Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 221956 RRA

National Weather Service Raleigh NC
355 PM EDT Thu Aug 22 2019

Bermuda high pressure will remain in control of our weather
conditions through tonight. A strong cold front will move south
through central NC Friday evening through early Saturday, bringing a
good chance of thunderstorms. The front will settle south of the
area during the day on Saturday with cooler high pressure building
into region through Monday.


As of 250 PM Thursday...

Plentiful sunshine throughout the morning/mid afternoon hours has
allowed for ample heating and destabilization to occur across just
about all of central NC. RAP analysis shows SB CAPE values ranging
from 3500 J/KG (West) to 4500 J/KG (East) with additional
destabilization likely over the next few hours. Convective
initiation is ongoing across much of the area, especially along an
old MCS outflow boundary, now progressing east of the Blue Ridge and
through the Foothills. Out ahead of the primary band of storms,
additional convection has fired along a remnant outflow boundary
situated along/just north of the US-64 corridor across the western
Piedmont, in the vicinity of the lee side trough. Convective
coverage and intensity will likely increase over the next few hours,
especially as the more organized band of storms progress east into
the Triad region (likely between 3 - 6pm) and eventually the
Triangle (4 to 8pm) before continuing into the eastern Coastal
Plain. These areas, especially toward the northern Piedmont zones,
will be better suited for a possible damaging wind gust threat.
Here, DCAPEs range in the 1100 - 1300 with slightly lower values as
you progress south. PWAT values in the 1.6 to 2.1 inch range (Higher
east) will also support some very heavy rainfall embedded with these
storms, keeping a low end urban and small stream flood threat
active, especially with a 20 - 25kt storm motion. Storms will likely
continue into the overnight hours, becoming elevated and feeding off
of residual ML CAPE which should remain impressively high,
especially east of the Triangle, until areas get worked over.
Coverage should diminish after midnight, allowing skies to clear and
patchy fog/stratus to develop, primarily in vicinity of large water
bodies in areas that see appreciable rain Thursday evening.
Overnight lows will settle into the lower 70s.


As of 330 PM Thursday...

Friday and Friday night: a cold front associated with an
unseasonably strong positively-tilted northern stream trough diving
SE through the NE US and Mid-Atlantic states will provide the focus
for widespread showers and storms Friday afternoon and into Friday

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 of 2000-3000 J/Kg and
a belt of strong westerlies of 25 to 35 kts of effective south
dropping south into the area will support the threat for an
organized line or cluster of strong to severe storms Friday
afternoon and evening. CAMs support the above scenario, advancing a
well-organized cold pool dominated convective line segment into the
area between 4 pm to midnight, with damaging thunderstorm wind gusts
the primary threat, especially from US Highway 64 north. Convection
should weaken and the severe threat should diminish from loss of
daytime heating Friday evening as the line progresses into southern
half of the forecast. If the cold front slows down/briefly stalls
across the area Friday night/early Saturday as indicated by the GFS,
pockets of heavy rainfall could lead to some localized flash
flooding, especially in low-lying or poor drainage area. Average
rainfall amounts are expected to range between 1 to 2 inches.

It will be another warm and humid day ahead of the front. Highs
Friday afternoon ranging from mid to upper 90s north to lower 90s
south. Min temps across the area Friday night will depend on how far
south the cold front progresses. But the airmass will feel noticeably
cooler across the north with lows ranging from mid 60s north to
lower 70s south.

Saturday and Saturday night:

A shallow of wedge of cooler and stable air will attempt to advect
south through the day on Saturday as the cold front progresses south
into SC and GA. While convection should remained anchored to  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 evening/night. What a difference day makes wrt
temperatures on Saturday with expectations of a wedge regime setting
up north of the front. Weak overrunning atop the low-level NELY flow
will lock in the low clouds and produce spotty rain showers and/or
drizzle. Have lowered forecast highs on Saturday. Highs ranging from
lower 70s(a 15 F degree difference from Friday) to lower 80s south.
Lows Saturday night in the lower to mid 60s


As of 255 PM Thursday...

Extended portion of the forecast remains relatively on-track, with
uncertainty remaining high, especially early next week. Have
adjusted POPs slightly upward Sun - Tue as the front lingers/drifts
north and hints of some tropical/sub-tropical influences begin to
appear. Models continue to trend wildly with these features, so
thought it best to linger CHC/SCHC POPs into the middle of next
week, with a gradual warming trend likely.

Previous valid discussion...

Meanwhile, it`s worth noting as mentioned in the latest TWOAT, the
trough of low pressure located over the central Bahamas is progged
by several models, including the latest ECMWF, to lift north and
merge with the aforementioned cold front remnants off the SE coast
on Sunday.  Fortunately these models keep any low pres development
well off the coast, but should this happen, central NC may actually
benefit from this pattern by way of increased N/NE flow which would
advect drier air into our area from the north.  On the other hand, a
front that stalls closer to the immediate coast would warrant PoPs
across our eastern zones for Sunday. But the trends have tended to
favor the drier option.  Finally, such vigorous NE flow would keep
temps below normal for daytime highs.

That system will exit to our NE on Monday, with mainly dry weather
continuing for our area.  The next short wave will then approach
from the west Monday night and Tuesday, with climo or higher PoPs
needed for Tuesday and perhaps into Wednesday with the passage of
this short wave.


...As of 100 PM Thursday...

24 hour TAF period: VFR conditions continue to remain dominant
across central NC. This trend should continue through the early
portion of the afternoon/evening hours, with deteriorating
conditions (periods of IFR/MVFR) possible with thunderstorm
development. Timing of evening convection has been a bit difficult
to pinpoint, but expecting to see a general NW to SE movement with
any multicellular clusters that do form. Thus, have introduced VCTS
along with TSRA tempo groups across the Triad terminals (KINT, KGSO)
between 21z/5pm and 02z/10pm, with an hour or so delay as you head
east. Can`t rule out some isolated airmass storm development out
ahead of this main line, but will account for that with amendments
as needed. Any storms that do impact terminals will result in
periods of reduced visibilities, ceilings, and gusty winds up to
40kts. Tonight, storm coverage diminishes from west to east, leading
to steadily clearing skies. Can`t rule out some patchy fog/stratus
formation, especially in areas that see appreciable rainfall
Thursday evening.

Looking ahead: Beyond 18z/Friday through Wednesday... Showers and
thunderstorms are expected to develop Friday across northern NC
early in the afternoon before dispensing across the area into Friday
night. Sub-VFR CIGs/VSBYs will be common with any storms that do
impact terminals. A cold front is set to pass through the region
Friday night into Saturday, introducing widespread SUB-VFR
conditions that could linger much of the weekend, especially across
the southern terminals. Can`t rule out some brief breaks in the
CIGs/VSBYs late Saturday/Sunday across the northern terminals, but
expecting deteriorating conditions once again as the front lifts
back north into the first half of next week.





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