Area Forecast Discussion
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FXUS61 KPBZ 212331

Area Forecast Discussion
National Weather Service Pittsburgh PA
631 PM EST Sat Jan 21 2017

Mild weather will continue through the weekend. Rain will move
back over the area on Sunday.


Mid and high clouds have overspread the region as of 5 PM, with
model indications of a fair amount of lift entering with a mid-
level wave generally at or above 20 kft. However, a chasm in the
RH field below this level has yielded little but virga as of this
hour, and as the wave ejects northeastward this evening, all signs
are pointing at this being the most likely continuing scenario.
Thus, showers were replaced with sprinkles in the forecast.

Behind this early evening wave, satellite imagery indicates a
brief but rather abrupt period of clearing is translating
northeastward. Hi-res guidance has some handle on this with sky
cover backing off considerably centered around 03z. Skies were
thus cut for all locations for a period of 2-4 hours from SW to NE
as this area of clearing moves through. With still high surface
dewpoints, this may yield a decent period of radiational cooling
and thus some patchy fog development as it does so likewise.

Behind the clearing, mid and high clouds look to increase again,
however decent lift really does not enter the picture from the
south again until well into the daytime on Sunday, so the
remainder of the overnight will be dry. Fries


Model depictions of the cutoff low pressure set to dig toward the
Upper Ohio Region have deviated from previous days solutions with
a more westward track of the center. Increasing ascent via deformed
flow and warm advection with increased moisture through the mid
levels all support escalating precip probabilities to categorical
numbers for the period.

With the northern branch of split flow holding colder air north and
west of the region, and with warm advection into the mid levels
of the cutoff, this looks to remain a rain event although the
latest NAM has developed sub-zero air over the region at the 850mb
level despite moist advection and an otherwise warm column.

QPF was progged using a RFC/WPC blend for the period. An inch to
inch and a half is anticipated for the early week with limitations
imposed by low pressure progression and mid level dry slotting.

Relative warmth will continue, although the diurnal temperature
spread will be restricted by clouds and precip.


The trend of the a slow low-pressure-exit on Tuesday continued in
todays deterministic and ensemble depictions. Colder air wrapping
into the western flank of the departing low may thus turn rain to
snow, before ridging ensues in response to plains low pressure.

That system is forecast to interrupt the resulting temperature
moderation via a cold front across the Upper Ohio, which will
drive temperature back to seasonal readings for the latter half of
the week.


General VFR conditions are expected tonight as weak shortwaves
maintain mid and high level clouds across the area, though a
brief period of clearing could result in patchy MVFR to local IFR
fog overnight/early Sun morning as temperatures drop near the dew
point. VFR is expected Sunday until the approach of a stronger
shortwave later in the day results in a deterioration to MVFR
conditions in rain.

Restrictions are likely through Tuesday with slow moving low
pressure, and again with a Thursday cold front.




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