PBZ issues Area Forecast Discussion (AFD) at Jan 23, 6:50 PM EST262
FXUS61 KPBZ 232350 AAA
AFDPBZ
Area Forecast Discussion...UPDATED
National Weather Service Pittsburgh PA
650 PM EST Thu Jan 23 2025
.SYNOPSIS...
Below normal temperatures will continue the remainder of the week
with a chance of light snow showers tonight. Periodic snow
chances continue Friday north of I-80 and in the ridges.
Temperatures will moderate some for the weekend but still remain
below normal.
&&
.NEAR TERM /THROUGH FRIDAY/...
KEY MESSAGES:
- Light snow possible tonight with minimal accumulation.
-----------------------------------------------------------------
Snow showers and flurries continue to move across Ohio ahead of
a surface cold front and digging shortwave trough. Most of the
snow is light and expect it to remain that way as the boundary
crosses. Have updated overnight PoPs using a blend of CAM/hires
guidance as ensembles are not picking up on these mesoscale snow
showers very well. Snow accumulations were also updated as well
as overnight lows.
Previous discussion...
The best chances for snow showers locally will likely come
tonight along and ahead of a weak boundary as opposed to the
impulse.
Latest guidance still offsets the passage of the wave and the
boundary, so the overlap of forcing will be displaced. Better
moisture return will come just ahead of the boundary with a more
saturated DGZ, but the 12z CAMs all suggest different flavors of
moisture depth. The NAM seems to be the most bullish with saturation
through -18C and producing efficient snow showers. Other CAMs are
less interested in total boundary layer moisture content. The
current snow squall parameter is elevated across Indiana with
reductions to visibility dipping down to 1-2 miles and putting
a quick coating on surfaces, but the RAP/NAM weaken the
parameter as it progresses east to ~1; with loss of daytime
heating, low level lapse rates likely weaken by the time it
reaches here. At this point, HREF hourly probabilities for
measurable snowfall reach 30-60%, highest in the higher
elevations. The more persistent snow will be across the ridges
with orographic ascent, but the moisture issue will still
remain. Bumping up to the HREF 90th percentile accumulated QPF,
totals outside of the ridges don't eclipse about 0.05", so
assuming an 18-20:1 SLR, highest totals within the snow showers
should be an inch or less with potentially up to 1-1.5" or so in
the ridges. Timing looks to be after the evening rush, so peak
travel shouldn't see a notable impact, but with road
temperatures at or below freezing, a quick burst of snow within
scattered showers will easily accumulate on surfaces.
&&
.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
KEY MESSAGES:
- Widely scattered light snow showers Friday with slightly colder
temperatures.
- Light snow north of I-80 Saturday with moderating temperatures.
--------------------------------------------------------------------
A weak shortwave will slide through on Friday as surface high
pressure progged across the Carolinas extends ridging locally. Can't
entirely rule out very low-end snow shower chances in the afternoon
as low-level lapse rates steepen to near dry adiabatic, but dew
point depressions are anywhere from 7-12 degrees, so it may be a
struggle to reach the ground. HREF probability for reaching a half
an inch is <10% in our area. Highs will take a hit back down a few
degrees as some colder air works in in west-northwest flow. The
steepening lapse rates with cold advection at the 850 mb level will
allow us to tap into elevated flow aloft. Momentum transfer isn't
all that impressive with only 15-20 knot flow, so wind may take on
more of a 8-12 mph sustained nature with sporadic gusts. Depending
on the amount of clearing we can get overnight, lows may have a good
shot (currently 50-70%) at dropping into the single digits again
across most of the area. Rising heights and intruding dry air looks
to erode the clouds, and despite still noted spread within the NBM
25th/75th cloud coverage, the upper end of the distribution
indicates clearing, so once confidence becomes higher, lows may need
nudged down a bit.
Heights will rise a bit on Saturday behind the departing wave as the
surface high expands across the Carolinas with ridging still in
place locally. Temperatures will rebound in southwest flow aloft
which will pump 850 mb temps to -7 to -9C, but heating will be
stunted by lingering cloud coverage. A trough may spark off some
snow showers north of I-80 in the afternoon, but the chance for
>0.5" is ~20% with weak forcing available. Highs will reach the low
30s with better confidence across the board which is still a few
degrees below average.
&&
.LONG TERM /SUNDAY THROUGH THURSDAY/...
KEY MESSAGES:
- Below average temps are expected to linger through early next
week.
- Rounds of weak disturbances will produce scattered snow chances.
--------------------------------------------------------------------
The better snow chance, albeit with low accumulation potential, will
come on Sunday. Ensembles have come into better agreement on another
flat shortwave making a quick fly by during the morning hours as
surface low pressure drags a weak front through. All clusters hug
the best chances (~20%) for >1" of snow to I-80 and north with lake
enhancement, though boundary layer flow is progged to be more
westerly than northwesterly, so said enhancement may stay north out
of our area. Chances for >0.5" extend a bit further south down the
ridges with favorable flow for orographic ascent, so the highest
accumulations will be found in these locations, but still looks to
be low impact. The primary issue will be available moisture as the
lack of amplification with westerly mid-level flow isn't pulling
from a great moisture source region.
Sunday's highs will be very similar to those of Saturday in the
upper 20s/low 30s; probabilities to eclipse the freezing mark sit
around a 50-70% chance as far north as Pittsburgh but are trending
cooler.
Ensembles eject yet another shortwave late on Monday through the
Great Lakes. Noted disagreement still comes into play with the
latest ensembles regarding the progression of the upper levels; two
clusters, with which has the most evenly distributed membership,
exhibit a slower and deeper trough ~10 dam below the mean. Two
others suggest a less amplified solution. Better moisture return,
both low and mid level, comes from the more amplified solutions as
expected with a slight enhancement in 24 hour QPF. The latest
ensembles have trended toward a more amplified solution, but weaker
ones are still in play. Total QPF is distributed anywhere between
minimal on the low end to a quarter of an inch on the higher end.
Accordingly, the chance for >1" of snow is highest along the PA
ridges and north of I-80 around 30-50%, but will have to wait
for better agreement to hone down more details as overall
confidence is low.
&&
.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
This period will be mostly defined with borderline low VFR and
high MVFR cigs with temporary drops expected in snow through
roughly 06Z to 10Z as a quick hitting wave crosses the area.
Periods of snow will likely be temporary, and drops to high IFR
and low MVFR visibility have been noted to the west. The timing
of this has been captured in tempo groups. Behind this wave,
there may be a temporary lift to VFR, but this is only 60%
confident for southern ports, decreasing to the north. MVFR is
then likely 50% to 70% for daybreak through the afternoon
tomorrow, before chances decrease from southwest to northeast.
There is a 40% to 60% chance far southwestern ports maintain
VFR for this same period.
Outlook...
VFR returns late Friday through early Saturday under high
pressure. Restrictions and snow showers are then possible
through Sunday with a crossing cold front. VFR returns Monday
under high pressure.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...MLB
NEAR TERM...MLB
SHORT TERM...MLB
LONG TERM...MLB
AVIATION...Milcarek
Source:
PBZ issues Area Forecast Discussion (AFD) at Jan 23, 6:50 PM EST---------------
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