Severe Thunderstorm Warning 221, NWS Raleigh, NC

AREA FORECAST DISCUSSION
National Weather Service Raleigh NC
325 PM PM EDT Wed Aug 21 2019

.SYNOPSIS...
Weak subsidence will filter into central NC on the backside of a
departing upper level disturbance exiting east off the coast. In its
wake, a surface trough will begin to sharpen across the Appalachian
foothills with the Bermuda high strengthening off the southeastern
Atlantic Coast. This will help strengthen onshore flow ahead of a
strengthening upper-trough and associated cold front, likely to pass
through central NC Friday night and Saturday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 220 PM Wednesday...

Latest water vapor imagery depicts a weakening shortwave now
departing off the Mid Atlantic coast with a secondary shear axis,
associated with a weakening mid level shortwave, now beginning to
cross the Appalachian Mountains. At the surface, Bermuda high
pressure continues to keep southwesterly flow dominant across the
eastern seaboard with a developing leeside trough beginning to
sharpen along the Appalachian foothills. Full boundary layer heating
and mixing was slow to come by this morning thanks to a stubborn
low level stratus deck in place across much of the CWA. This feature
finally decayed around lunchtime, allowing at least modest
heating/destabilization to occur. Current SPC mesoanalysis depicts
2500 - 3500 J/KG SB Cape beginning to form across portions of
central NC, with ML CAPE nearing 2000 J/KG across the eastern
Piedmont/Coastal Plain zones where surface dewpoints remain in the
middle 70s. Some subsidence/capping is evident across the area,
which should be ample enough to limit airmass thunderstorm
development until much later in the period. CAMs continue to back off
on tstorm coverage this afternoon and evening, depicting only
isolated development along the residual MCV/Outflow boundary draped
NE/SW just east of the I-95 corridor, where coverage should be
prominent earlier in the period (between 2pm- 6pm); and across the
northwest Piedmont where weak mid-level forcing is expected to cross
east of the foothills with residual daytime heating to work with. In
response, have trended POPs slightly higher than CAM guidance this
afternoon and evening, with isolated to scattered storms possible,
especially across the northwestern zones. Any storms that do form
could produce locally heavy rainfall thanks to slower storm motions
(15 to 20kts) and localized damaging wind gusts from embedded wet
microbursts. Showers and storms may linger briefly into the
overnight hours, but should weaken drastically with the setting sun.

Even with the late start, afternoon temperatures remain on track,
likely to top out in the upper 80s to lower 90s in the next few
hours. Overnight lows will settle in the lower 70s, with only patchy
fog/stratus formation possible.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
As of 325 PM Wednesday...

Our region will remain primarily under the influence of the Bermuda
high during the short-term period, keeping a warm/moist surface flow
regime in place. Strengthening westerly flow aloft will help to
sharpen and maintain the lee side trough over the western Piedmont
zones, which will likely serve as the primary forcing mechanism for
afternoon showers and thunderstorms. Skies should remain mostly
clear for the early part of the day, allowing plenty of
destabilization to occur. Expect SB/ML CAPE values equivalent if not
slightly higher than on Monday thanks to slightly higher sfc
dewpoints and plentiful surface heating. While most CAMs aren't very
bullish in terms of convective coverage, expect at least airmass
type thunderstorms to be scattered to low end numerous, especially
along and west of US-1. Primary threat will once again be locally
damaging winds and heavy rainfall due to the slow moving nature of
the convection. Storms should follow diurnal climo guidelines, and
diminish rapidly after sunset, leaving only patchy fog/stratus.

Daytime high temperatures will be comparable, if not a degree or two
warmer than what was witnessed Wednesday, topping out in the upper
80s (NW) to lower 90s. Overnight lows will settle into the low to
mid 70s.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
As of 250 PM Wednesday...

A cold front associated with an unseasonably strong northern stream
trough/upper low passage through the NE US and Mid-Atlantic states
will provide the focus for widespread showers and storms at the
start of the period. The timing of this front through the area
continues to be a source of large model spread and high forecast
uncertainty. General timing brings the cold front into the area
from the NW late Friday afternoon and will slip slowly south through
the overnight, before finally settling south of the area on Saturday.

Low-level frontal convergence underneath the right entrance region
of a ~90 kt upper jet streak associated with the upper trough moving
through the region will result in deep/strong ascent across the
area. Precipitable waters of 2.0-2.25", MLCAPE ~2000 J/Kg and a belt
of strong westerlies of 30 to 35 kts dropping south into the area
will support the threat for organized strong to severe storms, along
with the threat of heavy rainfall of 1 to 2 inches with localized
flash flooding possible Friday afternoon and evening.

If latest model trends verify, a shallow of wedge of cooler and
stable air will advect south through the day on Saturday as the cold
front progresses south into SC and GA. While convection should
follow the front, models suggest a a period of ana-frontal precip
lingering across at least southern portions of the forecast area
INVOF of the h8 trough axis Saturday afternoon and  evening,
possibly lingering into Saturday night. As the cooler and drier
continues to filter into the area from the north Saturday night and
into   Sunday, courtesy of a strong ~1030 parent sfc high over New
England, rain chances should decrease, leaving behind primarily
cloudy and cool conditions Sunday and Monday. Rain chances return as
early as Tuesday and Wednesday with the approach of a shortwave
trough from the WSW.

Finally, the NHC is monitoring an area of disturbed weather located
over the central and northwestern Bahamas. Some slow development of
this system is possible with a low-20% chance of becoming a tropical
cyclone over the next several days. As such, it is incredibly too
early to forecast what impacts, if any, are possible across central
NC. Make sure you follow a trusted weather source for all of your
weather information.

&&

.AVIATION /17Z WEDNESDAY THROUGH MONDAY/...
As of 115 PM Wednesday...

Conditions have been slow to return to VFR across central NC this
morning thanks to a stubborn stratus deck which has finally shown
some decay. VFR conditions should prevail and persist over the next
24 hours as mid level subsidence fills in behind the departing upper-
level system, now progressing off the Mid Atlantic Coastal Plain.
Even shower and thunderstorm coverage should remain isolated at best
this afternoon, with CAMs continuing to come in less impressive with
each sequential model run. Still can't rule out a passing
shower/storm across the Triad terminals between 4pm - 9pm this
evening, but elsewhere, probability remains too low to include in
the forecast for now. Fog and stratus shouldn't be as widespread
tonight into Thursday morning thanks to drier air in place.

An unsettled weather pattern is expected to end the week as a
moisture builds ahead of a cold front, likely to pass through the
region Friday night into Saturday. This will increase shower and
thunderstorm coverage along with increase the likelihood of
widespread fog/stratus across the region each morning. There could
be a brief period of favorable aviation conditions on the backside
of the front Saturday evening into Sunday, however, the front is
expected to return precipitation chances back North as early as
Sunday night, likely to bring additional rounds of showers and
storms into early next week.

&&

.EQUIPMENT...
Repairs out at the GSO Upper-Air observation site have been
completed. Flights will continue, as weather dictates, beginning
again tongiht (00z Thursday Observation). National helium shortages
continue to impede normal observation schedules.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

SYNOPSIS...JJM
NEAR TERM...JJM
SHORT TERM...JJM
LONG TERM...CBL
AVIATION...JJM
EQUIPMENT...RAH

Hazardous Weather Outlook
National Weather Service Raleigh NC
439 AM EDT Wed Aug 21 2019

NCZ007>011-021>028-038>043-221000-
Person-Granville-Vance-Warren-Halifax-Forsyth-Guilford-Alamance-
Orange-Durham-Franklin-Nash-Edgecombe-Davidson-Randolph-Chatham-Wake-
Johnston-Wilson-
439 AM EDT Wed Aug 21 2019

This Hazardous Weather Outlook is for central North Carolina.

.DAY ONE...Today and tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

An isolated severe thunderstorm will be possible Thursday afternoon
with the main severe weather hazard being damaging wind gusts.

A very moist and unstable atmosphere will lead to the development of
scattered thunderstorms Friday and Saturday. The thunderstorms will
produce brief periods of torrential rainfall, possibly resulting in
minor flooding. Additionally, a few of the storms may become severe
producing damaging wind gusts and marginally large hail.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

NCZ073>078-083>086-088-089-221000-
Stanly-Montgomery-Moore-Lee-Harnett-Wayne-Anson-Richmond-Scotland-
Hoke-Cumberland-Sampson-
439 AM EDT Wed Aug 21 2019

This Hazardous Weather Outlook is for central North Carolina.

.DAY ONE...Today and tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

An isolated severe thunderstorm will be possible Thursday afternoon
with the main severe weather hazard being damaging wind gusts.

A very moist and unstable atmosphere will lead to the development of
scattered thunderstorms Friday and Saturday. The thunderstorms will
produce brief periods of torrential rainfall, possibly resulting in
minor flooding.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

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