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FXUS61 KPBZ 121720

Area Forecast Discussion
National Weather Service Pittsburgh PA
1220 PM EST Thu Dec 12 2019

Dry and relatively warmer weather will continue today into Friday
before a low pressure system returns rain to the area Friday night
and Saturday.


At 650 AM, made some minor updates to sky cover and hourly
temperatures. Generally quasi-zonal flow aloft will bring patches of
high clouds across the area through the afternoon, with plenty of
sunshine still figured.  

Previous discussion...

Strong, southerly return flow will begin today with eastward
progression of the sfc high and a digging trough across the upper
Midwest. WAA will drive temperatures up nearly 10 degrees higher than
Wednesday, though still a couple degrees below seasonal average.
Otherwise, quiet and mostly sunny(!) weather continues.


Continued warm, moist advection will slowly saturate the column
overnight in advance of a mid-level shortwave trough, resulting in
increasing cloud cover once again this evening. A warmer airmass 
along with the effects of cloud cover will result in minimum 
temperatures Friday morning around 10 degrees warmer than those of 
this morning and close to seasonal norms. 

The aforementioned mid-level wave will force weak cyclogenesis 
across the Southeast US tonight, resulting in a sfc wave lifting 
north as an eventual coastal low on Friday. While the low will remain
to our east while sliding up the coast, warm/moist air will overrun 
a cold sfc layer in the ridges Friday late morning into the 
afternoon. The complicated thermal profile will be favorable for 
freezing rain and possible sleet mixing in with rain and snow in 
portions of the higher elevations until the sfc warms to above 
freezing. Additional freezing rain will be possible Friday evening as
locations in the ridges drop back below freezing with warm air 
aloft. Some ice accumulation will be possible, especially across 
Garrett County, MD, and headlines may be needed.

Outside of freezing rain, a decent rainfall event is likely with 
this system as QPF values from 0.5 to 1 inch are expected over much 
of the forecast area through the weekend. 

Warm advection will push afternoon temperatures on Friday and
Saturday into the 40s.


Rain will transition to a mix of rain/snow or all snow Saturday 
night into Sunday morning as temperatures cool with cold advection in
the wake of the low. At this time, do not expect much in the way of
snow accumulation outside of light amounts in the ridges and higher
elevations given lack of deep moisture in the dendritic growth zone.
Continued westerly flow will also be unfavorable for typical, post-
frontal upslope snow.

Considerable model uncertainty continues with respect to the 
potential winter system Monday into Tuesday. Latest EC keeps the 
weak, crossing low to our south, keeping most of the CWA in the cold 
sector and in play for winter weather for much of the event. GFS, 
however, pushes a more robust low to our west and north across Lake 
Erie, limiting winter p-types to brief overrunning with the warm 
front. Given the uncertainty at this time, the winter weather threat
for snow and ice will need to be monitored over the next several

Expect temperatures to remain near seasonal average Sunday through 
Tuesday, depending on the track of the sfc low, with a cool down on


VFR conditions are expected through the forecast period. High clouds
are expected through most of tonight, with lowering/thickening clouds
after 12Z Friday as moisture increases ahead of an approaching
shortwave. Light precipitation will arrive after that time as well,
with freezing rain possible along the ridges and a rain/snow mix to
the west.

Restrictions are likely this weekend with crossing low pressure.



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