Area Forecast Discussion
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FXUS61 KPBZ 200424

Area Forecast Discussion
National Weather Service Pittsburgh PA
1224 AM EDT Tue Mar 20 2018

A wintry mix of precipitation is expected today with low
pressure. The mix should change to snow tonight before gradually
tapering off Wednesday.


Precip chances should increase overnight and today as low
pressure tracks across the TN and OH Valley regions. Warm
advection ahead of the low should result in mainly rain or a
rain snow mix S of I 70 outside of the higher terrain. Rain and
snow is expected N of I 70 as precip further overspreads the
region today, along with a brief potential for freezing rain.
Little if any accumulation of ice or snow is expected with
marginal temperatures and the high March sun angle.

The higher terrain of WV and MD should see snow and freezing
rain transition to mainly snow today. Will maintain the Winter
Storm Warning for the expected snow and ice accumulations, with
temperatures more favorable for accumulation with cold E upslope


Quiet a turn around in the forecast with the 12Z operational
runs with respect to pending snowfall late tonight into
Wednesday then Tuesday night into Wednesday. NAM went bonkers
with QPF and thus snowfall for much of the region, however given
its March and daytime accumulation is very tough, did not buy
and cut about 50 percent off its snow forecast. This allows us
wiggle room if the 00Z runs flop back to a low QPF / low snow
amount forecast. Of course, it would not be surprising to see a
few counties get upgraded to a winter storm warning if a band
sets up.

A two pronged attack with one surface low passing to our south
late tonight. There will be a warm layer aloft that could lead
to a short period of a wintry mix before switching to a rain
snow mix as the pocket cools. Any accumulation of ice should be
confined to parts of eastern Tucker and Garrett county before
switching over. A prolong period of cold advection with
northeast to easterly winds will lead to wet snow over the
lowlands. Believe omega fields are overdone due to warm
advection, which has yielded a lower QPF than most models. One
thing we don`t have to worry about is being dry slotted since
mid level low will pass over the central Appalachians.

The time frame for heaviest snowfall will be Tuesday night into
Wednesday especially for the higher elevations. The ensembles
have a -2 to -3 standard deviation of the U wind component
which is decent /not great/ for significant snowfall. Given it
will be at night, and snowfall rates can overcome a warm
ground, expect most of the accumulation during this time frame
over the lower elevations. During the day Wednesday snowfall
will persist over the higher elevations given colder surface
temperatures, snow covered ground, and 1-2" per hour snow
rates. BUFR soundings support wet snow given a near isothermal
profile in the sounding. Overall storm total snow amounts are
close to the CIPS analog for this event which puts double digit
numbers in the mountains and a 2-4 range over lower elevations.

Confidence is high enough in 8" in 24 hours of wet snow for the
higher terrain to upgrade the winter storm watch to a winter
storm warning. In addition, since this will be an elevation
dependent event, included the ridges of eastern Fayette county
and higher elevation of eastern Mon and western Preston county.

Elsewhere added a winer weather advisory in collaboration with
adjacent offices. We reside on the lower side of the numerical
guidance for snowfall, basically between NAM and GFS, which
aligns more with ensembles. The warning was a little tricky
since there will be a break in the snow Tuesday afternoon and
evening before the second strong wave crosses producing a bulk
of the snowfall and tried to capture that in the text ontop of
the gridded forecast.


Benign weather to commence this period with high pressure
building south from the western Great Lakes. This will bring
cool northerly winds into the region as daytime highs will
remain in the 30s given H8 temps around -8C. Diurnal cumulus
clouds will be abundant given cold air aloft. The surface
anticyclone continues to drop slowly southeast overtop the Ohio
headwaters Friday. Aloft there will be a passing shortwave
trough, which will generate passing clouds, but given BUFR
soundings illustrate dry low levels, do not foresee any
measurable precipitation Friday.

Mid level heights build this weekend, however at the expense of
the warm advection, a warm front will cross the region Saturday
afternoon / evening. Thermal profiles suggest there could be a
wintry mix at the onset especially in the mountains and north of
I-80 despite it beginning late afternoon / evening, which is a
warmer part of the day. For now, will keep things a rain / snow
mix with high pops. With the surface low track staying to our
south on several models Saturday night into Sunday, far
northern locations could stay all snow and see several inches of
snow accumulation. At this time, locations south of I-70 should
mix over to rain and stay rain for much of the event.
Confidence is not high enough for a mention in the hazardous
weather outlook per control run and ensemble spread, but as the
week goes and we get through the mid week weather event,
messaging can start for this /if needed/.

With differences amongst long range guidance, this affects
text bulletins and gridded temperature plots. MEX is about 5-8
degrees warmer than ECE guidance given track of weekend system.
Will side with the cooler envelope that is not fully captured
in the super blend guidance.


VFR conditions are expected through the predawn hours although
high cigs wl lwr with the approach of low pressure. Condition
deterioration will occur from the south after daybreak as that
low shunts moisture over the region. Expect general MVFR with
pockets of IFR visibility, with continued degradation to
widespread IFR during the afternoon.

Restrictions, albeit with gradual improvement, are likely into
Thursday morning.


MD...Winter Storm Warning from 5 AM early this morning to 8 PM EDT
     Wednesday for MDZ001.
OH...Winter Weather Advisory from 8 PM this evening to 8 PM EDT
     Wednesday for OHZ039>041-048>050-057>059-068-069.
PA...Winter Weather Advisory from 8 PM this evening to 8 PM EDT
     Wednesday for PAZ013-014-020>023-029-031-073>075.
     Winter Storm Warning from 8 PM this evening to 8 PM EDT
     Wednesday for PAZ076.
WV...Winter Weather Advisory from 8 PM this evening to 8 PM EDT
     Wednesday for WVZ001>004-012-021-509.
     Winter Storm Warning from 5 AM early this morning to 8 PM EDT
     Wednesday for WVZ512>514.
     Winter Storm Warning from 8 PM this evening to 8 PM EDT
     Wednesday for WVZ510-511.


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