PBZ issues Area Forecast Discussion (AFD) at Dec 31, 7:30 AM EST242
FXUS61 KPBZ 311230
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
730 AM EST Tue Dec 31 2024
.SYNOPSIS...
Low pressure will move up the Ohio River valley today bringing
showers and a potential thunderstorm. Colder air in the wake of
this low will change the precipitation to snow late tonight and
continue through Wednesday night. Colder weather is then
expected to take hold through early next week.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
KEY MESSAGES:
- Period of showers today and tonight with another low pressure
system.
- Potential for a strong storm or two this afternoon.
--------------------------------------------------------------------
The near term will feature a low pressure center tracking
across the OH Valley and eventually into the Upper OH Valley.
Here the 850MB low is progged to track through central IN and
into NE OH through the coming day. At this time, the amount of
cloud cover will increase through the early morning hours before
dawn. This will give some insulation to the area, effectively
slowing down the radiative cooling. The forecast challenge to
determining how cold surface temps get before the onset of the
coming low. Currently, there is a 80% chance of freezing
temperatures from Butler and north. This is pretty much early in
the night and will likely start to see some warm advection as
the low pushes closer to the forecast area. Thus, the most
likely scenario is that the onset of precip from this
approaching low will be a 12Z to 14Z timeframe and may result in
a couple instances snow in the OH counties before temperatures
warm well above freezing. Given the somewhat convective nature,
there likely wont be any impact during the morning commute. As
well, the timing seem to be after sunrise and temperatures
should be well above freezing by then.
Heading into the coming day, the approach will feature the
development of the warm front and will lift through the area in the
14Z to 18Z timeframe. During the time, a pseudo-warm section
will allow temps to warm somewhat into the afternoon allow for
marginal destabilization as the HRRR does give some 500 J/Kg
surface based instability and plenty of shear. While some
assisted lift from the ridges may be needed, the proximity of
the parent low and forcing associated with this disturbance, a
marginal (1 of 5) has been issued for the southern forecast area
mainly over the ridges, but this seems likely over a larger area
but south Allegheny County. That said, the main threat would be
winds and possibly hail although, the hail threat is less
likely.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY/...
KEY MESSAGES:
- Snow showers expected Wednesday/Wednesday night, with the
ridges most favored for accumulation.
- Winter Storm Warning issued for the West Virginia Ridges.
--------------------------------------------------------------------
After the passage of a strong cold front, cold advection starts
to really kick in later tonight with 850 mb temperatures
starting out above freezing and then crashing to about - 6C by
Wednesday morning, with continued cold advection taking them
closer to -10C by Wednesday evening. With west to WNW flow this
will set up a Lake Effect event mainly north of the forecast
area but brushing the northern portion of the I-80 counties with
perhaps 2-3" of snow. The ridges will also get 2-4" of snow
with 5-9" over eastern Preston and Tucker Counties in WV. That
along with 50 mph wind gusts has prompted an upgrade to a Winter
Storm Warning issued for the WV ridges. The probabilities from
the NBM highlight just above 80% for warning criteria in the WV
ridges and the HREF suggests near 90%, thus giving substantial
confidence to go warning. The PA ridges and I-80 counties will
most likely need a Winter Weather Advisory as the NBM probs
would suggest some 50% to 60% chance of getting 3 inches or more
in the I-80 counties and also the PA ridges. Thus, the advisory
decision will hold off until the 12Z and 18Z model data comes
in to refine the amounts and overall advisory area.
The impacts of the snowfall will continue through Wednesday night
but a wind shift to more westerly winds or even WSW winds will bring
an end to the snow accumulation over the PA and WV ridges. The
counties along I-80 will continue to see snow showers through
the day on Thursday. This also poses an interesting decision on
the advisory, potentially having the I-80 counties continuing
under the advisory longer than the PA ridges. Winds will still
be gusty through the day on Thursday with a 90% to 100% chance
of wind gusts over 35 mph on the ridges and even a decent 50% to
60% in the lower elevations on Thursday.
&&
.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
KEY MESSAGES:
- Late week pattern shift will favor below normal temperature,
periodic snow, and high terrain gusty wind.
--------------------------------------------------------------------
All the teleconnections point toward sustained cold through the
middle of January. MJO is in phase 7 and moving into phase 8,
which is a cold signal for our area. GEFS show PNA heading more
positive, while blocking near Greenland and high heights has the
NAO going negative and suggesting west coast ridge and east
coast trough. AO showing a negative trend towards a mean of -4
So the dreaded pattern of being well below normal in the coldest
part of the year after two mild winters will make this feel
relatively bad.
Another trough expected with roughly 100 meter height falls
around Friday, which helps drop 850 mb temps from -12C range
toward -16C. Given the flow turns more NW, expect another round
of lake effect snow showers over more of the forecast area.
Saturday looks to be the coldest day this week with temperatures
likely to be about 10F below normal.
Uncertainty then increases substantially for Sunday and Monday.
Timing and details of what looks to be a potent trough
somewhere from the southern Plains to the Great Lakes area will
obviously lead to a snow or ice event that could be significant.
Because there is a wide spread of temperatures and QPF it is
hard to know yet how much impacts there will be in the Upper
Ohio River Valley.
&&
.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
An approaching low pressure system will pull a warm front north
across the region late this morning into early afternoon, with
a band of rain and deterioration to MVFR expected along the
front. Local IFR is possible in heavier rain.
MVFR and scattered to numerous showers are expected after the
passage of the warm front this afternoon, with a thunderstorm
also possible with limited instability generally south of PIT.
Gusty winds will be possible in any storms that form. Further
deterioration to IFR is expected this evening as the low tracks
across OH and NW PA, dragging a cold front across the region
overnight (after 06z).
Southeast winds will increase this morning, with a few gusts up
to 20kt possible. Higher gusts are expected at LBE where
downsloping wind will accelerate. Expect a shift to the W after
the cold front pushes through tonight, with gusts up to 30kt
possible thereafter through the remainder of the TAF period.
Outlook...
Restrictions are likely Wednesday as rain changes to snow. Lake
and terrain enhanced snow showers could result in restrictions
lingering Thursday north of I-80 and along the ridges, with
improvement to VFR possible elsewhere. Snow and widespread
restrictions return Friday with a crossing cold front, and
Saturday under cold NW flow off of the Great Lakes.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...Winter Storm Warning from 1 AM Wednesday to 4 AM EST Thursday
for WVZ512>514.
&&
$$
SYNOPSIS...Shallenberger
NEAR TERM...Shallenberger
SHORT TERM...Craven/Shallenberger
LONG TERM...Craven
AVIATION...Cermak/WM
Source:
PBZ issues Area Forecast Discussion (AFD) at Dec 31, 7:30 AM EST---------------
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