Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Pittsburgh PA
1225 am EST Thursday Nov 27 2014
another shot of cold air will move through later today and
tonight. This will provide the risk for snow showers. High
pressure will improve conditions on Friday.
Near term /through today/...
overnight period looks to be in good shape. Will mention areas of
dense fog overnight. Plenty of residual moisture near the
surface...due to earlier snow melt and lack of flow. Drier surface air
to the west...as seen with the lower dewpoints...is trying to
nudge into the region...which will help to prevent a widespread
event. Will keep an eye on visibilities overnight...and if the
dense fog becomes widespread enough...will consider an advisory.
Temperatures have been adjusted with latest guidance.
Colder air will slowly spread across the area today. Flow at the
middle and upper levels will remain light and unorganized through
much of the day...providing little push...thus the slow change in
air-mass. By this afternoon...mixing heights will rise as large
scale Ascension increases ahead of shortwave trough. Colder air at
850 mb will March eastward and the flow aloft will increase and
become better organized. Mixing heights will rise enough to reach
the snow growth zone. Will slowly increase probability of precipitation this afternoon...
with the highest numbers across the north and ridges.
Short term /tonight through Saturday/...
a series of weak upper level waves will cross the area tonight.
This will drive The Heart of the cold air southeastward across the
region. Flow will remain generally out of the northwest tonight...so
will expect snow showers to continue until the second wave exits
around dawn Friday. Again...will focus the highest probability of precipitation across the
north(lake influence)...and ridges(orographic effect).
Shortwave will quickly exit to the east Friday morning...
inversions will lower and subsidence will increase as broad high
pressure moves eastward across the lower Ohio Valley. Any
lingering snow showers will quickly end Friday morning. The cold
pool aloft will slowly eject to the north which will allow for a
decrease in cloud cover.
Warm air advection will continue Saturday night as a warm front crosses the area
and flow in the lower and middle-levels becomes south-southwesterly.
Models are hinting at the development of isolated snow showers
developing with the passing front. Will include schc probability of precipitation Friday
A weak disturbance will cross on Saturday...which will temporarily
put The Breaks on the warm air advection. Could see a passing shower Saturday
afternoon...but deep moisture is lacking and the wave is
unimpressive. For now will leave Saturday dry.
Temperatures will moderate on Saturday.
Long term /Saturday night through Wednesday/...
shower chances will increase Sunday...ahead of the next cold
front. Front will clear the area early Monday morning. Showers
will end but temperatures will cool down on Monday. High pressure will
provide a dry and chilly Tuesday...with temperatures moderating on
Aviation /05z Thursday through Monday/...
although most sites have IFR conditions as of 00z...difficulty
with tonights forecast is determining when conditions will lift to
MVFR. Think that many sites will see some improvement by
midnight...while others will likely remain IFR until morning. Low
level moisture is shallow...but with very light flow...there may
not be much of a mechanism to prevent air from stagnating.
Scattered lake effect showers are expected to develop by Thursday
afternoon...but coverage will not be not high enough to warrant taf
mention at this time.
Outlook.../Thursday night through Monday/...
non VFR developing Thursday in -shsn...lingering across northwest PA into
Friday. Non-VFR Sunday in -shra with the next front.