HEAVY WARM-SECTOR RAINS FROM ILLINOIS TO MIDDLE ATLANTIC COAST, MAY 26–28, 1956

Abstract

During the weekend of May 26-28, 1956, a large, cold, high pressure system moved slowly off the east coast of the United States followed by a broad current of moist tropical air from the Gulf of Mexico. As this moist current overran the retreating cold air mass, widespread warm-frontal-type rains spread over the northeastern United States. The warm front was poorly defined at the surface, with a broad zone of gradual transition. Aloft the front was somewhat more distinct but still quite broad. To the rear of this frontal zone the warm sector air mass extended uninterruptedly to the Gulf of Mexico. The synoptic situation on May 27 is presented in figure 1A. It was in the extensive belt of mT air that the rains described in this paper occurred. The rains began in Illinois early Saturday morning of May 26 and spread eastward with the warm front to the coast by Sunday morning. Surprisingly enough, however, they continued throughout a broad strip from central Illinois eastward all day Saturday, Sunday, and part of Monday. The rain over most of this area ceased when a southwardmoving cold front brought dry air and subsiding currents to the region. East of the Appalachian Mountains the warm-sector rains were accompanied by a deepening trough at the surface and aloft. Here the rains began when the flow at the 700-mb. level became cyclonic and ended when the warm-sector flow lost its cyclonic curvature after passage of the 700-mb. trough. The principles relating surface precipitation to cyclonic curvature aloft [l] have long been recognized so the rains occurring in this part of the country were not unusual. Development of heavy rains which fell farther to the west in the warm sector were not so easily explained. The upward motion in the warm sector must have been quite widespread to produce such a large area of precipitation and therefore should be subject to detection or computation by several of the methods which have been developed for the study of vertical motions. The cause and distribution of these vertical motions will be investigated in this paper by examination of each of the following types of charts: 1. precipitable moisture, 2. Showalter stability index, 3. differential advection, 4. surface isobaric convergence, 5. sea level pressure change (Laplacian of), 6. curvature of flow pattern at 700 mb. and higher, 7. jet stream, 8. tropopause, and 9. JNWP vertical motion computations (900400 mb.).