January 18, 2002
First measurable snow of the season for the Philadelphia metro area on the horizon? It is certainly looking like some frozen precip is going to affect the region during the day on Saturday. Guidance is in very good agreement tonight, in regards to the track of low pressure during the day on Saturday. The low is generally forecast by guidance to head ENE from the lower Tennessee Valley to off the Mid Atlantic Coast around the Southern tip of the DelMarVa Peninsula on Saturday and early Sunday. Overall, guidance has trended farther North with the track of the low the past few runs, and the QPF accordingly has followed. As far as 500mb vorticity(atmospheric 'energy'), we have two general camps. One which keeps Northern and Southern streams separate, and the other, which phases the two sometime late Saturday or early Sunday leading to potentially more snow for us and parts of New England. I will say upfront that I am leading towards a wintry solution for our area, possibly mixing with sleet or rain for a period, and ending as snow.
This storm does not look like a classic setup for a monster snowstorm for the entire Northeast, but QPF certainly looks high enough and the 850 temps and thickness levels low enough to keep our area right on the border between heavy wet snow and a mix or rain, potentially up to 6" or more of snow. Again, I am leaning towards snow, possibly to sleet or a mix, and ending as all snow. As far as timing, light snow looks to break out in the late morning or very early afternoon on Saturday and continue through most of the day. It could be heavy for a period and even mix with sleet or rain, but no piece of guidance changes us over to plain rain at this time(although I still feel we see some sort of a change during the height of the event). One reason I am a bit skeptical to go with all snow at the moment is the Northward trend seen on guidance the past few runs. This has happened a few times already this winter and each storm has been just a bit too warm for our region in regards to snowfall. However, if the area of low pressure can strengthen a bit faster than progged, and conditions are right aloft, there is plenty of cold air ready to be tapped in Canada, which could keep us all snow.
Anyway, my breakdown is that we see phasing between jets, but not really a full phase, just enough interaction between the two to draw the low pressure just a bit farther North. That leads me to believe that DCA/PHL/ and perhaps NYC should be right on the line between moderate->heavy wet snow and mix or rain. Going with accumulating snow mixing with sleet then back to snow for the Delaware Valley. Total accumulations 2-5" for metro PHL and very nearby suburbs. As always, there will be another update prior to the storm to make any last minute adjustments if necessary. For now, my confidence is above average with this forecast. We should have a winter storm watch up before midnight Friday night for the area with advisories South and East of the city, provided of course the next run or two of guidance continue to merge on the forecast, as it seems is ocurring. Interestingly enough, I actually have used the NGM tonight as my model of choice, so-to-speak, as it's upper level features and jet structure looks quite reasonable through 48 hours. Not many speak fondly of the NGM these days, but it continues to do well in the 24-36 hours range this winter. That's all for now, more later. Take care!!!
S.B.
