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STORMcast Alert Update 2017-06 (Updates in red)

Issued: 13Mar17

Time:  1730 EDT  

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 and local media outlets for appropriate actions during these events.

 

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

BLIZZARD WARNING -
8 PM Mon. to 6 PM Tues. for points north and east of Phila.  Snow totals of up to 18"-24" and sustained winds of over 25-35 mph.

WINTER STORM WARNING
- 8 PM Mon. to 6 PM Tues. for most of interior PA, northern DE and inland southern NJ.  6"-12" of snow with a sleet mix Tues. morning.

COASTAL FLOOD WARNING
- 7 AM to 3 PM Tues.  for Atlantic coast and Delaware Bay.  Moderate flooding expected on the morning high tide.  12'-18' foot seas with 6'-9' waves along the Atlantic beaches.

HIGH WIND WARNING -
6 AM - 6 PM Tues. for coastal NJ.  Expected NE winds in excess of 20-30 mph with 60 mph gusts along the immediate coast.

(Map courtesy of NWS)

Freedom toast Tuesday...and maybe Wednesday too

The STORMcasts pass 100,000 hits.  Wow...

Hi everybody.  I apologize for not updating earlier but those of you who know me are aware of the recent distractions so I hope everyone will bear with me.  If you've been around awhile, you are well aware that I refuse to declare "spring" until we get through the teens of March and this is why.  I still have memories of the Superstorm of '93 and its record breaking low pressure reading (and 17" of snow) so this storm potential does not really surprise me all that much.  Although these late season storms are uncommon, I would not go so far as to say they are "unusual".  Remember that to make a really decent snowstorm in Philly, we need a careful balance between temperatures cold enough to make snow and warm enough to maximize the water content in the air.  Usually, we get one or the other but, every now and then, nature delivers a treat when they both come together at the same time.

As opposed to tropical systems that thrive on warm ocean water for fuel, extra-tropical storms like this upcoming Nor'easter thrive on temperature differences so the greater the difference in temperature from the cold and warm components, the greater potential for a showstopper.  In March, increased sun angle and longer days begin to provide warmth and water vapor to the atmosphere but significant cold shots from the north are still possible.  Our temperatures over the last two days have been a clear indicator of that and the cold ground will help increase the snowfall amounts.  Based on what I am seeing, this will not be a repeat of the Superstorm of '93.  That was created by a rare convergence of two jet streams and that is not the case with this event...this is a strong, but not atypical Nor'easter.  We're just seeing it late in the season.  However, a highly amplified trough is going to start digging way to the south over the next 12-24 hours and will interact almost perfectly with a surface low that is expected to spin up at the base of the trough.  Timing and placement look pretty ripe for a major event.  In my mind, it's really just a matter of tracking at this point.  The deepening trough is now evident on imagery and pressures are dropping fast at the surface.  A large, disorganized swatch of precipitation is now covering much of the SE and will be edging up the coast over the next 6 hours when it will hit air cold enough to freeze it.  As the surface low deepens, and swings out over the ocean near the DelMarVA, the storm will rapidly intensify and, uh, the fun will begin.  Timing is still more or less on track for the begin of festivities in the midnight to 2 AM timeframe.

Development and Model Guidance

By tomorrow evening around dinner time, the upper level trough should be digging down almost to the Gulf Coast and we'll probably see the first signs of dropping pressure at the surface in the Tennessee Valley.  By midnight, that surface low should track off the coast near Cape Hatteras and the upper level trough will begin to take on what is known as a "negative tilt".  The upper air feature is actually a bit more complicated than that with an extra little ripple in it (I found this intriguing) but basically the surface low will be just ahead of the tilted trough axis, a perfect spot for rapid intensification.

Snow should overspread the region in the midnight to 2 AM timeframe and winds will gradually pick up as well as the low tracks out over the ocean and strengthens.  If the current track comes to fruition, we should be well into it by the morning rush with heavy snow and wind.  The storm will accelerate offshore through the day and be out of the area by late afternoon so worst case scenario is about a 15-18 hour event.

Upper air (left) and surface analysis (right) showing the interaction between the negatively-tilted upper level trough (yellow line) and the associated coastal surface low at 6 AM Tues. morning.

I would have to say the models have not been as "nervy" as they can sometimes be and I've been casually watching them since Thursday.  Right now, the GFS has been showing the highest consistency over the past few days and the NWS likes what it sees as well.  The biggest difference between our two main operational models (the GFS and the NAM) is exactly where the highest amount of precipitation is expected to fall.  The GFS has the low a bit farther out to sea than the NAM so the focus is along the I-95 corridor.  If this track pans out, we're probably looking at about 12" of snow.  The NAM, however, has the storm a bit closer to the shore which will do two things.  First, it will push the focus of the highest snow totals inland towards the Lehigh Valley where the extra moisture along with some orographic lift from elevation could conceivably drop as much as 18"-24".  I gotta say that's kinda impressive.  Also, this scenario will mean warmer air could push inland towards Philly adding some rain and sleet to the event which will knock down our total accumulation.  If that occurs, I think it'll show itself in the noon to 4 PM timeframe.  Looking at some consensus models, they do seem to suggest this might happen but it might also be towards the end of the event once most of the snow has already fallen so we'll have to see.  At this point, the NAM still has a storm track closer to the coast compared to the GFS by about 50-75 miles and the NWS is basically shooting the middle. 

Today's models are again pretty consistent and the only real thing to wrestle with is how close to the coast the center of the storm tracks.  The GFS still has the greatest swatch of precipitation moving through the Lehigh Valley where the 18"-24" of snow seems like a very real possibility.  Parts of the Poconos, northern NJ, NYC and even parts of upstate NY could be looking at even more if this storm track plays out.  The GFS track will also mean less sleet/rain for the metro area and a bit more snow so Philly and suburbs could be looking at up to about a foot.  If the NAM track holds, there will be some sleet introduced into the metro area around daybreak as warmer air pushes inland.  This will limit the overall snow depth near the city and shift the deepest snowfall inland to the west of the Lehigh Valley and into the eastern side fo the Appalachians.  Difficult to say for sure which one will occur but no matter where you are in the region, it looks like a major shoveling operation tomorrow.  If the sleet kicks in, it will also be a wet, heavy, "heart attack" kind of snow so take your time shoveling.  The NAM solution should bring about 8-10" of snow with ice in the middle.  This period of closest approach will also conincide with the highest wind speeds so sustained winds of 25-30 mph and near white-out conditions are likely during the morning hours.  A morning commute could be near impossible given the expected timing.  Personally, I'm waffling a bit but I still like about 8" in Philly with some sleet keeping tht total accumulations down a bit.  There could be lots of drifting with the wind though so snow totals could vary wildly.  I feel better about deep snow in the outlying counties especially towards the Lehigh Valley.  

As the storm passes (in either model solution), cold air wrapping around the center will reintroduce another shot of pretty much all snow into the region into the afternoon before tapering off towards nightfall.  Cold air that is about 15oF below normal should remain in place for at least another 24-48 hours before moderating. 


Here's a comparison between the Mon., 2 PM GFS (left) and NAM (right) model runs.  Both position the center of the storm just off Cape May around 11 AM Tues. but the NAM is hinting at a slightly lower central pressure and closer pass to the coast than the GFS.  This difference in storm track will be the main impactor on any mixing of precipitation with the snow.  The NAM scenario favors a period of sleet or even rain pushing in as far as the Philly metro area around the time of the storm's closest approach. 

Snow day on Tuesday?

Although the accumulation totals are highly dependent on the storm's track and the fickle nature of Nor'easters is notorious (remember a few years ago when we got a complete miss less than 12 hours before it was supposed to start), there has been enough consistency in the models to make me think this will be a matter of how much rather than if.  Based on the Sunday morning models, I think around 8" is a decent middle of the road estimate.  If the storm tracks inland, less, and if it swings out just a bit, more.  Any you look at it, Tuesday looks like a real mess at this point with snow removal based on total amounts.  This will likely be the biggest storm of the season.  I will try to update things tomorrow if it makes sense but I've got a busy day ahead of me and might not be able to get back to an update so keep an eye on things folks and be careful.  With a little luck, it'll be "Freedom Toast Tuesday"!  There is really no indication this storm will miss so it's really just a matter of how much.  Given the total amount of snow possible, interruptions are to be expected into Wed. as well if everything pans out.  Behind the storm, I will add that there is a chance of a much smaller event Saturday which could bring another short-lived shot of winter precipitation but lets get through this first storm before we go talking about that one.
 

Final Note

What has taken up most of my time the last few days will come to a somber conclusion tomorrow for my dad who was about as far from a meteorologist as one could be.  He even tried to convince me once that we might be able to quell hurricanes by towing icebergs into them.  In spite of this though, I know he was a big fan of what I have learned over the years as a teacher and a scientist and was proud of all that I've accomplished so this STORMcast is dedicated to him. 

Rest easy dad, I've got it from here.  Until we see each other again...



School Day Impact for Tuesday (as of 1750 hrs. Mon.)

Normal Day....................0%
Day Off............................100% - Significant snow removal issues could also affect Wed.
2 Hr Opening..................0% 
Early Dismissal.............0%

rs

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