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Area forecast discussion 
National Weather Service Pittsburgh PA
623 PM EST Sat Dec 20 2014

Synopsis...
high pressure will be in control into Monday. Expect increasing
precipitation chances Tuesday. Storm system brings rain and wind
Christmas evening...and snow showers Christmas day.

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Near term /through Sunday/...
with 615pm update...lower clouds remain scattered out from
Pittsburgh to Latrobe and Morgantown...but do not expect this to
persist through the evening. Minimal changes were needed to hourly
temperature trends. Previous discussion follows.

Weaker inversion across portions of southern PA has allowed lower
clouds to break up somewhat...although they are hanging tough so far
across the northern counties near fkl/duj as well as in southeast
Ohio. Expect these to reform tonight as inversion sharpens up again
in very weak flow...thus going mostly cloudy most areas. Going to hang
on to slight chance probability of precipitation in the southeast with shortwave trough
moving across and slightly better moisture profile...but snow
accumulation is not expected. This possibility ends by 09z. Only
minor modification to generally seasonable temperatures.

Inversion is maintained Sunday morning...but by afternoon expect
stratus to mix out into a cumulus field and then scatter out by evening.
A bit more sun should allow for afternoon high temperatures around 5
degrees warmer than today. Cl

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Short term /Sunday night through Tuesday night/...
benign weather initially with high pressure to our east and in
control. Southeast winds in response to surface anticyclone
position will facilitate development of stratus over eastern
portions of Tucker and Garrett counties Monday into Monday night.
Precipitation onset has slowed down from previous cycles...but there remains
a very low probability of freezing rain and/or sleet over
aforementioned areas Monday. For now...trended forecast with precipitation
commencing during the afternoon which reflect latest numerical
models. With this scenario...low level sub-freezing air should
scour out yielding an all rain ptype.

Rain shadow effect takes places Tuesday with southeast winds
widespread over the region. An isolated shower reaching eastern Ohio
and far northwestern PA is not out of the question...so kept a slight
chance. Would not be surprised to see breaks in the clouds which
could allow for a few locations to exceed 60 degrees. Record highs
for Pittsburgh...Morgantown...and Zanesville all reside well into
the upper 60s...so at this juncture record warmth is not foreseen.
Precipitation associated with the surface low pressure system ejecting from
the Gulf Coast region reaches southeastern areas of the forecast area that
evening.

A blend of the ongoing forecast and bias corrected European model (ecmwf) were used
for temperatures.

&&

Long term /Wednesday through Saturday/...
highlights:
- a wet Christmas evening
- temperatures return back to normal values Christmas day
- slightly above normal temperatures return by Friday

Strong low pressure system that has been much advertised given the
time period which it affects our area is still on track. Long range
ensemble prediction systems and operational models are converging on
a track through ind / ill then north into Canada. This will make
for all liquid precipitation on the front end. Temperatures will
soar to nearly 20 degrees above normal Wednesday with 50s common.
Coupled with influx of moisture from the Gulf of Mexico...soundings
illustrate elevated instability which pushes temperature differential
between 800 mb and 500 mb to nearly 0c. This could lead to isolated
thunderstorms ahead of the baroclinic zone. For the time
being...opted to leave thunder out of the forecast and wait for
better continuity given this still is five days out. Despite precipitable waters
nearly two Standard deviations above normal...combination of
katabatic winds and dry conveyor belt will limit quantitative precipitation forecast potential to
negate any flood threat. Long range hydrologic models show very
little rise in area rivers...which correlates well to the projected
quantitative precipitation forecast of 0.50 to 1.00".

With the track of the cyclone due north...cold air injected into the
area will be of Midwest origin...so temperatures will retreat back to
seasonal benchmarks Christmas day. For those who are looking for
snow on Christmas...a chance of snow showers exist...but given
trajectory from the southwest any accumulation associated with the
upper level system will be light at best. Historically...there is
only a 17 percent chance of at least one inch of snow on the ground
at Pittsburgh Christmas morning.

800 mb winds push 50kts Wednesday night into Thursday and highest
anomalies reside along the Lake Shore...so expect highest gusts
would do so as well. Nonetheless...period from Wednesday night into
Thursday strong isallobaric flow could yield gusts in excess of 40
miles per hour especially in the mountains of western Maryland and northern WV.

Balance of the extended is characterized by uncertainty as one would
expect in the day 6 -7 time frame. European model (ecmwf) has a stronger system
crossing Friday night versus GFS which is much weaker. Either case
ptype should be rain with snow showers on the backside. No plunges
of Arctic air are forecast in the next 10 days. Maximum/min
temperatures were a blend of the forecast and European model (ecmwf) bias corrected
data set.

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Aviation /23z Saturday through Thursday/...
while fkl...duj...and zzv are expected to remain under MVFR ceilings
overnight...remaining terminals may continue to have lower clouds
scatter out leaving a middle deck temporarily. By midnight all
terminals are expected to return to uniform MVFR ceilings. Cannot rule
out some ceilings below 2kft but did not have confidence to include
them. Uncertainty remains as to when skies should scatter out on
Sunday. The scattering trend should move from southwest to
northeast...but current forecast does not eliminate ceiling from
Pittsburgh and locations northeast until 00z Monday.



Outlook.../Sunday night through Thursday/...
next storm system will begin to impact the region on Tuesday and
result in widespread rain...cig/vis restrictions...and periods of
low level wind shear into Thursday.

&&

Pbz watches/warnings/advisories...
Maryland...none.
Ohio...none.
PA...none.
WV...none.

&&

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