PBZ issues Area Forecast Discussion (AFD) at Dec 29, 8:26 AM EST005
FXUS61 KPBZ 291326
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
826 AM EST Sun Dec 29 2024
.SYNOPSIS...
A low pressure system will lift northeast through the Upper
Ohio Valley today bringing strong winds and rainfall. There is a
potential for a strong storm or two this afternoon. A second
disturbance will track across the area on Tuesday.
&&
.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:
- Rain continue through the morning and into the afternoon.
- Wind Advisory in effect this afternoon through this evening.
- Unseasonably warm daytime highs of 20 to 25 degrees above
normal.
- Strong storms possible this afternoon and evening.
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Near-term grid updates were mainly cosmetic in nature,
freshening up the hourly PoPs and hourly T/Td grids. Did elect
to add a small chance of thunder today. Although overall
instability does appear fairly weak and shallow, pockets of
higher SBCAPE/MUCAPE could allow for some modest lightning
generation. Stratiform rain will continue to lift across the
region this morning, with a reduction in coverage in the warm
sector behind the warm front as some drier air wraps in
temporarily.
The 13Z SPC convective outlook update trimmed the marginal
severe risk area to eastern Ohio. This makes sense, as this is
where the HREF has consistently placed modest instability from
midday into the afternoon ahead of the surface low and upper
trough. With temperatures rising into the lower and mid 60s this
afternoon, a few hundred J/kg of MLCAPE remains possible.
Combined with strong low-level shear/SRH, there remains the
potential for downward momentum transfer of strong low-level jet
wind (60-70 knots at 850mb) in the stronger showers/storms, as
well as a brief tornado if low- level instability and SRH can be
realized. The overall threat does appear to be localized, as
low-topped showers/storms along the cold front should remain
fairly scattered. Best timing appears to be in the window from 1
PM to 7 PM or so. Further east into PA/WV, the threat is less as
instability will be much lower.
The Wind Advisory will be maintained, as stronger gusts will
begin to be realized as the low-level inversion starts to break.
The higher gusts may be realized more with showers/convection
during the afternoon and especially west of Pittsburgh, but the
terrain may still experience 45 MPH+ gusts as well.
Previous discussion...
The forecast features a low pressure system now positioned
tracking through the Mid MS Valley area this morning. The low
will continue to lift north through the day. As this happens the
warm front, currently positioned along the OH river in southern
OH. This will continue to lift stratiform rain north into the
forecast area. Trends with the ensembles suggest the main axis
of moisture has shifted more to the west and thus, the rainfall
threat is a bit less impactful than once thought. In fact, the
NBM only gives a 25% to 40% chance of an inch over the PA
portion of the forecast area. But the OH counties over the
northwest portion of the forecast area shows the best
probability getting an inch within 24 hours, roughly 60%
probability. As the low lifts north through the day, the low
level jet will begin to develop just after 15Z. The winds just
above the surface (roughly 850MB) will be between 60 and 70
knots. Its very possible that the strongest winds mixing to the
surface can occur with only a couple extra degrees of warmth and
allowing deeper mixing. With all ensembles pointing to a higher
max temperatures through the coming, the 90th percentile for
high temperatures today give near records across the area. The
90th percentile for winds gives Wind Advisory criteria. The NBM
probability gives a greater than 40 mph wind gust value of 40%
over the OH counties but its 20% or less over the PA counties,
except for the ridges where the probs are higher. All of that
said, the NBM is not handling the winds very well so went with
the 90th percentile for winds as it seems likely that somewhere
in the lowlands will see criteria. Thus a Wind Advisory was
expanded over the entire area.
The surface low and upper trough will continue to lift
northwest of the region tonight, with cold advection and
residual vort advection supporting scattered to numerous
showers, generally favoring areas north and west of Pittsburgh
after midnight and into Monday morning. Wind gusts will slowly
diminish overnight as the surface low pulls away.
&&
.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
KEY MESSAGES:
- Another period of rain likely Tuesday/Tuesday night and into
Wednesday with crossing low pressure.
--------------------------------------------------------------------
On Monday, the area will be under post frontal cold advection
with some assist from the lake. This will result in some
lingering showers through the day but mainly over the northern
counties. Cloud cover will be slow to scatter out through the
day. Dry weather likely returns (briefly) for Monday night amid
drier northwest flow and influence of surface ridging centered
over the Carolinas. Though cooler, temperature will remain
around 10 degrees above the daily average.
The next surface low pressure system will likely advance from the
Lower Ohio Valley Tuesday morning, crossing our region and reaching
the Middle Atlantic/southern New England area by 12Z Wednesday.
Ensemble guidance shows generally good agreement with this scenario,
although minor differences in trough timing remain, likely affecting
precipitation begin and end times. Rain is expected at onset given
warm profiles, with a gradual mix with/change to snow through
Wednesday as colder air pours in behind the system. The best
chance of accumulation through Wednesday night will be in the
higher terrain (50-90% chance of an inch or more over 24 hours),
with a secondary, lower probability region north of I-80.
&&
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
KEY MESSAGES:
- Late week pattern shift will favor below normal temperature,
periodic snow, and high terrain gusty wind.
--------------------------------------------------------------------
The extended period will stand to be a bit more active than
previously thought. The cluster analysis shows relatively high
confidence that a period of upper troughing will continue
Wednesday night into Saturday. Differences remain among the
various members with depth, axis timing, and shortwave details.
However, below- normal temperatures can be forecast with high
confidence. NBM probabilities of sub-32 degree max temperatures
are 60-100% areawide through the period. A surface pressure
gradient of varying intensity may also lead to gusty winds,
especially during the afternoon hours each day. Ensemble
probabilities continue to favor locations above 2000 feet for
potential advisory-level gusts (50-90% probability), but gusts
of 25 to 35 MPH appear possible across the rest of the Upper
Ohio Valley. Periods of snow showers can also be expected in
this pattern, with northwest PA and the higher elevations most
favored for accumulations.
&&
.AVIATION /13Z SUNDAY THROUGH THURSDAY/...
VFR is still noted at most terminals across the region at this time.
As expected DUJ/FKL are the exceptions to this with the former
experiencing LIFR conditions in fog and the latter just dropping
to IFR itself. CIGs are expected to begin to deteriorate most
notably along a ZZV-BVI-FKL line over the coming hours.
Stronger winds aloft above an inversion will create potential
LLWS conditions until the surface cold front crosses later in
the day and mixes out the lower levels. With the increased
mixing and the tightening surface pressure gradient, most sites
should gradually return to low VFR, but surface winds will
increase to 35kts on average later this afternoon and into the
evening. Instability is limited, though an embedded
thunderstorm will be possible in a broken line of showers that
develops along the front. Confidence is too low for thunderstorm
inclusion in the TAF at this time, though any showers have the
potential for strong wind gusts.
Outlook...
Restrictions and gusty winds are likely early Monday with a
crossing upper trough. VFR should return late Monday and Monday
night under weak high pressure. Periodic restrictions return
Tuesday through Thursday with crossing low pressure, and
subsequent upper troughing. Rain should also gradually change to
snow by mid week.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Wind Advisory until 10 PM EST this evening for PAZ007>009-
013>016-020>022-029-031-073>078.
OH...Wind Advisory until 10 PM EST this evening for OHZ039>041-
048>050-057>059-068-069.
WV...Wind Advisory until 10 PM EST this evening for WVZ001>004-012-
021-509>514.
&&
$$
SYNOPSIS...Shallenberger
NEAR TERM...CL/Shallenberger
SHORT TERM...CL/Shallenberger
LONG TERM...CL
AVIATION...88/AK
Source:
PBZ issues Area Forecast Discussion (AFD) at Dec 29, 8:26 AM EST---------------
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