Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 230713
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
310 AM EDT Tue May 23 2017

.SYNOPSIS...
A series of low pressure systems will cross central NC through mid-
week, causing periods of showers and thunderstorms. Improving
weather conditions are anticipated by late in the week.

&&

.NEAR TERM /today through tonight/...
As of 310 AM Tuesday...

...Flood watch in effect today through this evening...

Coverage of showers and embedded storms has fallen to a relative
minimum early this morning, however slow-moving convective cells
with 1-2"/hr rates have produced pockets of high totals over parts
of the Piedmont in the last several hours. Areas of showers and
storms will continue to spread over central NC from the SW through
the day, as mid level perturbations track northeastward in the
cyclonic flow around an expansive low centered over MN/IA and
covering much of the central/eastern CONUS. The surface pattern is
quite messy this morning, muddied by subtle synoptic-scale features
and convectively-induced boundaries and mesoscale highs within a
light low-level flow. There appears to be two main dewpoint
discontinuities, one just NW of the forecast area and another to the
south across central GA and central SC. PW values along and south of
the latter frontal zone are close to daily record values, and these
(as well as the front itself) are expected to surge northward into
central/eastern NC through the day. Forecast soundings continue to
depict a deep saturated warm layer (LCL-0C of nearly 4 km) over
central NC later today, favoring warm rain processes, and the
initial weak flow will lead to slow-moving and poorly organized
cells through this morning, until the steering flow picks up a bit
toward midday. Most models with both parameterized and explicit
convection generate areas of heavy rainfall totals today through
this evening, although they differ on location, likely a function of
the murky and diffuse surface pattern and resultant variations in
low level mass convergence that are tipping the scales to heavy rain
production. With the high water content and strengthening deep layer
forcing for ascent as the most prominent mid level wave tracks NE
through the Carolinas today, expect likely to categorical pops
areawide today, with a risk of high rain totals and pockets of
flooding possible anywhere. Based on the pace of movement of the
rain area now extending back across GA/AL and southern MS and the
model projections of the movement of the upper divergence maxima
across the Carolinas today, expect the peak threat of flooding rains
to occur late morning through mid afternoon, gradually tapering down
and pushing to our ENE this evening. Will issue a flood watch in
effect from 5 am this morning until 2 am tonight, with storm total
amounts of 1-3" likely and locally higher totals probable. Will
taper down pops west to east tonight as the mid level wave and upper
jet core shift to our NE, although a moist and stable pool overnight
will lead to lingering areas of drizzle and thick stratus. Temps
have a high error potential today, as any amount of sunshine in the
SE CWA could send readings soaring. Will go with highs from the
upper 60s NW to the upper 70s SE. Lows tonight 58-65. -GIH

&&

.SHORT TERM /  /...
As of 340 PM Monday...

To be updated.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
As of 300 PM Monday...

The upper-air pattern at the beginning of the long term period
features a high amplitude trough extending southwesterly from
the Great Lakes into the MS Valley with an embedded close low
centered near MO/IL. The upper trough axis will slide east on
Thursday and take on an increasingly negative tilt with both the
GFS and EC moving the long wave trough axis off the mid-
Atlantic coast by 12 UTC Friday. The southwesterly flow ahead of
the trough strengthens with 75 to 85kts progged at 500 mb over
the eastern Carolinas early Thursday. While NWP guidance is
similar in the large scale pattern there are differences in some
smaller scale features that will play an important role in the
forecast details.

At the surface, the pattern is rather complex with an existing
weak CAD wedge in place across central NC on Wednesday morning
with dew points in the upper 50s near the VA border and the
upper 60s in the southern Coastal Plain. Guidance is probably
too aggressive in driving the warm sector northward to the VA
border by mid-afternoon, a more reasonable approach would lift
the wedge front more slowly, eventually reaching the VA
Wednesday evening as a surface low develops along the front in
upstate SC Wednesday evening and moves to southwestern VA
Thursday morning. A trailing cold front will then sweep east
across the Carolinas and Virginia Thursday morning.

Given the strong forcing for ascent ahead of the upper trough, low-
level warm advection, localized ascent with the wedge front and then
the cold front along with precipitable water values climbing in
excess of 1.75 inches, another robust precipitation event is
expected Wednesday morning through midday Thursday with general
rainfall amounts of 1 to 1.5 inches with localized amounts in
excess of 2 inches. Currently SPC has our region outlooked in
general thunder and a marginal severe weather risk for the
Wednesday and Wednesday night period. Initially, the severe
weather risk should be confined to the southeast portion of the
area but as the warm sector expands north and with the strong
low to mid level flow producing 0-6km bulk shear values in
excess of 40-50kts with a weak to perhaps borderline moderately
unstable air mass. An expanding severe weather threat is
expected, initially in proximity of the wedge front and then
ahead of the advancing cold front.

The significant precipitation should be over by Thursday but
scattered showers and possibly some storms are expected ahead of
the upper trough that will be located near the Appalachians.
The combination of a relatively moist environment combined with
steep low- level lapse rates should support scattered convection
and fairly widespread cumulus field. All of the precipitation
should end late Thursday night as the air mass stabilizes and
larger scale subsidence develops. Dry weather is expected on
Friday and Friday night.

The pattern becomes less clear over the weekend as troughiness
begins to develop over the mid and upper MS valley and a warm front
lifts north across the region on Saturday and then stalls near or
just north of NC for late Saturday and Sunday. A limited threat of
convection persists into Monday but confidence in the forecast over
the end of the holiday weekend is limited.

Highs on Wednesday and Thursday will be muted by cloud cover and
precipitation and modulated by the placement of the warm front.
Highs should range around 70 in the Triad to around 80 across
the southeast. Highs should moderate into the upper 70s to lower
80s on Friday and the mid to possibly upper 80s on Saturday
through Monday. -Blaes

&&

.AVIATION /06Z Tuesday through Saturday/...
As of 1255 AM Tuesday...

Lower than usual confidence this morning. Aviation conditions
currently vary from VFR at INT/RDU/RWI to MVFR at GSO and IFR at
FAY, and this variation will continue through 09z this morning,
after which time we should see a trend to prevailing MVFR/IFR at
INT/GSO/RDU and perhaps RWI, with MVFR to VFR conditions at FAY this
morning. A large area of showers and isolated storms is expected to
spread over central NC throughout the day, lasting into the evening,
and a trend back down to prevailing IFR conditions are anticipated
at all sites by late morning, with low cigs and occasional MVFR to
IFR vsbys in heavy rain. Unfortunately, details regarding the
specific timing of this heavy rain and storms is uncertain, but we
do have confidence in poor aviation conditions dominating through
tonight, as low (mainly IFR) cigs are expected to persist overnight
even as the heavier rain departs from west to east between 00z and
06z. A surface front oriented SW to NE across central NC will hold
in place today as low pressure tracks northeastward along it,
leading to erratic winds at central NC terminals, mainly from the
SE/S shifting to SW at FAY, and mainly from the NE elsewhere through
tonight.

Looking beyond 06z late tonight/early Wed morning, adverse aviation
conditions are likely to hold Wed through Wed night as a deep mid
level trough and surface cold front approach from the west, bringing
another round of heavy showers and a few potentially strong
thunderstorms. Improvement to mostly VFR is expected Thu, although
scattered afternoon showers generating MVFR conditions are possible.
VFR conditions should return for Fri/Sat as a weak high pressure
ridge extends into the area. -GIH

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for NCZ007>011-021>028-
038>043-073>078-083>086-088-089.

&&

$$
SYNOPSIS...Badgett
NEAR TERM...Hartfield
SHORT TERM...
LONG TERM...BLAES
AVIATION...Hartfield



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