Scientific Forecaster Discussion
fxus61 kpbz 220001 aaa
Area forecast discussion...updated
National Weather Service Pittsburgh PA
701 PM EST Thu Nov 21 2019
generally light rain will accompany a cold front which will cross
late tonight and Friday morning. Another system will provide rain
and snow shower chances over the weekend. Seasonably chilly
temperatures can be expected.
Near term /through Friday/...
early evening update will include minor changes to the pops and a
refresh of overnight temperatures. A very dry atmosphere remains in
place, and it will take some time for the lower levels to saturate.
Because of this, and the lack of large scale Ascension to speed up
this process, have pushed back the onset of showers.
overnight, upper support for precipitation remains fairly weak
initially ahead of the advancing cold front. Towards morning, the
approaching mid-level shortwave will provide a better opportunity for
lift. Areas near and south of the Mason-Dixon line will also be
closest to right entrance region jet dynamics, enhancing
precipitation here a bit, and thus the highest pops were place here
towards morning ahead of the cold front, which will lie somewhere in
the County Warning Area at 12z. Temperatures will remain quite mild initially ahead
of the front, before dropping into the upper 30s to mid 40s toward
Short term /Friday night through Saturday night/...
precipitation coverage will begin to decrease Friday morning as the
cold front exits along with the support for lift. Low-level ridging
quickly builds in behind the front with moisture evacuating as well,
so despite low-level northwest flow, there will not be much opportunity for
lingering lake-induced showers. The boundary layer may cool in time
to allow a few snowflakes to fall just before precipitation ends,
but this is not expected to have any impact. All precipitation will
cease by later Friday afternoon, with temperatures dropping a bit
below normal once again.
A brief period of ridging will provide dry weather Friday night and
into Saturday morning. The next mid-level wave will then ride up the
Ohio Valley through Saturday night. The NAM keeps this wave further
south than most other guidance and is discounted. This system will be
able to scoop up better moisture than the previous one as
cyclogenesis occurs near the Gulf Coast, with southwest flow bringing
a 0.75-1.0 inch precipitable water air mass into our southern counties by Saturday,
which will help push quantitative precipitation forecast to around a quarter-inch.
Colder air wrapping in from the west behind the surface low will
allow a mix with or change to snow Saturday night. However, with the
dendritic growth layer remaining in the mid-levels and with a
relatively warm ground, accumulation should be limited to a dusting
in scattered cases. Overall, temperatures will remain fairly seasonable
for Saturday and Saturday night.
Long term /Sunday through Thursday/...
some precipitation will linger into Sunday before ridging takes hold.
Minor upslope enhancement could occur in west-northwest flow during the morning,
but lake enhancement appears unlikely due to insufficient lake/air
Delta-T values. Any additional very minor snow accumulation will be
limited to the higher ridges.
Surface ridging will provide mainly dry weather Sunday night through
Tuesday. Temperatures will rebound back to warmer-than-normal levels
in southwest flow. The next system will likely impact the region
during the Tuesday night-Wednesday period, with a low pressure center
crossing the Great Lakes. Much uncertainty remains with the track and
intensity of this low, with the ensembles providing a wide range of
solutions. Relied on the model blends for now, with precipitation
chances lingering into Wednesday night. A mostly dry and seasonable
Thanksgiving day is currently in the forecast, with high pressure
Aviation /23z Thursday through Tuesday/...
a low-level jet ahead of the approaching trough will create a period
of low level wind shear at all sites, primarily btwn the warm and cold frontal
MVFR/IFR conditions with light rain showers is expected with
approach and passage of the sfc cold front late this evening and
through dawn on Friday. The passage of the shortwave trough and
increasing sfc high pressure in the wake of the cold front Friday
will end showers and bring a return to VFR conditions.
Wind gusts will increase Friday morning and continue through most of
the afternoon. West-northwest gusts of 20 to 25mph can be expected.
restrictions are likely with the next low pressure system this