PBZ issues Area Forecast Discussion (AFD) at Nov 29, 10:35 AM EST074
FXUS61 KPBZ 291535
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
1035 AM EST Fri Nov 29 2024
.SYNOPSIS...
Isolated snow showers today with highest chances in the ridges
and north of I-80. A prolonged cold period sets in with
occasional snow shower chances likely through the beginning of
December.
&&
.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:
- Isolated snow showers in the lowlands.
- Minor accumulations with upslope snow in the ridges and lake
effect north of I-80.
- Temperatures gradually falling through the day.
-------------------------------------------------------------------
Mid morning update made to make only minor adjustments to PoP
and temps based on latest obervations and trends. Some light
snow showers continue in the meager moisture across the lowlands
as a shortwave embedded in the upper trough pivots across the
region. Strengthening cold advection and cloud cover will keep
temperature from warming much, if at all today.
Previous...
Cold advection really gets cranking with probability for 850 mb
temperatures <-10C all but a guarantee by the afternoon hours
as the core of the thermal trough passes overhead. With Lake
Erie water temperature observations in the low 50s, the lake-
effect machine will be hard at work. Effects for most of our
neck of the woods are looking limited though with 925-850 mb
flow nearly due westerly. This should orient lake-effect
streamers such that most remain out of our area, save for
extreme norther Mercer, Venango, and Forest where CAMs wiggle a
lake effect band around this morning. It will be a very tight
gradient for accumulation, but if the band sags far enough
south, those areas could see a couple hour period of heavier
snow this morning. Significant lake effect snow accumulation is
expected off to our north; if your post-Thanksgiving travel
takes you along I-90 anywhere from Cleveland to Buffalo, please
visit the NWS Cleveland or NWS Buffalo web pages or social
media for more information.
Upslope driven snow in the PA and WV ridges also expected today
as an upper wave moves through with low level flow out of the
WNW to W. Aided by non-zero SBCAPE, some minor snow
accumulations are possible on the order of 1-3 inches, limited
by a shallow moist layer within the dendritic growth zone and
drier sub-cloud layer air. Through tonight, HREF probabilities
for 3" are up to ~40% in localized areas given a scattered
mode. Elsewhere in the lowlands, we'll be struggling with a
similar drier sub-cloud layer with Td depressions of 5-10
degrees. Low-level lapse rates will surely steepen with 850 mb
cold advection, but coverage is likely to be very isolated, so
will carry a slight chance snow shower mention.
Friday night will be the first night of a couple with lows
likely dipping into the teens with 80-90+% probability outside
of the immediate urban areas. Efficient mixing will promote
gusts to 25-30 mph today (potential for 40-45 mph in the WV
ridges) and continuing overnight tonight, so wind chill values
early Saturday morning likely reach single digit readings
(potential for zero or a couple degrees below in the WV ridges).
With this being the start of the first notable cold spell in an
otherwise warm fall, be sure to bring pets indoors and dress in
layers if spending time outdoors.
&&
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
- Some lake effect snow possible north of I-80 on Saturday.
- Well below-normal temperatures filtering in for the weekend,
with near-zero wind chills Saturday and Sunday morning.
-------------------------------------------------------------------
500 mb height rises into the day on Saturday with increasing dry air
in the wake of departing shortwave energy and a brief transition to
more quasi-zonal flow will keep most of the area dry but still cold
with only 20-30% probabilities of highs reaching above freezing
both days. It looks as if some of the area, especially
Pittsburgh and south, could actually see some sun on Saturday
with drier air building in. Will introduce some snow shower
chances on Sunday driven by pieces of shortwave energy pivoting
the upper low. The best lift is progged south of Pittsburgh
along with the continuing lake effect, but flow becoming more
south of west should help orient the bands further out of our
area. NBM probs for 1" south of the Mason Dixon line sit around
30-40%.
Kept the temperatures for the weekend period below the
deterministic NBM due to its warm bias as we enter the cold
season. Steep lapse rates will promote continued mixing of wind
gusts with a 60-80% chance of >30 mph gusts Saturday relaxing
some on Sunday.
&&
.LONG TERM /MONDAY THROUGH THURSDAY/...
KEY MESSAGES:
- Much colder temperatures remain into next week.
- Rounds of shortwaves maintain snow chances.
-------------------------------------------------------------------
The first couple days of the long term period will be driven by
ECONUS troughing as an upper low across far northern New England
absorbs back into the flow. Temperatures will remain below normal
with high confidence through at least the middle of next week with
the core of the 500 mb thermal trough still moving overhead; there
is still a 70-90% chance of below freezing daytime highs through
next Wednesday. The lake effect snow will still be cranking come
Monday as a series of shortwaves dives through the Great Lakes into
early Tuesday, and 850 mb flow may take on a bit more of a
northwesterly component albeit not the most favorable direction for
snow bands to dip south into our area. Better upslope across the WV
ridges would be favored and the chance for measurable snowfall bumps
up to 60-80%, highest around and north of I-80. The goal posts are
still generally wide up there with nearly nothing on the low end and
up to 2-3" on the high end dependent on a more potent shortwave
largely driven by the Canadian ensemble.
Thereafter, ensemble clusters all show the low and trough axis
absorbing back into the flow. Building surface high pressure Tuesday
night into Wednesday is favored to shift flow more southerly with
relatively high confidence which should put an end to the lake
effect, but a clipper system may reintroduce snow chances. More
uncertainty come Wednesday had been driven by the depth of another
potential wave diving through northwest flow aloft up to this point,
and what a surface low reflection does. The trend has been toward a
weaker solution with latest guidance. That said, it still looks to
induce enough lift as it and a surface boundary slide through to
provide a greater than 50/50 shot for more widespread measurable
snowfall. Exactly how much is still uncertain and dependent on the
strength of forcing. Through the rest of the week, temperatures are
favored to moderate some as flow becomes more zonal allowing for
weak warm advection.
&&
.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
An exiting shortwave within Great Lakes troughing will push
morning convective snow showers east while westerly boundary
layer flow offers a mix of MVFR to low VFR overcast conditions
this morning.
Persistent cold advection plus additional shortwave movement
is expected to promote isolated to scattered snow showers mainly
between 15z-22z; confidence on terminal impacts and timing is
low, but heavier snow showers have potential for brief
reductions of visibility to less than 2 miles. Best chances for
accumulating snow favor areas north of FKL/DUJ and the higher
terrain east of LBE/MGW. Little variation is expected in cloud
heights through this period, hovering in the MVFR to low VFR
range. Steeper lapse rates and residual pressure gradients will
support gusty winds through 03z out of the west, occasionally
reaching 25-30kts.
Loss of deeper mixing and weak subsidence will erode snow
showers for most locations aft 00z while area wind lowers
slightly overnight.
Outlook...
Cig restrictions are possible at FKL through Saturday morning.
Restrictions and scattered snow showers are then possible Sunday
through Tuesday as a series of shortwave troughs cross the Great
Lakes and Upper Ohio Valley region.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...MLB
NEAR TERM...34/MLB
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...Frazier
Source:
PBZ issues Area Forecast Discussion (AFD) at Nov 29, 10:35 AM EST---------------
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