Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
404 am EST Thursday Dec 18 2014
it will be cloudy with near seasonal temperatures through the end
of the week.
Near term /until 6 PM this evening/...
temperatures this morning have stagnated as low level cold air advection has ceased and with
abundant cloud cover blanketing the area. Light upslope snow showers
and flurries across the north will continue to dwindle this
Lake enhanced moisture...saturating and deepening the boundary
layer...will return over our northern 1/3 of the County Warning Area by middle-morning with
chances of snow showers persisting throughout the near term. This
afternoon hi-res models keep lake effect snow production active
despite NAM bufr soundings show inversion heights lowering and a
slight warming of the mid-levels. Will maintain chance probability of precipitation across the
northern tier of the County Warning Area.
Elsewhere...clouds will continue to be ever-present...but
insufficient moisture will preclude any precipitation. With lack
of sunshine and no real change to the air mass to speak of...high temperatures
will remain below average with most locations flirting with the
freezing mark. Winds could make it feel about 5-10 degrees colder.
Short term /6 PM this evening through Saturday night/...
another weak wave will pass through the area tonight deepening the
boundary layer across the northern 1/2 of the County Warning Area by injection of
upstream lake moisture. 00z NAM shows 850 mb moisture pushing south
across Lake Erie late tonight embedded in the broad weak trough axis.
A cold pool coincident with this feature will bring 850 mb temperatures back
to near -11c. While not optimal...the colder temperatures aloft should aid
in snow generation with the weak wave. Will keep likely probability of precipitation
confined to the area north of I-80 where Lake Erie enhancement will
occur...but do expand probability of precipitation south nearing the Pittsburgh metropolitan area
and south along the Laurel and Chestnut ridges. Little to no
accumulation is expected.
High pressure will finally squash any remain chance of light snow
Friday. The low level inversion will hold steady through Friday with moisture
remaining trapped underneath...but erosion of that moisture does
occur. This should be the first chance to have at least some
sunshine return to the area.
The next system is poised to skirt our south and east Sat in the
form of an open wave. Models remain split on the northern extent of the
precipitation as deeper moisture will be fighting high pressure
through its progression through the Tennessee Valley. Backed off probability of precipitation and
slowed timing to fall in line with the latest GFS/ECMWF. As of this
issuance...even the sref mean has fallen more in line with the
aforementioned models...bucking the northern track it clung to earlier.
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 /09z Thursday through Monday/...
a persistent low level deck of stratocumulus will keep sites
under MVFR ceiling restrictions today...with sites south of Interstate 80
occasionally improving to VFR while sites north of Interstate 80 may
see IFR at times with occasional light flurries.
Outlook.../Tuesday night through Sunday/... periodic ceiling
restrictions will continue until boundary layer flow backs to the
southwest on Saturday ahead of an approaching middle-level trough of
low pressure. &&