Area Forecast Discussion
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FXUS61 KPBZ 101010
Area Forecast Discussion
National Weather Service Pittsburgh PA
510 AM EST Sat Dec 10 2016
Lake effect snow will wind down north of I-80 this morning with a
few breaks in the clouds across the area. Cold air will remain in
place in advance of the next system that will affect the area
tonight through Monday with snow, rain, and the potential of a
wintry mix before much colder air arrives late in the week.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --Fairly unstable lapse rates remain through the dendritic layer
this morning with near saturation also noted in RAP soundings
through the layer. This is occurring as a weak wave seems to be
passing through on water vapor imagery as well. The net result is
flurries pushing eastward through the area, which should
dissipate as the morning goes on and drier air encroaches.
Likewise farther north, lake effect snow bands that had resided
over Mercer and Venango County all night due to persistent NW flow
through the dendritic layer even with southwesterly surface flow
have gradually moved northward as 850 mb flow turns more westerly
and eventually southwesterly today. This should end accumulating
snow in these areas, and the previous lake effect snow advisory
was thus dropped.
Some breaking up of clouds will be likely during the daytime today
as time height sections reflect a drier column with surface flow
favoring a southerly component. With that in mind, high
temperatures may manage to muster a bit more upward mobility than
in previous days even as no areas seem poised to challenge
The next system starts to affect the area after midnight tonight
with isentropic ascent increasing in the 285-295K layer.
GFS/NAM/SREF reflections of lift all favor a band of modest
moisture transport in the layer focused first over the southern
periphery of the area, then transiting slowly northward as warmer
air advances northward into the Ohio Valley. Consensus QPF favors
roughly a tenth of liquid equivalent as this crosses with a solidly
below freezing column and an impressive depth of saturation and
lift through the dendritic layer. Thus a general inch or two of
snowfall seem likely as this area of isentropic ascent in advance
of a northward migrating warm front seems likely overnight tonight
into Sunday morning for most areas. Fries
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.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
A complex weather scenario will unfold over the latter part of the
weekend, stretching into Monday. While there are still some unknowns
with this system, confidence in several features including the warm
trend is growing. Increasing southerly flow on the back of the
surface high will shift the warm advection snow band northward
Sunday. Models still differ on just how far north this band gets,
with the NAM bullish on fully clearing the northern periphery, which
seems a bit hasty, given the strongest southerly flow is still well
back to the west. Regardless, most operational models show a carved
out warm sector drawn into the area, so PoPs south and east have
been trimmed accordingly. The result is categorical light snow
continuing north of Pittsburgh Sunday afternoon, while what
lingering precipitation that exists south of Pittsburgh transitions
to a rain/snow mix as the warm push begins.
By Sunday night, the nearly stacked low will move through the Great
Lakes. The center of the low this far west favors strong warm
advection overnight into Monday. Pushed by a stout low-level jet in
the warm sector, 800-900mb temperatures will increase quickly.
Surface temperatures will gradually warm through the pre-dawn hours,
but will lag behind the warming mid-levels. The result will be a
relatively complex precipitation type forecast. Current forecast
calls for pure rain making it to Pittsburgh by the morning rush,
rain/snow north to I-80, and snow persisting north of that.
Southeasterly flow near the ridges will hold the cold air in the
eastern facing slopes, ultimately favoring a freezing rain or sleet
profile before flow veers southerly.
Given the recent warm trend seen in the models, will carry just a
chance of freezing rain, and hold off on any FZRA headlines. In
terms of snowfall, the best estimate for totals from Saturday night
through Monday morning is still about 4-6 inches north of I-80 in
PA, with only an inch or two across Pittsburgh. Length of snowfall
duration and melting of accumulating snow will preclude any snow
related headlines at this time.
The warm push continues well into Monday, transitioning all
precipitation south of I-80 over to rain. This should quickly cut
into any laying snow that had accumulated in the previous 24hrs.
Strong moisture transport along the trailing cold front will bolster
rain efficiency through Monday afternoon. Will continue to carry
categorical PoPs with the frontal passage. Showers will taper down
Monday evening as the front clears our eastern edge and upper-level
moisture evacuates. A mostly west wind behind the cold front will be
detrimental to any continued lake-effect or upslope snow, but enough
low-level moisture could maintain patchy drizzle.
.LONG TERM /TUESDAY THROUGH FRIDAY/...
High pressure in the wake of the early week system will
continue the dry trend through Tuesday. An arctic cold front will re-
enforce the cold west-northwest flow Tuesday night. Dry high
pressure at the surface will cut-off any moisture feed along the
front, thus limiting precipitation potential with it`s passage.
Another shot of cold then comes with a passing shortwave trough
Wednesday night. All told, by Thursday temperatures will be well-
below average with high temperatures expected to be 5-10 degrees
below our average lows. Next best chance of widespread precipitation
comes with a system next early next weekend. At this point, models
showing fairly good agreement with the developing stages.
Temperatures will begin to moderate by the weekend as the upper-
level flow amplifies ahead of this next system.
.AVIATION /10Z SATURDAY THROUGH WEDNESDAY/...
Lake effect snow bands will continue to impact KFKL and KDUJ into
the dawn hours. Elsewhere, MVFR cigs will gradually improve to VFR
today as drier air arrives from the SW. The next system will roll
into the area tonight with snow overspreading all sites near the
end of the TAF valid period.
Widespread restrictions are likely Sunday and Monday, and again
Wednesday, with crossing areas of low pressure.