Area Forecast Discussion
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FXUS61 KPBZ 200056 AAB

Area Forecast Discussion...UPDATED
National Weather Service Pittsburgh PA
856 PM EDT Mon Mar 19 2018

Winter weather makes a return as low pressure passes to our
south. Snow accumulation outside of the mountains is difficult
during the day in March. Dry weather takes hold Thursday and
Friday, but all week temperatures will be below normal.


900pm update...continue to make minor adjustments to the
overnight period in coordination with the latest hires model

Previous discussion...
A return of the winter kitchen sink to the region tonight into
Tuesday morning for parts of the region. Low pressure crossing
the southern Appalachians and mid level confluence atop our
region ahead of a incoming cyclone will spread QPF toward
daybreak Tuesday. Forcing remains relatively weak so not a lot
of snow expected for locations where columns is cold enough. The
bigger impact would be a period of freezing rain in favored
southeast upslope regions of Garrett and eastern Tucker county.

Ongoing forecast of around a tenth inch of ice is on track with
latest operational runs, so will hoist a winter weather advisory
from 6Z - 18Z Tuesday. Until adiabatic winds can pool enough
cold air to remove the warm layer aloft which won`t be too long
precip will switch over to snow.

The bigger question of the forecast is there a period of
freezing rain as far north as US 422 that would include the
Pittsburgh metro region? By the time precipitation arrives in
the aforementioned region it will after daylight and given the
run of days above 50, any ice won`t have a great /if any/
impact on road surfaces. Surface temperatures will be climbing
well into the 30s and perhaps lower 40s so any snow would be
confined to grassy surfaces if any accumulates.


Heavy Wet Snow in the Allegheny Front Tuesday night into

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.


- No sign of Spring
- Quiet to start - active end
- Can`t say we have seen the last of snow

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 TAF period as high
pressure slides east. Clouds should increase today/tonight
ahead of approaching low pressure, with precipitation
beginning mainly south of Pittsburgh prior to 12Z. The form will
be rain initially, but some freezing rain and snow may mix in

Restriction potential returns with low pressure late Mon night
thru Wed.


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


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