Wind Advisory 17, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
244 AM EDT Fri Apr 19 2019

.SYNOPSIS...
An approaching strong cold front from the west will bring
strong to severe storms Friday. Cooler temperatures arrive
behind the front for the weekend, with lingering showers across
the mountains on Saturday. Temperatures will rise above normal
again next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
As of 1045 PM...The latest radar mosaic now shows the QLCS entering
NW GA. The northern half of the line has weakened quite a bit, and
probably will weaken further, as it runs out ahead of the front and
better instability. Meanwhile, guidance is way overdone with pre-
frontal upslope-driven precip that was expected along the S/SE-
facing escarpment. Have trimmed the PoP thru the next few hours to
mainly slgt chc, until the convective line enters around 5z.
Otherwise, forecast looks on track.

Looking at the synoptic setup, an area of low pressure centered near
Lake Huron is carrying a cold front through the Ohio Valley and
southwest to Texas attm. This is associated with a highly amplified
pattern with a virtually full-latitude trough over the Plains, and a
deep ridge just off the East Coast. Moist southerly flow in our
area, along with seasonably warm/moist sfc conditions, maintains a
shallow cu field across most of the CWFA. PoPs will develop first
along the Blue Ridge Escarpment, but expand to more of the area as
the synoptic setup continues to develop tonight into early Friday.

Overall, this setup continues to point to a significant
high-shear/low-CAPE QLCS threat for a large chunk of the
Southeast. The shortwave will continue to dig and sharpen up
across the Mississippi Valley overnight, with a closed upper
low developing near Memphis by morning, with the whole trough
shifting east thereafter. The resultant midlevel flow is strongly
meridional as the trough crosses the region, which is important
per HSLC research. Surface-based instability is expected to be
effectively nil for most of the night, but is shown to develop
very quickly in the morning once diurnal heating begins. The tight
gradient ahead of the trough will be associated with a 50+ LLJ in
the early morning, and 40-50 kt 850mb winds persisting through
the day, along with an amplifying jet streak and well-defined
region of upper divergence traversing the area. 0-1km bulk shear
will easily reach 40 kt, a value also representative of severe
HSLC events. Once the instability starts to develop, the best
overlap between it, the LLJ, and dynamic forcing associated with
the trough appears to be in the western CWFA; activity will become
increasingly likely to form into a QLCS with time. CAM consensus
shows this happening around midday in the central CWFA, propagating
east of the CWFA by late afternoon. HREF output depicts a number
of updraft helicity swaths during peak heating mainly along/east
of a line from Hickory NC to Union SC. The threat mainly will be
damaging wind but a couple of 1 to 2 inch hail events could occur
in the eastern CWFA if we do realize the 1000+ J/kg as depicted on
the HREF mean and operational NAM. A few short-tracked tornadoes
certainly will be possible at times through the day, but especially
in the eastern zones. That said, the NAM/WRF family of guidance
(which of course includes the HREF members) are a bit scarier
than the GFS. The latter shows the 0-1km shear tapering off a bit
following the diurnal loss of the LLJ, plus it is not as unstable.

We have evaluated hydrologic threats for this event, and given
the anticipated exceptionally high PWAT values and robust moisture
flux, at least isolated flooding will be a concern in the near term
period. Perhaps the most likely concern (as usual) would be the
south-facing Escarpment, with terrain anchoring and enhancement
most likely to produce excessive runoff. However, with the event
being relatively short duration, rises on the rivers of the French
Broad basin still are not expected to push them to flood stage. For
the remainder of the area, the forcing and moisture will produce
very high rainfall rates, but storm motion is likely to be quick
enough to keep the threat localized at best. Also as usual, the
Charlotte metro area may be the exception to this rule. All this
said, the areas most likely to flood are small and isolated enough
that a Flood Watch or Flash Flood Watch is not really appropriate.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/...
As of 230 pm EDT Thursday: The numerical models feature excellent
agreement on a 542 dm (and deepening) closed 500 mb low center
approaching the southern Appalachians Friday evening. This should
keep numerous showers going across the mountains as lapse rates
steepen, with more scattered showers across the foothills and
piedmont. Snow levels will gradually descend to uncover the higher
peaks around the Smokies through the late evening and overnight
hours. The increasingly stacked system will bring the base of
the associated trough axis across the forecast area through
Saturday. At least scattered showers will continue under the
trough, especially over the mountains, where snow levels will
continue to fall. Some accumulating snow is likely above 4000
feet, with a couple of inches possible above 5000 feet. There is
very little pure orthogonal northwesterly upslope flow of
moisture into the spine of the mountains until late Saturday
into Saturday night, and by then the low level moisture is
becoming quickly shallower. Thus, do not anticipate any winter
hazard products with the package this afternoon, but the high
elevation potential in and around the Smokies will bear close
watching.

As the low pressure system lifts slowly east through the mid-
Atlantic region the latter half of the weekend, the cold advection
northwest flow gradient will pick up a bit. The resulting breezy to
windy conditions will help inhibit frost formation across the active
zones (which is everywhere but the northern NC mountains.)
Anticipate temperatures warming back into the 70s in many areas east
of the mountains as insolation improves and downslope warming
continues.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 245 PM EDT Thursday: Upper low over the Mid-Atlantic will
continue to lift north, with high pressure building in and
increasing thicknesses. Temperatures will increase through the
extended, 5 to maybe even 10 degrees above seasonal. A shortwave
spinning around the upper low may bring some showery activity or
even possible thunderstorms Tuesday afternoon, GFS a little stronger
with this idea than ECMWF. Next system gearing up over the Rockies
will push off into the Plains sometime midweek, but there are some
very significant timing differences between the global models, so
frankly opted to keep a fairly generic blend through the end of the
period and into the new day 7. As a note to readers, depending on
how near-term activity shapes up tonight, the extended forecast may
not be updated until Friday afternoon.

&&

.AVIATION /07Z FRIDAY THROUGH TUESDAY/...
At KCLT and elsewhere: Rapidly deteriorating conds expected between
12Z and 18Z associated with a dynamic storm system to move through
the region. Model guidance shows that the strong line of showers and
thunderstorms (QLCS) approaching across the deep south will hold
together and even intensify east of I-26 and especially near the I-
77 corridor, including KCLT. Brief SW/W gusts in excess of 40 kts
is possible with the QLCS. Light to occasionally moderate LLWS is
possible, especially between 15Z and 20Z. LLWS issues may linger at
KAVL behind this time as winds veer to the SW. A secondary impulse
of showers and embedded thunderstorms associated with the leading
edge of the upper low are possible just about anywhere between 18Z
and 00Z.

Outlook: Improving conditions are expected early Saturday. Spotty
restrictions cannot be ruled out over the mtns as an upper low
passes by on Saturday.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  95%     High  88%     Med   75%     High  92%
KGSP       High  88%     High  91%     Med   78%     High  85%
KAVL       High  89%     Med   69%     Med   74%     High  80%
KHKY       High  84%     High  84%     High  84%     Med   75%
KGMU       High  88%     High  89%     Med   73%     High  85%
KAND       High  89%     High  84%     Med   74%     High  80%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

SYNOPSIS...Carroll
NEAR TERM...ARK/Wimberley
SHORT TERM...HG
LONG TERM...TDP
AVIATION...Munroe

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
503 PM EDT Thu Apr 18 2019

GAZ010-017-NCZ033-048>053-058-059-062>065-501-503-505-507-509-
SCZ001>003-192115-
Rabun-Habersham-Avery-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-
Graham-Northern Jackson-Macon-Southern Jackson-Transylvania-
Henderson-Caldwell Mountains-Burke Mountains-McDowell Mountains-
Rutherford Mountains-Polk Mountains-Oconee Mountains-
Pickens Mountains-Greenville Mountains-
503 PM EDT Thu Apr 18 2019

This Hazardous Weather Outlook is for the mountains of northeast
Georgia, western North Carolina, and Upstate South Carolina.

.DAY ONE...Tonight.

Gusty winds from the south will develop across the high elevations
of the southern Appalachians tonight. Wind gusts of 40 to 50 mph
will be possible, mainly on exposed peaks and ridge tops. A band of
heavy rain showers and embedded thunderstorms is expected to move
into the mountains of the western Carolinas and northeast Georgia
between midnight tonight and daybreak Friday. The primary threat
from these showers and storms will be from very heavy rainfall
leading to rapid water rises on small streams and in areas of poor
drainage. Localized flash flooding will be possible. In addition,
brief, damaging wind gusts cannot be ruled out.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Gusty south winds and the threat for localized flash flooding will
continue into Friday. The potential for localized damaging winds
associated with heavy rain showers and thunderstorms is expected to
increase a bit during the daylight hours Friday.

.SPOTTER INFORMATION STATEMENT...

Spotters may need to forward reports of damaging winds or flooding to
our office on Friday.

GAZ018-026-028-029-NCZ035>037-056-057-068>072-082-502-504-506-508-
510-SCZ004>014-019-192115-
Stephens-Franklin-Hart-Elbert-Alexander-Iredell-Davie-Catawba-Rowan-
Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-Union-Greater Caldwell-
Greater Burke-Eastern McDowell-Greater Rutherford-Eastern Polk-
Greater Oconee-Greater Pickens-Greater Greenville-Spartanburg-
Cherokee-York-Anderson-Abbeville-Laurens-Chester-Greenwood-
503 PM EDT Thu Apr 18 2019

This Hazardous Weather Outlook is for the foothills and piedmont of
extreme northeast Georgia, western North Carolina, and Upstate South
Carolina.

.DAY ONE...Tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

A band of heavy rain showers and embedded thunderstorms is expected
to move across the foothills and Piedmont from early Friday morning
through the afternoon. Isolated severe thunderstorms capable of
producing locally damaging wind gusts and isolated brief tornadoes
will be possible, especially across the Piedmont, from mid-morning
through the afternoon. In addition, localized excessive rainfall may
cause flash flooding of small streams and urban areas.

.SPOTTER INFORMATION STATEMENT...

Spotters may need to forward reports of damaging winds, tornadoes,
or flash flooding to our office on Friday.

HG