Time: 1700 EST (UPDATES in red)
The STORMcasts shown here are intended for the students of UDHS's AGS and AES geoscience programs and are for educational/informational purposes only. They frequently combine scientific concepts along with a sense of humor that often pokes fun at our behavior during weather events. While intended to be informative, the STORMcasts should never be used to make life and death decisions during severe weather events. Follow the appropriate bulletins posted by the National Weather Service for appropriate actions during these events.
Nope, I didn't forget. But, what did you see each of the last 6 times you checked the news about snow accumulation...and don't even try to convince me you haven't been looking. The snow totals kept changing...shocker.
Coastal storm people! This is par for the course and snow totals could still swing significantly since Philly is dead center in the area where temperatures will be hovering around freezing. Combine that with the usual jiggling these storm tracks undergo in the models and you should see why I didn't see much point in putting out a STORMcast until now.
Here's Where We Are
The low that will whiz up the coast this evening is currently getting its act together over LA where it is drawing in Gulf moisture and creating a hefty squall line of heavy rain and thunderstorms in the deep south. We're already transitioning into a spring pattern that far south so the thunderstorms are sometimes combined with tornado potential which seems pretty likely today. However, we're still dancing with sub-freezing temperatures especially at night and decent sized snowstorms in March are not that out of the ordinary.
Although we're obviously not experiencing them here in Philly tonight, the south (LA, AL, MS and GA) have been experiencing heavy thunderstorms with several confirmed tornadoes.
By late this afternoon or early evening, the low will begin a rapid track up the coast where it should move offshore somewhere around the entrance to the Chesapeake Bay. It's at this point that the precipitation should really pick up as the storm recharges with Atlantic moisture. As it slides past us offshore, we'll see respectable amounts of rain changing to snow before it ejects out of the region overnight. By morning, it should largely be over.
The low is currently positioned along the GA/SC border and heading towards the NE. As it approaches the Atlantic, water content will increase as should precipitation rates.
The Snowfall Challenge
Very challenging with this one for a couple of reasons:
1. With the exception of interior PA where the temperature profiles are much "safer" to estimate, most of us will be hovering right around the freezing mark through the event. The morning model run is actually quite interesting since both the NAM and GFS have temperatures remaining above freezing. From what I can see, we're largely relying on evaporative cooling ahead of the storm to take us into the snow range and that can be pretty tricky to nail down precisely for any specific location.
The cooling did its job (and quickly too) but now that the atmosphere is saturated, there really won't be much more. Currently in DelCo, its 32F and there's a possibility of some warm air aloft sneaking in over the colder stuff at the ground later tonight. Can't say for sure but if the layer is thick enough, some late night sleet might further cut down on totals.
2. Storm track is always a player here. Snowfall gradients are notoriously tight in these storms so a distance of only 20 miles or so could mean the difference between feast or famine when it comes to snow totals.
Based on current projections, I think a solid 4" is a pretty good bet for the immediate metro region. I know the morning diagram from the NWS says more than that, but I have some doubts. I would not discount it outright but I want to see if the afternoon projection stays the same before I throw chips down on it. Once you move away from the urban heat island, places like ChesCo and Upper MontCo. should see a bit more in the 5"-7" range. Bucks should be about the same but some of that county gets a bit of elevation lift too. Delco, DE and south Jersey will probably see this start as rain before changing over to snow. The ice potential is pretty low this time and the transition should be quick. Interior PA...it'll be all snow, that's pretty easy to see on this one. The GFS has the heaviest swatch of precip. centered just over the river in NJ while the NAM prefers a swath right up I-95. Again, that's only a difference of about 30 miles so you can see the challenge in getting it right.
Like I said earlier today, nailing the snow totals will be very tricky with this system and now that I've had a chance to look at some soundings based on the afternoon model run, it looks like the city and immediate suburbs might see a bit of warm air injected aloft around midnight. Provided this happens, we could see a brief period of sleet which would knock down the snow totals a bit before everything switches back to snow and ends towards morning. It's quite tough to tell exactly how far inland this warm air will nose in and the NWS is trying hard to figure this out too. In response to this possibility, NWS has also backed off a bit on their snow totals especially in NJ. I'll add the NAM has shifted east a bit and has come into agreement with the GFS as of the 18Z model run with the heaviest swath of precipitation centered over south NJ. In the bigger scheme of things, it won't change all that much except for possibly helping to better understand where the major rain/snow line will be. I'm still not convinced about the deeper snow totals but will just have to ride it out and see what happens. At this point, no additional analysis will reveal anything I haven't already discussed.
Most of the region should see the onset of precipitation by around 4-5 PM. Again, don't be surprised by a sudden drop in temperature this afternoon shortly before the whole thing starts so just because you're up around 40F right now doesn't mean you're going to stay there. Max precip. should hit later in the evening in the 8-10 PM range and it could be pretty impressive for awhile once the big, wet flakes start coming down. The event should taper off around 2-4 AM and be completely over by daybreak depending on your exact location...this is a fast mover and won't really be around for all that long.
Room For Error
I'm hesitant to hang my hat on any precip. totals on storms like these because of the big differences in precip. totals that can be created by minute differences in the actual storm track. All indications are that this will be a medium impact event and the timing is right to really mess things up in the morning if it all pans out. However, I always reserve a bit of caution for a last minute course correction that just brings some heavy rain to the south and light snow to the north and west. This is not a huge storm by the usual metrics but it's one of the first of the season to sweep to our east rather than west so we always get excited about those. The truth is that this type of storm moving at the speed it is supposed to really only has the ability to drop a max. of about 8"-10" of snow before it speeds out of the region so I guess hope for the "best" but don't be surprised if you get a bit less than what you hopes for.
School Impact Mon.
Unless the temperature profile winds up being a complete bust (not unheard of), it looks like a three day weekend for most of us. Seniors, if you had a two hour late opening Friday but were "sick" (yeah...blah, blah, whatever you say), you'll probably get this one free of charge.
Day off - 70% (esp. in outlying areas with heavier snowfall potential)
Late Arrival - 30% (I'm leaving some room for a bust)
Early Dismissal - 00%
Full Day - 00%