Area Forecast Discussion
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FXUS62 KRAH 201925
Area Forecast Discussion
National Weather Service Raleigh NC
325 PM EDT Mon Mar 20 2017
High pressure centered over central NC this morning will shift
offshore the Southeast coast this afternoon, in advance of a broad
low pressure system and associated cold front approaching the
mountains from the WNW. The aforementioned low pressure system will
push southward through the Carolinas as a backdoor cold front
Tuesday evening into Tuesday night. Canadian high pressure will
build into the Carolinas from the northwest Wednesday into Thursday.
.NEAR TERM /Through Tonight/...
As of 100 PM Monday...
Today: High pressure centered over the area this morning will move
offshore this afternoon as a broad surface low and associated cold
front approaches the mountains from the west. As the sfc high shifts
offshore, warm advection will commence from west-east this aft/eve
as initially calm or light/variable winds become W/WSW at ~10 knots.
Clear skies this morning will become mostly cloudy this afternoon in
the Western Piedmont (and at least partly cloudy in the east) as a
thick veil of cirrus emanating from a mesoscale convective complex
(MCC) over IN/OH/KY advects downstream into the Carolinas via NW
flow aloft. Thickening mid/upper level ceilings could affect
afternoon temps by a few degrees in the W/NW Piedmont. With the
above in mind, expect highs 5-10F warmer than yesterday, ranging
from the lower 60s (NE Coastal Plain) to mid/upper 60s elsewhere,
warmest in the Sandhills/SW Piedmont.
Tonight: Low-level moisture will increase from the W/WNW tonight as
the aforementioned front progresses slowly south/east into TN/WV/VA
and low-level flow strengthens downstream in the Carolinas. Model
guidance is fairly unanimous in showing measurable precipitation in
the N/NW Piedmont late tonight (06-12Z Tue). Model guidance is most
likely picking up on remnants of the MCC, e.g. the eastern portion
of the complex as it progresses S/SE through WV into southwest VA
overnight. Alternately, strengthening low-level warm advection may
result in marginal/elevated destabilization and a potential for
elevated showers at the leading edge of a 925 mb warm front lifting
NE from Upstate SC through the Foothills/Western Piedmont of NC
between 06-12Z Tue morning. Given the dry airmass (PWAT 0.20-0.40")
currently in place east of the mountains, confidence remains low
with regard to whether or not sufficient moistening/elevated
destabilization and/or sufficient forcing will be present to
maintain lingering MCC remnants or support the development of
elevated showers late tonight. At this time, will continue to
indicate a ~20% of showers after midnight across the W/NW Piedmont.
Given broken/overcast cloud cover and a SW breeze, expect lows Tue
morning to be much warmer than this morning, in the mid to upper
40s, coldest in the E/NE Coastal Plain. -Vincent
.SHORT TERM /Tuesday and Tuesday Night/...
As of 216 PM Monday...
A broad surface low and associated cold front progressing SE into
the Appalachians tonight will track slowly southward through central
NC as a backdoor cold front late Tue aft/eve into Tue night, in the
presence of small amplitude shortwave energy traversing the region
in NW flow aloft. Aside from elevated convection that may develop
over or propagate into portions of central NC early Tue morning,
expect dry conditions to otherwise prevail through mid Tue afternoon
given weak forcing and a strong capping inversion associated with a
pronounced elevated mixed layer (H7-H5 lapse rates ~8c/km) advecting
into the Carolinas from the west. Expect highs Tue afternoon in the
mid/upper 70s to lower 80s. By late Tue afternoon, convection is
expected to develop over and/or propagate into central NC from the
west as the cap weakens via diurnal heating and forcing strengthens
as the low/cold front progresses southward into central NC and
shortwave energy traverses the region in NW flow aloft. The latest
guidance suggests the potential for convection will be greatest
along/south of Hwy 64 between 21Z Tue afternoon and 06Z Wed morning.
Lows Tue night will be driven by cold advection in the wake of the
front, ranging from the mid 40s (N) to lower 50s (S).
Severe Potential: Very steep mid-level lapse rates, Spring
insolation and seasonable low-level moisture (dewpoints rising into
the mid/upper 50s) will yield as much as ~1000 J/kg of MLCAPE late
Tue aft/eve, in the presence of deep layer shear sufficient for
supercellular organization. With the above in mind, deep convection
that develops and/or propagates downstream into central NC Tue
aft/eve will have the potential to become severe, with a primary
threat of large hail and damaging winds. The threat for severe
weather will be greatest (relatively speaking) south of the
Highway 64 corridor between 5pm and Midnight. -Vincent
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
-- Changed Discussion --As of 325 PM Monday...
Surface cold front and attendant axis of deeper moisture will push
south of the area by daybreak. Rain/showers should be exiting
southern portions of the forecast area by the morning rush hour
commute, with NW-SE post-frontal clearing during the late
morning/early afternoon, as the leading edge of the strong Canadian
high builds in from the north. Highs Wednesday a good 15 to 20
degrees cooler than Tuesday, ranging from lower to mid 50s NE to
lower 60s SW.
With the cP airmass centered over the area Wednesday night through
Thursday night, temperatures through the period will average a good
10 to 15 degrees below normal. Thursday morning looks to be the
coldest morning, with most area at or below freezing, lows in the
upper 20s to lower 30s. Highs in the 50 to 55.
The Nam is a wet outlier in the depiction of spreading precip into
the area Wednesday night and into the day on Thursday, in response
to shortwave disturbances moving through the area. The remainder of
the NWP guidance keeps this overrunning precip suppress south of the
area and thus will keep forecast dry.
The modified Canadian parent high will shift offshore on Friday, with
southerly return flow allowing for a quick moderation to 70 degree
readings by Saturday with ridging aloft keeping it dry through
Friday night and possibly through much of the day on Saturday.
Strengthening warm moist air advection on the eastern periphery of
the closed mid/upper level cyclone moving east-northeast into the
Ohio Valley will result in increasing chance of showers and possibly
a few thunderstorms late Saturday and into the day on Sunday. The
closed cyclone is forecast to gradually weaken as it moves east-
northeast, and thus the threat for severe weather is minimal at this
time. Continued mild with highs in the 60s and 70s. Lows in the
upper 40s and 50s.
-- End Changed Discussion --
.AVIATION /18Z Monday through Saturday/...
As of 100 PM Monday...
24-hr TAF Period: VFR conditions will rule through the majority if
not all of the TAF period, with light/variable winds becoming SSW/SW
at 5-10 knots late this afternoon and evening. MVFR/IFR ceilings
will be possible at northern terminals (INT/GSO/RDU/RWI) late
tonight, however, confidence is too low to warrant mention at this
Looking Ahead: Expect a good chance for sub-VFR conditions
associated with showers/storms in assoc/w a backdoor cold frontal
passage late Tue afternoon into Tue night. VFR conditions will
return in the wake of the front on Wed. Winds will become N/NE at 10-
15 knots with gusts up to 25 knots in the wake of the front late Tue
night. Gust potential will decrease during the day Wed. VFR
conditions will prevail Wed/Thu as Canadian high pressure builds
into the Carolinas from the north. Sub-VFR ceilings will be possible
Friday when southerly return flow develops as high pressure moves
offshore the Southeast coast. Widespread adverse aviation conditions
will be possible late this weekend in association with a potent
upper level disturbance, attendant low pressure system and
associated cold front approaching from the west. -Vincent