Severe Thunderstorm Warning 158, NWS Blacksburg, VA

Area Forecast Discussion
National Weather Service Blacksburg VA
312 PM EDT Sat Aug 31 2024

.SYNOPSIS...
A cold front over the Ohio Valley will cross the Appalachians
this weekend...promoting scattered showers and thunderstorms.
Passage of the front will introduce cooler and less humid
weather for next week.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 245 PM EDT Saturday...

Key Messages:

- Highest threat for severe thunderstorms this afternoon and
  evening from central West Virginia into central Pennsylvania

- Warm and muggy weather continue through Sunday

Two main areas of scattered to numerous thunderstorms this
afternoon and evening. One area was in the mountains that have
had more sunshine and have a more unstable air mass for storms
to develop. The second was in the warm sector ahead of the cold
front across the Ohio Valley. These storms will have the
potential to be more organized and produce strong to damaging
wind through sunset.

Convective allowing models bring the storms ahead of the front
into mountains in a weaker form overnight. The clouds and
precipitation, along with the mild dew points will result in
low temperatures about 10 degrees above normal.

The leading cold front crosses the region on Sunday. This moves
the most active area for thunderstorms into the foothills and
piedmont Sunday afternoon. Temperatures will be starting in the
70s in some locations, and it won't take much heating to warm
well into the 80s.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 1230 PM EDT Saturday...

Key Points:

1. Trending cooler.
2. Lingering showers/storms Sunday evening.
3. Isolated showers/storms near the Blue Ridge Monday afternoon.
4. Dry Tuesday.

A look at the 31 Aug 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows the axis of an upper trough situated from just east of
Hudson Bay, extending southwest into the Lower Ohio River basin on
Sunday night. By Monday night, the axis shifts east to just east of
the spine of the Appalachians. By Tuesday night, the axis is over
the Canadian Maritimes, and a broad upper ridge is strengthening over
the central US. At the surface, a cold front is expected to be east
of the region by Sunday evening with high pressure centered over the
Upper Mississippi Valley. Monday through Tuesday, the center of high
pressure expected to head eastward across the Great Lake region and
then into New England. An associated ridge axis is expected to
become situated along the lee of the Appalachians.

Output from the 31 Aug 00Z Ensemble Situational Awareness Table
shows 850mb temperatures Sunday night falling through the night,
reaching roughly +13C to +15C by Monday morning. On Monday, the
value range dips just slightly to +13C to +14C. On Tuesday, values
fall even more, with numbers ranging from roughly +11C to +13C. The
lowest values will be in associated with the lee side ridge. The low
end of this range will be flirting with the 2.5 to 10 percentile of
the 30-year climatology for this time of year. Precipitable Water
(PW) values are expected to decrease quickly Sunday night. Around
sunset, values will still be around 1.75 to 2.00 inches across the
southern and southeastern sections of the forecast area. These
values correspond to the 90 to 97.5 percentile of the 30-year
climatology for this time of year. Northern sections are expected to
be drier, closer to 1.25 inch. By daybreak Monday, values are
expected range from just shy of 1.00 inch in the north to 1.50 inch
across the southeast. For Tuesday much of the region is expected to
see PW values under 1.00 inch.

For the above weather scenario, showers should remain a possibility
Sunday night until the upper trough axis is east of the region.
Chances will be highest across southeast sections where PW will
remain the highest the longest. Enough instability may still be
around for a few storms across southeast section Sunday evening.

Monday through Tuesday high pressure builds north of the region.
Since the center will be north of us, anti-cyclonic flow will
provide for a general easterly fetch of air into the region. The
combination of upslope easterly flow, and any locations across the
southern sections that manage to hold on to PW numbers close to
1.50, will have the potential for abundant cumulus development and
possibly some isolated to scattered showers/storms.

Temperatures during this portion of the forecast will trend to
values slightly below normal for this time of year.

Confidence in the above weather scenario is moderate to high.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 1230 PM EDT Saturday...

Key Points:

1. Cooler than average during the period.
2. Generally dry Wednesday and Thursday.
3. Better chances showers/storms Friday and Saturday.
4. Low confidence in precipitation timing/location from Thursday and
onward given large deviation in individual model solutions.

A look at the 31 Aug 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows a ridge of high pressure moving east to over Ohio and
Tennessee Valleys by Wednesday and Wednesday night. By Thursday and
Thursday night, the ridge axis shifts to over the East Coast and
flattens a bit. Concurrently, a trough will move over the Upper
Mississippi River Valley. By Friday into Friday night, there is not
a lot of eastward movement regarding the trough. However, it is
expected to amplify. Southwest flow aloft will be over our region.
On Saturday, expect little change in the overall synoptic pattern as
compared to conditions expected on Friday. At the surface, a ridge
of high pressure is expected to remains situated over the lee of the
Appalachians. Low pressure will be near Hudson Bay with an
associated cold front extending southwest into the Central Plains
States on Wednesday. For Thursday, the lee side ridge axis remains
in place. A cold front make gradual progress eastward to the Great
Lakes region. By Friday, the ridge continues to hold fast along the
lee of the Appalachians. An inverted trough/cold front is positioned
over the Lower Ohio and Tennessee Valleys. By Saturday, ensemble
averaging depicts a slightly less defined configuration of the
pattern from Friday. The general idea however remains ridging just
to our east and inverted troughiness just to our west.

Output from the 31 Aug 00z Ensemble Situational Awareness Table
shows 850mb temperatures are expected to range from roughly +11C to
+13C on Thursday, with the lowest values associated with the lee
side ridge. On Friday, expect only a small increase to +12C to +14C
range. On Saturday, expect similar values. By Sunday, look for
values of +13C to +14C. For Precipitable Water on Tuesday, much of
the region is expected to see values under 1.00 inch. Numbers are
only slightly higher on Thursday with a range of 1.00 to 1.25 inch.
For Friday, PW values will be slightly higher at 1.25 to 1.50 inch.
Saturday, numbers are down slightly to 1.00 to a little over 1.25
inch.

Given the above weather pattern, high pressure will initially be to
our north, but work its way eastward. Its associated ridge axis is
expected to take up residency along the lee of the Appalachians. Low
level flow should gradually veer east to southeast. While we cannot
rule out a stray shower or storm along the crest of the southern
extent of the Blue Ridge Wednesday and Thursday. On Friday and
Saturday, precipitation chances are expected to greater across the
area with the placement of an inverted trough immediately to our
west. Chances should be best in the west nearest the instability
aloft.

Confidence in the portion of the forecast is low. The above
narrative and forecast is based upon the ensemble average solution.
However, there is a wide variety of solutions within the individual
members that comprise that ensemble, and also with other
deterministic model solutions.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
As of 300 PM EDT Saturday...

Two main areas of scattered to numerous thunderstorms this
afternoon and evening. One area was in the mountains that have
had more sunshine and have a more unstable air mass for storms
to develop. The second was in the warm sector ahead of the cold
front across the Ohio Valley. These storms will have the
potential to be more organized and produce strong to damaging
wind through sunset.

Convective allowing models bring the storms ahead of the front
into mountains in a weaker form overnight. Expect MVFR to IFR
ceilings to return overnight with MVFR to IFR fog in the typical
river valleys west of the Blue Ridge.

The leading cold front crosses the region on Sunday. This moves
the most active area for thunderstorms into the foothills and
piedmont Sunday afternoon.

Wind speeds will be less than 10 knots, but strong and gusty
near thunderstorms.

Average confidence for ceiling, wind, and visibility.

EXTENDED AVIATION OUTLOOK...

A second cold front will cross the region Sunday night
resulting in a change in airmass by Monday.

Scattered to numerous showers and thunderstorms are expected
until the front clears south of the Mid Atlantic region. Through
early Monday, expect a mix of flight conditions with low
confidence on specific flight categories due to showers/storms
and nighttime stratus and fog.

Conditions improve, and dry weather is expected Tuesday through
Thursday. Local TAFs sites will still be subject to overnight
valley fog.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

SYNOPSIS...AMS/PM
NEAR TERM...AMS
SHORT TERM...DS
LONG TERM...DS
AVIATION...AMS/PM

Hazardous Weather Outlook
National Weather Service Blacksburg VA
303 PM EDT Sat Aug 31 2024

NCZ001>006-018>020-VAZ007-009>018-022>024-032>035-043>047-058-059-
011915-
Ashe-Alleghany NC-Surry-Stokes-Rockingham-Caswell-Watauga-Wilkes-
Yadkin-Tazewell-Smyth-Bland-Giles-Wythe-Pulaski-Montgomery-Grayson-
Carroll-Floyd-Craig-Roanoke-Botetourt-Rockbridge-Patrick-Franklin-
Bedford-Amherst-Henry-Pittsylvania-Campbell-Appomattox-Buckingham-
Halifax-Charlotte-
303 PM EDT Sat Aug 31 2024

This Hazardous Weather Outlook is for north central North Carolina,
northwest North Carolina, central Virginia, south central Virginia,
southwest Virginia and west central Virginia.

.DAY ONE...This afternoon and tonight.

There is a marginal risk of severe storms this afternoon and
evening. Damaging wind and heavy rain are the main threats.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

There is a marginal risk of severe storms Sunday afternoon.
Damaging wind and heavy rain are the main threats.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

VAZ019-020-WVZ042>044-507-508-011915-
Alleghany VA-Bath-Mercer-Summers-Monroe-Eastern Greenbrier-
Western Greenbrier-
303 PM EDT Sat Aug 31 2024

...SEVERE THUNDERSTORM WATCH 666 IN EFFECT UNTIL 7 PM EDT THIS
EVENING...

This Hazardous Weather Outlook is for west central Virginia and
southeast West Virginia.

.DAY ONE...This afternoon and tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Severe Thunderstorm Watch.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

There is a marginal risk of severe storms Sunday afternoon.
Damaging wind and heavy rain are the main threats.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

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