Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Pittsburgh PA
153 PM EDT Tuesday Mar 11 2014
low pressure will track across the upper Ohio Valley region tonight
into Wednesday evening bringing rain...snow...gusty winds...and much
Near term /until 6 PM this evening/...
bumped up maximum temperatures a couple degrees based on current readings and
expected decent amount of sunshine.
Surface ridging should keep dry weather across the region today...though Maryland and
high level clouds will increase through the day. Temperatures are starting out
warm...with highs 10-15 above average expd.
Short term /6 PM this evening through Friday/...
00z NAM surface low track is about 100 miles northwest of GFS/European model (ecmwf)
consensus...and have therefore not used the NAM in this forecast.
Stationary front will be draped across the region this
evening...with majority of precipitation remaining farther to the
north. Precipitation will not begin in earnest until after
midnight...with bulk occurring Wednesday morning. With recent
models having trended slightly farther to the northwest...have
also inched up overnight temperatures as well...especially with
southerly winds continuing through the time period. Still expect
that all precipitation will start as rain...with mix possible
across the north late tonight and into Wednesday morning. Think
that changeover to snow may not occur until Wednesday afternoon in
most locations...although one fly in the ointment is that models
are also showing a dry slot punching into the region during the
afternoon hours as well. This could cut down on snow
accumulation...as precipitation may actually transition to
freezing drizzle for a couple of hours with warm cloud top
One unexpected threat may be of severe weather...although the threat
appears limited right now. The surface low should pass near
Morgantown Wednesday afternoon...and high-resolution models indicate
that there could be a line of thunderstorms extending south along
the cold front. Although buoyancy will be limited...moderate
low-level lapse rates and wind shear could allow for some
thunderstorms across the southern most portions of the forecast
By late afternoon cold air will come roaring into the
region...turning all precipitation to snow. Cannot rule out the
possibility of a flash freeze with rainfall from earlier in the day
freezing in place with the rush of cold air. In addition...very
strong winds up to 40 miles per hour are also expected...and cannot rule out
the possibility of a Wind Advisory being needed. Temperatures will
plummet Wednesday night with lows dropping into the single digits
nearly everywhere. Wind chills will approach advisory criteria of
-10 Thursday morning north and southeast of Pittsburgh.
For most of the forecast area...did not change much when it comes to
snow totals...although a lot of uncertainty remains. At this point
it appears that the bulk of the precipitation will fall during
Wednesday morning when temperatures will support rain. Although
Wednesday afternoon would be the primary time for any heavier
snowfall...potential for dry slot and freezing drizzle limits how
high to go with snow totals. In addition...liquid precipitation
equivalents seem a bit high considering European model (ecmwf)/GFS both show an
open wave at 500mb and 700mb low closing off as the system
departs. Did bump up snowfall amounts north of Interstate 80
during afternoon hours. Upslope snow potential will be the best
Wednesday evening...but inversion heights will drop quickly after
midnight. Current forecast thinking is still 1 to 3 inches of snow
for most locations...with 3 to 6 inches possible north of US 422.
This would remain below warning criteria...and with there still
being a lot of uncertainty...did not feel the need to issue a
Winter Storm Watch for the northern counties.
By Thursday morning snow should be limited mostly to upslope or lake
effect snow...although additional accumulation should be limited by
low inversion heights. Even as high pressure ridge moves in and
skies clear out...temperatures will still end up some 20 to 30
degrees below normal. Increasing cloudiness Thursday night will act
as a bit of a thermal blanket and prevent as cold a night as
Wednesday night...but lows from the upper single digits to the middle
teens will remain well below normal. Strong warm air advection will bring temperatures
back to near normal Friday.
Long term /Friday night through Tuesday/...
rain and snow chances will rtn Friday night and Sat with the approach and passage
of a cold front. Genl East Coast trofg will keep below average temperatures through
Monday...though any significant systems are prognosticated to stay to our southeast. Flow
becomes more zonal by Tuesday with temperatures approaching seasonal avgs.
Aviation /18z Tuesday through Sunday/...
VFR conds expd into tonight as Maryland and high level clouds increase ahead of
approaching low pressure. This low will spread MVFR conds in rain across the
region Wednesday morning. An isolated thunderstorm is possible well S of pit but chances too low
for taf inclusion at this time. Experimental a change over to snow from north-S Wednesday afternoon.
Gusty north-northwest winds will develop as the low passes later Wednesday...with a strong x
wind expd at pit.
Outlook.../Wednesday night through Sunday/...
rstrns will continue Wednesday night as low pressure exits. Rstrns are again possible
Friday night into Sat with a crossing cold front.