Area Forecast Discussion
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FXUS62 KRAH 210232
Area Forecast Discussion
National Weather Service Raleigh NC
1030 PM EDT Mon Mar 20 2017
A warm front will push northward through the area tonight, bringing
mild air for Tuesday. A cold front will push slowly southward
through the area Tuesday evening through Tuesday night. Cool high
pressure will build into the region from the north Wednesday through
.NEAR TERM /Through Tonight/...
As of 1030 PM Monday...
Earlier update is on track. The earlier MCS over S OH has weakened
considerably as it has crossed WV and become farther removed from
the best wind fields and moisture flux. The residual area of showers
(and a couple of isolated embedded storms, although these too will
weaken) will continue a slow decrease in coverage and intensity,
following the HRRR and WRF-ARW, but still expect enough showers
across the far N and NE CWA to keep chance pops, highest near the
Kerr Lake and Lake Gaston areas. Lows from the upper 40s east to
lower 50s west. -GIH
Earlier discussion from 730 pm: Forecast was updated to account for
greater shower chances across the north and NE tonight, along with
greater cloud cover (earlier increase from NW) and warmer temps
tonight. The MCS covering southern OH (including a bow echo tracking
along the Ohio River) is on track to cross WV into VA over the next
several hours, following the mean steering flow, although as it does
so, it will move out of both the low level theta-e ridge and low
level jet nose, and this loss of support should result in a decrease
in intensity and coverage of showers as it approaches far north-
central and northeast NC late tonight. That said, however, the
HRRR/RAP/WRF-ARW, as well as the latest GFS/NAM/ECMWF, all show a
batch of scattered showers crossing the NE CWA very late evening
into the overnight hours. Have raised pops late evening through
tonight from the Triangle N and E, with good chance near Kerr Lake
and Lake Gaston. This should be mostly showers by that point,
especially given the lingering lower dewpoints and cooler temps over
the NE CWA, where mid level lapse rates will still be under 6 C/km.
With a faster and greater increase in cloud from the west and NW
this evening, hindering radiational cooling, have bumped up lows to
around 50 / lower 50s, except for upper 40s east of Highway 1. -GIH
Earlier discussion from 100 pm: 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/...
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.
.AVIATION /00Z Tuesday through Saturday/...
As of 745 PM Monday...
24 Hour TAF Period: Medium confidence of VFR through the TAF period
with SSW/SW winds becoming more variable and light overnight.
Scattered to broken clouds near 5 kft can be expected into the
overnight hours giving way to higher ceilings later Tuesday
afternoon. Light precip at the northern terminal sites is possible
between 6 and 12Z Tuesday morning as a disturbance moves along the
VA border. No thunder is expected with the precipitation at this
time. Other than a brief period of gusty winds (15-20 kts) at KFAY
late tomorrow morning into early afternoon, winds at most terminals
should remain variable between 5-10 kts.
Long Term: There will be another chance for precipitation at KFAY
just after the TAF period as a disturbance passes just south of the
forecast area. Otherwise expect mainly VFR conditions through much
of the period.