The presidents' day cyclone of 18-19 February 1979: synoptic overview and analysis of the subtropical jet streak influencing the precyclogenetic period

The Presidents' Day cyclone of Feb. 18-19, 1979, was an intense and rapidly developing storm that produced heavy snowfall along the East Coast of the U.S. An analysis of the cyclone is presented which isolates three jet streaks that appear to have played important roles in the development of two separate areas of heavy snow. One area of heavy snow developed prior to cyclogenesis and is linked, in part, to an increasingly unbalanced subtropical jet streak (STJ) and a noticeably ageostrophic low-level jet (LLJ). The second area of heavy snow developed in conjunction with the explosive cyclogenesis off the East Coast as a polar jet streak and midtropospheric trough propagated toward the coastal region from the north-central U.S. This paper examines the STJ in detail. The maximum wind speeds associated with the STJ increased by 15-20 m sec super(-) super(1) between 1200 hr GMT Feb. 17 and 1200 hr GMT Feb. 18, 1979, as the jet propagated from the south-central toward the eastern U.S. During the 24-hr period, the flow in the STJ became increasingly supergeostrophic and, apparently, unbalanced. Ageostrophic wind speeds increased to >30 m sec super(-) super(1) , 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, ageostrophic flow, and divergence along the axis of the STJ are linked to the increasing confluence in the entrance region of 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 moisture transport associated with the LLJ, are found to be important factors in the development of the first area of heavy snow.