12-6-2011 Technical Discussion

Technical Discussion

Synopsis:

                A series of low pressure systems will develop along a lingering stationary front running across from the Northeast States to Gulf of Mexico. A final low, the strongest, will bring periods of rain and snow to much of the Commonwealth Wednesday night. The low pressure system will slowly intensify as it slides up along the eastern seaboard on Thursday, leaving some colder air behind its passage. The weather will remain calm with some isolated lake-effect snows over the extreme northwestern PA. The next cold front is expected to pass the region over the weekend.

Short-term (Tuesday night-Wednesday night)

                A stationary front that has lingered over the Northeast has produced periods of light to moderate rain showers to the State Pennsylvania. This slow moving front will eventually slide southeastward enough so that the region is located on the colder side of the air masses. A very sharp and squeezed 500-mb trough located over the lower Midwest will gradually become negatively tilted. As it does so, it will accelerate westward toward Tennessee Valley and gain curvature vorticity. The positive vorticity advection ahead of the 500-mb shortwave will create a favorable zone for cyclongenesis. With the lingering stationary front extending into that favorable zone for cyclongensis, a series of low pressure systems will develop and slide northeastward along the front. As the 500-mb shortwave a! pproaches the pre-established stationary front, a weak primary low will form late Tuesday night. The low pressure system will slowly intensify as it moves northeastward due to the continuing negative tilting of the upper-level trough and baroclinic instability. To the south of the warm front, the precipitation type will be all rain. However, to the north of the warm front some snow is expected, especially over higher elevations.  This should by no means be a major winter storm as the surface and mid-level temperature profile will be too warm for any substantial snow accumulations. Most of the models, including the GFS, NAM, and European all indicate a rather warm temperature profiles for the region, therefore, only isolated areas will see snowfall more than 4 inches.  The precipitation should be the heaviest Wednesday night, but the quick moving nature of the system will limit the amount! s. In addition, this is not an ideal synoptic setup for a majo! r Northe ast snowstorm since the evolution of the storm does not involve the phasing of troughs and jet streams.

Mid-term (Thursday-Friday)

                A high pressure system will build into the region on Thursday in the wake of the storm. The skies will clear out, but the temperatures across the Commonwealth will remain cold as cold air advects southward. Locations nearby Lake Erie may see isolated snow showers due to cold air passing over the relatively warm lake water.  Any lake-effect snow should quickly come to an end as the winds shift to coming from the southeast later Thursday. Conditions will remain calm on Friday before the next frontal passage on Saturday.

Long-term (Saturday-Sunday)

                A stronger cold front will sweep through the region on Saturday. The temperatures will drop across the region to below normal values for the first time in a while. With a stronger cold air advection and winds, more substantial lake-effect snow bands should set up, giving regions close to the Great Lakes periods of light to moderate snow showers.

D. Wang