Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
713 PM EST Thursday Jan 29 2015
a cold front will cross the area tonight...followed by building
high pressure Friday and Saturday. Low pressure may bring more
wintry weather Sunday.
Near term /until 6 am Friday morning/...
The Kitchen sink and potpourri of goodness that has been falling
from the sky over the past roughly 12 hours is already showing
signs of transitioning toward all snow to our immediate north and
west. Surface observations and dual pol signatures suggest this is
occurring...and model soundings indicate rapidly steepening lapse
rates through the evening...even though surface temperatures may
take a few hours to fall off precipitously. As such...while all
but the northern tier of the County Warning Area is likely raining at the
moment...the trend toward frozen precipitation will be rapid
through the evening hours.
That said...freezing surface temperatures are only really noted in
surface observations from Cleveland to the north and west. As
such...several hours of snow are likely during the evening with
marginal temperatures for accumulation at best. As such...the
initial onslaught of snow that will be likely during the evening is
fairly unlikely to be an efficient accumulator. However...things
will drastically change overnight.
A deeper and much more thermodynamically abrupt short wave and low
level frontal boundary combo will move through the area between 06
and 12z tonight. As this occurs...the dendritic growth zone Will
Tank down into the saturated layer and given frontogenetical
lifting...this will likely lead to a period of fairly efficient
snow showers for all locations given steep lapse rates and large
scale forcing for ascent. In the ridges...this large scale ascent
will be overlaid with unidirectional northwesterly upslope flow
working on very unstable low level lapse rates. While moisture
flux into the unstable layer from Lake Erie will likely be minimal
due to abundant ice cover...saturation below the inversion level
will not be dependent upon this...and with
instability...upslope...and large scale ascent all overlaying
each other in the overnight hours...snow showers will likely be
heavy efficient dendrite producers. As such...while most locations
will only see 1-3 inches of snow overnight outside the
ridges...locations in the higher elevations seem more likely to
end up with 4 to 8 given ratios surging toward 25 to 1 overnight.
All advisories will remain in effect for the overnight and into
Friday. Heavy snow showers will likely be handled individually
with Special Weather Statement statements through the night. Fries
Short term /6 am Friday morning through Saturday/...
extended the Winter Weather Advisory through the day Friday for the
ridges...though with a Frozen Lake Erie will allow for the northwest PA
portion expire at Friday morning. Upslope snow showers should continue through the
day in the ridges before thinning moisture in the snow growth region and
lowering temperature inversions end the precipitation. A crossing upper trough should
allow for some scattered snow showers outside of this area as well.
Building high pressure should bring dry weather Friday night into Sat.
Below seasonal average temperatures were forecast using the latest sref numbers.
Long term /Saturday night through Thursday/...
models continue to joust in handling of low pressure projected toward the middle
Atlantic region from Saturday night into Monday. Persistence remains
prudent with latest GFS in genl line with current thinking outside
of timing issues. Meanwhile...European model (ecmwf) is forecasting a southward
track to surface slow center and precipitation shield. Both models continue to depict
an open middle level wave.
Bottom line is that an extended period of light...to moderate...but
accumulating snow is likely...with confidence building for areas
alng the Interstate 70 corridor.
Colder weather will follow as a series of shortwaves again dig an eastern
Continental U.S. Upper trough...details for which will be refined with time.
Aviation /00z Friday through Tuesday/...
condition deterioration is underway and degradations can be
expected into Friday as mixed precipitation spreads over upper Ohio terminals
and changes to snow with cold front passage this evening. IFR can be
expected in snow showers this evening...and in widespread stratus which
will accompany the cold surge over the region.
Outlook.../Friday night through Tuesday/...
improvements can be expected as high pressure builds later on
Saturday...however...snow and restrictions are likely with Sunday
Maryland...Winter Weather Advisory until 7 PM EST Friday for mdz001.
PA...Winter Weather Advisory until 10 am EST Friday for paz007>009-
Winter Weather Advisory until 7 PM EST Friday for paz074-076.
WV...Winter Weather Advisory until 7 PM EST Friday for wvz023-041.