November 25, 2002
A white Thanksgiving this year for Southeast PA? It is looking more and more likely with each successive run from guidance. A piece of upper level energy will be diving South Eastward out of Central Canada and into the Central US. The strength of the upper low will aid in the development of a weak surface low along an existing cold front(better reflected at the 700mb layer as vertical lifting) that will head through VA and off the coast early on Wednesday. Here's how we think it will play ouy:
Later this afternoon and evening, a cold front will be pressing Southeastward through the area. The coldest air of the season will be settling in behind the front and you will notice a significant cooldown on Tuesday as high temperatures only reach into the low 40s for the high under variable skies. Temperatures will likely continue to drop as the day goes on. Clouds will increase Tuesday night as temperatures continue to dip into the lower 30s for the overnight low.
Precipitation will overspread the region from West to East starting late Tuesday night into early Wednesday morning. Precip may begin as plain rain or even a mix in some areas but will changeover to all snow shortly thereafter. The commute to work on Wednesday morning (and for early holiday travelers) will be messy, but not impossibe. Two saving graces that will keep this "storm" (as stated earlier, really just a weak wave associated with the upper energy) from becoming significant are the boundary layer and surface temperatures and the speed of the disturbance.
With temps getting into the 50s today and the 40s on Tuesday, surface temps will still be a bit warm to allow for significant accumulations on anything but grassy surfaces and maybe your car. However, roads will probably still get a bit messy, but not what we are used to in the middle of the winter months. With the quick forward speed of the storm, accumulations again will be kept relatively low.
With that said, guidance is ranging anywhere from .20 QPF all the way up to .75 QPF for our area. In snowfall terms, that would generally be a range from 1-8" from a low ratio system. We believe the ETA is overdone with it's higher end totals, yet we feel the MM5 may be slightly underdone. The GFS, in terms of QPF, is actually a very nice compromise in this situation and it has done relatively well in this range. A total of 1-2" (an area or two may squeeze out up to 3") should be the general totals for most of Southeastern PA. Confidence in the forecast is higher than normal, but may need to refer to subsequent guidance runs to see if forecasted snowfall totals continue to look accurate or need tweaking. For now, feel that 1-2" snow is a solid call for the general area. In all, WSWs should not be needed but a WWA will likely be issued in the PM on Tuesday. Will update tonight if needed, else out of town until Tuesday night and will update at that time. Take care.
S.B.
