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STORMcast Advisory 2018-13

Issued: 01Apr18

Time:  1500 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 for appropriate actions during these events.


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

WINTER WEATHER ADVISORY - For much of the forecast region from midnight tonight until 1 PM Monday.  3"-5" of snow possible with locally heavier amounts especially to the north and west of the metro area.

HAZARDOUS WEATHER OUTLOOK - For southern NJ and DE.  Low possibility of widespread hazardous weather Mon. morning.

(Map courtesy of NWS)
Winter is coming...again.

If you were ever wondering whether meteorologists had a sense of humor, all you have to do is look at tomorrow's forecast because I guarantee that they're all laughing about it.

In just the next chapter (note I did not say last) of "The Winter That Never Started but Now Will Never End," we're setting up for yet another round of snow that could cause some problems for the metro area's morning commute. 

While this storm will not be all that impressive, it's timing overnight is a key factor in its disruptiveness so here's the scoop.

Tonight

After nightfall, high pressure will settle in over southeastern Canada which will rotate cold air down into the Mid-Atlantic dropping our temperatures close to freezing.  With the high in place, a low pressure center generated along a rather minor dip in the jet will then slide over our area before moving offshore and intensifying just off the NJ shore.  Once again, it's NJ's fault.  As usual, this ocean influence will be the source of most of the water that will drop as wet snow as it crashes into the descending cold air. 

  Timing and Who Gets What?

Looking at the morning models, most areas should be experiencing precipitation somewhere between midnight and 4 AM.  It is likely to start as rain for many but a fairly quick transition to snow should occur in the 4 AM- 6 AM timeframe.  With the exception of southern NJ where this could be largely a rain event, Philly and the 'burbs should see snow between about 5 AM and 11 AM as the low moves offshore and taps into Atlantic water.  Peak snowfall is expected around the morning rush (oh goodie).  The whole affair should end quickly by around lunchtime with daytime heating perhaps capping the event as some more rain.  With daytime temperatures in the metro area expected to be in the mid 40s, whatever is on the ground could literally be gone by the end of the day.

At the moment, it looks like the Lehigh Valley and the lower Poconos could get the brunt of this storm where temperatures, water amounts, and a bit of elevation are just right for a respectable snowfall.  Anywhere from 3"-5" with some heavier bands is possible.  The metro area will be hovering right around freezing as in the March storms and the ground is a bit warmer so accumulations will be more limited, perhaps 2"-4".  These snowfall totals are still within the parameters for a WWA which was issued this morning.  In spots, amounts might even be deep enough for a Winter Storm Warning but I think that it will be spotty and, with most if not all of the snow literally disappearing by evening, I think the NWS will probably stick with the WWA. 

Whether the snow sticks to road surfaces will mainly come down to how fast it falls.  Since a lot of this storm will occur prior to typical April daytime heating, it will all come down to snowfall rates.  If we get lucky, everything is on the yard but there are some concerns about banding again that could overwhelm ground temperatures' ability to melt the snow.  If that is the case, road conditions could be impacted for just a few hours in the morning especially further away from the city.  Tough to say for sure and I've got other things to do tonight so time will tell on that one.  If you see White Walkers on the Schuylkill, you'll have your answer.       

 
IMorning model imagery shows the line up for tomorrow's winter event.  Tonight, high pressure falls into place and directs a stream of colder air into the region (left).  By morning (right), a low pressure system will spin up out over the NJ coast and supply the cold air with a steady shot of moisture before daytime temperatures have a chance to turn it to rain.

Later in the Week


A more respectable storm is expected to sweep to our west on Wed. and could bring a decent amount of precipitation with even some embedded thunderstorms during the day.  However, since we'll remain in the warm sector of the low, this one looks like all rain.  Finally, Saturday could see another quick shot of snow towards daybreak as a late season clipper flies through from the west but it's a bit early to tell trajectory on that.

For the Statisticians

Granted it has been awhile but decent-sized March-April snowstorms are not out of the question.  With some quick NWS research, here are some April analogs...

Latest date with measurable snow - April 27 (1967), .1"
Latest date with >1" of snowfall - April 19 (1983, 1.3"
Latest date with >6" of snowfall - April 9 (1917), 7"
Largest April snowfall on record - April 3 (1915), 19" 
Last time we had measurable snow in April - April 7 (2003), 1.8"

 
School Impact on Monday

Day off.................... 05% Perhaps a few in outlying districts closer to the sweet spot.  Lehigh Valley?
Early Dismissal....  00% All will be over by lunchtime.
Late Opening.......   30% If snowfall rates are high enough, morning commute problems could occur.  Outer ChesCo,
                                  MontCo, and BucksCo?
Normal Day..........   65% Not sure it will snow heavy enough to cause a major problem.  Immediate metro area?




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