Area Forecast Discussion Issued by NWS Pittsburgh, PA
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000
FXUS61 KPBZ 181823
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
223 PM EDT Mon Mar 18 2024
.SYNOPSIS...
Scattered snow showers are expected on and off through Wednesday
with a crossing upper trough. Dry weather is then expected to
return as temperatures gradually warm through the end of the
week.
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.NEAR TERM /THROUGH TONIGHT/...
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KEY MESSAGES:
- Convective snow showers/graupel today.
- Minor accumulation on grassy/elevated surfaces north of
Pittsburgh.
- 1-2" is possible with a localized 3 inch total in the ridges.
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Expect the axis of an upper trough to continue to pivot through
the area today with cold advection in northwest flow dropping
850 mb temperatures to as cold as -11C and 500 mb heights fall.
Shallow instability will develop into this evening as the
coldest air moves overhead and sufficient diurnal heating from
an increasing mid- March sun angle resulting in convective snow
showers intensifying into this evening. Hi res soundings show
depth of the moisture reaches to the top of the dendritic growth
zone as the lapse rates steepen which will allow for heavy
bursts of snow/graupel.
Coverage will be reinforced in the early overnight hours
despite the loss of instability as a surface trough slides south
across the area. This may be the best shot for some minor
accumulation up to an inch to two, especially in any areas where
snow showers persist, north of Pittsburgh as the surface cools.
Probabilities are showing roughly 50% to 70% for an inch of
accumulation in the counties along I-80 and in the higher
terrain. Overall, outside of the ridges, accumulation will be
highly variable and dependent on where the snow showers develop,
but with a warm ground at this time of year and unfavorable
afternoon timing, expect that accumulations will be confined to
grassy/elevated surfaces and bridges and overpasses. In the
ridges, 1-2" is expected with aid from upslope flow. Given the
instability and upslope component, they may be an isolated 3
inch amount in the higher terrain from this snow shower event.-- End Changed Discussion --
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.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
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KEY MESSAGES:
- Scattered snow showers Tuesday and Wednesday with best chances
for I-80 corridor.
- Light accumulation possible.
- Potential for strong wind gusts Tuesday night - Wednesday.
- Low RH and elevated winds possible for Tuesday.
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The upper trough axis gets an assist from a trailing shortwave
and pushes out of the area on Tuesday allowing surface ridging
to briefly build from the south. Another upper wave dives out of
the Great Lakes later Tuesday and associated low pressure will
slowly meander by to our north Tuesday into Wednesday. The best
forcing and moisture will remain off to our north with low end
snow shower chances mainly north of I-80 as ridging keeps the
rest of the area dry through the daytime hours. A surface trough
will sag through the area Tuesday night with vorticity
advection streaming along with it and bring snow shower chances
further south into the PA and WV ridges overnight with the
probability for measurable snowfall highest in the WV ridges at
50% to 60%. This stands to be the best area with snowfall
chances. In addition, outside of areas that get the heavier snow
showers such as the lower elevations and south of I-80, expect
drier air to infiltrate the areas in OH and WV and into some of
SW PA. This will lead to low RH and elevated winds. An SPS will
likely be needed for Tuesday afternoon.
Colder air aloft moves in on Wednesday as the upper trough sags
south with several shortwaves rounding its base. Low level lapse
rates will steepen as a result and with the passage of a cold front,
we`ll see snow chances continue on Wednesday again primarily for the
I-80 corridor with the best moisture. Forecast soundings show some
marginal instability with ensemble probability of >50 J/kg around
50% to 70% there, so the potential for some heavier banded snow
showers is there and supported. Given the unfavorable diurnal
timing and higher March sun angle, would be tough to accumulate
efficiently on surfaces and result in more of a visibility
concern, but any showers with higher rates may overcome the
warmer ground and put down a quick accumulation. That said, the
higher end of the distribution for snowfall totals seems
reasonable for Wednesday, but it will be a day where the higher
totals are localized to areas that see heavier snow showers,
most likely north of Pittsburgh.
Wind gusts will also increase with the passage of the front as the
gradient tightens with highest gusts expected on Wednesday. Ensemble
probability of exceeding 30 mph gusts is 80-90% areawide and will
have to monitor the WV ridges for potential wind headlines with
probabilities of advisory criteria increasing for Wednesday where
gusts to 40-50 mph are possible.-- End Changed Discussion --
&&
.LONG TERM /THURSDAY THROUGH MONDAY/...
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KEY MESSAGES:
- Below average temperatures Thursday rebounding to near normal for
the weekend.
- Rain/snow chances return Friday.
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By Thursday, the upper trough pivots out through Nova Scotia as a
brief bout of surface ridging noses in from the north. This should
shut off any lingering lake effect precipitation Thursday morning
and return dry weather for a day. A few ensemble members linger the
trough a bit longer which keeps low probability snow shower chances
around into the late morning on Thursday, but any additional
accumulation would be minimal.
Things then become a bit more uncertain with timing and strength of
a flat shortwave sliding through the Great Lakes on Friday in
relatively zonal flow returning the chance for snow showers mainly
for the I-80 corridor and north. Lack of injection of cold air ahead
of it with the progressive nature of the wave should keep any
accumulation on the lower side.
Going into the weekend, a southern stream shortwave slides through
the Southeast and attempts to phase with the northern wave. Low
pressure looks to develop across the Southeast and turn up the
coast. The most likely ensemble solution at this time is a phasing
offshore of New England keeping the low out of our hair, though a
low probability chance exists for quicker phasing which would result
in higher precipitation chances continuing from Friday into the
first half of Saturday. Models and ensembles seem to also
provide a bit of consistency to the Sunday and Monday forecast
bringing in high pressure to the area with the potential of a
few upslope showers.-- End Changed Discussion --
&&
.AVIATION /18Z MONDAY THROUGH SATURDAY/...
Ceilings remain VFR during the early afternoon, although
stratocumulus has overspread the region as a shortwave trough
crosses from the northwest. Low-level dry air should keep
ceilings above the MVFR threshold through the afternoon into the
evening.
The main issue into the evening will be scattered to numerous
snow showers, which have increased in coverage with the arrival
of the shortwave and with a modest increase in instability due
to daytime heating. While terminals will experience VFR
visibility for the majority of the period, a few drops to IFR
will be possible in heavier snow showers. Included TEMPOs into
late afternoon at several terminals to cover this. Wind gusts
of up to 20 knots can be expected with the mixing as well as a
decent surface pressure gradient.
Snow shower intensity will drop off as daytime heating is lost,
although some modest accumulation will become more likely after
the sun sets. Ceilings should drop slightly into the MVFR range
overnight as the boundary layer cools and moistens a bit, and as
the inversion level drops. A few snow showers may linger north
of I-80 past 12Z Tuesday, with dry conditions otherwise. Wind
will be gustier by midday, with peak values of up to 25 knots
out the west-southwest.
.Outlook...
MVFR conditions may linger into Wednesday with a new shortwave
and burst of colder air from the north. VFR returns Thursday as
high pressure builds over Ohio.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...Shallenberger
NEAR TERM...Shallenberger
SHORT TERM...MLB/Shallenberger
LONG TERM...MLB
AVIATION...CL