mechanism", by an adjustment for moisture. This assumption is probably most realistic when considering all-season PMP. Having to spread the storm sample throughout the 12 months weakens the assumption and we compensate by more liberal envelopment (par. 3.6). 3.3.2 Atmospheric Moisture The best measure of atmospheric moisture through depth can be obtained from radiosonde observations. 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City Province Lat. Long. (hr) (in.) 1/15-17,1958 76 Liverpool NS 44°08 64°56 24 60 5.8 10.0 3/31-4/2,1962 77 Alma NB 45°36 64°57 24 9.3 5/25-28,1961 78 45°47 66°45 6 24 72 4.0 9.5 11.8 5/30,1961 79 Buffalo Gap Sask. 49°07 105°17 6 10.5 6/29-7/1,1935 80 Tilson Man. 49°22 101°18 60 13.0 9/20-23,1942 81 Stellarton NS 45°34 62°39 72 13.4 therefore published (Ho and Riedel 1979) the maximum observed semimonthly precipitable water (w or depth of liquid equivalent of the water in a column of air) for more than 100 stations for the period of record. This information was useful guidance. However, radiosonde data alone cannot be used for moisture maximization for several reasons. First, many extreme storms occurred before the radiosonde network was established. Second, the radiosonde network is too sparse to detect narrow tongues of moisture that are important to many storms. The solution was to use surface dew points, which are observed by many stations, as indices to atmospheric moisture. A saturated pseudo-adiabatic atmosphere is assumed, tied to surface dew points, which fixes the moisture and its distribution with height in the atmosphere. Tests have shown that the moisture thus computed is generally an adequate approximation to atmospheric moisture in major storms or for high dew point situations (Miller 1963) . For a saturated pseudo-adiabatic atmosphere, tables have been prepared (U.S. Weather Bureau 1951) giving w values based on 1000-mb dew points. Two dew points are required for moisture maximization. One is the dew point representative of moisture inflow during the storm. The other is the maximum dew point for the same location and time of year as the storm. Both storm and maximum dew points are reduced pseudo-adiabatically to 1000 mb to normalize for differences in station elevations. Both storm and maximum dew points are usually taken as the highest value persisting for 12 hours. Instantaneous extreme dew point measurements may not be representative of inflow moisture over a significant time period, Also, taken over a duration, the effect of possible erroneous instantaneous dew point values is reduced. 15 3.3.3 Representative Storm Dew Point Dew points are selected in the warm moist airflow into the storm. Both distance and direction of the dew points from the rainfall center are record- ed. An average dew point value from several stations is considered to give the best estimate. Care must be used to insure that dew point observations are taken within the moist tongue involved in the heavy precipitation. The time sequence of dew points from each station is reduced to 1000 mb before averaging. After averaging, the highest persisting 12-hr value is selected. 3.3.4 Maximum Dew Point Maximum dew points are generally the highest dew points observed for a given location and time of year. These dew points are based on seasonal and regional envelopes of maximum observed surface dew points that have persisted for 12 hours, reduced to 1000 mb, at many stations (Environmental Data Service 1968) . We adjust the storm to the maximum dew point 15 days from the storm date into the warmer season except for one case, the Hale, Colo., storm of 1935 which was accompanied by unusually cold air judged to be dynamically signi- ficant to the rainfall. Moisture maximization adjustments are increased by up to 10 percent by the 15-day seasonal transposition into the warm season. In the cool season (December-February) , the 15-day leeway usually does not change the moisture adjustment. 3.3.5 Moisture Adjustment Moisture maximization is accomplished by multiplying observed rainfall by the moisture adjustment, which is the ratio of w for the maximum 1000-mb 12-hr persisting dew point to the w for the storm 1000-mb 12-hr persisting dew point. Theoretical justification for this adjustment is found in HMR No. 23 (U. S .W.B. 1947) . This maximization expressed mathematically is: w _, „ p (maximum) . , . , . _ . , P X -. r — = moisture adiusted rainfall w (storm) P where P = observed rainfall w = precipitable water. (Maximum) refers to enveloping highest observed w and (storm) refers to the storm w . (Both dew points are for the same location.) 3.3.6 Elevation and Barrier Considerations Where there is a significant mountain barrier between the moisture source and rain location, or the rain occurs at high elevations, a refinement is sometimes applied to the moisture adjustment. In such cases, mean elevation of the barrier ridge, or elevation of the rainfall rather than the 1000-mb surface, is used as the base of the column of moisture. For the region of 16 our study, location of representative storm dew points (usually toward a coast and at lower elevations) and restrictions to storm transposition (par. 3.4.2) generally eliminated the need for using elevation in the moisture adjustment. 3.4 Transposition 3.4.1 Definition Transposition means relocating storm precipitation within a region that is homogeneous relative to pertinent terrain and meteorological features. 3.4.2 Transposition Limits Topography is one of the more important controls on limits to how far storms can be transposed. If observed rainfall patterns show correspondence with underlying terrain features, or indicate triggering of rainfall by slopes, transposition should be limited to areas of similar terrain. Identi- fication of broadscale meteorological features is important, e.g., surface and upper air high- and low-pressure centers that are associated with the storm, and how they interact to produce the rainfall. Thus, useful guidance to determining transposition limits are storm isohyetal charts, weather maps, storm tracks, rainfalls of record for the type of storm under consideration, and topographic charts. The more important limits to storm transposition for this study were: a. Transposition was not permitted across the generalized Appalachian Mountain ridge. b. Tropical storm rainfalls were not transposed farther away from nor closer to the coast without an additional adjustment (par. 3.4.4) in cases where the maximum dew point charts showed no variation. c. In regions of large elevation differences, transpositions were re- stricted to a narrow elevation band (usually within 1000 feet of the eleva- tion of the storm center) . d. Eastward limits of transposition of storms located in the Central United States were the first major western upslopes of the Appalachians. e. Westward transposition limits of storms located in Central United States were related to elevation. This varied from storm to storm but in most cases the 3000- or 4000-ft contour was used to set the limit. f . Southward limits to transposition were generally not defined since other storms located farther south usually provided higher rainfall values. g. Northward limits were not defined if they extended beyond the Canadian border (the limit of the study region). We used a simplification in transposing by making decisions for each critical storm on whether or not to transpose to each of 20 grid points 17 covering the study region. These points are shown on figure 2. A similar set of points was used as a test in the all-season study (HMR No. 51) for transposition rather than setting outer bounds. After regional smoothing, thus extending the influence of major storms, the results of the two tech- niques are similar for the area size of this study. 3.4.3 Transposition Adjustment The transposition adjustment applied to relocated rainfall values is the ratio of w for the maximum 12-hr persisting dew point for the transposed location to that of the storm in place. The maximum dew point is for the same distance and direction from the transposed location as the storm representative dew point is from the storm location (par. 3.3.3). 3.4.4 Distance-From-Coast Adjustment for Tropical Storm Rainfall The general decrease in tropical storm rainfall with distance inland is well known. It is attributed to the difficulty of maintaining the same rainfall intensity as distance from the moisture source increases, and to the deterioration of the tropical storm circulation with increasing distance inland. A study (Schwarz 1965) developed a relation showing the distance in tropi- cal rainfall with distance up to 300 n.mi. inland. The relation was based on both observed and moisture-maximized tropical rainfall data for several area sizes and durations. Figure 3 shows this variation along with its extension for distance farther inland. It shows no decrease in rainfall for the first 50 n.mi. inland from the gulf coast, a smooth decrease to 80 percent at 205 n.mi. inland, and 55 percent at 400 n.mi. inland. We applied the adjustment for distance-from- coast to tropical storm rainfall (all durations) transposed within the region where the maximum 12-hr persisting dew point charts (Environmental Data Service 1968) indicate no variation. When transposing tropical storm rainfalls farther from the coast, the values are decreased. In the same way, they are increased when trans- posed nearer to the coast. Of the major rains shown in table 2 we applied the distance-f rom-coast adjustment to six storms. These storms are identified with a double asterisk by the storm number. 3.5 Total Storm Adjustment Table 2 includes the grid points to which the selected major storm rain- falls were transposed, the grid point where the storm was most important, and the total adjustment to the rainfall for those grid points. This total ad- justment is the product of the adjustments for maximum moisture and for storm transposition; the latter was determined either from maximum dew points or from the distance-f rom-coast relation. 18 o CO o to K « CO s CO S o i i cq Cfl I 19 % z o < o 03 u o a, £ 05 o <3 Of -u> u_ Sh LU .n u «K z •P o < 1— o to & CN +i o o 1SVOD IV 3mVA dO !N3DH3d 20 3.6 Envelopment Moisture maximization and transposition of major storms to the 20 grid points set the very lowest value of PMP for each month at these points. How much to envelop these values and give consistent PMP from place-to-place, month-to-month and duration-to-duration is the major portion -of this study. 3.6.1 Durationally By this we mean smooth curves of rainfall depths extending from one dura- tion to another. Such smooth curves imply that the storm record has given more extreme depths for certain selected durations than for others. 3.6.2 Seasonally Seasonal envelopment, in much the same context as in durational envelop- ment, assumes the storm record does not provide equally extreme depths for all months. We thus draw smooth seasonal curves for a selected grid point, enveloping all of the data for some months. 3.6.3 Regionally We assume that except for topographic and coastal influences we should have a smooth regional pattern of PMP. 4 . ANALYSES 4.1 Introduction We have set the stage by describing the available data and the basic con- cepts that bear on the results and on how these results will be reached. We now give details of the data analyses leading to our goal. One additional consideration entering into our procedures is how to show results concisely but fully, that is, how to show PMP for durations for 6 to 72 hours for 10 mi 2 for each month for the study region. Tests have shown that for a point east of the 105th meridian when PMP values for 6, 24, and 72 hours are plotted on linear graph paper (duration vs. depth) and joined by a smooth curve through the point of origin (0,0) the curve adequately defines PMP by 6-hr increments to 72 hours. We decided to present maps covering the region for each month showing PMP depths at midmonth in inches, for 6, 24, and 72 hours. (For July and August, the mapped values turn out to be identi- cal to the all-season PMP of HMR No. 51 and to each other for the entire study region. Similarly, the PMP values for January and February are the same) . In our approach, we need to take care that the smoothing procedures give PMP values that are not unrealistically extreme: each smoothing step, if not done with care, could give a cumulative envelopment that results in an unreasonably large final product. This concern must be balanced with the basic need — that at the least, all known observed rainfall depths maxi- mized for moisture should be enveloped. Further, our results should, to the best of our capability, give extremes that will not be exceeded by future storms. 21 4.2 Minimum PMP at 20 Grid Points Seasonal plots were made for each of the 20 grid points (fig. 2) of the greatest moisture maximized and transposed from depths for 6, 12, 24, 48, and 72 hours. We then drew tentative smooth enveloping curves for these data for 6, 24 and 72 hours on each plot. An example of these plots and in this case the final smooth enveloping curve is shown in figure 4. The storms on the figure (identified by a number near the X axis) can be identified by storm number in table 2. The remainder of chapter 4 gives analysis of various rainfall data that was helpful in decisions on how to obtain a consistent set of PMP values for all durations, months, and locations. 4.3 Statistical Computations of Taped Rainfall Data (1948-73) Using computer programs we determined a variety of products that could influence or give guidance to our study. The first simple product, of course, is a tabulation of the greatest observed depths for a specified duration for each month of observation. Such maxima lend themselves to statistical analysis. From the rainfall for each station recorded on tape with 20 or more years of record, the maximum values for each month for a duration of interest were put into series of all January maxima, all February maxima, etc. To each series we fitted the Fisher-Tippett type I distribution by the Gumbel fit- ting procedure. This statistical distribution is used almost exclusively by the National Weather Service, for precipitation frequency analysis (Hersh- field 1961 and Frederick et al. 1977). From the fits, the rainfall amounts with 0.04 and 0.01 chance of being equalled or exceeded for each month at each station were abstracted (here- after referred to as 4 percent and 1 percent probability level rainfalls) . These rainfall amounts for all stations located with each 2° latitude and longitude quadrangle were then averaged. This was done for quadrangles overlapping (both in the north-south and east-west direction) by 1° latitude and longitude, respectively. This averaging was a smoothing step. Table 4 summarizes the statistical analyses for 6, 24, and 72 hours. The sets of 4 percent and 1 percent probability level and maximum values which were computed and plotted on maps using a computer program are indicated in table 4, part 1. Figure 5a is an example of the 6-hr 4 percent probability level values for November and their analyses. Part 2 of table 4 refers to maps of the ratios of monthly values to the maximum value for any one of the 12 months. These ratios are guides to the corresponding ratio of monthly PMP to all-season PMP. Figure 5b is an analyzed example of these ratio maps for the 4 percent probability level amounts for 6 hours for November. The values shown in this figure came from combining the ratios in each quadrangle for the month& of November and December. The highest ratio of the two months was selected and plotted. L 1 i i I i i i I i i i 1 i i ^3 -f-i CO I r-H O is I CV J ,-v.co *?^ ■ s ;!s Ps HJ> CND 23 Table 4. — Statistical analyses for 1948-7 S station precipitation on magnetic tapes ■k 4% *1% maximum Part 1 12 maps, each 6 hr X X X duration, in 12 hr X X X inches 24 hr X X X 1 day X X X 2 day X X X 3 day X X X Part 2 12 maps, each 6 hr X X X duration, % 12 hr X X X of highest 24 hr X X X month 1 day X X X 2 day X X X 3 day X X X Part 3 2 maps, each 6 hr X X X duration of 12 hr X X X month of 24 hr X X X maximum and 1 day X X X and mini- 2 day X X X mum 3 day X X X Part 4 12 maps, each 6/24 X X X rain ratio 12/24 X X X 72/24 X X X *4% and 1% probability values based on Fisher-Tippett Type I distribution fitted by the Gumbel procedure. 24 03 I CO 3 « Ml I I a 10 <3i «4 25 Q CO Q) l« CD Cfi o is 0) 0) o ft S o "a 1 to Sh o 3 *o « » Sj +i V ^ •t» D * rCl !/> O Sh • ftj to 0) &S s ^ t-i r-S? ca -pi Sh Si O •^ fc. 26 .Or 6 r l3 O? Jp\ C9^ ©Nq i i i 1 1 1 8 10 8 12 1.0 r l2 o ^^ ^Oj jg/o .8 CO 2 .6 °\4 .2 1 1 1 1 1 6 8 MONTH 12 4 6 8 MONTH Figure 5o. — Seasonal variation of ratios of monthly maxima to the highest monthly 4% probability values for 6-hr duration (arid points Z 3 6 3 12 3 13 3 17 3 20). 27 o i o S o i i s &4 28 This procedure, applied to all pairs of successive months, was designed to avoid anomalous month-to-month variations. For the 20 grid points (fig. 2) values were taken from the analyzed ratio maps and plotted on seasonal charts, one for each of the three durations. Figure 5c shows some examples of the results for 6 hours. Equating the highest value to all-season PMP and multiplying this depth by the percents of the maximum, gave us an estimate of PMP for each month based only on the 4 percent probability variation. The analyzed maps showing the months of maximum and minimum months for the 4 percent, 1 percent and maximum of record levels (part 3 of table 4) also provided helpful information. Figure 5d is an example of the maps showing the month of maximum 4 percent probability level values for the 6-hr duration. Part 4 of table 3 shows that maps were prepared giving the ratios of 6/24-, 12/24-, and 72/24-hr and 3-day/l-day rains (4 percent, 1 percent, and maximum of record levels). Such ratios are a guide to maintaining depth- durational consistency in the final product. 4.4 Maximum Observed Rainfall Values Under par. 3.2 we discussed maps of maximum rainfall based on all the data sets listed in table 1. Such maps were developed for 6-, 12-, 24-, 48-, and 72-hr durations. Smooth regional analyses were made of data on each of these maps, taking into account in at least a gross manner, the maximum values on adjacent maps so that there would not be unrealistic changes from month to month. This resulted in a few extreme storms being undercut. Of course without moisture maximization and transposition these analyses give values that are too low for PMP. Figures 6a, 6b, and 6c are examples of maps of the extremes with analyses for 6-hr, 24-hr, and 72-hr durations, respectively, for November. These maps show storm depths with numbers in parentheses, in some cases, that correspond to the storm numbers in table 2. From the map analyses, values were read for the 20 grid points (fig. 2). Smooth seasonal curves were drawn to these monthly maxima for each grid point separately. Another set of monthly curves were developed by express- ing each month's depth in percent of the maximum ( of the 12 months). The resulting smooth curve is, then, a seasonal variation of percents of the maximum month (100 percent) based only on the analysis of observed data. Figure 6d is an example of the seasonal variation of the ratios for 6-hour rainfall for 6 selected grid points. 4.5 Maximum Atmospheric Moisture 4.5.1 Precipitable Water in Soundings Since we adjust record storms to maximum moisture (w ) , guidance to our work here is maximum observed moisture at upper-air sounding stations. Such w„ values have been computed for twice-a-day soundings for the period of reliable records for all U.S. stations and the maximum values extracted on a 29 semimonthly basis (Ho and Riedel 1979). In that publication, such values have also been plotted on maps and analyzed for our study region. These analyses were smoothed seasonally as well as regionally. Figure 7a is an example of such maps taken from that publication. 4.5.2 Surface Dew Points In par. 3.3.4 we briefly described maps of maximum 1000-mb 12-hr persisting dew points (T,). These maps, being an index to moisture in the atmosphere, are also clues to smooth seasonal and regional patterns of extreme rainfall. An example of these maps is shown in figure 7b. 4.5.3 Seasonal Variation of Maximum Moisture We determined the seasonal variation of both maximum w and maximum T^ (see figures 7a and 7b for examples) by expressing each month or half-month value as a percent of the highest of the year for each of the 20 grid points. Figure 7c shows examples of these smooth seasonal curves. 4.6 Seasonal Variation of Rainfall at Selected Long Record Stations (1912-61) Daily precipitation amounts of some 50 selected stations with 50 years of record (1912-61) are recorded on magnetic tape. This record was processed in the same way as the 1948-73 tape data (par. 4.3), except that the data were not spatially averaged. Seasonal variations of these computed parameters were plotted for each station separately. Figure 8 shows the unsmoothed and computer-produced plots for Duluth, Minn., and Kansas City, Mo., for the 3-day duration. For each month, the maximum observed value, the 1 and 4 percent probability values, and the ratio of each month's 4 percent probability value to the maximum monthly value for the year are given . 4.7 Rainfall Depth-Duration Relations 4.7.1 Within Storm Depth-duration relations of rainfall in major storms and their variations within the region and seasonally, give guidance to depth-duration relations for PMP. The 6- to 24-hr (6/24) rain ratios together with 72- to 24-hr (72/24) rain ratios quite adequately define depth-duration relations from 6 to 72 hours. We computed these two sets of ratios for the major storms for 6, 24 and 72 hours given in Storm Rainfall in the United States for each month. High 6- and high 24-hr depths were both considered for 6/24 ratios in order not to bias the results. Similar data selection was carried out for the 72/24 ratios. Of course, for the storm to be counted, it had to last at least 24 hours for a 6/24 ratio and 72 hours for a 72/24 ratio. 30 £ o r-o> ft I > _^--<\ \^-^^\ \ ^-V"^ \ \ \--~-~~^ — " 1 o Ml s \^-' 1 ^-"^t \ 0\. \ — •— " — \ \ L~-— — ""*" \ \ \ L— — — - "* N -o •o -<\ \ \ JtZTM £\ V ^V^\ \ JU--- — \ \ \ - S- \ "JPa^WrA^ \Jwr" \V\J W^A-A^i \ \ I— • 5 " ■8 g . § i- i ^ \ \ 3^- J > i 'r^.~ — A 2^ y o -o \—^^\ s V^x\ '^^V^C^A \ \ \ \ \ \^r- "^T \ J- ~-\ 10 Y~~-^~\ \ \ \&¥ A ~\ \ \ """-"A\ i V^-Vn^r^Vfi^ br CO 00 „ J^~ \ r\) Aa^cT HY *A \ \ \ ,'iL-U — Trir~~~/T O- \ ^ \£__ — nf \ \ 1-+- z- - vl , sJLM \T \\ ■ / tv \ ^J^^^Hjks. ) ^AV — tni \ I \ ! C 1 JL ___-i- — —^ ffj i z^tS^rri — \ I \\ ! " l \ \| U__\__J — 1 \ ^^l^--^ ! ? ::: w~^\ \\ \ i\ Ji-L^-^i — ^I^Tl'"" # rs n — \ ?W / \ 4- — Vi \ r^-, li 1 I 00 as \ /-W \?LU — — \ \ ' 1 ? V l \ A. • 3 V" R f A" P* Ml __U 4* — iTTT — W io— 4 J___l— — 1— — T __ o- o> n - ^" "X/u \ y ~ff<— •. . 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UJ en ^ u- -H --.I o < rO [< >l ^- 1 ID -3 CD rO ^ 1" ** •>l z CM K. '1 Lr «>J >- < O rO 1^ -*1 nJ cc CL < cc < 2 L-" LEGEND -RANGE ^AVERAGE >] •O CQ UJ U_ N 1^ «U > l 1 z < CD I 1,1,1,1 ,i,i, > l 1 > 1 i 1 o Q CM O O CD O 1^- O CD O lO O o rO o o CM —. o o 01 IVil UHbZ/ZL +s K CO s is o +i co Pn o E Ph •s. CO O Ph « is I "Stl CX] \ Cs. Pn O CO £ rg O o Pn a o CO « . CO o 0) Ph s C35 CO &4 44 < Or < Or Or X 2.30 2.20 2.10 — 2.00 1.90 1.80 .70 .60 1.50 1.40 1.30 1.20 1. 10 1. 00 LEGEND XDEC ■ ■ FEB OMAR - MAY • JUNE - AUG ASEP - NOV / INDICATES TWO STORMS XA A — O X X • od x x A« A ° o A ^0 A. O A V * AO 00, •. -%?..*• O • Q X A •/» o • "A x„<5 A o» • A C A XA°0 A m F X A A AO A^X A OA o 'A" • ~~b ^..A.A. A £ A 3 5.0 10-0 15-0 20-0 25.0 72 HR RAIN CIN.) 30.0 35.0 Figure IS. — Within storm 72/24-hr rain ratios vs. magnitude of 72-hr depths. 45 12 24 36 48 60 72 1.5 12 24 36 48 60 72 6 /x XT 6 * 8 r 1 _ —j l i'20 PMP . 1. c fc'° UJ Q n -^r^TT67 •64 •65 (STORM #) 1 I 1 68, 12 24 36 48 60 72 5 20 17 . ll t hi 1 - / ^^70 • 70 70« - /6°. • • • /• 67 67 • 67 67" /• J 67 fill 1 1 5 1.0 5 cc 12 24 36 48 60 72 DURATION CHR) 12 24 36 48 60 DURATION CHR) 72 Figure 14. — Depth- duration plots for November (grid points 3, 6 3 12, 13, 17, 20). Upper curves are the ratios of rainfall for various durations to the 24-hr values (+ from PMP; X from 4% probability values). Lower curves show PMP values. Maximised rainfall values transposed to the grid point are shown with storm number (table 2). 46 We found an inverse relation between the 6/24 ratio and that for 72/24. That is, if the 6/24 ratio is high, the 72/24 ratio is low, and visa versa. This appears to be meteorologically reasonable. For example, a high 6/24 ratio, expected in summer with brief thunderstorm type rainfall, is associ- ated with a low 72/24 ratio. 4.8 Regional PMP Gradients We have insisted ^MP should not show sharp demarcations or changes from one point to the next unless explainable by terrain effects. Thus, we have plotted the 6-, 24-, and 72-hr PMP depths against selected latitudes and longitudes, covering the region in order to eliminate sharp changes. Figure 15 is an example of such plots .... showing 6-hr PMP along longitude 91°W for latitudes 30° to 47°N for each month. 4.9 Some Observations on PMP Patterns The objectives or requirements of a) smooth patterns and gradients of PMP for each month and each duration (6, 24, 72 hours), b) smooth progression of increasing depths with duration, c) a smooth progression of PMP depths from month to month, and d) envelopment of moisture maximized and transposed storm rainfalls required numerous iterations. As one of the four objectives is approached, changes in analysis effect the other three. We should repeat a fifth objective uppermost in our thoughts during the study; this was to avoid undue indirect maximization and envelopment in achieving the objectives . Some specific indications from the guidance material that were incorporated in the PMP patterns are as follows : The semimonthly maximum w maps (see example in fig. 7a) indicate a grad- ual progression of moisture from the Gulf Coast northward in early spring. A ridge of high moisture extending from the Gulf coast to the Great Plains can be identified easily in the summer months. The maximum w maps indicate that moisture remains high through September. The maps of 4 percent probability rainfall also show higher values extend- ing inland from the Louisiana and Mississippi coasts during April and May than in adjacent months. Maps of greatest observed rainfall depths show maximum precipitation in June in the northwestern portion of our study region. This set of maps reveals that maximum rainfall occurs in September along the eastern seaboard and in the gulf states. Scattered high values also appear in early October in some coastal regions, especially in Texas. Some of the data, particularly the probability level values, show a longer season of maximum rainfall for the states bordering the Gulf and Atlantic coasts than for the interior regions. The plateau extends into September and early October. This can be explained by the greater opportunity for tropical 47 Id D « s K o £ O t-Jl S; <■£ I to » "o. 1 --^ \ \ \ \ o — O-i o -o . -o V*— j' *x ^T\^ PsX^^\ o o- ^- « r* o -o CN III 5 o < -si N. ^^ ■" 1 "" O-j """ o o o -o ^^v?-'-vA^ ^^TV \ Y\ W- oj <£» 0°\ ^ -^\\\ "£W ^___ svf- _x~— ~~^^/xj\ x^ l \ \v- «.\ *. to&A -H — )k \\h \ _ vr ^— IVi) v|— \"^ 1 \A/-—Pr-- o- K 1 /! 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For simplification the PMP for the midmonth in which the storm occurred is compared with the storm depth. For example, if a storm occurred on any day in March it is compared with PMP for mid-March. A March 1 storm would actually be a higher percent of March 1 PMP and a March 31 storm would be a lower percent of March 31 PMP. No comparisons were made for July and August, the months for which we accept the all-season PMP of HMR No. 51. Comparisons of observed rainfall to all- season PMP in HMR No. 51 are given by Riedel and Schreiner, 1980. Figure 47 shows a seasonal plot of the number of known storms that are >_ 50 percent of 10-mi PMP for 6, 24, and 72 hours. As discussed earlier, undoubtedly, many more storms have reached 50 percent of PMP than have been sampled by the sparse network. That there are fewer cases in winter than 82 1 1 1 1 1 1 1 1 1 o • Ixl - Q > • O 2 1- • O O Q. • LU c/> CD 3 < -J 3 ~3 2 • 3 -o >- • < 2 q: • q. < cc • < 2 CD • Ixl Li- 2 • < 1 1 1 1 1 1 1 1 1 —> Sh ,o, H-, QO A (^ \0 *£ r-J. ^ fc .<3 W v-o> £Lj +i s <» S "\.> to ^~ o si fc O rO 0) r« o '■§! C\] « 3 cs. rCj fei i 'tJ £ i X* « • CI IN. ^h 1 +s \H to W co 3 & CT5 3 i s t3}to ■^ •^ fe, o CM 00 <0 * CM O 00 CD 5f CM SIAIU01S JO d39IAinN 83 summer is in the right direction: fewer storms have been studied in the cool season and fewer surveys made after storm events to find extremes. Table 6 lists chronologically the storms that have observed depths >^ 50 percent of PMP for each month. Some of course are identical to the major storms of table 2. We only show comparisons for rainfall depths for 6, 24, and 72 hours. If more durations were added (between 6 and 72 hours) many more storms would reach 50 percent of PMP and the percentages shown would be higher. 84 Table 6. — Known storm rainfalls for 6, 24 and 72 hours that are within 50 percent of mid-month BMP for the month in which the storm occurred (July and August storms not included) Obs. COE Storm Dur. Precip. % of Assign. ' Date number Lat. Long. (hrs) (in.) PMP No. Source* Jan. 1-2, 1941 3 48°00 89°42 24 4.7 65 TP No. 16 Jan. 22-27, 1949 5 35°52 9 2°. 19 6 24 7.5 11.7 52 54 SW3- •10 STR Feb. 2-18, 1883 41°42 77°16 6 3.6 60 0R5- •11 STR Feb. 12,1886 41°54 71°23 24 7.9 56 STR Feb. 6, 1960 43°07 73°35 24 5.1 61 DTD Feb. 25-27,1969 12 44° 16 71°18 24 72 8.4 14.1 86 68 DTD Feb. 10-11, 1970 13 44° 16 71°18 6 24 4.7 10.2 89 100 DTH Feb. 1,1973 14 32°56 92°36 6 10.6 65 DTH Mar. 13, 1888 42°43 73°18 24 6.1 52 TP No. 16 Mar. 28, 1902 35°41 85°48 24 11.0 50 TP No. 16 Mar. 23-27, 1913 17 40°22 83°46 24 72 7.3 10.4 55 61 0R1- •15 STR Mar. 11-16, 1929 19 31° 25 86°04 6 24 72 14.0 20.0 29.6 73 65 74 LMV2 '.-20 STR Mar. 12,1936 44° 16 71°15 24 6.5 66 TP No. 16 Mar. 22, 1949 44°25 72°16 24 5.0 55 TP No. 16 (update) Mar. 31,1951 41°56 74°23 24 6.7 57 TP No. 16 (update) Mar. 25, 1964 21 35°37 84° 12 6 7.5 55 DTK Mar. 16-18, 1965 22 46°53 90°49 72 6.6 54 DTD Mar. 25, 1965 41°34 75°52 6 4.3 57 DTH Mar. 2-5, 1966 23 47°14 98°35 24 4.7 57 STR Mar. 14, 1973 24 44°21 103°46 24 5.7 71 DTD Apr. 11-14, 1933 26 43°08 70°56 6 4.9 52 NA1- -23 STR Apr. 3-4,1934 27 35°37 99°40 6 17.3 73 SW2- -11 STR Apr. 24-28, 1937 28 39°40 77°54 72 11.3 53 SA5- -13 STR Apr. 21, 1951 33°21 94° 30 6 14.2 53 DTH May 30- June 1, 1889 41°45 77°17 6 7.4 53 SA1- -1 STR May 30-31,1935 34 39°36 102°08 6 16.5 82 MR 3-28A STR 24 22.2 83 May 6-12,1943 35 35°29 95°18 72 24.9 56 SW2- -20 STR May 12-20,1943 35°52 96°04 6 15.9 56 SW2- -21 STR June 13-18,1886 38 31°19 92°33 72 29.0 53 LMV4-27 STR June 27-Jul.l, 1899 30°52 96°32 72 34.5 64 STR June 17-21,1921 39 47°18 105°35 6 10.5 55 MR4-21 STR 24 13.3 53 72 14.6 53 June 30,1932 30°01 99°07 24 31.7 75 GM5-1 STR June 19-20,1939 32°44 100°55 6 18.8 71 STR June 10-13,1944 41°52 97°03 6 13.4 53 MR6-15 STR June 23-24,1948 42 29°22 100°37 24 26.2 66 STR June 23-28,1954 43 30°12 101°35 6 16.0 61 SW3-22 STR 24 26.7 71 72 34.6 77 See notes at the end of the table. 85 Table 6. — Known storm rainfalls for 6, 24 and 72 hours that are within 50 percent of mid-month PMP for the month in which the storm occurred (July and August storms not included) (Continued) Obs. COE Storm ftrs) Precip. % of Assign. Date number Lat. Long. (in) PMP No. Source June 8-10,1962 44°12 103°31 72 14.9 62 DTD June 23-24,1963 45 41° 14 97°05 6 24 14.6 16.2 57 51 STR June 24,1966 46 47°21 101°19 6 11.1 53 STR June 9,1972 47 44°12 103°13 24 14.9 54 MR10-12 STR June 20-22,1972 48 42°05 78°10 24 72 14.3 18.5 52 58 NA2-24A STR Sept. 8-10, 1921 49 30° 35 97°18 6 24 72 22.4 36.5 37.6 74 84 72 GM4-12 STR Sept. 17-19, 1926 50 43°12 96°00 6 24 15.1 21.7 62 71 MR4-24 STR Sept. 14-18,1936 31°47 100° 50 24 72 26.0 30.0 68 65 STR Sept. 1,1940 52 39042 75°12 6 20.1 76 NA2-4 STR Sept. 2-6,1940 53 36°15 96°36 6 24 18.4 23.6 65 64 SW2-18 STR Sept. 3-7,1950 54 29°03 82°42 24 72 38.7 45.2 81 82 SA5-8 STR Oct. 7-11,1903 56 40°55 74°10 24 13.7 51 GL4-9 STR Oct. 17-22, 1941 57 29°48 82°57 24 72 30.0 35.0 73 66 SA5-6 STR Oct. 11-18,1942 58 38°31 78°26 72 18.7 52 SA1-28A STR 0ct.30-Nov.l, 70 30°41 81°28 24 22.0 67 DTD 1969 72 22.6 56 Oct. 10-11, 1973 63 36°25 97°52 6 16.9 77 STR Nov. 7,1915 65 48°54 103°18 24 4.0 56 TP No. 16 Nov. 2-4,1927 67 44°03 71°45 6 24 72 7.8 12.0 14.0 78 79 71 NA1-17 STR Nov. 22-25,1940 69 30°08 96°08 24 72 18.6 21.1 59 53 GM5-13 STR Nov. 1,1948 37°02 99°59 6 6.1 50 DTH Nov. 13, 1954 24°33 81°48 24 19.9 62 TP No. 2 Dec. 5-8,1935 72 29°54 95°37 24 72 18.6 20.8 66 57 GM5-4 STR Dec. 29- Jan. 1, 1949 73 42°40 73°19 24 72 8.1 12.6 62 73 NA2-18 STR Dec. 20, 1959 37°25 82°01 6 6.7 56 DTH Dec. 26-28,1969 74 44°16 71°18 6 24 72 3.3 8.6 10.4 55 77 70 DTD Dec. 26-28,1969 44° 40 70°09 24 72 6.0 10.0 51 71 TP No. 16 (update) COE: Corps of Engineers * : Source STR: Storm rainfall TP No. 16: Technical Paper No. 16 DTD: Data tape; daily precipitation DTH: Data tape; hourly precipitation 86 ACKNOWLEDGEMENTS The authors thank J. Miller, Chief, Water Management Information Division and Dr. Vance A. Myers, Chief, Special Studies Branch for reviewing the study. They also wish to thank meteorologists L. Schreiner, R. Watkins and R. Zehr, technicians Marion Choate, Teresa Johnson, Roxanne Johnson, Normalee Foat and Keith Bell for assistance and Virginia Hostler, Clara L. Brown, and Donna Smith for typing the numerous versions of this report. REFERENCES American Meteorological Society, 1959: Glossary of Meteorology. Boston, Mass. , 638 pp . Atmospheric Environment Service, 1961- : Storm Rainfall in Canada, Downsview, Ontario , Canada . Atmospheric Environment Service, Canada 1973: Canadian Normals, Volume 2 Precipitation 1941-1970, Downsview, Ontario, Canada, 330 pp. Department of Transport, Meteorological Branch 1970: Climatologioal Station Data Catalogue: The Prairie Provinces, CL14-70, 126 pp; Ontario CL15-70, 90 pp; Quebec CL16-70, 65 pp; The Atlantic Provinces, CL17-70, 46 pp. Department of Transport, Meteorological Branch — , Maximum Precipitation Reported on Any One Observation day, 1931-1958 (unpublished report). Environmental Data Service, 1968: Maximum persisting 12-hour 1000-mb dew points (°F) monthly and of record. Climatic Atlas of the United States, Environmental Science Services Administration, U.S. Department of Commerce, Washington, D.C., pp. 59-60. Frederick, Ralph H., Myers, Vance A., and Auciello, Eugene P., 1977: Five- to 60-Minute Precipitation Frequency for the Eastern and Central United States. NOAA Technical Memorandum NWS HYDRO-35, U.S. Department of Commerce, Silver Spring, Md. , 36 pp. Hansen, E.M. , Schwarz, Francis K. , and Riedel, John T. , 1977: Probable Maximum Precipitation Estimates, Colorado River and Great Basin Drain- ages, Hy drome teorological Report No. 49, National Weather Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Silver Spring, Md. , 161 pp. Hershfield, David M. , 1961: Rainfall Frequency Atlas of the United States for Durations From 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years. Technical Paper No. 40, Weather Bureau, U.S. Department of Commerce, Washington, D.C., 115 pp. Ho, F. P., and Riedel, John T. , 1979: Precipitable Water over the United States Volume 2: Semimonthly Maxima. NOAA Technical Report NWS No. 20, U.S. Department of Commerce, Silver Spring, Md., 359 pp. Jennings, Arthur H. , 1952: Maximum 24-Hour Precipitation in the United States. Technical Paper No. 16, Weather Bureau, U.S. Department of Commerce, Washington, D.C., 284 pp. Jennings, A.H. , (revised) 1963: Maximum Recorded United States Point Rain- fall for 5 Minutes to 24 Hours at 296 First-Order Stations. Technical Paper No. 2, Weather Bureau, U.S. Department of Commerce, Washington, D.C, 56 pp. > PENN STATE UNIVERSITY LIBRARIES ADDQQ3325T772 NOAA— S/T 80-65