Area Forecast Discussion
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FXUS61 KPBZ 230907

Area Forecast Discussion
National Weather Service Pittsburgh PA
407 AM EST Mon Jan 23 2017

Low pressure will spread rain over most areas, with some snow
as well in some locations this afternoon into Tuesday. The next
cold front arrives Wednesday.


Still an average to below average confidence in the forecast.
Models are fairly similar with the track of the upper low over
the next 24 hours, taking the center across the Carolinas and off
of the Delmarva by 12Z Tuesday. Categorical PoPs are still
appropriate with the main deformation band crossing during the
day providing plenty of ascent, with ample moisture in the form
of 0.75-0.9 PWAT values. Main questions center on precip type and
snow amounts. Still expect a mix with snow this afternoon and
tonight, with a full changeover possible tonight north of
Pittsburgh and along the ridges. The timing of that change and
resulting snow amounts will be tricky.

NAM is still the coldest in the column this afternoon and
tonight, quite likely due to an enhanced dynamic cooling effect
as compared to the GFS, which continues to show this to a lesser
degree. A scenario more like the NAM could result in a quicker
changeover and higher snow accumulations than currently depicted,
and could in theory require a snow advisory issuance in the far
northeast counties and down the ridges. However, even if dynamic
cooling results in a burst of snow this afternoon/evening, the
warm boundary layer and ground may help to cut down any
accumulations. By the time the boundary layer cools tonight,
precipitation will be on a downward trend as deformation lifts
north and a more showery regime takes hold in developing N/NW
flow. For now, will continue with relatively low accumulations
along the ridges, topping out in the 1-2 inch range. Will continue
the HWO mention due to the uncertainty, and cannot rule out an
eventual snow advisory issuance.

Will also monitor winds, as there are indications in model
soundings that mountain wave activity could lead to enhanced
downslope gusts west of the ridges, particularly in Westmoreland
and Fayette counties. For now, thinking ridgetop inversion may
not be strong enough to allow this to occur, but will continue to

Temperatures will likely slowly fall through the day as colder
air wraps in. Took a model blend for low temps tonight.


The vertically-stacked low pressure system will advance up the East
Coast Tuesday, loosening it`s grip on the Mid-Atlantic and Upper
Ohio Valley. As a result, deep moisture will erode eastward, leading
to a general decrease in shower activity through midday Tuesday. Any
lingering showers that do remain will be light, transitioning from
snow or a rain/snow back to all liquid as temperatures rise.

We`ll see a brief lull in precipitation Tuesday afternoon through at
least early Wednesday with a transient high pressure. The next low
will move through the western Great Lakes Wednesday evening, draping
a relatively dry cold front through the region. Will keep only
chance PoPs along the front as the low nears. The better chance of
precipitation areawide will come Wednesday night stretching into
Thursday with the passing upper trough axis, as cooler air pours
into our mid-levels. Steepening lapse rates and injection of Great
Lake aggregate moisture will allow for rain showers to eventually
transition to at least a rain/snow mix. Temperatures will remain
well-above average in through the middle part of the week.


A conglomeration of upper lows will lead to broad troughing of the
upper-level flow through at least the weekend. This will bring the
return to more seasonal conditions. Several impulses passing through
the mean flow, will result in periodic snow chances. Some
accumulation potential exists through the weekend, especially north
and into the high elevations. While the Great Lakes are still
generally ice-free, surface temperatures of Lake Erie, in
particular, have dipped into the mid 30s. This will help limit any
lake enhancement as colder air arrives.


Flight conditions will continue to deteriorate through the night
through MVFR, to IFR areawide for most of Monday. Upper level low
pressure tracking to our southeast will spread a band of rain over
the area for much of the day. Snow may begin to mix in north of
Pittsburgh and along the ridges during the late afternoon/evening
hours. Easterly winds will increase with time, and gust to between
25 and 30 knots during the day.

Restrictions are likely through Tuesday as low pressure moves off
the coast, and again late Wed through Fri with a crossing cold
front and subsequent upper troughing.




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