Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
641 PM EST Friday Dec 26 2014
..updated aviation discussion...
high pressure through Saturday with dry and mild conditions. A cold
front will bring the next chance of rain Saturday night and Sunday.
Near term /through Saturday/...
high clouds are quickly advancing into the County Warning Area this evening as middle
and upper level relative humidity values start to wander northward.
However...under those layers...fairly dry air remains firmly
entrenched. Additionally...with the inverted temperature
profiles...relative humidity values along the ridgetops continue to be well below
those in the lower elevations of the County Warning Area. That said...middle and high
clouds advancing northward should put a lid on huge temperature
falls tonight. While statistical guidance suggests much of the County Warning Area
should challenge freezing with lower 20s in the ridges...this
makes no meteorological sense. Inverted profiles should allow for
middle-slopes and ridges to be warmer than the valleys through the
night...particularly as 850 mb temperatures remain around +4 to
+5c. With that said...low temperatures were adjusted upward most
everywhere...particularly over Ohio and in the higher elevations.
Cloud expectations were likewise increased substantially through
the evening. Fries
Previous discussion follows for Saturday...
expecting another mostly sunny day Sat with highs well above average
under strong warm air advection ahead of the next cold front. 850 mb temperatures in the
5-8c range will engulf the area with mixing from modest srly flow.
A decent 40+kt jet will skirt our northwestern periphery Sat
afternoon/evening...but a Stout h9 inversion will hold most of
that wind back. Could see a few locations...especially
north...gust near 20kts. Tax
Short term /Saturday night through Monday night/...
high pressure becomes dislodged and pushed offshore from the
middle-Atlantic Sat night as a low pressure system moves through the
western Great Lakes. By late Sat night...moisture from another low
pressure system near the lower MS valley will move northward along a
shared boundary. This relatively weak boundary will approach our western
periphery as a cold front late Sat night into early Sun morning.
Rain showers will overspread the forecast area into the day
sun...although with limited moisture along the weak
front...precipitation is expected to be light.
Frontal progress slows as it gets stretched out and becomes parallel
to the midlevel flow. However...a fresh wave will ride
northeastward along the boundary during the day on Sunday...keeping
precipitation in the forecast. Best support/moisture clips the
southeastern corner of the County Warning Area..and kept categorical probability of precipitation here.
Behind the front Sunday night...colder air sags in slowly on initial
westerly flow. Expect most areas will see precipitation end before
any meaningful changeover to snow. A few showers will linger on
Monday in the southeast County Warning Area as the boundary finally pulls
away...while dry weather and some clouds linger for the bulk of the
area. While precipitation ends in the southeast Monday night...another
very weak boundary may try to sag into the northern counties through
Tuesday. Again...moisture and support will be very limited and
elected to maintain slight chance probability of precipitation here.
After above normal temperatures continue Sunday and Sunday
night...more seasonable values return Monday and continue thereafter.
Long term /Tuesday through Friday/...
the extended will largely be dry with a decreasing trend in
temperatures as a broad trough in the longwave pattern slowly sags
south across the eastern Continental U.S.. there is a slight chance of light
snow showers associated with a shortwave transitioning through the
flow develop mainly along and north of Interstate 80 during the
middle of the week...but moisture appears to be quite limited.
Further out...an area of low pressure previously closed off over the
southwest will eventually impact the region...but models vary widely
in timing of this feature...by a margin of several days.
Aviation /00z Saturday through Wednesday/...
Return flow quickly becoming re-established across the area early
this evening with ample high cloud expected to overspread the region
over the next several hours.
Within broader southwesterlies...models depict a shortwave impulse
moving through the area between roughly 09 and 15z late
tonight/Saturday morning. Convective middle cloud has been developing
across southern Ohio through WV over the past several hours...suggesting
this impulse may be a bit stronger than advertised.
Nonetheless...models show modest isentropic lift overspread the area
along with abundant relative humidity and cross-terrain flow to
support the notion of low end VFR if not locally mfvr ceilings
roughly in the 09-15z window across the terminals overnight. Not a
huge weather impact...but one Worth nothing. Additionally...given
the middle level support and low stability within the strengthening
warm advection regime...would not be surprised to see a few
scattered showers in this time frame as well.
Winds will be light from the south to south-southwest overnight...increasing
back to around 7-8 knots after 16z once the shortwave clears and we
break out into the true warm sector. Expect the cold front/attendant
precipitation to hold off until shortly after 00z tomorrow evening.
Outlook.../Saturday night through Wednesday/...
restictions possible with the cold front Saturday night and then
again with a passing wave across our south Sunday night.