Issued: 19Mar18 20Mar18
Time: 2130 EDT 2000 hrs EDT (No further updates will be posted)
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.
Just a word about models. One of the greatest forecasting difficulties with a storm like this one is that certain model types do not do well with things that are climatologically out of whack and this storm definitely qualifies. We could very well break one or more March records with this storm especially with regard to snow totals. Officially, the record March snowfall in Philly was the Blizzard of '93 at 12" and there is a good shot this storm will surpass that...might even crush it. With a blocking high in place and an upper air feature that continues to amplify, this could be a near perfect alignment of surface and upper air features to create a massive snowstorm. Just 48 hours ago, that was not the case so understand, in some regards, forecasting a storm we've never seen before is quite problematic.
Agreement or Not?
Still, no, not really. The big switch from yesterday to today was a pretty distinct westward shift in models including the pesky GFS that earlier in the week had the event largely missing us. Just about everybody shifted west and when that kind of signal is sent, you really can't ignore it. When the shift held after the afternoon model runs, the NWS alerts all went up together around 3:30 Mon. afternoon. And, not only did they post for the first part of this storm, they posted for the second which is why we've got some overlapping alerts right now. In particular are the WWAs and WSWs posted for the metro area. The WWA is not a lesser posting, it is simply addressing Tuesday. The same areas are also under WSWs for Wed. The NWS has even made it a point to say:
"...run to run discontinuity appears to be the only forecast continuity with the operational models for this event, which means that the forecast remains unusually low confidence even within 24 hours..."
So, is there still a chance this is all for nothing? Yes, honestly there is but that probability seems to be going down by the hour.
Part 1 - Tuesday
The first part of this double Nor'easter will start to affect us no later than lunchtime Tues. For much of NJ, this could be a mainly rain event with temperatures above freezing. However, remember my remark yesterday that "...The temperature profiles are really sketchy in this window right now but I think somebody in the area is going to be dealing with a lot of sleet..."? I'm sticking to my guns here with sounding forecasts supporting the idea of some serious sleet in the metro area in the afternoon. Exactly who gets it is too fine a detail for me to discern from models but we will be hovering right around the freezing mark for much fo the day and a shift of only one or two degrees could be critical. We could see frequent transitions between rain/sleet/snow throughout the day before things turn to all snow towards evening. Along with this sleet and heavy snow will also come the highest winds. They should remain below advisory levels near Philly but the shore could get lashed pretty hard. Still, the NWS is issuing some strong language about the possibility of widespread power outages and tree damage. This will especially be true if freezing rain mixes in. The afternoon commute will most likely be a disaster in this kind of scenario. Our best chance near the city is for just enough warm air to nose in and keep it as rain...I'm skeptical.
I think I hit this one fairly well with the sleet. The initial onset of the precipitation was thwarted for a few hours by a very dry lower atmosphere around lunchtime today (I was discussing it with my meteo. students) but the temperature supporting a mixed bag of ice and snow occurred in most locations. Saw a fair amount of graupel too (inside joke for some close friends here). Unfortunately, the atmosphere continues to hover around 32F and the ice threat will actually increase before it decreases into late this evening. There is still a wedge of warm air at about 5000 ft and forecast soundings have it remaining until after midnight before colder air erodes it and causes a transition back to all snow for the rest of the event. If we can keep the ice as sleet, that will be a plus as it does not stick to the trees and wires like freezing rain but the addition of heavy, wet snow tomorrow along with wind might negate any advantages we would see from continued sleet.
Part 2 - Wednesday
Well, it might be time to get out the lawn chair and head to Wawa to watch the follies. Originally, we were thinking there might be a lull between the two sections of this storm but now it doesn't really look like that will happen. As the reinvigorated low rapidly deepens offshore, it will draw down colder air from a blocking high to the north which will turn most of the precipitation to snow and keep the low from moving out. This means the storm will be hovering just off the NJ shore with nowhere to go and an entire ocean to draw water from. If the temperature profile holds, we will again see a significant blast of barely freezing air lob huge amounts of 8' snowflakes at us by the bucket full. With some areas experiencing short-lived explosive snow growth and even some more thunder snow, 6"-8" will be the norm but I expect we'll see locally heavier amounts near a foot in some places. I've seen a few graphics that seems to place the immediate Philly suburbs in this sweet spot. Again the NWS is issuing some pretty strong language:
"...Dangerous winter storm promises to produce major winter impacts for our forecast area Tuesday through Wednesday evening. Wind driven sleet/freezing rain and wet snow becomes drier from north to south late Wednesday but too late to forestall what may be the most power outages from any event in our forecast area this March..." If this scenario plays out, well, nothing is moving Wed.
I do not see us emerging from beneath the precipitation shield until well after dark Wed. night. If the damage is extensive enough, we'll need Thurs. just to put the pieces back together.
Snowfall expectations continue to rise as the Delaware Valley lines up with the sweet spot of this storm tomorrow and many areas could see a foot or more. Maximum water content with just enough cold air to freeze the entire air column should make this one memorable snowstorm. Recent model runs have also repeatedly amplified the jet and swung it to include a near perfectly placed negative tilt (I've talked about these in the past) with regard to the surface low off the coast. A comparison would be to a highly efficient chimney and fireplace. At ground level, the fire is blazing and looks great but with the flu closed, the heat and smoke have nowhere to go and the whole system clogs up filling the house with smoke. If the flu is opened and the chimney is clear though, convection is supported and the fireplace does its job as it was designed. What we have here is a perfectly placed surface low (the fire) and an upper atmosphere that is creating a rapidly spinning conduit of air above it (the flu/chimney). As long as the two remain connected (which they will do through tomorrow), the moisture laden air over the ocean will rise to great heights, condense and create precipitation which is then hurled inland in a counterclockwise direction from the center of the low. And, if the storm can't move due to obstructions out ahead of it, the same spots keep getting hit time after time. That's going to be us tomorrow. Winds will also increase noticeably tonight as the low deepens off the NJ shore. The main concern here is that peak winds should come around the same time we are transitioning from ice to snow overnight so if we've got buildup on the trees, you might be without power by morning. Peak snowfall should come shortly after this period be tomorrow morning into the afternoon. Some estimates have rates as high as 2"-3" per hour in places that could benefit from some elevational lift. This rate might not last long but it will be spectacular where it occurs. If a bit of lift gets added in, don't be surprised to hear some thundersnow or maybe even see a lightning strike.
Could it Miss?
At this point, I doubt it. I think this will just come down to how much/how bad. I feel pretty good about Tues. and the ice and just a bit less so on the snow totals. Throughout this entire event, our temperatures will barely budge out of the 30F-33F range which is unusual. Usually there's more of a day/night spread but the influences of this storm are so widespread, no other factors are really coming into play.
One For The Books?
I've always used the 1993 Superstorm (often called the Storm of the Century) as the benchmark for March storms when it dumped 12"-17" on the region March 12-14 but, if this one pans out with any amount of severity, it should be one of the latest major storms to ever hit the area. A quick look back revealed a 12"er on March 19-21, 1958 under similar circumstances but that's about as late as I can find. Funny thing there is that the big headlines for that storm were about power outages from heavy snow which we could be revisiting ourselves. We'll just have to wait and see how this one pans out. Good luck everybody. Get some batteries for the toaster.
It looks like this storm is shaping into a record breaker for March snowfalls. Again, snow of this magnitude in March has only happened about 6 times before in Philly and there are normal seasonal factors that almost always inhibit this kind of storm's formation. But, every once in awhile, things just come together for the perfect opportunity for a healthy intake of, or course, freedom toast! It is time. Don't mess around with this storm folks, it's going to be dangerous out there tomorrow. Stay home, be safe, and ride it out.
Oh, and by the way, there might be another one coming Sunday. I just thought I'd drop that bomb to see how many of you read this far.
This is still a possibility folks. No I'm not kidding. I know it's hard to look past the front walk right now but more winter precipitation could be on tap Saturday night. Let's get through this first.
School Impact on Tuesday
Day off.................... 15% (I think many will try to get it in. Too many other days off already in March)
Early Dismissal.... 75% (If the ice comes in, it'll be a definite...if rain, we'll make it)
Late Opening....... 00%
Normal Day.......... 10% (Only if a rain scenario)
School Impact on Wed.-Thurs.
Very high likelihood of disruptions both days if current scenario holds. Sticking to this...could even see two days off which will create many problems for districts already over their planned allowances for the year.