DOC. 4 „ C 55.13: NVJS25 UNIVERSITY OF ILLIF OiS LIBRARY AT UR3AI A CHAMPAIGN STACKS S 35 BOOKSTACKS- DOCUMENTS NOAA Technical Report NWS 25 4**°''* 3 a c \ <5& ^r ES of * Comparison of Generalized Estimates of Probable Maximum Precipitation With Greatest Observed Rainfalls Washington, D.C. March 1 980 d &os,to ry APRt 41QS0 SW8 ®^^ U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Weather Service NOA ivTS National I Service Ser The National (NWS) observes and measures atmospheric phenomena; develops and distrib- utes forecasts of weather conditions uings of adverse , collects and disseminates weather •rmation to meet the needs of the public and specialized users. The NWS develops the national meteorological service system and Improves procedures, techniques, and dissemination lei and hvdrologic measurements, and forecasts. NWS series of NOAA Technical Reports is a continuation of the former series, ESSA Technical Report Weather Bureau (WB). Reports listed below are available from the National Technical (ion Service, U.S. Depart- ment of Commerce, Sills Bldg. , 5285 Port Royal Road, Springfield, Va. 22161. Prices vary. Order by accession number (given in parenthesi ESSA Technical Reports WB 1 Monthly Mean 100- , 50-, 30-, and 10-Millibar Charts January 1964 through December 1965 of the 1QSY Period. Staff, Upper Air Branch, National Meteorological Center, February 1967, 7 p, 96 charts. (AD 651 101) WB 2 Weekly Synoptic Analyses, 5-, 2-, and 0.4-Mb Surfaces for 1964 (based on observations of the Meteorological Rocket Network during the IQSY). Staff, Upper Air Branch, National Meteorologi- cal Center, April 1967, 16 p, 160 charts. (AD 652 696) WB 3 Weekly Synoptic Analyses, 5-, 2-, and 0.4-Mb Surfaces for 1965 (based on observations of the Meteorological Rocket Network during the IQSY). Staff, Upper Air Branch, National Meteorologi- cal Center, August 1967, 173 p. (AD 662 053) WB 4 The March-May 1965 Floods in the Upper Mississippi, Missouri, and Red River of the North Basins. J. L. H. Paulhus and E. R. Nelson, Office of Hydrology, August 1967, 100 p. WB 5 Climatological Probabilities of Precipitation for the Conterminous United States. Donald L. Jorgensen, Techniques Development Laboratory, December 1967, 60 p. WB 6 Climatology of Atlantic Tropical Storms and Hurricanes. M. A. Alaka, Techniques Development Laboratory, May 1968, 18 p. WB 7 Frequency and Areal Distributions of Tropical Storm Rainfall in the United States Coastal Region on the Gulf of Mexico. Hugo V. Goodyear, Office of Hydrology, July 1968, 33 p. WB 8 Critical Fire Weather Patterns in the Conterminous United States. Mark J. Schroeder, Weather Bureau, January 1969, 31 p. WB 9 Weekly Synoptic Analyses, 5-, 2-, and 0.4-Mb Surfaces for 1966 (based on meteorological rocket- sonde and high-level rawinsonde observations). Staff, Upper Air Branch, National Meteorological Center, January 1969, 169 p. WB 10 Hemispheric Teleconnections of Mean Circulation Anomalies at 700 Millibars. James F. O'Connor, National Meteorological Center, February 1969, 103 p. WB 11 Monthly Mean 100-, 50-, 30-, and 10-Millibar Charts and Standard Deviation Maps, 1966-1967. Staff, Upper Air Branch, National Meteorological Center, April 1969, 124 p. WB 12 Weekly Synoptic Analyses, 5-, 2-, and 0.4-Millibar Surfaces for 1967. Staff, Upper Air Branch, National Meteorological Center, January 1970, 169 p. NOAA Technical Reports NWS 13 The March-April 1969 Snowmelt Floods in the Red River of the North, Upper Mississippi, and Mis- souri Basins. Joseph L. H. Paulhus, Office of Hydrology, October 1970, 92 p. (COM-71-50269) NWS 14 Weekly Synoptic Analyses, 5-, 2-, and 0.4-Millibar Surfaces for 1968. Staff, Upper Air Branch, National Meteorological Center, May 1971, 169 p. (COM-71-50383) NWS 15 Some Climatological Characteristics of Hurricanes and Tropical Storms, Gulf and East Coasts of the United States. Francis P. Ho, Richard W. Schwerdt, and Hugo V. Goodyear, May 1975, 87 p. (COM-75-11088) (Continued on inside back cover) """"fcaToK^ ,*- NOAA Technical Report NWS 25 Comparison of Generalized Estimates of Probable Maximum Precipitation With Greatest Observed Rainfalls John T. Riedel and Louis C. Schreiner Office of Hydrology Silver Spring, Md. Washington, D.C. March 1980 U.S. DEPARTMENT OF COMMERCE Philip M. Klutznick, Secretary National Oceanic and Atmospheric Administration Richard A. Frank, Administrator National Weather Service Richard E. Hallgren, Director Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/comparisonofgeneOOried g . CONTENTS NU>\ i £& Page Abstract 1 1. Introduction 1 2. Definitions 1 3. Sources of PMP values 2 4. Sources of greatest observed rainfalls 5 5. Procedure 5 5.1 Introduction 5 5.2 U.S. east of the 105th meridian 6 5.3 U.S. west of the Continental Divide 6 6. Results 15 6.1 Comments 15 6.2 East of the 105th meridian 19 6.3 West of the Continental Divide 21 7. Magnitude of PMP: West of the Continental Divide vs. east of the 105th meridian 21 7.1 Comparisons of PMP with maximum observed rainfalls. ... 21 7.2 Comparisons of PMP with 100-yr rainfalls 22 8. Summary 23 Acknowledgments 24 References 25 TABLES 1. Generalized PMP studies used in comparisons 4 2. Storms with rainfalls >^ 50 percent of PMP (6 area sizes and 5 durations) U.S. east of the 105th meridian 7 3. Storms with rainfalls >_ 50 percent of PMP (3 area sizes and 3 durations) U.S. west of the Continental Divide 16 4. Number of storms per area size and duration with rainfalls >_ 50 percent of PMP 20 5. Number of storms with rainfalls exceeding various percent- ages of PMP (10 mi 2 , 6 and 24 hours) 22 ^^^ ILLUSTRATIONS Page Regions covered by generalized PMP studies in comparisons Chart No. 1 2, 3, 4. 5, 6. 7, 8. 9. 10, 11, 12. 13, 14. 15, 16. 17, 18, 19, 20, Observed point rainfalls U.S. east of the 105th meridian 21 50 percent of all-season PMP for 6 hr/10 mi 2 , Same as chart 1, for 12 hr/10 mi' Same as chart 1, for 24 hr/10 mi' Same as chart 1, for 48 hr/10 mi' Same as chart 1, for 72 hr/10 mi .2 Observed areal rainfalls U.S east of the 105th meridian _> 50 percent of all-season PMP for 6 hr/200 mi 2 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 Same as chart 6 for 12 hr/200 mi . for 24 hr/200 mi 2 . for 48 hr/200 mi 2 , for 72 hr/200 mi 2 , for 6 hr/1,000 mi 2 , for 12 hr/1,000 mi' for 24 hr/1,000 mi' for 48 hr/1,000 mi' for 72 hr/1,000 mi for 6 hr/5,000 mi 2 .2 for 12 hr/5,000 mi' .2 for 24 hr/5,000 mi for 48 hr/5,000 mi' for 72 hr/5,000 mi' 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 iv Page 2 Chart No. 21. Same as chart 6, for 6 hr/10,000 mi 47 22. Same as chart 6, for 12 hr/10,000 mi ...... 48 23. Same as chart 6, for 24 hr/10,000 mi , 49 2 24. Same as chart 6, for 48 hr/10,000 mi 50 25. Same as chart 6, for 72 hr/10,000 mi 51 26. Same as chart 6, for 6 hr/20,000 mi 2 52 27. Same as chart 6, for 12 hr/20,000 mi 2 53 28. Same as chart 6, for 24 hr/20,000 mi 2 54 29. Same as chart 6, for 48 hr/20,000 mi 55 2 30. Same as chart 6, for 72 hr/20,000 mi 56 31. Observed point rainfalls U.S. west of Continental Divide > 50 percent of all-season PMP for 6 hr/10 mi 2 57 2 32. Same as chart 31, for 24 hr/10 mi 58 33. Observed areal rainfalls U.S. west of Continental Divide _> 50 percent of all-season PMP for 24 hr/500 mi 2 59 34. Same as chart 33, for 48 hr/500 mi 2 60 2 35. Same as chart 33, for 24 hr/1000 mi 61 2 36. Same as chart 33, for 48 hr/1000 mi 62 37. Ratios of 10-mi 2 PMP (HMR No. 51) to 100-yr rainfalls (T.P No. 40) for 6 hours 63 38. Same as chart 37, for 24 hours 64 39. Ratios of 10-mi 2 PMP (HMR Nos. 36, 43, and 49) to 100-yr rainfalls (NOAA Atlas 2) for 6 hours 65 40. Same as chart 39, for 24 hours 66 V COMPARISONS OF GENERALIZED ESTIMATES OF PROBABLE MAXI PRECIPITATION WITH GREATEST OBSERVED RAINFALLS John T. Riedel and Louis C. Schreiner Hydrometeorological Branch Water Management Information Division Office of Hydrology National Weather Service, NOAA Silver Spring, Maryland ABSTRACT. This study summarizes known storms of record over the United States east of the 105th meridian and west of the Continental Divide that have point or areal rainfall depths that are >_ 50 percent of PMP. More than 240 storms met this criteria. The storms are identified and percentages of PMP are shown on maps. Some judgement on the relative magnitude of PMP in the two large regions is given by comparison of the ratios of PMP to 100-yr return period rainfall. Such ratios for 24 hours range from 4 to 6 east of the 105th meridian. For the western mountainous states, these ratios are as low as 2 in the more mountainous locations and as high as 6 in the desert and sheltered spots. 1. INTRODUCTION Studies by the Hydrometeorological Branch of the National Weather Service giving generalized estimates of probable maximum precipitation (PMP) have now been completed for the United States east of the 105th meridian and for the region west of the Continental Divide. These studies are used extensively by Federal, State and local government agencies, as well as private companies and individuals as a standard in planning and designing water control structures. The purpose of this report is to list and show on maps those storms of record that are within 50 percent of PMP. Additionally, we show ratios of point PMP values to values for the 100-year recurrence interval. 2. DEFINITIONS PMP is defined as "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 Meteorological Society 1959). Realizing there are yet unknowns in our understanding of the physical process responsible for extreme rainfall, we usually refer to the PMP values as estimates. Procedures for developing PMP estimates are not discussed in this study. These are given in detail in the referenced nydrometeorological studies and summarized in Operational Hydrology Report No. 1, "Manual for Estimation of Probable Maximum Precipitation,, " (World Meteorological Organization 197 3) . Generalized PMP estimates provide results for large regions and are presented on a series of maps or a combination of maps and computational procedures. Thus, the user can obtain PMP estimates for any basin within the range in area sizes and durations now required or expected to be required in the future. Other estimates are at times determined for specific drainages. These may be termed site specific PMP estimates. Botn local or thunderstorm PMP and general storm PMP were determined for the western states. These are both needed since the most intense station or point rainfalls of record in these states occur locally, not in connection with large scale weather patterns that produce rains over large areas for durations of a day or more*. This differs from storm experience in the United States east of the 105th meridian where extreme point rainfalls occur within general longer duration rain situations covering large areas. Locales torm PMP is developed from storms that cover areas less than 500 mi 'and have durations less than 6 hours. These are either thunderstorms or intense convective showers. Examples of this storm category are the 6.75-in. value in 1 hour at Morgan, Utah (8/16/1958), and the 8.25-in. value in 150 minutes at Chiatovich Flat, California (7/19/1955) , with rainfall covering an area less than 100 mi . ^General storm PMP is based on storms covering areas larger than 1000 mi and lasting a day or more. They are generally related to broadscale synoptic weather patterns, as exemplified by the September 3-5, 1970 tropical storm in Arizona and tne January 19-24, 1943 storm in California. In these storms, 24-hour point rainfall amounts were 11.4 and 25.8 inches, respec- tively, and rain amounts over an inch covered areas of several thousand square miles. All-season PMP is the greatest PMP regardless of season. As an example for large drainages, the all-season PMP is a late fall or winter event in California but a summer or early fall event in states bordering the Gulf of Mexico and Atlantic Ocean. 3. SOURCES OF PMP VALUES Figure 1 outlines four regions for which generalized PMP estimates are available and table 1 lists the PMP studies and some pertinent information. Other generalized PMP studies are available but were not used in the comparisons. These include studies for specific large drainages, such as those for the Susquehanna River drainage (Goodyear and Riedel 1965) and the Tennessee River drainage (Schwarz and Helfert 1969). Valid comparisons with PMP in these reports would require an individual PMP estimate for the exact location of the isohyet encompassing the desired *The area of Oregon and Washington West of the Cascade Divide is an exception. Here rainfall climatology shows that the most extreme point rainfalls have occurred in general storm situations. area size. Since the average values of PMP in these studies are equivalent to that from Hydrometeorologioal Report (HMR) No. e Maximum .'tation Estimates, United States East of the 105th Meridian," (Schreiner and Riedel 1978) , we consider such comparisons unnecessary for the present report. We have not included comparisons for the region between the Continental Divide and the 105th meridian even though a generalized study covers this region (U.S. Weather Bureau 1960). That study provides estimates for durations to only 24 hours and area sizes to 400 mi-. This is a more restrictive range than in the other studies used in this comparison. In addition, the 1960 study provided estimates for the entire United States west of the 105th meridian, using a degree of general- ization not comparable with that used in the other studies. Table 1. — Generalized PMP Studies Used in Comparisons Ilydrometeoro logical Report Geographic Bounds Scope No. 36 Pacific coast drainage General storm PMP areas up to 5000 mi ; (U.S. Weather Bureau of California 1961 Revision, U.S. 6 to 72 hrs. Each Weather Bureau 1969) month, October-April No. 4 3 Columbia River and General storm PMP, areas up to 5000 mi ; (U.S. Weather Bureau coastal drainages of 1966) Oregon and Washington 6 to 72 hrs. Each month, October-June Local storm PMP, east of Cascades Ridge, areas up to 500 mi ; durations to 6 hrs. Each month May- September No. 49 Colorado River and General storm PMP, areas up to 5000 mi ; 6 to (Hansen et. al 1977) Great Basin drainages (also all of California 72 hrs. Each of the for local storm PMP) 12 months Local storm PMP, areas up to 500 mi ; durations up to 6 hrs. All season No. 51 U.S. east of the 105th Areas from 10 to 20,000 (Schreiner and Riedel meridian mi ; 6 to 72 hrs. All 1978) season 4. SOURCES OF GREATEST OBSERVED RAINFALLS Surveys made after extreme storm and flood events usually uncover greater rainfall depths than those measured at stations that report regularly. This is so since there is little chance that the most intense (or near most in- tense) rainfall in a storm will occur over a preselected rain gage. Many of the greatest rain catches from postflood surveys are used in storm depth-area-duration studies. Results of these studies giving maximum, areal rainfall depths are included in a published catalog titled Storm Rainfall in the United States, Depth-Area-Duration Data (Corps of Engineers 1945- ) . Other accounts of extreme areal rainfall events studied in less detail have been added to a more inclusive catalog (Shipe and Riedel 1976) . While this latter publication covers depths for selected areas between 100 and 10,000 mi , maximum depths for smaller areas down to station values and for larger areas are available in the data file used to prepare this publication. This augmented catalog is a comprehensive source for known maximum areal rainfall depths that have occurred over the contiguous United States. Several studies of maximum station rainfalls for regular observing sta- tions have been published. Weather Bureau Technical Paper No. 15 3 "Maximum Station Precipitation for 1 3 2 3 3 3 6 3 12 3 and 24 hours 3 " (U.S. Weather Bureau 1951-61) shows greatest depths for 1-, 2-, 3-, 6-, 12-, and 24-hr durations on a monthly basis for the period from 1940 to about 1950 for all regularly published recording gage stations in 33 states. U.S. Weather Bureau Technical Paper No. 2 3 "Maximum Recorded United States Point Rainfall for 5 Minutes to 24 Hours at 207 First-Order Stations, " (Jennings revised 1963) gives the greatest recorded depths through 1961 for various durations from 5 minutes to 24 hours at 296 first-order Weather Bureau stations. U.S Weather Bureau Technical Paper No. 16 3 "Maximum 24-Hour Precipitation in the United States 3 " (Jennings 1952) gives the greatest 24-hr or 1-day value of record through 1950 for each month for the regular reporting stations. This last report has been updated for the present comparisons through 1973 for all states. 5. PROCEDURE 5.1 Introduction Our procedure was restricted to comparing large observed values with the all-season PMP. An alternative, would be to compare storms with the PMP for the month of the storm. We chose also to compare 10 mi PMP with maximum station values, where these are available. In some cases, station values were not available and average depths over 10 mi were used. For many earlier storms, there were insufficient data to distinguish between the two. Therefore, in Storm Rainfall in the United States (Corps of Engineers 1945- ) , station depths are used sometimes as 10-mi depths. 5.2 U.S. East of the 105th Meridian Generalized all-season PMP estimates for this region are given iv\ HMR No. 51 in map form for 10, 200, 1,000, 5,000, 10,000, and 20,000 mi for durations of 6, 12, 24, 48, and 72 hours (30 maps). For each of these area size and duration combinations we have found all known storm depths that are 50 percent or more of PMP. These storm depths, in percent of PMP, are plotted in place of occurrence on charts 1 through 30. Table 2 lists these storms chronologically with index letters for identification on maps, a Corps of Engineers assignment number, if applicable from Storm Rainfall in the United States (Corps of Engineers 1945- ) , location of the storm center (by town, state and latitude/longitude), and chart numbers on which each storm appears. As an example, the 6/9-10/1905 storm, index AZ, Assignment No. UMV 2-5, centered near Bonaparte, Iowa, 40°42 'N latitude and 91°48'W longitude, has observed values ^_ 50 percent of PMP for: 6 hr/1,000 mi! 12 hr/1,000 mi 24 hr/1,000 mi 2' 2' 6 hr/5,000 mi , 12 hr/5,000 mi 24 hr/5,000 mi 2' chart 11 (56%) 6 hr/10,000 mi mi 2 mi mi 2 mi chart 12 (57%) 12 hr/10,000 chart 13 (52%) 24 hr/10,000 chart 16 (67%) 6 hr/2 0,000 chart 17 (63%) 12 hr/20,000 chart 18 (54%) chart 21 (66%) chart 22 (61%) chart 23 (52%) chart 26 (61%) chart 27 (54%) A total of 177 separate storms are listed in table 2. Major rain centers, separated by more than 200 miles were listed as separate storms even if they occurred on the same date. In some cases depth-area-duration data from Storm Rainfall in the United States (Corps of Engineers 1945- ) are given separately for different storm centers as well as for the entire storm area covered by individual centers. When this occurred, values for the different storm centers were compared if the centers were more than 200 miles apart. If the centers were closer together, comparisons were made for only the storm center giving the greatest rainfall depth. The areal rainfall depths were compared with the PMP at the location of the maximum observed point rainfalls. This approximation avoided deter- mining the actual location of the maximum depth, and determining the aver- age PMP for possibly an odd-shaped isohyet. Generally, if the location of the maximum areal depth is farther south than the maximum point depth, the true percentage of PMP is less than shown; on the other hand, if the loca- tion is farther north, the true percentage is greater than that shown on the charts. Except for unusually shaped isohyets these differences would be only a few percent. 5.3 U.S. West of the Continental Divide Topographic influences in the western states make it difficult to prepare simple mapped values of PMP as is done for the region east of the 105th meridian*. 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For example, say for a 1,000-mi maximum depth, the exact location of the isohyet encompassing this 1,000-mi area would need to be determined and the PMP computed for this location. The simplifying assumption (used east of the 105th meridian) that the center of this 1,000-mi area would coincide with that of the maximum point rainfall cannot be used. We therefore have compared in detail only generalized all-season PMP esti- mates for 10-mi for 6- and 24-hr durations with maximum observed station depths for these durations. For 10-mi the limitations on computing PMP are not as great. Generalized all-season PMP maps for these durations were based on PMP computations for each month specified in each of the three reports, (see table 1) on a quarter degree latitude-longitude grid for both the general storm and local storm. The greatest all-season value from the two storm types was then selected for each grid point. The local storm 10-mi PMP for 6 hours exceeds the 6-hr general storm 10-mi PMP over much of the western states. In fact, it exceeds the 24-hr 10-mi general storm values for some regions, and is therefore used for those cases in the comparisons with 24-hr observed rainfalls. We have made less detailed comparisons of maximum observed areal depths with PMP for 500 and 1000 mi for both 24 and 48 hours. For the most part, these comparisons used the limited storm samplings provided in Hy drome teoro- logioal Reports No. 36, "Interim Report — Probable Maximum Precipitation in California^ " No. 43, "Probable Maximum Precipitation, Northwest States, " and No. 49, "Probable Maximum Precipitation Estimates , Colorado River and Great Basin Drainages . " For each of the comparisons we estimated the location covered by the maximum depth over 500 and 1000 mi and computed PMP for each month for that location. The highest PMP or all-season value was then used. Charts 31 through 36 of the United States west of the Continental Divide show observed storm rainfalls* that are 50 percent or more of PMP. Table 3 lists the storms chronologically as in table 2. There are 66 separate storm events for these six combinations of area sizes and durations. Major rain centers separated by more than 200 miles, although with the same storm date, are listed as separate events. 6. RESULTS 6.1 Comments 2 Chart 1 for 6 hours, 10 mi shows two storms with percentages greater than 100 implying observed values greater than PMP. These are: a) The Smethport, Pa. storm of July 17-18, 1942 (observed 30.8 + inches in 4-1/2 hours — a world record for this duration) ; *The maximum station values for four storms are from isohyetal maps obtained by expressing observed storm depths in percent of mean annual precipitation and then through isopercental analyses obtaining a greater point depth. 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I (M 1 «tf rH CD W CN \ H ro \ cn CN H 1 W CO r^- 1 CM 1 \ rH rH rH i 00 1 1 H 1 1 \ O ro CM rH rH O rH o ifl \ CN 00 o 1 cn ■vD ro cn cn cm CM rH CM w rH \ ro OJ cn \ \ CN ^ \ CN \ H rH CN \ \ \ rH CN \ rH W \ rH CM \ \ CM \ o \ \ \ CN (T> rH rH rH CN rH ro OJ V0 rH H rH CM rH CN H H rH CN 18 0) 4J Cm c X CD O £ vO ro ^ in fO ^ ro LD ro * ^ ro m m m »x> rn * ^ ^ CN (N ro vD CN ro ro CN ro CN \0 ID ro ro m m CN m H ro H m CM CN (N ro H ro (N ro •^r h ro n <* m H m ro ro ro ro cm ^r rn rn CN H m ro ro n N cr cr cr ■ — i • — i -^r in H H ^ \ \ o\ cy> oo rH CN H \ I r~ r- i CN CN H i CT \ CN O H \ O rH ro O o o ( GO "vT H CN H H O CN ro CN ro h m io CN H O CN CO H r» -^ ko O 'd' ro ro ro ro (< < N < < U U n ■p >1 H H Pm > •H CO * X O in M-l c0 rO x: u ■rH > O XI U 3 x: -p co U cr> < PQ id n o in CT o in m o ro I \ m cr CN H CN CO <3* O 00 CN ro CN ro iH cn CN "3" H ro CN CN CN rH ro CTl O CN CO O ro CN ^ rH o r~ cn co <3< "^ ro ^ ^ < < < D C_> 3 U M U U x: CO 3 PQ 2 C rd 3 ro CN CN cjaw CmOkhid n n n onnniD CN ID cn CTl H cr in \ in CTl ro CT H H I oo r- h \ o 00 CN H CO I CN CN ro rr 1X> H h cn o m ro H CT CT O CN CN O O H 00 CN CT 00 CTl ro ^ ^ ro T U CQ cq ro CM I CN Cm CO ^ tJ S 2 O >x> CT CT ro CO CT CN *£> 1 rH (0 c rfl (0 O !H CU & ■H % 0) cn 3 S •H X cu r0 CD S CO * 19 b) The Cherry Creek, Colo., storm of May 30-31, 1935 (observed 24 inches in less than 6 hours) . 2 For both of these storms there are sufficient data to define 10-mi aver- age observed rainfalls distinct from the point values. The 6-hr 10-mi value is 24.7 inches for storm (a) and 20.6 inches for storm (b) . These 10 mi average 6-hr depths are 97 percent and 89 percent of 6-hr 10-mi PMP, respectively. The apparent greater than 100 percent of PMP values extend into the 12-, 24-, and 48-hr durations (charts 2, 3, and 4) in the case of the Smethport storm. Here, if 10 mi average values are compared, the percentages of PMP are 93, 94, and 86, respectively. For many individual storm events, numerous recorded station rainfalls are 50 percent or more of PMP. We have listed only the comparison of PMP with the one greatest observed value in each storm. This is particularly impor- tant for the western states. With sharp gradients in PMP, there is a strong probability of higher percentages of PMP for some of the lesser observed values in a storm. An example is the January 19-24, 1943 storm in southern California. The maximum observed point at Hoegees Camp, California, (24-hr value) is 25.8 inches (PMP=34.1 inches) which gives 76 percent of PMP. In the same storm 20.3 inches was observed at Mount Wilson Airway station which is located only a few miles west of Hoegees Camp. This 20.3-inch amount is 82 percent of PMP at the location where it occurred. A detailed time- consuming search for such higher percents in all storms would (a) uncover many more storms within 50 percent of PMP and (b) raise the percents, especially for those given on charts 31 and 32 for the western states. 6.2 East of the 105th Meridian Table 4A gives the number of storms that are 50 percent or more of PMP for each combination of area size and duration. In general, an increase in this count with increasing area size is noted. This is contrary to what one might expect if all other factors were equal since we have studied fewer rain storms with maximum average depths for 20,000 mi than for 10 mi . One factor that is not equal is that for small areas a few extreme point values, when considered over the region within which they could occur, i.e., transposed, are so much greater than most other storms. This reduces the storm count, as shown in table 4A for small areas. The larger the area, the less the effect of the point extreme; that is, differences between large area rainfalls and PMP are less. We also note in many cases an increasing count of storms with increasing duration. The same reason is given for this as for the increase with area size. In all except two instances (east of the 105th meridian) , the station, or 10 mi cases >50 percent of PMP came either from Storm Rainfall tn the United States (Corps of Engineers 1945- ) or Greatest Known Areal Storm Rainfall Depths for the Contiguous United States (Shipe and Riedel 1976) . Many of the rainfall analyses of the largest storms in these publications e.g., Cherry Creek, Colorado, May 30-31, 1935; Smethport, Pennsylvania, July 17-18-, 1942; Rapid City, South Dakota, June 9, 1972; Enid, Oklahoma, October 10-11, 1973; Kansas City, September 11-13, 1977; etc., are based on post storm surveys made to determine maximum rainfall amounts. This 20 c O /-N •H W 4-1 >-i OCNvJOOCNvOCN-d-OOCNvOCN^-OOCN Cd J2 H N <■ l~» H M ^ I s H N ■* N U ^~ ' 3 Q 00 a Ph u • Ph Q> cd o E £. a ^ O 4^> rC •^ +S a LO Q k f~) o •ri U ^g Cd X K • J-l w 9 <0 3 Q -l S ca < >-" cu CO o +S CO 4-4 O § fOo>OrHi-Hr^.m^->3-vDo>cn vo i-* oocriOiHcNon^-mvor^oooo r-Hi-ICNCNCNCNCNCNCNCNCNCNcn vOCNId-00CN\OCN (N cm ro cn (N rn n OCTiin^OrOrHO H(Sfi>*iri^oi^coaiOHMco^in r^ r^ s s 50 percent of PMP. Of these, the maximum values for at least 20 storms were recorded at regular reporting stations — that is, surveys which might have uncovered yet greater rainfalls were not made after the storm events. Because of sparse habitation in the Western States, there remains the possibility that greater values would not have been found, although they likely occurred. Probably few surveys have been made after storm events in the Western States because of the small likelihood that additional larger catches would be discovered. The distribution of storms shows that a large portion of the 66 events occurred either in California or in the coastal region of Washington and Oregon. We expect observed storms to approach PMP more closely in wet regions like the Sierra Nevadas than in dry regions. 7. MAGNITUDE OF PMP: WEST OF CONTINENTAL DIVIDE VS. EAST OF 105TH MERIDIAN 7.1 Comparisons of PMP with Maximum Observed Rainfalls A question is whether PMP estimates west of the Continental Divide (West) are comparable to those east of the 105th meridian (East) i.e., do the values represent the same degree of "conservatism." All other factors equal one should expect more values >^ 50 percent of PMP for the East because we have more observations there and the region is larger. That is, in the East there are currently about 6500 precipitation reporting stations in a region of almost 2 million square miles, while in the West there are only about 2100 stations in a region of about 800,000 square miles. We have also studied a much larger number of storms in the East, 673, while in the West we have studied only 139. Despite this imbalance of data, examination of table A shows 77 10 mi cases for the 6- and 24-hour durations combined that are _> 50 percent of PMP for the region west of the Continental Divide while for the East there are only 59 such cases. We believe this is due to the few most extreme values for the East that reduce the number of cases : 50 percent when these few are transposed (a point already discussed) . Some indication of this is found in the data. A count of the number of cases >_ 50-, >60-, >70-, >80-, and >90-percent of PMP for 10 mi for 6 and 24 hours, 22 East and West is shown in table 5. The total number of cases _> 50 percent of PMP is higher for the West, but when the criterion is >70 percent of PMP, there is a higher count for the East. In summary, obstacles to comparing the frequency of storm rainfalls >_ 50 percent of PMP in the East to those in the West, include difference in a) the number of storms analyzed, b) the number of post storm surveys, and c) the number and variety of record storms . Table 5. — Number of Storm Rainfall Cases Exceeding Various Percentages of PMP (10 mi . 6 and 24 hours). >^50% _>60% >70% >80% >90% East of 105th 59 32 19 7 3 meridian West of Conti- 77 39 13 4 nental Divide 7.2 Comparisons of PMP With 100-yr Rainfalls Some judgement on PMP in the East compared to that in the West can be made from examining ratios of PMP to 100-yr rainfalls. Charts 37 and 38 show ratios of 6- and 24-hr PMP (Schreiner and Riedel 1978) to 100-yr rainfall (Hershfield 1961) for 6 and 24 hours, respectively, east of the 105th meridian. The ratios for 6 hours vary from about 4 near the gulf coast to about 7 in the Great Lakes region. For 24 hours the range is about 4 to 6. Now let's look at the ratios west of the Continental Divide for 6 hours (chart 39) and 24 hours (chart 40) . The 100-yr values come from NOAA Atlas 2 (Miller et al. 1973) . Both maps show a greater variation in ratios from place-to-place than for east of the 105th meridian. This was to be expected. Mountain masses have a large effect in regional variation of rainfall magnitudes. We would expect, as shown, that for regions where there are frequent large rains occurring at or nearly at the same place because of orographic influences (e.g., Sierra slopes), the storm depths would more closely approach PMP (lower ratio of PMP to 100-yr). Similarly, highest ratios generally occur as shown in locations where heavy rainfalls are infrequent because of sheltering or distance from the moisture source (e.g., central valley of California or the Snake River Valley). Charts 37 and 38 show a similar trend to lower ratios in the eastern Appalachian Mountains and lowest ratios near the Gulf where the storm experience is greatest. 23 In the west for 6 hours (chart 39), the ratios vary from 2 to 8. For 24 hours (chart 40), the ratios vary from 2 to 6. In general, the western states show a wider range in ratios. The authors of NOAA Atlas 2, however, (personal communication) believe that if rainfall frequency studies were now made for the eastern mountainous regions with the attention to orographic factors used in NOAA Atlas 2, results would be less smooth with more centers of high and low values. If so, this could result in a greater range of PMP to 100-yr ratios. 8. SUMMARY This report provides some perspective to the user on the relation between PMP and maximum observed rainfalls. In the east, of 675 storms studied, 177 (about one-fourth) had a rainfall depth (for at least one of the standard area sizes and durations considered) that was >_ 50 percent of PMP. This comparison is from documented records of storm rainfalls that extend over approximately 100 years. These storm rainfall amounts are well distributed over the range of durations and area sizes. From the length of record and over this large region, we have a few storms, about 1 percent, that are within 20 percent of PMP. For the region west of the Continental Divide, the comparisons are more difficult. Our data sample is smaller and because of maps of PMP for a given area size and duration cannot be readily prepared, detailed comparisons analogous to those in the East cannot be made. In spite of a smaller storm sample in the West, we found there are more cases >_ 50 percent of PMP for 10 mi areas (6 and 24 hours) than in the East. This is due to the much fewer post storm surveys in the West. The many more eastern post storm surveys have resulted in a relatively few very extreme observed point rainfalls which, when considered over the region where they could have occurred, set the magnitude of PMP that exceeds by quite a margin the remaining maxima. The comparisons of PMP values with the values for the 100-yr occurrence interval indicate a rough comparability in PMP between the East and the West. It is, however, not possible to assign a recurrence interval to PMP, nor even to assume that locations, where the ratio of PMP to 100-year values are the same, have the same recurrence level. The PMP to 100-year ratios give general guidance to approximate PMP magnitudes. In general, they should be low in regions of frequent heavy rains and high where large amounts are uncommon. These trends are seen in charts 37 to 40. The Cascade-Sierra slopes, the Appalachians, Gulf of Mexico coastal region, etc., have the lowest ratios. The highest ratios are in the central valley of California, Snake River Valley, North Dakota, etc. The relative values examined in this study are, therefore, about what would be expected. 24 ACKNOWLEDGMENTS Our thanks to John F. Miller, Chief, Water Management Information Division and Vance A. Myers, Chief, Special Studies Branch for their reviews and helpful suggestions and to Roger Watkins for his assistance. We are indebted to Teresa Johnson, Roxanne Johnson, and Marion Choate for help in computations, plotting data and drafting our figures. We appreciate the typing of text and tables by Kathryn Carey, Virginia Hostler, and Clara Brown, 25 REFERENCES American Meteorological Society, 1959: Glossary of Meteorology . Boston, Mass. , 638 pp. Goodyear, H.V., and Riedel, J.T., 1965: Probable Maximum Precipitation, Susquehanna River Drainage Above Harrisburg, Pa. Hydrometeorologioal Report No. 40, U.S. Weather Bureau, Department of Commerce, Washington, D.C. , 70 pp. Hansen, E.M., Schwarz, F.K., and Riedel, J.T., 1977: Probable Maximum Precip- itation Estimates, Colorado River and Great Basin Drainages. Hydrometeoro- logioal Report No. 49, National Weather Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Silver Spring, Md., 161 pp. Hershfield, D.M. , 1961: Rainfall Frequency Atlas of the United States for Durations from 20 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. Jennings, A.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 Rainfall 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. Miller, J.F., Frederick, R.H., and Tracey, R.J., 1973: Precipitation-Fre- quency Atlas of the Western United States, Vol. 1: Montana, Vol. II: Wyoming, Vol. Ill: Colorado, Vol. IV: New Mexico, Vol. V: Idaho, Vol. VI: Utah, Vol. VII: Nevada, Vol. VIII: Arizona, Vol. IX: Washington, Vol. X: Oregon, Vol. XI: California. NOAA Atlas 2, National Weather Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Silver Spring, Md. Schreiner, L.C., and Riedel, J.T., 1978: Probable Maximum Precipitation Estimates, United States East of the 105th Meridian. Ey drome teorological Report No. 51, National Weather Service, National Oceanic and Atmospheric Administration, Silver Spring, Md. . 87 pp. Schwarz, F.K., and Helfert, N.F., 1969: Probable Maximum and TVA Precipita- tion for Tennessee River Basins up to 3,000 Square Miles in Area and Dura- tions to 72 Hours. Hydrometeorologioal Report No. 45, Weather Bureau, Environmental Science Services Administration, U.S. Department of Commerce, Silver Spring, Md., 166 pp. 26 Lpe, A. P., and Riedel, J.T., 1976: Greatest Known Areal Storm Rainfall Depths for the Contiguous United States. NOAA Technical Memorandum NWS HYDRO-33, National Weather Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Silver Spring, Md., 174 pp. U.S. Army, Corps of Engineers, 1945- : Storm Rainfall in the United States, Depth-Area-Duration Data. Washington, D.C. (ongoing publication). l.S. Weather Bureau, 1951-61: Maximum Station Precipitation for 1,2,3,6,12 and 24 hours, Part I: Utah, Part II: Idaho, Part III: Florida, Part IV: Maryland, Delaware and District of Columbia, Part V: New Jersey, Part VI: New England, Part VII: South Carolina, Part VIII: Virginia, Part IX: Georgia, Part X: New York, Part XI: North Carolina, Part XL1: Oregon, Part XIII: Kentucky, Part XIV: Louisiana, Part XV: Alabama, Part XVI: Pennsylvania, Part XVII: Mississippi, Part XVIII: West Virginia, Part XIX: Tennessee, Part XX: Indiana, Part XXI: Illinois, Part XXII: Ohio, Part XXIII: California, Part XXIV: Texas, Part XXV: Arkansas, Part XXVI: Oklahoma. Technical Paper No. 15., Department of Commerce, Washington, D.C. U.S. Weather Bureau 1960: Generalized Estimates of Probable Maximum Precipitation for the United States West of the 105th Meridian. Technical Paper No. 38, Department of Commerce, Washington, D.C, 66 pp. U.S. Weather Bureau, 1961: Interim Report Probable Maximum Precipitation in California. Hydro-meteorological Report No. 43, Environmental Science Services Administration, Department of Commerce, Washington, D.C, 228 pp. World Meteorological Organization, 1973: Manual for Estimation of Probable Maximum Precipitation. WMO No. 332, Operational Hydrology Report No. 1, Geneva, Switzerland, 190 pp. 27 107 103 Chart No. 1.— Observed point rainfalls U.S. east of the 105th meridian > 50 percent of all-season PMP for 6 hr/10 mi 2 . 28 Chart No. 2. — Same as chart 1, for 12 hr/10 m% 29 Chart No. 3. — same as ohart 1 } for 24 hr/10 mi . 30 107' 103° 99' 95* 91* 87* 83 79 Chart No. 4. -Same as chart 1 3 for 48 hr/10 mi . 31 100 100 260 300 400 KILOMETERS Chart No. 5. — Same as chart 1, for 72 hr/10 rm 32 STATUTE MIIES 100 200 30 100 200 300 <00 KILOMETERS 75* Chart No. 6.— Observed areal rainfalls U.S. east of the 105th meridian >_ 50 percent of all-season PMP for 6 hr/200 mi . 33 STATUTE MILES 100 100 200 300 1 i I i ' I i 1 1— ' 100 100 200 300 400 KILOMETERS Chart No. 7.— Same as chart 6 3 for 12 hr/200 mi' 34 STATUTE MILES 100 Q 100 200 30 00 100 200 300 400 KIIOMETERS Chart No. 8.— Same as chart 6, for 24 hr/200 mi 2 . 35 95 91 Chart No. 9.— Same as chart 6 3 for 48 hr/200 mi . 36 STATUTE MILES 100 200 30 00 260 300 400 Chart No. 10.— Same as chart 6 } for 72 hr/200 mi 2 . 37 Chart No. 11.— Same as chart 6 3 for 6 hr/l s OOO mi 1 38 107 103 STATUTE MILES 00 200 30 100 200 300 400 KILOMETERS Chart No. 12.— Same as chart 6 3 for 12 hr>/l 3 000 mi . 39 Chart No. 13.— Same as chart 6 } for 24 hr/1,000 mi' 40 Chart No. 14.— Same as chart 6 3 for 48 hr/l s 000 mi . 41 107 103 99 95 300 400 KILOMETERS Chart No. 15.— Same as chart 6 3 for 72 hr/l 3 000 mV 42 Chart No. 16.— Same as chart 6, for 6 hr/5 3 000 mi . 43 Chart No. 17.— Same as chart 6 3 for 12 hr/5 3 000 mi .2 44 .19 5 # 5 BHH 54 ^ r L 3 % • 56 45 ; co EM AP • FP 54 \ CK*J 55 52.^aS AJ 41 ° _. \ 57 54 AU AZ 52 f: 107' 103* -i r 99* >" 37" 5 . 3 BQ 5 J GC ^54 OQ 70 5 . 3 & 8 3 G> soft 57 & 55V W' 33° 51 ES ■-u 57 DT 52 1 1 - -•- ow ' DGV / !50cV«r-rr _. ^eTTn:* EQ*63 D * A C*D AQ 200 300 100 100 200 300 400 KILOMETERS 103 Chart No. 18.— Same as chart 6, for 24 hr/5,000 mi 1 45 Chart No. 19.— Same as chart 6, for 48 hr/5 } 000 mi . 46 Chart No. 20.— Same as chart 6 S for 72 hr/5 3 000 mi . 47 Chart No. 21.— Same as chart 6 } for 6 hr/10 3 000 mi . 48 Chart No. 22.— Same as chart 6, for 12 hr/10 3 000 mi' 49 100 200 300 i 1 r-4 .J |— < 00 100 200 300 400 Chart No. 23.— Same as chart 6, for 24 hr/10 3 000 mi' 50 107 103 67" 6? CO 45' 41° 8Q I J__ 33 G * <37° 50 i 5.3 FM 5 5 i FC 5 .52 FD 6J CVJ ficc !Jm 5.5 69h BGES •33' FA 5.1 t)G & S^^i 3 1 BU i ri - : i -1 • 59 1 D 1 CD . |> EV, tKX^ « 29° -JF.Q STATUTE MILES 1 00 Q 100 200 30 100 100 200 300 400 KILOMETERS 25' 83" 79° 75" Chart No. 24.— Same as chart 6 3 for 48 hr/10,000 mi .' 51 Chart No. 25.— Same as chart 6 } for 72 hr/10 3 000 mi . 52 -33" STATUTE MIIES 1 00 Q 100 200 30 100 100 200 300 400 KILOMETERS Chart No. 26.— Same as chart 6 S for 6 hr/20 J 000 mi .2 53 107 103 99 95 STATUTE MILES 100 200 300 —i — '-r — >*— i — ' 100 200 300 400 KILOMETERS Chart No. 27.— Same as chart 6 } for 12 hr/20 3 000 mi c 54 Chart No. 28.— Same as chart 6 3 for 24 hv/20 3 000 mi ' . 55 300 400 Chart No, 29.— Same as chart 6 3 for 48 hr/20 3 000 mi 1 56 100 200 300 — — 100 100 200 300 400 KILOMETERS Chart No. 30.— Same as chart 6 3 for 72 hr/20,000 mi .2 57 45* Chart No. 31. — Observed point rainfalls U.S. west of the Continental r2 Divide _> 50 percent of all-season PMP for 6 hr/10 mi . 58 Chart No. 22.— Same as chart 11, for 24 hr/10 mi .: 59 Chart No. 33.— Observed areal rainfalls U.S. west of Continental Divide > 50 percent of all-season PMP for 24 hr/500 m^ . 60 Chart No. 34.— Same as chart 33, for 48 hr/500 mi' 61 127 123 Chart No. 35.— Same as chart 33 s for 24 hr/1000 mV 62 127* 123* 119* 115* HI 107 as / / Chart No. 36.— Same as chart 33, for 48 hr/1000 mi' 63 107° 103° 99° 95* 91* STATUTE MILES 100 200 -l 1 l ' i H 1 — 100 100 200 300 400 KILOMETERS Chart No. 37.— Ratios of 10 rrri 2 PMP (HMR No. 51) to 100 yr rainfalls (T.P. No. 40) for 6 hours. 64 Chart No. 38. — Same as chart Z7 3 for 24 hours. 65 NENTAL DIVIDE 40- O Chart No. 39.— Ratios of 10 mi PMP (HMR Nos. 36 } 4Z 3 and 49) to 100-yr rainfalls (NOAA Atlas 2) for 6 hours. 66 -A:\fr^ CONTINENTAL DIVIDE Chart No. 40. — Same as chart Z9 3 for 24 hours. •CtU.S. GOVERNMENT PRINTING OFFICE: I 980-3 1 1 " 04 6 / 6 3 (Continued from inside front cover) NWS 16 Storm Tide Frequencies on the South Carolina Coast. Vance A. Myers, June 1975, 79 p. (COM-75- 11335) NWS 17 Estimation of Hurricane Storm Surge in Apalachicola Bay, Florida. James E. Overland, June 1975. 66 p. (COM-75-11332) NWS 18 Joint Probability Method of Tide Frequency Analysis Applied to Apalachicola Bay and St. George Sound, Florida. Francis P. Ho and Vance A. Myers, November 1975, 43 p. (PB-251123) NWS 19 A Point Energy and Mass Balance Model of a Snow Cover. Eric A. Anderson, February 1976, 150 p. (PB-254653) NWS 20 Precipitable Water Over the United States, Volume 1: Monthly Means. George A. Lott, November 1976, 173 p. (PB-264219) NWS 20 Precipitable Water Over the United States, Volume II: Semimonthly Maxima. Francis P. Ho and John T. Riedel, July 1979, 359 p. (PB-300870) NWS 21 Interduration Precipitation Relations for Storms - Southeast States. Ralph H. Frederick, March 1979, 66 p. (PB-297192) NWS 22 The Nested Grid Model. Norman A. Phillips, April 1979, 89 p. (PB-299046) NWS 23 Meteorological Criteria for Standard Project Hurricane and Probable Maximum Hurricane and Probable Maximum Hurricane Windflelds, Gulf and East Coasts of the United States. Richard W. Schwerdt, Francis P. Ho, and Roger R. Watkins, September 1979, 348 p. (PB-80 117997) NWS 24 A Methodology for Point-to-Area Rainfall Frequency Ratios. Vance A. Myers and Raymond M. Zehr, February 1980, 180 p. NOAA SCIENTIFIC AND TECHNICAL PUBLICATIONS The National Oceanic and Atmospheric Administration was established as part of the Department Commerce on October 3, 1970. 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