Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
639 PM EST Friday Nov 21 2014
high pressure will dominate tonight. A couple of weak disturbances
will cross the region over the weekend until low pressure returns
widespread showers Sunday night.
Near term /through Saturday/...
freezing rain chances increasing from Zanesville to Beaver to
DuBois and northward for Saturday morning and early afternoon...
High pressure will maintain control for the night tonight as
winds quickly subside to barely a southerly drift. Dewpoints over
the region are largely in the single digits...and while a bit more
moist air is poised to advect toward the region going toward
sunrise...this will do little to decrease the downward potential
of temperatures overnight...particularly in areas with snow on the
ground. That said...teens seem to be in store for most locations
except the scattered single digits in the cold spots with snow
cover. Temperatures may start to edge upward toward morning as
southerly flow starts to increase and cloud increase from the west
as well ahead of the next system.
Lift in the 290-300k layer quickly increases from the southwest
around 12z on Saturday. Condensation pressure deficits in the
layer generally bottom out around 10 mb in a corridor from
Zanesville to Beaver to DuBois and northward going into the latter
half of the morning hours. Additionally...the boundary layer below
the lifted layer seems to saturate sufficient. Warm advection in
the lifted layer seems likely to bring temperatures in the warm
layer aloft into the +2 to +5c range over a depth of about 5 kft.
This should be sufficient to melt all falling hydrometeors as they
fall. Given the extremely cold start to the day and weak surface
layer advection under the inversion...along with the recent very
cold weather...frozen ground...and snow cover in the northern
reaches of the forecast area...it does seem that light freezing
rain will be likely during the morning and even early afternoon
hours from Zanesville northeastward to the I-80 corridor.
Precipitation rates look to be very light...however restricted
upward mobility of temperatures due to wet bulb effects and a cold
start mean some ice accumulation will be possible.
Due to the fact that the atmosphere is likely to be very dry at
the onset of the event...as well as the fact that lift and
saturation is not particularly strong whatsoever...confidence on
widespread ice accumulation is not high at the moment.
However...confidence in at least some freezing rain north of the
aforementioned areas is sufficient to raise the probability of
freezing rain toward 60 percent. This is not sufficient at the
moment to issue a Freezing Rain Advisory...however it will likely
need to be looked at more closely as the precipitation develops.
However...the hazardous weather outlook was updated to outline the
area most at risk for some icing Saturday morning through early
Temperatures will increase by late afternoon at all sites enough
that any falling precipitation by that juncture should be plain
Short term /Saturday night through Monday/...
an upper trough is prognosticated to move into the central Continental U.S. Through the
weekend. Several weak shortwaves will rotate around the base of the trough and
advance across the Ohio Valley and Great Lakes region in SW flow. The first of these
is prognosticated for a Sat passage. The best moisture and upper spprt is
prognosticated to stay along I 80 and northward...so maintained shower chances in this
region. Did mention just a slight chance of freezing rain in this region as
critical thicknesses indicate liquid precipitation...and surface temperatures may
still be cold enough if precipitation would occur in the morning.
After a couple more weak waves Sat night and sun...a stronger shortwave
with deep moisture and upper spprt is prognosticated for a Sun night passage.
Maintained Cat shower probability of precipitation with this feature. As the main trough advns
toward the Great Lakes Monday...a cold front is expd to cross the region. With limited
moisture kept probability of precipitation in the chance category for now.
With warm advection through much of the period...temperatures should gradually warm to
several degree above seasonal levels by lt in the weekend.
Long term /Monday night through Friday/...
widespread showers end Monday night with colder air moving in
through the long range. Close to wpc forecasts. Fast moving shortwave
will bring a chance of snow showers Thursday.
Aviation /00z Saturday through Wednesday/...
VFR overnight with sky clear and light winds the rule. Warm advection
Saturday morning will bring a return of clouds...although ceilings
will initially remain VFR. Not included yet but low level wind shear is a
possibility as a SW low level jet Cranks up. As noted in near term section
above...freezing rain is a possibility across the northwest portion of the
forecast area. For now...handling via prob30/tempo groups for
zzv/fkl/duj. Confidence still a bit low for inclusion further
south but bvi/pit/agc will need watching. Ceilings will lower a bit
towards the MVFR/VFR threshold during the afternoon. Temperatures
should warm sufficiently to allow for remaining freezing rain to turn to
plain rain towards midafternoon. Wind gusts up to 20 knots from
the southwest can be anticipated. Cl
Outlook.../Saturday night through Wednesday/...
possible MVFR to IFR conditions Sunday night into Monday as the
next storm system approaches.