Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Pittsburgh PA
1231 PM EST Wednesday Feb 10 2016
snow showers this afternoon and tonight will gradually diminish
Thursday. Temperatures will continue to fall through the week with
bitter cold to start the weekend.
Near term /through Thursday/...
for the afternoon update changes to hourly temperatures and winds
were minor. Hi-res models continue to show band of snow along
Lake Erie transitioning south late afternoon and evening as trough
and wind shift passes. Previous discussion below.
Snow showers should continue this afternoon ahead of an advancing
shortwave trough. The trough will swing through this
afternoon/evening...further deepening the boundary layer and
giving rise to snow squalls. Near term models all show
favorability for snow squalls from 18-00z in the btv snow squall
parameter. This will have the potential to impact the evening
commute...especially for areas from Pittsburgh north where deeper
lake-enhanced moisture resides.
Lake-enhancement will continue in the wake of the departing upper
trough tonight. Current guidance continuing the trend of showing
multiple lake-to-lake snow bands setting up. Upslope snow will
continue and even strengthen tonight as the colder air pours in
and the dendritic snow growth zone descends through the column.
Expecting snow ratios to maximize this evening through
tonight...which will allow for efficient accumulation.
Temperatures will continue their descent into the well-below
normal range. Many locations will be reaching into the single
digits before sunrise Thursday.
A complicated headline configuration played out this
morning...including an extension of the current long-duration
Winter Weather Advisory in the ridge communities. In
addition...have opted to issue a new advisory for today through
early Thursday for Pittsburgh metropolitan north for the potential impact
to the evening commute and the prolonged efficient lake-effect
snows. Decided to keep areas south of Allegheny County out of the
advisory as snow showers should be more scattered in nature.
Short term /Thursday night through Saturday/...
a bit of a break in the action looks to be in store for Thursday
night into early Friday as low level flow backs to be a bit more
southwesterly. This will cut off the over-lake flow
component...allowing for the boundary layer to dry out and skies
to temporarily scatter out. Inversion heights also falling will
yield a clearing and cold night on Thursday night...however with
flow turning southwest...things high single digits seem a good bet
with new snow cover.
The next system in the northwesterly flow aloft looks to dive
southeastward toward our area for Friday evening or so. Model
timing with this system has been suspect over the past few
runs...and the trend on the 00z cycle was to bring it in a bit
later Friday than in previous cycles. That said...lapse rates look
to rapidly increase through the dendritic layer concurrent with
strong differential positive vorticity advection moving through the area in the left exit
of the upper jet streak. Low level frontogenesis maximizes ahead
of the vorticity maxima and a bout of snow squalls seems likely to move
southeastward through the area. Behind the round of squalls...deep
layer northwesterly flow should allow for lake effect snow showers
to resume with a strong upslope component into the ridges.
Accumulations will be likely most everywhere as this situation
Long term /Saturday night through Tuesday/...
as lake effect winds down and drier air moves over the area by
Saturday night...skies should start to break up. Temperatures at
850 mb on all models drop into the middle -20s celsius by Saturday
night...making this likely the coldest air for this winter to
wander across our area. A multi-model consensus favors Saturday
night as the coldest night of the pattern...with widespread
numbers below 0f at the surface likely overnight.
Shortwave ridging and warm advection follows the brunt of the cold
air going into early next week...however large scale cyclonic flow
will re-establish itself thereafter. This means periodic systems
moving through the area with generally normal or below normal
temperatures and chances of snow showers through the period. Fries
Aviation /18z Wednesday through Monday/...
while most terminals are bouncing back and forth between MVFR and
IFR in occasional snow showers...primary focus in the first six
hours is on a heavier snow band currently across the Lake Erie
shoreline that is supposed to move across the region later this
afternoon and evening. The northeast to southwest oriented band
has been moving slower than anticipated by model forecasts...and
the timing in the tafs still could be an hour or two earlier
considering recent trends. Have generally gone with 1 mile
visibility for a few hour window...but potential is definitely
there for visibilities to drop to 1/2 mile or less. Confidence was
not high enough to include that specific mention in 18z tafs. The
band may stay far enough north as to not impact the zzv/mgw
After the band moves through...winds should shift a little more
west-northwest...allowing lake effect snow showers to develop.
Strongest snow showers will be at fkl/duj...with IFR/LIFR
conditions expected. Elsewhere...think that intensity of snow
showers will be light enough and ceilings high enough to keep MVFR
flight categories instead of IFR flight categories. Coverage of
showers should decrease by Thursday morning...although snow will
continue at fkl/duj.
Outlook.../Thursday through Monday/...
occasional restrictions are expected through the period under
cold upper-level troughing. While Thursday and Sunday should be
generally dry...scattered to widespread snow showers are expected
Maryland...Winter Weather Advisory until 7 PM EST Thursday for mdz001.
Ohio...Winter Weather Advisory until 7 am EST Thursday for ohz041.
PA...Winter Weather Advisory until 7 PM EST Thursday for paz007>009-
Winter Weather Advisory until 7 am EST Thursday for paz013-014-
WV...Winter Weather Advisory until 7 PM EST Thursday for wvz510>514.