Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1104 am EST Thursday Dec 18 2014
a weak disturbance will cross the region this evening...followed
by building high pressure Friday.
Near term /until 6 PM this evening/...
clouds will continue today with morning radiosonde observations showing low level moisture trapped below a
temperature inversion. A few flurries or light snow showers are possible...but
with the shallow moisture layer nothing significant is expd. Temperatures should average
a few degree below the seasonal average.
Short term /6 PM this evening through Saturday night/...
a weak shortwave is prognosticated to advance across the region tonight. With the continued
low level moisture and temperature inversion just scattered flurries are expd...though a
few light accumulating snow showers are possible with some limited lake and
terrain enhancement north of I 80 and in the ridges. Building high pressure
should end any remaining snow showers and flurries Friday morning...though
inversion trapped low level moisture should keep clouds across the region.
A shortwave across the southeastern Continental U.S. Is prognosticated to bring low chances for light
snow well southeast of pit lt Sat...though models continue to trend further S
with precipitation from this system. Temperatures should average a little below the seasonal
Long term /Sunday through Wednesday/...
the longwave pattern will become more meridional Sunday through
the first half of next week as a trough of low pressure digs south
over the central US and deepens as it transitions eastward. Latest
guidance shows good continuity with this feature and overall are
in fairly good agreement. With that and latest rmop and modtrends
very similar to 24hrs ago...opted to adjust the extended to be more in
line with a blend of the 00z wpc...ec...and gefs....which
resulted in a fairly big increase in both low and maximum
temperatures towards the middle of the week with higher confidence
in timing. Currently...the brunt of precipitation looks to fall
overnight Tuesday through Wednesday morning as rain ahead of the
cold front under strong warm air advection and moisture advection...with latest
ensemble guidance indicating v-component anomalies by Tuesday
afternoon around 2 Standard deviations above normal. Temperatures
are forecast to be around 10 degrees above normal by Tuesday
afternoon...and staying relatively warm Tuesday night as stronger
southwest surface winds ahead of the system keeps strong warm air advection in
play and the lower levels mixed. While the finer details
certainly could change...latest model forecasts show cold air behind
the front fully into the forecast area by Christmas morning at
which time all precipitation would changeover to snow. Changeover
should happen relatively quickly...but there is some uncertainty
on how long we may see mixed precipitation based on latest prognosticated
critical thicknesses. In general...currently around one half inch
of liquid precipitation is expected at this time ahead of the cold
front with light snowfall on the backside. Nomads reanalysis based
on the latest 00z run gives around a 50 percent chance of an inch
or so of snow falling on the backside of the front the night of
the 24th and into the 25th.
Aviation /16z Thursday through Monday/...
a persistent low level deck of stratocumulus will keep sites
mainly MVFR through the day...with times of IFR in light snow and
fog. Weak shortwave crossing region tonight will bring MVFR light
snow and near IFR ceilings.
Outlook.../Friday through Monday/...
stratus may persist into the weekend.