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24 CONTRIBUTIONS TO HYDROLOGY, 1937 RAINFALL Causes and intensity The immediate cause or the flood wae a rainfall of cloudburst inten- sity 1n eaatern Colorado and western Nebraska during the night of May 30-31. During two periods earlier 1n the month ,general rainfall had oc- curred over the Republiopn-Kansae River Basin. In the ttrot period, May 12-22, trom 2 to 6 inchos of rain tell, and 1n the second period, May 26-29, there was 1 inch or les9 over the area 1n northeastern Colorado, 3 inches or more 1n southwestern Nebraska, and 6 tnchea at 80me placos in eastern Kansas. As a roault of these two periods of rainfall the rivers reached fairly high stages a few days before the time of the major flood, and the 8011 was 1n condition to afford a high peroentage ot run-oft tram the storm at May 30-31. In Nebraska and Kansas the flood was augmented by heavy rains that occurred during the next 2 days. (See pl. 10.) The daily weather maps for the period from Kay 28 to June 3 show that an area of high pressure (more than 30.1 inches) appeared over the prov- Lnoe of Alberta on Way 28. It gradually worked eastward and by Kay 31 was located over the Great Lakes, and the pressure had increased to 30.2 inches. On May 28 an area of low pressure (29.1 inches) wae present over Arizona, and by May 30 it had moved over New Mexioo, with a pressure of 29.65 inches. On the evening of Yay 30 the low area was over southwestern Colorado, and its trend was northeastward, toward the high-pressure area. As the circulation of air around a low-pressure area is counterclockwise. the air currents on the south side had an easterly and northerly direction, bringing in warm moisture-laden air from the Gulf reglon. At the same time the direction of the air currents from the south side of the high-pressure area was westerly and southerly, carrying moisture from the Lake region. There were thus two bodies of molsture.laden air moving in opposite di- rections, and at their line of contact heavy preoipitation oocurred, which on the night of May 30 was concentrated in eastern Colorado and western Nebraska. Experience has shown that the relative positions of the high- pressure and low-pressure areas at that time are those which are most tavorable for heavy preoipitation in southeastern Colorado. In Nebraska heavy rains that fell around Hayes Center and other points in the Republican River Basin added their quota to the run-off, which moved down the valley so rapidly that the successive rieee in river stages so noticeable in western Nebraska were obliterated before the flood reaohed FLOOD ON REPUBLICAN AND KANSAS RIVERS, KAY .I.ND .TUNE: 1936 26 the Kaneas-Nebraska State line about mldnight on June 1. Raine in Kansas .ere heavy enough to create local floods on many rivere. These floods oc- curred slightly in advance of the arrlval of the flood in the Republican Rlver Baain and eerved to rob the onruaning waters from Nebraska of the channel storage and disoharge capaoity of the river. Area of heavy rainfall The etorm that caueed the heavy rainfall In the Republican Rlver Basin during the night of Yay 30-31 began just east of the ~ountains in the fore- Doon of Kay 30, and in that area local reeidents meaBUred, chlefly during the afternoon, as much a8 20 inchee (inoluding some hail) in stock tanks. This storm followed a general northeasterly direction aoross the head.aters of the Republican River and ended a few miles east of Curtis, Nebr., on May 31. The air-llne distance from the head of the Republioan Rlver In northeastern Colorado to Curtis, is 215 1n11es. Within th1e area the rain- fall was ooncentrated chiefly in the basin of the South Fork of the Re- publioan River but extended along the low ridge dividlng that baain from the basin of the Arikaree River nearly to Benkellllan, Nebr. Outside this ooncentrated area there were undoubtedly small areas of intensive raln- fall, as shown by the reoord of 11 inches In S80. 24, T. 6 S., H. 55 W., but only 1 inch 2 miles farther south. The aooompanying isohyetal map (tig. 6), shOll'S the distribution of rainfall from the oloudburst storm ot 11&'1 30-:51, as plotted from data obtained by local residents. In the Yioinity of McCook, Nebr., a tornado appeared on the afternoon of May 31 at the time or the flood crest, and a strong east .ind brought olouds of dust to that area, Unfortunately, the areao of heaviost rainfall contained no Weather Bureau preoipitation etations, and it was therefore necessary, in deter. mning the looation and approxlmate amount at rainfall, to obtain from local residents suoh information as could be fUrnlshed by those who had measured the rainfall in improvlsed rain gages. Reports of such measure- mente in northeaetern Colorado, eouthwestern Nebraska, and north.estern t~sas, together with data reported by the Weather Bureau from ltB sta- tione in those areas, are listed in the following tablel