PBZ issues Area Forecast Discussion (AFD) at Feb 8, 12:09 PM EST905
FXUS61 KPBZ 081709
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
1209 PM EST Sat Feb 8 2025
.SYNOPSIS...
A wintery mix with impactful ice is possible for parts of the
area through tonight, followed by dry and seasonable weather
Sunday and Monday. An unsettled pattern with more snow chances
is forecast Tuesday through Thursday.
&&
.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:
- Wintry mix of precipitation for portions of the region.
- Ice Storm Warning for the PA ridges.
- Winter Weather Advisories remain in the lowlands with an expansion
to a tier of counties south.
-----------------------------------------------------------------
12pm Update ..
The aforementioned band of precipitation appeared to have not
been making it to the ground, consistent with the dry air below
the 700mb layer.
Otherwise, observations are coming in cold north of Pittsburgh
and right around average to the south of the city. This is more
concerning for ice potential north of the metro area. Because
ensemble temperature forecasts often under-do the north-south
temperature gradient in mixed-precipitation systems, this
temperature behavior is as expected. Accordingly, temperatures
were lowered north of Pittsburgh and raised a bit south of
Pittsburgh. This resulted in a few changes in the ice forecast.
First, sleet and snow is more likely at onset north of I-80 with
a more prolific surface cold layer. Chances of 1" or greater
north of I-80 are up to 50% to 80%. If the 700mb warm advection
is more prolific, some of this may fall as ice pellets.
South of I-80 and north of Pittsburgh, the duration of sub-
freezing temperatures has increased to the point where HREF is
signaling up to a 75% chance of 0.25" of ice accretion on an
axis around Grove City, Butler, Parker, Indiana, and
Punxsutawney. While this assumes prefect ice accretion, it is
still quite concerning for ice accumulation potential, as this
is 50% to 100% increase from the 00Z run to 12Z run. This may be
prudent for an upgrade, though will continue to hold given one
factor: the ice accretion coefficient. A 2016 study showed ice
accumulation ratios were mostly around 0.2 to 0.4 (amount of
liquid compared to the amount of ice on a rod or branch). This
has accumulated ice between 0.1" and 0.3" given HREF QPF of 0.5"
to 0.75", but we suspect accumulation will be towards the lower
then of this distribution given the moderate precipitation
rates and daytime arrival.
9am Update ...
Band of radar returns associated with weak isentropic lift are
being seen near/south of I-70 this morning. Noted an earlier
mPING report of freezing rain just southeast of Lancaster,
although not confident all of the precipitation is currently
reaching the ground, particularly in north-central WV (where a
"null" mPING report was received under the band of returns).
Still, elected to issue an SPS for spotty freezing rain
potential across our southern tier of counties, given
subfreezing road temperatures. Added low PoPs in this region as
well for a few hours ahead of main precipitation event later.
No other changes to the forecast at this time, as temperature
structure remains similar to earlier expectations for now. Will
of course be monitoring closely over the next few hours and
can't rule out a further southward expansion of the advisory if
warming ends up slower than currently anticipated.
Previous Discussion ...
Cloud coverage will thicken and lower this morning as precipitation
associated with approaching low pressure slides up the Ohio Valley
and surface high pressure migrates off to the east. 925-850 mb flow
will back to the southwest as warm advection aloft ensues with an
approaching warm front. Meanwhile, surface flow remains locked
in out of the east-southeast. This will be an important aspect
of the system in regard to precipitation type; the NBM has
notoriously struggled with handling surface temperatures in this
regime, most notably with the system last weekend, as it eroded
the cold air much too quickly and too far north. Continued to
undercut highs and hourly temps from NBM to slow the warming
through the day to better capture the more likely trend based on
recent events. The result is going to be increasing confidence
in a period of freezing rain impacts.
Precip will move into northern WV after 13z as the warm nose
advances further north atop pre-existing cold air and road
temperatures. Surface temps will nudge up a couple degrees this
morning initially, but as precip begins to saturated the lower
levels characterized by dew point depressions as high as 10F,
dynamic cooling will allow temperatures to fall to their wet-bulb
values which will be sub-freezing at precip onset. This points
toward a period of freezing rain for most of the area from
Pittsburgh on north, and especially in the ridges, where coldest air
temperatures will linger. There's some more question how quickly the
warm nose can punch north to the I-80 corridor, so a thump of wet
snow is possible there at onset with a 40-60% chance of an inch
before changing over to freezing rain. The 00z HREF came in notably
more aggressive with QPF amounts and freezing rain amounts within
the current hazard areas, as well as increased probability further
south to the Pittsburgh metro. Increased precip rates may point
toward less efficient ice accretion, but elevated wind should help
to offset that effect. Probability of at least 50% for measurable
ice extends down to Coshocton County arcing east through a tier of
counties south of the current advisory, and have accordingly opted
to expand the advisory to cover this area. A noted jump in
probability as high as 80% for warning criteria in the ridges
has justified an upgrade to an Ice Storm Warning for the higher
elevations of Fayette, Westmoreland, and Indiana Counties. Can't
rule out the potential for the warning to expand across the
US-422 to I-80 corridor where probabilities also increased, but
confidence in cold air locking in longer in that area is lower
at this time.
Warm advection should finally win out and change precip type to all
rain south of 422 while the freezing rain threat will continue
further north and in the ridges into the evening hours. Hazardous
travel is expected Saturday afternoon and evening. It does not
take much ice accretion to cause issues... only 0.03" of ice at
KDUJ last weekend resulted in numerous vehicle accidents.
Total QPF will be highest across northern West Virginia where
already saturated soils from recent snowmelt and rainfall exist.
Probability for >0.5" is up to 80% and >1" around 50%. Some minor
rises on area rivers and streams are likely, but none are forecast
to reach action stage. WPC has placed areas south of I-70 in a
marginal risk for excessive rainfall.
As the low exits east and northwest flow prevails, precip will shut
off from west to east. Lingering snow showers/light freezing drizzle
will continue in the ridges and along I-80 with minor additional
accumulation (less than an inch) expected. The cold front will drag
through northern West Virginia this evening and CAMs suggest a
focused line of showers accompanying it. Despite strong low-level
flow not far off the surface, the warm advection driven inversion in
the warm sector is most likely to keep this elevated so not
expecting any gusty wind concerns with it. But, HREF soundings
suggest 100-200 J/kg MUCAPE, so if there's enough depth to the
moisture at that point, there could be some rumbles of thunder
accompanying the line.
&&
.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
KEY MESSAGES:
- Turning dry Sunday and Monday with residual cloud cover.
- High temperatures a couple degrees below normal.
--------------------------------------------------------------------
Strong low-level cold advection will drop 850 mb temperatures to -8
to -10C by early Sunday afternoon while steepening low-level lapse
rates as moisture traps under sinking subsidence. Backing boundary
layer flow will shift any lake-enhancement out of the area
leaving mostly cloudy skies and patchy drizzle for the remainder
of the day. Some residual upslope snow showers and/or freezing
drizzle continuing from Saturday night is still possible in the
ridges through early afternoon as model soundings hold shallow
moisture cut off below -10C and surface temps below freezing.
Quasi-zonal mid-level flow takes hold on Monday while surface high
pressure across the Midwest extends ridging into our area. A stalled
boundary from the weekend system will park off to our south across
the Carolinas with residual moisture locally providing some cloud
coverage that should dwindle some as the day progresses with highs
around normal.
&&
.LONG TERM /TUESDAY THROUGH FRIDAY/...
KEY MESSAGES:
- Tuesday disturbance returns accumulating snow chances but
with low confidence at this time.
- Another system moves through Wednesday night.
-------------------------------------------------------------------
The pattern then becomes more active on Tuesday as ensembles
develop surface low pressure along the aforementioned stalled
boundary and lift it back north as a warm front with a
progressive nature as the mid-level flow remains zonal. The
track and strength of the low will be of importance for what
kind of potential winter weather impacts we see locally.
Ensembles are in relatively good agreement that the low tracks
up the East Coast but differ on exact track. Strong surface high
pressure across the Dakotas will slide toward the Great Lakes
and its positioning will be a primary driver of said path.
Latest ensemble runs have remained farther south with the
overall QPF footprint, and the EPS continues to be the more
aggressive outlier not budging quite as far south. Chances for
at least an inch of snow are still elevated across most of the
area, but probability for advisory criteria (3 inches) has
decreased spatially and is highest at 50-70% in the ridges but
less in the lowlands (20-30%). Warning criteria (6 inches) is
still highest in Eastern Tucker County at 50%. The 90th
percentile/high end of the goal posts has trended downward as
well with the 6"+ amounts confined to the ridges, so we're
perhaps beginning to get a better grasp on the potential spread
in low end/high end amounts. Will still have to await better
agreement before honing in on totals.
The unsettled pattern continues through the week as ensembles eject
a deeper trough out of the Four Corners region with uncertainty on
its depth and speed. The grand ensemble mean brings another round of
low pressure moving up the eastern seaboard similar to the Tuesday
one and ramps up precip chances Wednesday night, though again track
and strength is going to be of importance for local effects. Another
round of snow looks possible but this time with the higher amounts
to the north.
&&
.AVIATION /17Z SATURDAY THROUGH WEDNESDAY/...
VFR with increasing mid and upper level cloud coverage can be
expected through the morning ahead of low pressure approaching
out of the midwest. As the mentioned low nears, cig and vis
restrictions are expected at all ports during the afternoon and
the remainder of the TAF period for all sites. Cigs will fall
rapidly to IFR from south to north with increasing low- lvl
moisture. Precipitation south of PIT will likely be rain, while
areas PIT and to the north will likely see a period of freezing
rain.
Precipitation onset is expected to be early afternoon before
turning heavier this evening, as the center of the low
transitions over western Pennsylvania. Low- lvl wind shear
(35kt to 40kts) will be possible late afternoon / early evening
south of KPIT.
Outlook...
Precipitation will diminish from west to east overnight under
increasing subsidence, but widespread IFR conditions are likely
through Sunday morning with saturated low levels.
Restrictions and snow are likely Tuesday with crossing low
pressure. Another in a series of low pressure systems will
maintain restriction and snow chances through Wednesday.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory until 1 AM EST Sunday for PAZ007>009-
013>016-020>022-073-077.
Ice Storm Warning until 1 AM EST Sunday for PAZ074-076-078.
OH...Winter Weather Advisory until 1 AM EST Sunday for OHZ039>041-
048.
WV...Winter Weather Advisory until 7 PM EST this evening for
WVZ511>514.
&&
$$
SYNOPSIS...Milcarek
NEAR TERM...MLB/CL/Milcarek
SHORT TERM...Milcarek
LONG TERM...MLB/Lupo
AVIATION...88/AK
Source:
PBZ issues Area Forecast Discussion (AFD) at Feb 8, 12:09 PM EST---------------
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