PBZ issues Area Forecast Discussion (AFD) at Nov 26, 8:00 AM EST537
FXUS61 KPBZ 261300
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
800 AM EST Tue Nov 26 2024
.SYNOPSIS...
Dry weather returns Tuesday into Wednesday, followed by
additional rounds of rain and snow from Thanksgiving through the
weekend. Confidence is high that an Arctic airmass will impact
the Ohio River Valley late this week into the weekend.
&&
.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:
- Rain showers depart this morning; dry weather and clearing
skies expected this afternoon through tonight.
------------------------------------------------------------------
Rain showers depart to the east this morning in the wake of a
cold front that is pushing through the area during pre-dawn
hours. A cool and dry west-northwest flow sets up in the wake of
the front, helping to erode clouds from west to east during the
day, primarily across lowland areas south of I-80 where the
HREF is showing high probabilities (70-90%) for scattered or
better conditions generally after 2 PM. Meanwhile, weak lake-
enhancement north of I-80 and upslope flow along the ridges
could help keep cloud cover around in those areas through most
of the day, with HREF probabilities lower (30-50%) until closer
to 7 PM, after which some clearing is expected.
Despite clearing skies, cold air advection keeps temperatures in
the mid to upper 40s, which in combination with breezy winds
will result in wind chills hovering in the upper 30s throughout
the day. Weak cold air advection and efficient radiative
cooling help temperatures drop quickly into the upper 20s to low
30s overnight before clouds begin to filter back in from the
west, mitigating the latter process and potentially serving to
moderate temperatures towards sunrise Wednesday.
&&
.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
KEY MESSAGES:
- Drier conditions are likely through Wednesday evening,
followed by increasing rain chances again Wednesday night as
another low pressure system approaches.
-------------------------------------------------------------------
Mostly zonal flow dominates aloft across the Ohio Valley and
Great Lakes on Wednesday under the southern periphery of a broad
upper low centered over eastern Canada. This allows weak high
pressure to builds into the area as it slips south of a strong
Hudson Bay low, maintaining dry weather across the local area
through at least the first half of the day Wednesday.
Meanwhile upstream, a weak shortwave trough digs quickly from
the northern Plains southeastward across the upper Midwest and
over the Great Lakes. A surface low deepens in response over the
Tennessee Valley and a broad precip shield forms across the Ohio
Valley to its north. Timing among ensembles still differs
slightly in how quickly this precip makes it into our area, with
the NBM still showing probabilities for measurable rainfall
holding off until after 7 PM after which they increase quickly
to 60-80% areawide. Breaking guidance down a bit further, some
of the faster ensembles (notably the ECMWF-ENS and GEPS) have
the upper shortwave progressing more quickly eastward across the
Great Lakes and Ohio Valley during the day Wednesday, resulting
in probabilities for measurable precipitation increasing to
60-80% after 1 PM Wednesday. Meanwhile, the GEFS is closer to
the NBM with a slower shortwave progression and most likely
precipitation onset time after 7 PM. In order to reflect this
uncertainty, the current forecast shows 20-30% PoPs over
eastern Ohio and far northwest PA starting at 1 PM, increasing
quickly to 70-90% areawide after 7 PM. The big takeaway here is
that, while there may be some minor uncertainty in exact onset
timing, there is high confidence that precipitation overspreads
the area after 7 PM and continues through the rest of Wednesday
night.
Latest thermodynamic profiles suggest some snow could mix in
with rain from the onset along and north of I-80, with all rain
falling elsewhere. Little accumulation would be expected
initially, given surface temperatures above freezing. However,
temperatures drop closer to freezing late Wednesday night, at
which point some minor accumulations could be possible over the
northeastern fringe of the local area (Venango/Forest/Jefferson
counties in PA).
&&
.LONG TERM /THURSDAY THROUGH MONDAY/...
KEY MESSAGES:
- Rain and snow chances continue Thanksgiving Day through the
weekend.
- Confidence is high that an Arctic airmass will impact the Ohio
River Valley Friday through the weekend, possibly into early
the following week.
-------------------------------------------------------------------
A cluster of long range ensembles suggest the center of a
surface low will be positioned over the Pittsburgh region by late
Thursday morning. If this scenario plays out, areas south of
Pittsburgh will continue to experience rain while areas north
would experience a wintry mix and/or snow if northeast flow
persists, with the greatest chance of that occurring being along
and north of I-80.
Overall, probabilities of measurable snow are still low, with
the NBM depicting probabilities for 1 inch of snowfall being
limited in the 20-40% range in the highest ridges of northern
WV and the northern half of Venango/Forest counties. This is
likely due to snow mixing with rain and times, and snow ratios
remaining low (between 5:1 to 8:1). Precip tapers off late
Thursday and becomes more showery with the greatest focus
occurring along the ridges due to persistent upslope flow and
north of I-80 where some weak lake enhancement could occur.
A significant pattern change is expected early Friday morning
as a long wave trough deepens across the Eastern CONUS. Below
average temperatures and periodic rounds of snow are expected
through the weekend as individual shortwaves rotate through the
base of the trough, aided by lake and terrain enhancement.
The NBM continues to depict high probabilities (>70%) for
afternoon temperatures remaining below freezing across much of
the area Saturday through Tuesday (including the entire area
Sunday and Monday). Meanwhile, probabilities for overnight lows
dipping below 20 degrees are also high (>70%) each night from
Friday night through the end of the long term, with the only
exception being the Pittsburgh metro where urban influences are
helping keep probabilities slightly lower, in the 50-70% range,
though even in those areas the chances for low temps below 25
degrees is still high (>90%).
&&
.AVIATION /13Z TUESDAY THROUGH SATURDAY/...
Conditions across the area feature lingering MVFR and IFR cigs
across the area with post frontal cold air advection. Expect the
cigs to persist through mid morning before beginning to clear
out. Assisting with the clearing conditions, winds will be from
the west through the day with gusts up to 25 knots. A look at
the probability of cloud cover shows a 60% to 80% of cloud cover
below 25% by 21Z. Expect clearing by the end of the day with VFR
conditions likely by noon.
Outlook... Additional showers and restriction chances are
expected late Wednesday into Thursday with a new large-scale
disturbance. Restrictions and rain/snow may continue into the
weekend.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...Cermak/Shallenberger
NEAR TERM...Cermak/Shallenberger
SHORT TERM...Cermak/Shallenberger
LONG TERM...Cermak/Shallenberger
AVIATION...Shallenberger
Source:
PBZ issues Area Forecast Discussion (AFD) at Nov 26, 8:00 AM EST---------------
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