Area Forecast Discussion
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FXUS61 KPBZ 191738

Area Forecast Discussion
National Weather Service Pittsburgh PA
138 PM EDT Mon Mar 19 2018

Low pressure will bring a wintry precipitation mix to the Upper
Ohio Region tonight, Tuesday, and Wednesday. Below normal
temperatures set up shop the balance of the work week.


Yet another sunny morning, which will be the last this week.
Passing cirrus clouds this morning will give way to increase
clouds from west, meanwhile stratus shall continue to expand
along and east of the mountains and could reach Garrett and
eastern Tucker counties this afternoon.

There are indications of sprinkles across parts of Tucker
county this afternoon from high resolution guidance, however
given forecast low level moisture fields think any measurable
QPF will hold off until after 0Z so removed pop mention for
areas south of the mason dixon line late this afternoon.
Otherwise, the remainder of the forecast is on track.


*** Heavy Wet Snow Possible in the Allegheny Front Tuesday night
 into Wednesday ***

Confidence is increasing for potential for 8" or more of
snowfall over 24 hours across the higher terrain of western
Maryland and northern West Virginia, so a winter storm watch was

A weather-plethora, winter-spring battle is setting up to engage
over the Upper Ohio Region for the short term period. Problems
include model discrepancy and run-inconsistency as well as
incidents of apparent feedback with overblown omega in the
developing warm advection regime.

Obs. and model world do agree that a southern stream low will
dig over the Lower Ohio shunting relative warmth and moisture
across WV and MD tonight, and finally over PA early on Tuesday.
The warm surface (after another 50+ degree day) will be an
inhibiting factor for travel issues for most areas in the event
of light snow, or should warm advection be sufficiently strong
for light freezing rain.

The focus for problems thus remains on the ridges where dynamic
cooling via falling pressure and southeasterly upslope is
likely to support a temporal combination of freezing rain and
snow development. As the aforementioned warm advection into the
mid levels should inhibit dendritic growth potential, have
limited potential snow amounts to advisory levels. In addition,
the warm surface and eventual mid March sun angle should limit
ice accumulation, especially after daybreak Tuesday; have thus
lowered the ice from QPF ratio for the tonight/early Tuesday
time frame. The bottom line is that advisory level winter precip
is anticipated for the ridges, with much lesser confidence for
other areas. A Hazardous Weather Outlook mention was thus
included as per collaboration with neighboring NWS offices.

For at least a portion of Tue, shortwave ridging in advance of
the digging Lower Ohio storm system should diminish PoPs, while
temperature rises into the 40s amid warm advection ahead of low
pressure encroaching upon the Lower Ohio. Thereafter, the Lower
Ohio low-center is progged to trundle slowly across the Mid
Atlantic region through Wednesday, with coastal surface
development enhancing boundary layer cold advection over the
immediate area. Periods of light snow and snow showers are thus
envisioned as the Upper Ohio is enshrouded on the northern
flank of the mid level low, with inhibitions provided by daytime
sun angle and a lack of mid level frontogenesis in a weakening
wind field.


- 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 overnight,
especially along the ridges. Currently, a trend towards rain is
forecast past 12Z west of the ridges, with a mix continuing in
the higher elevations. Ceilings and visibilities will sink to
MVFR during this time, with spotty IFR possible as well. Some
uncertainty remains in precip timing and type, and we expect
changes to the forecast going forward.

Restriction potential continues through Wednesday with
precipitation expected to change to all snow Tuesday night and
continuing into Wednesday.


MD...Winter Storm Watch from Tuesday afternoon through Wednesday
     afternoon for MDZ001.
WV...Winter Storm Watch from Tuesday afternoon through Wednesday
     afternoon for WVZ512>514.


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