Wednesday Afternoon Technical Discussion

Synopsis:

High pressure will be in control until Thursday evening. A clipper system (currently located in the Dakotas moving to the ENE) will move through on Thursday night producing snow. High pressure will move through on Friday while a Low approaches from the Southwest. The low will affect our area on Friday night and exit within a day. Snow will fall on the entire State at first, but for areas south of I-80 (especially along and S of the Turnpike), the snow won't last long. After the system exits on Sunday, skies look to remain mostly cloudy as we wait for a cold front to move through.

Models Used: 15Z SREF; 12Z NAM, GFS & European

Short-term (Wed. Night through Friday AM):

The GFS and NAM are consistent: A mix of clouds and sun for Thursday; PA gets about an inch of snow overnight Thursday into Friday; and skies are mostly cloudy between the clipper system & Saturday's snow.

Possible differences in amounts: (1) the high-res. NAM shows a snow squall affecting central PA and (2) the low's track is to the north.

Mid-range (Friday evening through Saturday afternoon):

The positions of the 540-line & 850-mb 0°C isotherm coincide but each model places those lines differently. The European model and half the SREFs place the line in Nrn PA while the GFS, NAM and the other half of the SREFs place it in Srn PA.

Here are the consistencies: (1) Large temperature gradient; (2) temperature affected by positions of High pressure over the Western Lakes & Low pressure S of the Mason-Dixon line; (3) All snow to start; (4) Northern PA and the mountains remain snowy; (5) Central PA's valleys see a changeover to sleet/ freezing rain Sat. AM; (6) Southern PA (especially near Pittsburgh and Philadelphia) see a changeover to rain during the wee hours Saturday; AND (7) wintry precipitation that occasionally mixes in cannot be ruled out. This is subject to extreme changes at times.

Long-range (2^nd half of the weekend into early next week):

A cold front will affect PA during this period.

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GOOD LUCK forecasting Friday night's storm.

-Jaron Breen