Time: 1850 EST
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Too Little, Too Late
The potential of a storm first popped up on Tues. when models started to show a possible convergence of the sub-polar and sub-tropical jet streams somewhere over the Mid Atlantic. When this happens, we can see "phasing" in the atmosphere where the energy contained in one stream of air positively combines with the energy of the other and you get a bigger storm. Saturday, it does look like the two jets will interact in this way but we're missing something very important to make this event something special, amplitude.
As I've said in the past, the main problem with winter storms in this region is that they rely on two ingredients that come from two different places. We need cold air from the west or northwest but we need water from the south or southeast to make this whole thing work like a respectable Nor'easter.
Structurally, this storm will be pretty strong as the combination of energy from the two jet streams combines over WV, MD and southern PA but the overall flow is pretty much east-west which will cut off the storm from tapping into necessary moisture from the south to make it a major snow producer. The "short wave" that will result will spin up a low pressure system off the DelMarVa Saturday night but the east/west flow of the upper atmosphere will rapidly push it out to sea. The upper atmosphere trajectory is all wrong for a kickin' snowstorm and the whole affair should be over in about 8-12 hours.
Now, in that 8-12 hour period and once the temperatures drop below freezing after nightfall, snowfall could be pretty impressive but it will tend to come in bursts and again, by Sunday morning, the whole affair should be over.
All soundings that I looked at suggest rain in the metro area coming in around 3-4PM but a transition to snow in the 7 PM time frame seems likely in the western suburbs. A rain/snow mix looks more likely in the immediate metro area. To be honest, I've got some temperature predictions that keep the city at just about freezing through the whole affair and much of south NJ above freezing. If that happens, there could be very little snow at all in those locations. I'm not sold on this since 2 meter temperature predictions are always problematic but it's a possibility.
Likely NWS Upgrades
Within the next few hours as the overnight model runs come in and assuming there is not much variation in the expected snow totals, I think we will see an upgrade to Winter Storm Warning for many of the areas currently under the Watch. The threshold for a Warning in the metro counties is only 4" and I think we'll get that so do not be surprised by this upgrade.
For Delco and Philly, a Winter Weather Advisory seems likely if the temperature profile doesn't change although it might not be that big of a deal unless you've got plans for later in the evening. We might see some widespread Winter Weather Advisories issued by morning but this really doesn't feel like a huge impact event to me at this point. Falling back to my earlier temperature discussion, this could start as rain in Philly and only turn to snow later on. I'm not very confident about this so don't take my word literally here. Ground temperatures are just really hard to predict down to the degree. Regardless, I do not see snowfall totals above 2"-3" in the city.
And "Poof", It's Gone
I think it's worth mentioning that, unlike after our typical winter storms when the weather gets cold and blustery on the back end, this one won't really produce those results. The jet pretty much stays on an east/west trajectory before a prominent high pressure develops over the entire eastern US by Monday. As that high slides to our east, a strong southwesterly flow will take hold and push much of the region to near record highs by Tuesday and Wednesday and we could come real close to seeing 70F. Frolic while you can, the snow doesn't stand a chance past Sunday.