PBZ issues Area Forecast Discussion (AFD) at Dec 14, 12:34 AM EST835
FXUS61 KPBZ 140534
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
1234 AM EST Sat Dec 14 2024
.SYNOPSIS...
Dry conditions with a warming trend expected through Saturday.
A wintry mix of ice, rain, and snow is possible Sunday morning
followed by all rain in the afternoon. Occasional rain chances
with above average temperatures next week.
&&
.NEAR TERM /THROUGH TODAY/...
KEY MESSAGES:
- Cold Friday with highs climbing closer to normal Saturday.
- Strong high pressure maintains dry weather.
--------------------------------------------------------------------
Evening Update...Updated the forecast to account for the diurnal
trend of temperatures and dew points around the around heading
into the overnight. The rest of the forecast is good to go.
Dayshift Discussion...Upper troughing is making its departure
this afternoon allowing for subsidence underneath anomalously
(+3 SD) strong high pressure that will establish overhead today
and tomorrow. 24 hour height rises by 12z Saturday sit around +5
to 8 dam, but despite increasing thicknesses, low-level cold
air will not erode very quickly. 850 mb temperatures still sit
in the -7C to
-14C range with easterly flow not initiating strong warm
advection, so sub- freezing highs are high probability north of
I-70; to the south, we should see some locales break the 32
degree mark. Regardless, another chilly December day today.
Increasing warm air is expected to advect aloft from the
southwest throughout today and tomorrow. Despite this,
decoupling overnight is expected. Light to calm winds to drop
lows around 10 degrees below normals, though wind chills will
remain close to air temperatures. Lows tonight will again be in
the teens north of Pittsburgh with 20s more likely to the south
with clearing skies and light wind.
More modification to the airmass is probable on Saturday with
increasing warm air aloft as 40s work their way in from the
southwest. Aside from some upper level cloud coverage, strong high
pressure keeps us dry. With easterly flow off of the ridges, dew
points may mix out a bit on Saturday afternoon.
&&
.SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/...
KEY MESSAGES:
- Medium probability, moderate impact potential for light ice
and snow accumulations Sunday morning.
--------------------------------------------------------------------
Ensemble clusters are in good agreement on a weak wave traversing
the central CONUS Saturday night into Sunday fostering surface
cyclogenesis and weak low pressure sliding up through the Great
Lakes. This will introduce precipitation chances for our area late
Saturday night. Timing has trended slightly towards Sunday morning
for the highest chance of precipitation onset, though initial
precipitation will have to battle a lot of dry air at onset.
Ensemble clusters have come into good agreement in timing and
relative intensity of the system. Strong warm advection will be
increasing 850 mb temperatures to 1-3C across the area by Sunday
morning with cold air still locked in at the surface. This type of
pattern regime is suggestive of freezing rain potential. There is
still uncertainty in exacts with regard to precip type, but the
thinking is that this is going to be a predominantly rain/freezing
rain event given the thermal profiles. If the elevated warm nose
proves less sharp than modeled, some snow is possible, but lower
probability at this time. The general thinking is that the
areas that have the best chance to see freezing rain impacts are
the I-80 corridor and especially the Laurels down to WV ridges
as the column saturates top down (medium probability). This is
due in part to 1) colder air temperatures in place Saturday and
Saturday night ahead of precipitation allowing for ground
temperatures to hold below freezing and 2) for the ridges,
southeasterly flow is favored to persist behind departing high
pressure. This setup typically makes it tougher for cold air to
erode in a warm advection regime, and not near as quickly as the
NBM wants it to. Combine this with drying downslope effects and
a wet-bulbing scenario and this could help to keep near-
surface temperatures colder for longer. Regardless of ensemble
flavor, elevated probabilities for measurable ice increase
Sunday morning, highest in the ridges. Some of the forecast
soundings available at this point are suggesting that Eastern
Tucker/Preston, Forest, Clarion, and Jefferson may hold on to
sub-freezing temperatures for a good bit of the day on Sunday
raising concerns for more prolonged freezing rain potential.
As for the lowlands and south of I-80, conditions will depend on the
exact onset of precipitation. Before sunrise, radiation will not
warm surfaces above freezing. After sunrise, roads will warm in
surface radiative processes, but limbs and branches may be more
reliant on air temperature. Lowland ice still remains lower
probability dependent on onset timing.
Winter headlines primarily for ice are becoming higher probability,
and subsequent updates will address that aspect as the finer scale
details become clearer.
&&
.LONG TERM /MONDAY THROUGH FRIDAY/...
KEY MESSAGES:
- Moderating temperatures into next week in an unsettled pattern.
- Rain chances return again Monday night.
--------------------------------------------------------------------
There is high confidence in the continuation of progressive flow
with weak embedded waves through early next week allowing rain
chances to continue, but ensemble clusters may wash out shortwave
positioning uncertainty which will affect exact precipitation
chances. The wave responsible for Sunday's event will exit on Monday
morning with relatively high confidence as brief shortwave ridging
builds behind it. This should lend to lower precip chances for the
daytime hours on Monday, though there is a lower probability chance
of a slower wave departure and lingering higher precip chances into
the day.
Uncertainty then ramps up quickly primarily with timing, and to a
lesser extent the depth/tilt, of a second, deeper trough approaching
from the central CONUS late Monday. Most of the question is a result
of the strength of the downstream ridge. Surface cyclogenesis
is favored somewhere across the Northern Plains while additional
areas of low pressure may develop along an elongated boundary
extending well south from the parent low. This, combined with
diffluence from the upper trough, will foster a quick return of
widespread rain chances late Monday into Tuesday. Some clusters
exhibit a more negatively tilted, faster progressing trough
which clears precip out of here earlier on Tuesday. Others hold
it back some even keeping it positive to neural overhead on
Tuesday morning and not ridding us of PoPs through the day on
Tuesday.
All clusters then suggest flat ridging behind the departing trough
ahead of yet another weaker wave approaching for the latter
half of the week, again bringing another round of precip chances
and gradually falling temperatures.
&&
.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
High pressure in the area will result in a high confidence VFR
forecast through the 24-hr TAF period with passing cirrus
throughout the day Saturday and winds remaining light and
generally out of the northeast.
Coverage in high clouds thickens and winds gradually veer to
more southeasterly towards the tail end of the period as the
next weather system approaches from the west.
Outlook...
Restrictions are likely to return on Sunday with the next
disturbance and will persist through Monday morning. There is a
chance for freezing rain Sunday morning with only a glaze or couple
hundredths of an inch accumulation expected, though locally higher
amounts may be observed in the ridges and along/north of I-80.
Temperatures warm up by early afternoon Sunday, resulting in a
transition to all rain. DUJ may be the only exception, as
ensemble guidance suggests a 30-50% chance they remain at or
below 32F throughout the day, which would increase the chance
for longer duration of freezing rain and greater accumulation.
We will continue to monitor conditions and provide updates if
there are any changes.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...MLB/Milcarek
NEAR TERM...MLB/Shallenberger
SHORT TERM...MLB/Milcarek
LONG TERM...MLB
AVIATION...Cermak/Shallenberger
Source:
PBZ issues Area Forecast Discussion (AFD) at Dec 14, 12:34 AM EST---------------
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