301
FXUS61 KPBZ 231749
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
1249 PM EST Mon Dec 23 2024

.SYNOPSIS...
A quick hitting system brings rain and snow to the area
overnight Monday night with snow accumulation north of
Pittsburgh and a freezing rain threat to the West Virginia
ridges on Tuesday. Temperatures moderate for the remainder of
the week with a slight chance of rain on Christmas Day.

&&

.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:

- Quick shot of precipitation returns tonight with accumulating
  snow in the I-80 corridor and ridges.
- Rain or rain/snow mix more likely elsewhere.
--------------------------------------------------------------------

High pressure meandering off of the New England coast will provide a
dry day today. Warm advection with strengthening southerly surface
flow will push highs 10-15 degrees warmer than Sunday into the 30s
further north and 40s to the south. Cloud coverage will be on the
increase from the west as moisture increase ahead of an approaching
quick-moving system that will bring rain and snow to the area
tonight.

A shortwave will cross just to the north of our region tonight as a
weak attendant surface low tracks east across the Great Lakes
pulling a boundary through as it passes by. Most likely onset timing
of precip looks to be after 10pm into eastern Ohio and
northwest Pennsylvania. With the timing in the overnight hours,
probability of dominant precip type to be snow is high north and
east of Pittsburgh coincident with the colder air where
overnight lows <32F are 90-100% (30-60% further south and in the
urban areas). This will be a fast moving system as the primary
shield of precip will likely be out of the area by sunrise, save
some lingering upslope in the ridges into Tuesday.

In terms of snow, the profiles and ambient temperature along the I-
80 corridor are favorable for some accumulation. Antecedent dry
near-surface air will have to wet-bulb at precip onset, though
with little consequence here with cold enough temperatures as
is. Despite this being a quick hitter, a couple inches of
accumulation is higher confidence. Total liquid amounts are
pointing toward the 0.15-0.25" range, most or all of which will
fall as snow. Also of note will be the potential for high hourly
snowfall rates. Profiles show a saturated dendritic growth zone
with plenty of lift in that layer favoring efficient snow
growth and rates. HREF is suggesting a 70-80% chance of
0.5"/hour rates and even a 30-50% chance of 1"/hour rates. This
will accumulate easily on roadways whose temperatures are
currently sitting in the teens and aren`t likely to rise
substantially given that forecast highs on Monday are only right
around the freezing mark. All told, storm total probability for
at least 1" is very high at 90+% across Forest, Clarion,
Venango, and Jefferson Counties. Stepping up to >2", those
chances dip a bit to the 40-80% range, and <20% when you get to
>3". Wouldn`t be shocked to see highly localized amounts hit 3"
if the strongest forcing can maximize amounts, but the fast
moving nature will otherwise put a cap on them; generally the
distribution is favoring a most likely 1-3" range.

Snow accumulations will drop off further south with up to an inch
possible down to US 422 and into the PA ridges. For Pittsburgh metro
and south, temperatures become more marginal for snow and may
favor more of a rain/wet snow mix. QPF also looks lower as well
with probability of >0.10" QPF only around 40-60%. The most
uncertainty in precip type likely lies right within the
Pittsburgh area as to whether or not it`s cold enough for snow.
Akin to further north, ambient dry air will need to wet-bulb
initially, and soundings show those temperatures right around
freezing. Dynamic cooling may allow for a burst of wet snow, but
even at that, modeled positive snow depth change is low,
suggesting that the ground may just be too warm for accumulation
outside of grassy areas. West of I-79 and south of I-70, this
is more likely to be a rain event with perhaps some wet flakes
mixing in.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
KEY MESSAGES:

- Chance for a glaze of ice in WV ridges and possibly I-80 region.
- Winter Weather Advisory in effect for WV ridges.
- Warmer on Christmas Eve with afternoon highs challenging the
  chances of a white Christmas.
- Low precipitation chances on Christmas Day.
--------------------------------------------------------------------

Cloudy skies will be the theme for Tuesday with weak cold advection
aloft and moisture feed of of the lakes. On the back side of the
departing precip shield, another weak impulse in the flow is progged
to move through in the morning hours. Guidance is suggesting some
light precip in the form of snow lingering as the depth of the
moisture shallows, though this may be deceiving as a look at
ensemble soundings does not support much snow production with
moisture cutting off around -5C. This could point toward an
opportunity for freezing rain/drizzle along the I-80 corridor
and, with higher confidence, in the WV up to PA ridges where
temperatures will be coldest; HREF probabilities for measurable
ice accumulation are elevated in the ridges where weak lift
is acting the longest on a saturated supercooled liquid layer.
Confidence at this point is high enough to go with a Winter
Weather Advisory to cover this threat. An expansion into the
higher elevations of Fayette is possible, but confidence wasn`t
high enough with this package.

This talk of snow begs addressing the white Christmas potential.
Temperatures on Christmas Eve are very likely to warm above
freezing, so some melting is certainly probable during the daytime
hours. Snow depth may be challenged as could a white Christmas (1"
of snow on the ground at 7am on Christmas Day). The highest chance
will be for those who have the most snow on the ground from this
past weekend`s event and what falls Monday night.

In the wake of the Christmas Eve disturbance, ensembles converge on
a brief ridging solution downstream of another quickly approaching
trough across the Midwest that is favored to weaken and flatten out
as it approaches locally on Christmas Day. Some uncertainty comes
into play with potential phasing of a northern stream wave
traversing north of the Great Lakes with the weakening southern wave
which also presents a quicker arrival of said disturbance, but this
isn`t suggested by the majority of the clusters and the others
suggest higher heights and even point toward ridging holding on.
This would be a non-trivial solution though should it come to
fruition and provide greater moisture return in a bit more amplified
flow compared to other solutions that suggest only very low-end
probability rain showers in eastern Ohio. For now, will opt
toward a more optimistically drier solution. If precip does
occur, profiles aren`t favorable for snow with probability near
100% to exceed 32 degrees for highs.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
KEY MESSAGES:

- Warming trend through the end of the week.
- Uncertainty in the pattern by next weekend.
--------------------------------------------------------------------

For the latter half of the week, all clusters suggest building
ridging by Thursday, again save one solution that hangs back the
upper disturbance and associated precipitation chances into early
Thursday morning. Regardless, ridging will finally take hold with
570+ dam heights presenting as increasing probability. Deep layer
southerly component flow will support a warming and moistening trend
as highs sneak up to the 50 degree mark of Pittsburgh with a 40-60%
chance on Friday and incrementally stepping up further north
into the weekend. Confidence in a warming trend is increasing.
Another disturbance is possible come next weekend, but
significant deviation from the grand ensemble solution increases
in regard to precipitation chances.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
VFR conditions and south winds around 7-10 knots continue
through the rest of the afternoon at all terminals. Increasing
cirrus clouds moving in from the west gradually lower late this
evening to low-end VFR ahead of an approaching disturbance. A
quick round of snow moves through overnight from the northwest,
reaching FKL/ZZV around 03Z, PIT/AGC around 05Z, and MGW by
around 08Z. A quick deterioration to high IFR/low MVFR is
anticipated once precip begins. Low-end IFR or even LIFR will
be possible north of PIT where snow is also most likely to be
the dominant precip type. Southern terminals, from PIT south,
are more likely to see a rain/snow mix, while ZZV may remain
all rain. Precipitation departs to the east by or shortly after
pre-dawn hours Tuesday, around 09Z for FKL/ZZV and later for
terminals farther south and east. The exception is LBE/MGW/DUJ
where weak upslope could promote lingering drizzle or light rain
into the day Tuesday.

Outlook...
Restrictions, predominantly in the form of low ceilings, are
likely to linger from the end of the current TAF period through
at least Tuesday night. Ensemble guidance indicates a 60% or
greater probability of MVFR at all terminals during that time,
though IFR probabilities drop to 20% or lower.

Improvement to VFR will be possible Wednesday, when guidance
indicates probabilities for MVFR ceilings finally drop to 20
percent or lower at all terminals. Any improvement would be
short-lived, however, as probabilities rebound to the 30-50%
range (especially north of PIT) Wednesday night into Thursday.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...Winter Weather Advisory from 4 AM to 7 PM EST Tuesday for
     WVZ511>514.

&&

$$

SYNOPSIS...MLB
NEAR TERM...MLB
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...Cermak