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STORMcast Advisory UPDATE 2018-18

Issued: 14Nov18

Time:  1930 EST

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.


Current National Weather Service advisory map as of STORMcast issuance...

WINTER STORM WARNING - For interior PA and the Appalachians from 4 AM Thurs. to 4 AM Fri.  Heavy mixed precipitation with 2"-5" of snow/ice accumulation.

WINTER WEATHER ADVISORY - For the greater Philly metro area, inland and upstate NJ and Poconos from 7 AM to 3PM Thurs.  Mixed precipitation of snow/ice up to 1" before turning to rain.

WIND ADVISORY - For coastal NJ from 1 PM to 11 PM Thurs. 25-35 mph winds with gusts up to 50 mph.

HAZARDOUS WEATHER OUTLOOK
- For parts of southern NJ and DE.  Low lying flooding possible through Friday morning.

(Map courtesy of NWS)

UPDATES IN RED

Hi everybody and happy fall...er...winter...er...whatever.  Like the seasons even make sense anymore.  If you're wondering about the delay, you know how cranky I get with long range winter weather predictions so I've been earnestly waiting to see if any kind of agreement or consistent trend developed between models since I first started paying attention to this yesterday.  Based on what I am seeing now, I decided to go to Advisory status on this STORMcast but am still leaving an opening for a better scenario although that window seems to be decreasing in size.

736 hits on the first winter STORMcast of the season...and 56 more just while I was typing up the update.  I'm flattered.

Here's the overall setup...

A fairly robust upper level trough currently exists just to our west that helped feed the heavy rains we received last night and as the trough shifts east, it looks like a piece of energy associated with the far southern branch of this trough will break off from the main flow and form a new closed low over the Southeast.  This closed low will still have some steering from the main jet but before it follows this guidance up the East Coast, it will tap into Gulf moisture and then become further augmented by the still relatively warm waters of the coastal Atlantic Ocean.  And...you know what that means...Nor'easter!

The major forecast issue for the Philly region will be the precise temperature profile (and I mean precise) and what impact that will have on the PTYPE (precipitation type) through the event.  And, as usual, it looks like a mixed bag so the "wintry mix" phrase will be on everybody's lips as the temperature profile wanders dangerously near the freezing mark during one of the busiest times of the day for traffic.  Oh goodie.

This will still be the crucial factor for everybody except the coastal counties where temps. should stay above freezing through the event.  Just about everybody else will have to contend with some level of mixed precipitation during dayloght hours and that is going to be the immediate problem.

Model Trends

We generally rely on two main operational models when looking at temperature profiles, the GFS and the NAM.  For this event, the NAM has been consistently just a tad colder than the GFS solution but both are now trending to a slightly cooler solution than what they looked like this morning which means the initial rain/snow line keeps moving further to the south and east.  This is still the case with the Wed. evening model runs but the NWS is leaning more heavily on the NAM since it has a better track record resolving these storms.  Unfortunately, forecast soundings are literally right along the freezing mark which means going into a shallow valley might be enough to drop temperatures from 33F to 32F and freeze ground surfaces.  It's going to be THAT close.  At the current time, it looks like the freezing mark at ground level cuts Chester, Montgomery, and Bucks counties right in half right about the time things start while Delco and Philly stay just above freezing.  Just remember that the ice decisions tomorrow will actually be made at about 5000-7000 ft rather than at the surface so I would not necessarily go banking my safety on a thermometer mounted in a rearview mirror.

Based on the current evening model runs, precipitation should overspread the region (from south to north) some time between about 10AM - 12PM and in most spots, it will be snow.  There, I said it M.D., snow.  NJ will likely remain rainy but most of PA now looks cold enough to be snow.  Heaviest precipitation should arrive after lunch and persist through the evening rush and it is in this window that things get dicey.  Although the NAM model is still a bit colder than the GFS, both hover very close to the freezing mark with the NAM transitioning to a snow/sleet mix by around 4PM while the GFS looks to do this a bit earlier.  I still think the timing is on track for a 10AM-12PM start.  I've been looking at some relative humidity indicators and the atmosphere is pretty dry right now so it's going to take some time for moisture to saturate everything before things will be able to reach the ground but that's not all good.  This low humidity also means that any early precipitation will cool the atmosphere as it evaporates and with us playing with the freezing mark, it could put us over the edge so be careful what you wish for.  Heaviest precipitation is still expected in the afternoon and it could be pretty heavy as this storm is really chocked full of moisture for a Novembere event.  Total water equivalent could be between !"-2" so there's a flooding conern expecially Thurs. evening once most of us transition to all rain.   If I lean on the NAM, I think we're looking at wet snow around lunch, a veritable smorgasbord of mixed ice/snow/rain by 4 PM and moderate to heavy rain by 7 PM. 

Current disposition of the upper air features that will guide the upcoming storm according to the Tues. evening GFS.  At far left (1 PM Wed.), the closed low begins to form over Ark-La-Tex and taps into Gulf moisture.  By 7 AM thurs. (center), the surface low has spun up off the Carolinas and begins its track up the East Coast assisted by the closed low.  At right (1 AM Fri.) the center of the storm is just off the NJ coast and at about peak intensity.  Again, thought this means ample moisture for high amounts of precipitation, it also means the injection of warm air so much of the region will experience rain by this point.

The reason for the ice transition (at least at this point) will be due to the expected path of the storm.  In Nor'easters, the closer to the coast the center travels, the more moisture we get but that also means the introduction of warmer air that increases the chances of a not-all-snow event.  Both models seem to favor this trajectory right now with a significant amount of warm air overriding the colder air at the surface.  Warm air aloft means high altitude snowflakes melt into rain but then refreeze as sleet as they get close to the ground and enter cold air again.  If there is a silver lining, it looks like the temperatures at ground level should remain above freezing so roads might have a difficult time freezing over.  By later in the evening, both models show a strong temperature inversion with lots of warm air aloft that should be thick enough to chang everything over to rain for most of the overnight.  The finally blast might once again include some snow squalls though as the departing storm wraps around cold air in its wake.  The whole show should be over by Friday morning(ish).

Uncertainty

If I've said it once, I've said it a hundred times that the path of Nor'easters is very fickle and slight jiggles can have big impacts on PTYPE so my best advice at this point is to prepare for a messy commute Thursday evening.  Timing of this storm is also up for grabs and my current time table is based on Wed. evening model runs so it could wind up getting adjusted tomorrow.  Higher latitudes and areas away from the immediate suburbs could see longer snow/ice mixes before a rain transition so the concern of accumulating ice is higher there.  I'm just not convinced we're goign to see it in Philly.  That warm, coastal air will be coming in pretty quickly. 

NWS ice forecast as of Wed. 8:30 PM.
The storm remains on track to inject large amounts of warmer air into the metro area through Thurs. evening which will unload the rainfall but further inland where the cold air will have more of a chance, there is a growing ice threat especially for the Lehigh Valley and points north and west of Philly.  Special caution should be exercised in these areas as driving could remain hazardous for extended periods of time this evening especially around the afternoon rush.

The Back End

As the storm passes and wraps cold air towards its core, there's a chance we could fall within the storm's dry slot which should help lower rainfall totals late Thurs. night.  Right now, I think this favors south Jersey a little bit more than Philly but the immediate suburbs could fall under it as well so hope for the best there.  Immediately to the west of that slot however will be some wraparound precipitation which could very well turn back to snow as the last of the water vapor hits incoming cold air in the wake of the storm.  Right now, the focus of that seems to more towards Lancaster or Harrisburg but a little jiggle could create a final burst of snow during the overnight.  As the storm intensifies offshore, the pressure field will also tighten so be prepared for some gusty winds overnight that could create some power outages.  With all of the rain and or ice, it seems like a distinct possibility.

I'll likely be back tomorrow with an update.  Have a nice night everybody.

Potential School Impacts

A day off seems a bit apocalyptic at this point unless you're under the Winter Storm Warning so I'm going to lean away from this scenario for just about all of the metro area.  Lehigh Valley has a chance but for now, let's say about 0%-10%.  Late opening just doesn;t make sense at all.  Now, as for an early dismissal, that's a distinct possibility if things arrive when we think they will.  Outlying counties that will experience the ice and snow the hardest stand the best chance but once the dominoes starts falling (we all know how THAT goes), I think we'll see some more widespread effects.  Right around the immediate Philly area, its a toss up since we could see a quick transition to rain in which case we'll just all get wet.  Let's say 60%-70% for outer districts and I'll go with a 30%-40% shot closer to the city.  There might even be a few late openings Friday if we get the snowburst at the end of the storm but I think those will be very limited in number if at all.

 

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