Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 230214

Area Forecast Discussion
National Weather Service Raleigh NC
1010 PM EDT Mon May 22 2017

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.


As of 1010 PM Monday...

Convection over the south-central and southeast part of NC late
afternoon and this evening has been slowly spreading out into a
large showers/isolated thunderstorm area. This area of convection
has been slowly building northward with time in response to the mean
steering flow aloft. The main front has stalled over the region
where convective outflows and ongoing convection has essentially
made it nearly impossible to find. We turn to the current data and
the blend of the Hi-Res models, which generally support continued
development of new convection to the north of the current large
"blob" of showers/isolated storms. We will place the high POP at
categorical overnight for this region including up into the Triangle
area. This activity may weaken in the next 3-6 hours, with
additional showers/isolated thunderstorms expected across the south
and west later tonight and Tuesday morning. The main threat
overnight should be locally heavy rain and possible urban flooding.
The severe threat is very low. Lows generally near the current
readings in the mid-upper 60s.


As of 340 PM Monday...

...Locally heavy rainfall likely Tuesday...

The lead upper disturbance and attendant sfc wave will lift across
central NC Tuesday morning. In its wake, there may be a temporary
lull in the widespread showers and scattered storms before another
disturbance and sfc wave lift northeast into our region Tuesday
afternoon/evening. The available moisture coupled with decent
convergence along a sfc front bisecting central NC southwest-
northeast and divergence aloft supplied by a 300mb jetlet lifting ne
along the spine of the Appalachians should maintain categorical PoPs
through the day into the evening. These showers should dump another
half to one inch of rain. This additional run-off will quickly fill
creeks and streams, likely requiring the issuance of urban and small
stream flood statements. If rainfall rates are greater, a flood
watch may be necessary.

Effective bulk shear projected to be stronger Tuesday afternoon
though widespread rain showers will likely limit available buoyancy.
If any locations have a threat for a strong/locally severe storm, it
would be over coastal plain counties which should be on the eastern
periphery of the rain showers.

High temperatures will vary from near 70 NW to near 80 SE.

Showers will gradually diminish in coverage and intensity Tuesday
evening as the upper level support wanes. Min temps near 60 NW to
the mid 60s SE.


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

As of 840 PM Monday...

24 Hour TAF Period: Scattered to widespread showers and storms will
be possible across southern portions of the area this evening,
mainly at KFAY. Sub-VFR conditions in association with the
convection is expected at KFAY as well. We may see some of this
activity spreading northward to the northern TAF sites late this
evening into early Tuesday morning. However, another area of low
pressure is expected to lift northeastward tonight from the Deep
South and cross central NC Tuesday morning. This will spread
widespread showers and isolated storms across the area Tuesday
morning. Another disturbance will follow quickly on the heels of the
first helping to continue to spread scattered showers and storms
across the area on Tuesday afternoon. Sub-VFR conditions are
expected to develop early Tuesday morning, generally IFR/Low end
MVFR. IFR cigs are expected to persist at KGSO/KINT and maybe even
KRDU (where a stalled frontal zone will be very near). Locations to
the east and south of the stalled frontal zone may see some slow
improvement in cigs Tuesday afternoon, but will keep cigs in the
MVFR range at KRWI and KFAY.

Outlook: Adverse aviation conditions are expected to continue
through Wednesday night, with a continued threat of precip and sub-
VFR conditions. Lingering precip chances and sub-VFR conditions will
be possible through Thursday, before VFR conditions return for good
on Friday.




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