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fxus61 kpbz 112345 
afdpbz

Area forecast discussion
National Weather Service Pittsburgh PA
645 PM EST Mon Nov 11 2019

Synopsis...
rain will increase from west to east this evening, changing to snow
overnight with the passage of a cold front. Snow accumulations of
around an inch are expected for most, with the usual areas near I-80
and higher elevations receiving upwards of 4 inches. Record-breaking
low temperatures may occur tomorrow night.

&&

Near term /until 6 am Tuesday morning/...
evening update...

Adjusted timing of pops slightly for the next several hours to
account for slightly earlier onset across western zones.
Temperatures were refreshed with current obs and high-res blends to
adjust timing of falling temperatures behind the front. Otherwise,
the overnight forecast is on track.

Previous discussion...

GOES water vapor imagery shows a significant upper-level low digging
across the north-central Continental U.S.. at the surface, a frontal boundary
currently extends from the Pittsburgh area southwestward through the
Ohio Valley, with precipitation across eastern Ohio and just beginning
to push into western PA.

Mesoanalysis shows that a frontal wave has developed and is
currently positioned in the Ohio Valley, somewhere east of Columbus.
As the aforementioned upper trough digs into the Great Lakes,
cyclogenesis of this wave is anticipated under the right-entrance
region of the upper jet as it shifts northeastward up the valley.

A strengthening temperature gradient and frontogenetical forcing in
conjunction with the upper-level divergence will promote a growing
precipitation field along and on the west/upstream of the frontal
boundary and the developing surface low. Precipitation should
overspread eastern Ohio and far northwest PA by this evening, eventually
reaching Pittsburgh in the late evening, and ridges shortly
thereafter.

Precipitation type will initially be rain at the onset of precipitation,
but will transition to snow as very cold air filters in behind the
departing and strengthening low as it ejects to the northeast.
Forecast soundings suggest that the surface will see snow when the
surface/low-levels can support snow... so expect a transition when
the surface temp gets within a few degrees of the freezing mark. This
will likely be in the late evening for Franklin southwest towards
Zanesville, midnight or early am for the Pittsburgh Metro, and by 3am
or so for everywhere else.

Snowfall will come from two mechanisms. The first of which is the
aforementioned frontal-precip. The second is that of lake-effect snow
through the day Tuesday. Snowfall accumulation associated with the
frontal-precip will likely end by dawn Tuesday as the system departs.
Accumulations of somewhere between 0.5 and 1 inch is expected for
lower elevations and south of I-80, while areas north of I-80 and the
ridges may see upwards of 2 inches by this point. Snowfall
accumulation at onset may be delayed by warm ground temperature, but
should quickly begin to accumulate as the very cold, Arctic air
filters in. The second wave or mechanism, the lake-enhanced
snowfall, will be talked about in the undermentioned short-term
section of this afd.

The bulk of frontal precipitation will exit shortly after the
thermal profile becomes cold enough to support snow. In fact, some
parameters suggest that there will only be a very short window for
snowfall associated with the frontal system. For this reason, only
expecting an inch or so at most in portions of eastern Ohio and have
not issued a Winter Weather Advisory for those counties.

&&

Short term /6 am Tuesday morning through Wednesday/...
by Tuesday morning, most snowfall associated with the frontal system
will have cleared the County Warning Area. Strong cold air advection of an Arctic
airmass will be underway, leading to a very cold (potentially record-
breaking) subsequent 48 hours across the forecast area, with lake-
enhanced showers persisting through the first half of that period.

Strong northerlies carrying -15 to -16 degrees celsius air at 850mb
over a ~10 degrees celsius Lake Erie will create steep lapse rates,
yielding cape in excess of 500 j/kg. This will promote robust and
fairly widespread lake-effect / lake-enhanced convection into the
area through much of Tuesday.

Between the frontal precipitation and subsequent lake-enhanced
snowfall, accumulations of around an inch are to be expected area-
wide. The outer snowbelt counties near/north of Interstate-80 may
receive between 2 and 6 inches, while the higher elevations from The
Laurels south through West Virginia will likely receive between 2 and
4 inches. Therefore, a Winter Weather Advisory has been issued for
these counties tonight into tomorrow. A few bands of isolated higher
amounts throughout the County Warning Area are possible with some of the more robust
snow streamers that mesoscale models are hinting at. Due to low
confidence in exact placement, have went with a general north to
south gradient (and elevation gradient) in snowfall accumulation,
ignoring any possible mesoscale snow streamers.

Shower coverage will decrease Tuesday night as the upper trough axis
passes and high pressure builds into the lower levels. The resultant
backing of wind from northwest to west will quickly end snow shower coverage
for the County Warning Area.

Highs on Tuesday will struggle to reach 30 before dropping into the
teens by Wednesday morning. A few cities may break or tie their
record daily low temperature.

Cold weather will continue on Wednesday as the low-level ridging
becomes centered across the County Warning Area. Expect another day where the
temperature remains below the freezing mark W/ increasing upper
clouds moving in from the west.

&&

Long term /Wednesday night through Monday/...
another quick-moving shortwave will move north of the region
Thursday. With the path of the wave and a dry atmosphere in place,
appears any light snow showers will remain north of the region.

Looking ahead, temperatures will gradually moderate as the synoptic-
scale wave pattern becomes less amplified. The next chance of
widespread precip will likely be at the end of the weekend or early
next week.

&&

Aviation /23z Monday through Saturday/...
VFR conditions is expected to prevail into the early evening hours,
with the exception of fkl, as a low pressure system pivots across the
Ohio River valley and pushes in drier air from the south.

With the help of a digging upper-level trough in the mid-west, the
frontal boundary will finally pass through our entire region late
this evening. Expect rain showers and widespread IFR conditions just
before midnight. Rain should transition to snow by late eve/overnight
in cold advection after frontal passage. Gusty northwest wind is expected this
eve/overnight with a tightening pressure gradient and mixing behind
the Front.

Lake-enhanced snow showers tomorrow may bring periods of reduced
visibility; however, confidence is too low at the moment to pinpoint
which sites and time-frame specifically.



Outlook...
with building high pressure Tuesday night into Wednesday, VFR
conditions shall return. Fkl may experience restriction until
Wednesday night if lake-enhanced snow showers prevail.

&&

Pbz watches/warnings/advisories...
Maryland...Winter Weather Advisory from 1 am to 7 PM EST Tuesday for mdz001.
Ohio...none.
PA...Winter Weather Advisory until 7 PM EST Tuesday for paz007>009.
Winter Weather Advisory from 1 am to 7 PM EST Tuesday for paz074-
076.
WV...Winter Weather Advisory from 1 am to 7 PM EST Tuesday for
wvz510>514.

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