Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 261900
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
300 PM EDT Fri May 26 2017

.SYNOPSIS...
High pressure will extend across the Southeast states today into
Saturday, as a weak trough extends down through western and central
North Carolina. An upper level disturbance will cross the region
late Saturday through Saturday night, followed by another
disturbance Sunday afternoon.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1000 AM Friday...

Lingering cyclonic flow aloft over the ern U.S., behind a powerful
upper low now lifting up the New England coast, will flatten and
become more zonal/Wly as a shortwave ridge upstream over the MS
Valley dampens.  Skies will be mostly sunny, with just a smattering
of flat cu at the base of a stout subsidence inversion forecast to
develop down to about 6-7 thousand ft. A swly breeze will become
gusty into the 20-25 mph range through early afternoon, in the MSL
pressure gradient between departing, sub-1000 mb surface low
pressure accompanying the aforementioned upper low, and a weak 1017
mb ridge centered over the ern GOM and FL. Warm, with highs mostly
80 to 85 degrees, but with low humidity values courtesy of dewpoints
in the 50s.

Tonight: An MCV over IA will likely help initiate, amidst a broad
low-mid level WAA regime, another cluster of showers and storms
across the upper Midwest and OH Valley today, with some of this
activity drifting in a weakening state into the central and southern
Appalachians by daybreak Saturday. Associated mid-high level debris
cloudiness may overspread central NC after midnight, with lows
generally in the lower 60s.

&&

.SHORT TERM /SATURDAY AND SATURDAY NIGHT/...
As of 420 AM Friday...

In generally Wly flow aloft, around sub-tropical high pressure
forecast to migrate across the Gulf coast states, a large plume of
hot and dry air (EML), well-sampled by the 00Z RAObs over the ern
Rockies and central-srn Plains (see Albuquerque and Amarillo), will
have overspread the southeastern U.S. by 12Z Sat. While this will
result in mainly hot and dry conditions over central NC, it will
also prove a favorable thermodynamic environment for severe storms
where moisture streaming in WSWly 850 mb flow beneath it is lifted
into it, in a WAA regime and across a weak frontal zone across the
central Appalachians. That frontal zone will have already been
convectively-modulated by overnight convection that may have drifted
into the VA`s, as noted above.

This pattern, similar to that of a couple of weeks ago on May 11,
will be supportive of the development of a cluster or two of storms
to and propagation across the central Appalachians and (probably) VA
and nrn NC Sat afternoon into Sat night. 35-45 kts of bulk shear,
and 150-250 0-3 km SRH, in the presence of moderate instability
fueled by the EML, will be supportive of an organized convective
mode that may include a small forward propagating MCS with embedded
supercells. A risk of all severe weather hazards would be possible
with such storm modes, but with tornado potential focused along and
immediately north of the effective frontal zone that will likely
extend just to the north of the RAH forecast area, or perhaps settle
into the nrn Piedmont and nrn Coastal Plain. The precise frontal
location remains of low predictability, however, given 1)
uncertainties in the amplitude of upstream MCV`s and how they may
dampen/amplify the flow and 2) probable convective/diabatic
modulation of the effective front. The storms will likely weaken
with time and srn extent as they move away from the frontal zone/low
level moist axis and into the hot and dry, capped EML. That dry air,
and associated high DCAPE, will favor strong evaporational cooling
potential and relatively significant outflow winds (35-45 mph)
surging well ahead of the dying storms, however.

Hot, with highs in the upper 80s to lower 90s. Lows, in an
increasingly humid regime, in the middle to upper 60s.

&&

.LONG TERM /Sunday through Friday/...
As of 300 PM Friday...

A short wave trough over the TN valley Sunday morning will move east
and across the Carolinas by late day.  Scattered showers/tstms are
expected along and ahead of it, developing over the mountains around
mid-day and moving east across our cwa during the late afternoon
into early evening.  With 2000+ J/kg cape and 40+ kt of deep layer
shear forecast in the models, the environment would appear to
support at least isolated strong to severe tstms.  Given the
position of an old boundary along and north of hwy 64, look for the
best chance for such activity to be across the northern half of our
CWA.

Short wave ridging behind the aforementioned short wave will provide
a break in the weather Sunday night into Monday morning, but the
next short wave moving through the longwave trough centered over the
Great Lakes will move across the Carolinas late day Monday into
Monday night, helping to push a weakening cold front across our
area.  This system will provide the next chance for showers and
tstms Monday afternoon through Monday night.

For the rest of the long term period, will keep pops near or above
climo each day as the aforementioned cold front will stall across
the region and a series of mid level short waves embedded in the
broad cyclonic flow move across the region.  Above normal temps
forecast through the period.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
As of 141 PM Friday...

Strong subsidence, behind the departing upper level trough will
result in VFR conditions through the 24 hour TAF period. Breezy west-
swly winds will gradually subside as pressure gradient relaxes
through the afternoon/early evening, with flow becoming SWLY with
the development of pre-frontal surface trough in the lee of the
mountains.

Outlook: A series of upper level disturbances along a back-door cold
front forecast to sag  south into the area over the weekend will
support a chance of showers and storms late Saturday
afternoon/evening, and again on Sunday afternoon and evening owing
to the presence of a moist and unstable airmass. With the front
stalled across the area, the threat of mainly afternoon/evening
showers and storms will continue on Monday, and again on Tuesday
with the approach of another cold front from the west.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...Hartfield
NEAR TERM...CBL/MWS
SHORT TERM...MWS
LONG TERM...np
AVIATION...CBL


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