The President's Day Cyclone of 18-19 February 1979: Synoptic Overview and Analysis of the Subtropical Jet Streak Influencing the Pre-Cyclogenetic Period

摘要

The Presidents' Day cyclone of 18-19 February 1979 was an intense and rapidly developing storm whichproduced heavy snowfall along the East Coast of the United States. An analysis of the cyclone is presentedwhich isolates three jet streaks that appear to have played important roles in the development of two separateareas of heavy snow. One area of heavy snow developed prior to cyclogenesis and is linked, in part, to anincreasingly unbalanced subtropical jet streak (STJ) and a noticeably ageostrophic low-level jet. The secondarea of heavy snow developed in conjunction with the explosive cyclogenesis off the East Coast as a polar jetstreak and midtropospheric trough propagated toward the coastal region from the north-central United States.This paper examines the STJ in detail. The maximum wind speeds associated with the STJ increased by15 to 20 m S-I between 1200 GMT 17 and 1200 GMT 18 February 1979 as the jet propagated from thesouth-central toward the eastern United States. During the 24 h period, the flow in the STJ became increasinglysupergeostrophic and apparently unbalanced. Ageostrophic wind speeds increased to greater than 30 m S-I,with a significant cross-contour component directed toward lower values of the Montgomery streamfunction,as the flow along the STJ became increasingly divergent with time. The increased wind speed, ageostrophicflow, and divergence along the axis of the STJ are linked to the increasing confluence in the entrance regionof the jet streak and the decreasing wavelength of the trough-ridge system in which the jet streak was embedded.The upper level divergence and upward vertical motion near the axis of the STJ along with the moisturetransport associated with the LU are found to be important factors in the development of the first area ofheavy snow.