893
FXUS61 KPBZ 220005
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
705 PM EST Thu Nov 21 2024

.SYNOPSIS...
Strong and cold upper low moves across area tonight and then
moves into eastern Pennsylvania Friday. This system will pull
away from the area on Saturday leading to dry weather by Sunday.
Unsettled weather is then expected next week including
Thanksgiving Day.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
KEY MESSAGES:

- Areas of snow continue this evening.
- More widespread snow spreads from north to south late tonight.
------------------------------------------------------------------

No appreciable changes needed with the evening update...

Deep and cold upper low with sub 5300 meter heights at 500 mb
was evident on water vapor imagery over central Ohio and it is
expected to move right over Pittsburgh area this evening. It
will then jump to New York City area by late tonight.

Initial weakening occluded low over Indiana will give way to
deepening low backing west across southern New England. Areas of
snow tonight under the cold upper low in sub -15C 700 mb air
will continue to be locally heavy given the decent lapse rates
available. However, steadier precipitation is expected to wrap
southward from the deepening New England system into our area
late tonight, starting in the I-80 corridor first and then
dropping across forecast area by daybreak.

The best chance for accumulating snows in the low lands will be
very late tonight and first thing Friday. For most part, that
will be an inch or less and mainly on grassy surfaces. Not much
impacts expected.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/...
KEY MESSAGES:

- Winter Storm for the higher terrain
- Snow and rain continues elsewhere
-------------------------------------------------------------------

Deep occluded low completes a circular loop on Friday as it
heads from New York City area back to the Poconos during the
day, and then turns back eastward Friday night and south of Long
Island over the Atlantic.

Main precipitation shield in northerly flow on west side of low
will be in place Friday morning for the forecast area, but then
will shift southward and focus in the upslope area of the ridges
Friday afternoon and Friday night. Warm advection from the north
will be turning the snow to rain over the lower elevations
Friday morning, but the snow will continue to fall heavily above
about 2,500 feet. Adjusted the winter weather advisory to end at
18z Friday as all the low lands should be turning to rain during
the mid to late morning hours.

850 mb temperatures will start out around -6C Friday morning in the
south and gradually warm in strong NW flow during the day to -2C
by evening. A deep layer of 45 to 55 knot winds just above the
surface will induce strong orographic lift especially for the
West Virginia Ridges and will result in heavy snow accumulations.
Although we are going with winter storm, wind gusts in the
40-50 mph range at the highest elevations suggest that near
blizzard conditions will have to be monitored for possible
upgrade.

NBM indicates snowfall rates will peak in the late morning
around 1.5"/hour along I-76 tunnel/pass and then shift to
eastern Preston/Tucker Counties by the early afternoon hours.
With the addition of high resolution guidance through Friday
night, the QPF and snow accumulations have increased and we have
expanded the winter weather advisory to the adjacent lowlands
from Morgantown WV area to Latrobe PA.

Amounts in the ridges are up as well. Using 13z NBM guidance,
initial snow totals in Davis area of Tucker County came out
around 28" with about 2.2" of liquid. That would be at the high
end of the potential, given that NBM Snow to Liquid ratios run
around 15-17:1 for much of the period of heavy precipitation.
Taking a look at the thermal profiles and vertical motion, it
seems that much of the lift will be in area below -10C.
Therefore, tried to limit the SLR to the 12-15:1 range or lower
and ended up with more reasonable amounts in the 18-24" range.

Using a local study, an average ratio between MGW and Davis COOP
is 5.38 to 1, and the current 3-4" at MGW versus 20-24" in Davis
area is very close to that climatological ratio.

While most of the precipitation will shut off Friday night as
upper low pulls away, the continued moderate northwest flow of
30-40 knots in the 850-700 mb layer will continue decent
orographic enhanced precipitation in WV ridges so the winter
storm warning going well into the night in northern ridges and
all day Saturday in eastern Tucker County WV looks good.

&&

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

- Dry on Sunday
- More unsettled weather returns Monday through Thanksgiving
-------------------------------------------------------------------

Upper ridge is overhead Sunday with decent agreement in the
ensemble guidance, with a surface ridge bringing dry weather to
area.

The next upper trough with surface low over Great Lakes is shown
on Monday which would bring rain chances to area with above
normal temperatures in the 50s.

Front comes through with cooling and drying and about a 10
degree drop in temperatures on Tuesday into the 40s.

Uncertainty in the global ensembles increases by Wednesday as it
often does past Day 5. There is a progressive westerly flow
pattern, but disagreement on timing of next trough which would
potentially bring precipitation chances either Wednesday or
Thanksgiving Day to the area. To give an idea of possibilities,
the 90th percentile NBM High for Thanksgiving is mid 50s, while
the 10th percentile is mid 30s. Therefore there is some chance
for wintry precipitation in the Upper Ohio Valley area for the
Wednesday/Thanksgiving busy travel period.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Scattered snow showers continue this evening, rotating around an
upper low dropping across the Upper Ohio Valley. MVFR or low VFR
ceilings will persist for a few hours, and most of the snow
showers will produce no worse than MVFR visibility, although a
couple of isolated instances of IFR are possible. A relative lull
in activity is expected during the late evening/early
overnight, but more widespread precipitation, associated with a
mid-level shortwave and wrapping around surface low pressure to
our east, is expected to drop into the region from north to
south overnight. ZZV is still expected to largely miss this
steady precipitation, although a few snow showers remain
possible.

Although precipitation will start as snow initially, a
changeover to rain will occur from north to south Friday morning
as warm air wraps around this mature system. Ceilings will drop
through MVFR to IFR around 12Z with high confidence (HREF probs
of 70-100% at all TAF sites). Visibility will likewise drop to
IFR in most cases (with possible occasional drops to LIFR) with
the rain/snow mix, with lingering mist during precipitation
lulls. Although flight conditions will remain largely IFR
through the afternoon, visibility may begin to slowly improve as
the rain changeover occurs, although ridge areas are more
likely to remain snow longer.

Wind gusts will also pick up Friday morning with the tightening
pressure gradient and improved mixing; 20 to 30 knot westerly gusts
are likely at most terminals through the day.

Outlook...
Restrictions are expected into Saturday as a series of low
pressure systems spin across the NE CONUS, bringing periods of
rain and snow showers to the area. VFR should return Sunday as
high pressure builds in.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 1 PM EST Friday for PAZ009-016-
     073-075-077-078.
     Winter Storm Warning until 7 AM EST Saturday for PAZ074-076.
OH...None.
WV...Winter Weather Advisory until 1 PM EST Friday for WVZ021-509.
     Winter Storm Warning until 7 AM EST Saturday for WVZ510>513.
     Winter Storm Warning until 7 PM EST Saturday for WVZ514.

&&

$$

SYNOPSIS...Craven
NEAR TERM...Craven/MLB
SHORT TERM...Craven
LONG TERM...Craven
AVIATION...CL