Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
751 am EST Wednesday Jan 28 2015
high pressure will provide a dry but cold day. Crossing low
pressure will bring a wintry mix Thursday.
Near term /until 6 PM this evening/...
no major alterations were needed for the dawn update with pockets
of clouds and flurries expected to rapidly dissipate.
strong ridge will move into the Midwest today and surface high
builds over the upper Ohio Valley. Cold pool aloft will slowly
dissipate...but it will still be plenty cold here. Would expect to
see a good deal of sunshine...as drier air continues to drift
southward under northerly flow aloft.
Even with sunshine temperatures will still be well below normal.
Short term /6 PM this evening through Friday/...
Ridge continues to drift eastward tonight and warm air advection will be ongoing
ahead of the upper level system. Still a chilly night with clear
to partly cloudy skies and widespread snow pack. Temperatures will start
to slowly rise as daybreak approaches.
Upper level ridge will flatten late tonight and Thursday morning
as shortwave trough swings through the Great Lakes. Next system
will slowly make its way across the upper Midwest Thursday morning
reaching western Ohio by midday. Surface low associated with cold
front will remain well north of the region. Precipitation will
increase in coverage ahead of the front late Thursday morning
through the afternoon. This is when the forecast gets real tricky.
Warm air advection continues ahead of boundary as low level jet will pull warmer
air northward. Low and middle-level thickness quickly rise to levels
too warm to support frozen precipitation. Depending on which model is
correct...this warming may envelop the whole area. Temperature
will rise Thursday...but with a very cold ground and large snow
pack the surface itself will be slow to respond. There is a high
threat Thursday for a period of ice across all of the area. The
real headache comes from the fact that sensors may show temperatures
above 32...but with the aforementioned ground conditions...rain
will freeze as it contacts the surface. Will use a blend of the
GFS and NAM for precipitation type...but the key will be to figure out
the surface temperatures...which will be no easy task.
Cold front quickly clears the area Thursday evening and much
colder air will plunge back into the region. Low and middle-level
thickness quickly respond and expect an atmosphere cold enough to
support all snow by 06z Friday. By this time...most...if not all
of the precipitation associated with the front will be to the east.
However...with strong cold air advection and the main shortwave trough swinging
through...expect snow showers to continue into Friday even with
lowering inversions and a Frozen Lake Erie. Snow growth zones rapidly
drop Friday...so until the lower layers dry out...the threat for
snow showers will continue through much of the day. Will focus
highest probability of precipitation on the ridges and far north.
Expect high temperatures on Friday to occur in the early morning with a
steady decline throughout the day.
Long term /Friday night through Tuesday/...
cold Arctic high shifts east into Saturday with snow showers in
northwest flow Friday ending Friday night. With 850mb temperatures
prognosticated near -20c by 00z Saturday temperatures Friday will be
steady or fall. Clouds should begin to break as center of high
moves into region Friday night...with lows below zero north and
single digits south. If conditions go clear and calm Friday night
may go below zero across the entire region. Temperatures recover
to mainly the 20s Saturday. A significant storm looks possible for
Sunday into early Monday before Arctic air returns to start the
Aviation /13z Wednesday through Sunday/...
areas of MVFR/isolated IFR ceilings are expd to dspt this morning as sbdnc
increases under building high pressure. VFR should continue into Erly Thursday until
low pressure brings rstrns and a wintry mix of precipitation.
Outlook.../Thursday night through Sunday/...
rstrns are likely through Erly Friday as a cold front exits. Rstrns are
likely again with advancing low pressure on sun.