Area Forecast Discussion
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FXUS61 KPBZ 231904

Area Forecast Discussion
National Weather Service Pittsburgh PA
304 PM EDT Thu Mar 23 2017

A period of above-normal temperature and increasing chances for
rain is expected through early next week.


A weak, low-amplitude disturbance will interact with the
strengthening low-level jet overnight to generate light
precipitation, particularly late tonight and early Fri morning.
Although the focus will be in northern counties, latest hi-res
models support broader PoP coverage across the forecast area.

The strong warm air advection and increasing clouds tonight will
inhibit the typical diurnal temperature cycle. Initial cooling
and residual dry air may be enough to support wet-bulb values
below freezing in very isolated locales for a brief window late
tonight at the onset of any precipitation (e.g. a few of the
coldest sheltered valleys in the ridge zones).

In general, temperatures are expected to remain above freezing,
but a brief period of light freezing rain may occur in those
sheltered valleys before warm air advection wins the thermal
battle. However, given the northern placement of the upper
disturbance, there is a lower confidence in precipitation with
southward extent, so it could be that little precipitation
occurs in the areas most thermally-favorable for any freezing
precipitation. This concern will be highlighted in the HWO
until the conditional threat is abated with confidence.


The disturbance responsible for overnight precipitation will
move east Friday morning, leaving the remainder of the day
mostly dry and relatively warmer. Another weak disturbance will
affect primarily the northern zones Friday night, given the
perseverance of an amplifying east-coast upper ridge.

A series of compact shortwave troughs will develop in the swrn
CONUS and eject newd beginning Saturday. As the first wave
acquires a negative tilt, shortwave ridging will amplify over
the Great Lakes region, leading to a likely-nwd displacement of
precipitation on Saturday and Saturday night.

Steady height falls will begin to spread across the region
Sunday morning as the ejecting wave nears. By afternoon, the
associated H5 jet maximum will cross the forecast area, likely
invigorating deeper convection with its passage amid modest

This wave will be exiting the region by Monday morning, with a
downward trend in precipitation expected by late Sunday night
as shortwave ridging develops in its wake.


In the wake of the Sunday wave, a brief respite can be expected
on Monday amid shortwave ridging before a second trough ejects
from the Plains. This second wave will be favorable for
widespread rain showers as it crosses on Tuesday as warm, moist
air is drawn poleward ahead of the system.

Behind the wave, seasonable air is expected to arrive mid-week
with highs in the 50s and lows in the 30s. Given ensemble
spread, there is considerable spread with the placement of the
pattern of upper-level troughs and ridges across the country
during this time, so the forecast details during this time
remains lower-confidence.


High pressure will keep a majority of clouds at bay through
tonight. A shortwave trough crossing along a warm front will
bring an increase in clouds and eventually light rain showers by
Friday morning. Kept VCSH mention for Pittsburgh and south while
northern ports stand the best chance to see borderline MVFR

Light wind will shift around to the south and gradually
increase through the period. An increasing low-level jet will
bring a chance for LLWS before mixing commences Friday morning.

The next chance for widespread restrictions is not expected
until the approach of low pressure early on Sunday.





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