Scientific Forecaster Discussion
fxus61 kpbz 220329
Area forecast discussion
National Weather Service Pittsburgh PA
1029 PM EST Thu Nov 21 2019
light rain showers will accompany the approach and passage of a cold
front across the region on Friday morning. Rain and snow shower
chances will persist over the weekend with passage of low pressure.
Near term /through Friday/...
overnight, upper support for precipitation remains fairly weak
initially ahead of the advancing cold front. Towards morning, the
approaching mid-level shortwave will provide better lift. In
addition, areas near and south of the I 68 corridor will also be
closest to right entrance region jet dynamics, thus enhancing
precipitation here a bit, and thus the highest pops were place here
towards morning ahead of the cold front.
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 /04z Friday through Tuesday/...
MVFR/IFR conditions with light rain showers is expected with
approach and passage of the sfc cold front through dawn Friday. The
passage of the mid level trough will end showers and bring a return
Wind gusts will increase Friday morning as cold advection off the sfc
deepens the mixing layer. West-northwest gusts of 20 to 25mph can be
restrictions are likely with passing low pressure this weekend.