Area Forecast Discussion
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FXUS61 KPBZ 212255 AAB

Area Forecast Discussion...UPDATED
National Weather Service Pittsburgh PA
655 PM EDT Mon May 21 2018

Active weather through Tuesday night with a series of showers
and storms moving through. A dry mid to late week is forecast.
Unsettled weather returns the back half of the upcoming holiday
weekend. Above normal temperatures are predicted the next seven


Evening Update:

Showers and isolated thunderstorms will continue to move east
south of I-70 for the next few hours. A Svr Thunderstorm Watch
has been issued to the east of region in Central Ohio. A line of
storms is expected to intensity in that vicinity before tracking
east with the sfc low. This line is still expected to reach our
eastern counties around midnight. The main threat would be wind
from some isolated storms.

Previous Discussion:

Warm front that extends from CVG to southwestern VA will lift
northward this evening. Isentropic upglide and daytime heating
will fire up scattered showers and storms over West Virginia and
southern Ohio before a more defined line develops across
northern Ohio. This will track east and perhaps be severe, but
it will be lacking surface instability and rely on 40kt low
level jet and modest mid level lapse rates. This area of showers
and storms should cross between 4Z- 8Z mainly north of I-70.
Brief heavy rain is possible, along with a very low risk of
damaging winds. Current SPC convective outlook looks reasonable
with greatest threat in our western zones of east central Ohio
closer to instability max.

Cloud forecast is tricky Tuesday through early afternoon because
of overnight precipitation and left over residual moisture. If
the sky clears and daytime heating occurs another round of
showers and storms would move through during the afternoon hours
ahead of a mid level piece of energy. These storms could
contain gusty winds which may attain severe levels. The marginal
risk in the latest SWODY2 as expanded to cover the entire
region. Given low confidence in amount of daytime heating this
risk level seems appropriate. H7-H5 lapse rates push 6.5Ckm-1
during the afternoon thus if surface instability is present
there will be more to tap as air parcels rise. Continued with
likely pops for the afternoon hours over all of western
Pennsylvania, northern West Virginia, and western Maryland.


Broken line of showers and thunderstorms should be through the
mountains by 3Z owing to dry weather the balance of the night.
Any fog development will be confined to northern areas where
boundary layer winds shall be lighter. Not to keen on widespread
fog development given BUFR soundings still show a fair amount
of moisture in the lowest few thousand feet, so decided to leave
fog out of the weather grids at this time.

High pressure builds down from the eastern Great Lakes
Wednesday. With the northerly wind component this will be the
coolest day of the next seven, but temperatures still should
climb well into the 70s given amount of sunshine and progged 10C
temps near the top of the mixing layer. As the surface
anticyclone settles on top of the area, Wednesday night into
Thursday morning is the best time window for fog potential.
We are in the stretch of the longest daylight hours of the
year, so confidence is too low for inclusion.

Diurnal cumulus develops downwind of Lake Erie Thursday other
than that a splendid day with lots of sunshine. Boundary layer
temps increase 2C, so daytime highs will be around 5F warmer
than Wednesday.


- Dry weather to start
- Trouble in the tropics
- Wet second half of Memorial Day Weekend

High pressure provides dry weather through the first half of
Saturday as mid level ridging takes control over the eastern
United States. With H5 heights around 582dm and H8 temps around
16C daytime highs should be well into the mid 80s outside of the
mountains and perhaps north of I-80. Plenty of sunshine and warm
temperatures between H6-H7 will put a cap on any cu from
growing into a shower or storm across the higher terrain.

The forecast gets messy Saturday afternoon through Memorial Day.
A potential tropical system will slowly spin along the Gulf
Coast states, while moisture is pumped north from the gulf on
the windward side of the Appalachians. This in conjunction with
an active jet stream brings several disturbances across during
the aforementioned time frame. PWATS peak around 1.7" while warm
cloud depths run near 12KFT. This spells potential for heavy
rain and potential flooding if an any gets hits repeatedly.
Right now, its too hard pinpoint location of showers with
embedded thunder so nothing that meets confidence for insertion
into the hazardous weather outlook. Severe thunderstorm threat
appears low given the amount of clouds forecast.

GEFS/GEPS remain in good agreement on the large scale synoptic
system outside small location differences near the Gulf Coast.
The timing of the shortwave troughs place highest QPF values in
the mountains Saturday evening through Memorial Day. NAEFS pwats
are in the low end of the probability of extremes using CFSR
climo, thus for now do not see a high end event.


VFR conditions and dry weather are generally expected into the
overnight hours before widespread rain moves in from the west.
Ceiling restrictions are not expected until Tuesday morning, and
primarily across the north. Widespread visibility restrictions
are not expected, although MVFR/IFR conditions could be possible
in isolated heavier rain. Isolated thunderstorms will be
possible overnight, and again Tuesday afternoon.

Restrictions in showers/thunderstorms are likely into Tue eve
with a crossing cold front.




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