December 7

Synopsis:

Two split fow longwave troughs will move east over the next 48 hours. The southern stream trough extends from the Midwest to northeastern Textas while the northern stream extends from the Hudson Bay to the Great Lakes. High pressure builds in Thursday and Friday. Over the next 48 hours, a shortwave digging from Western Ontario will round the base of the northern trough, amplifying and nearly phasing with the southern stream trough. As it propagates eastward, ridging builts over the Atlantic , extending westward into the East Coast Friday. Very cold 850mb are associated with the shortwave so it deepens as it moves over the relatively warmer Eastern U.S.  As the trough approaches the coast Friday night and Saturday, the upper level jet will intensify to 180kt at 300mb and lift northeastward as ridging continues to build in the Atlantic. Meanwhile, a series of low level shortwaves will round the base of the trough between Friday night and Saturday, instigating cyclogenesis just east of the 40N/70W benchmark. With the upper level jet still over the northeast, the low is not expected to rapidly develop. Most of the precipitation that develops will be consolidated around the resulting frontal boundary. However, the jet entrance/ext regions may be close enough to the low to force some of the precipitation that develops along the resulting frontal boundary to the north and west. There is still a much uncertainty regarding the proximity of the upper and lower level features. Just less than half of the GEFS and SREF ensemble members project a brush of light snow from this storm while GFS has been wobbling east and west with the jet axis over the past several runs.