Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 212356

Area Forecast Discussion
National Weather Service Raleigh NC
756 PM EDT Mon Aug 21 2017

A decaying stationary front extending across northern South Carolina
into Georgia will dissipate tonight. Weak surface high pressure near
the Mid-Atlantic coast will move offshore tonight and Tuesday as a
lee trough develops in the Piedmont of Virginia and the Carolinas. A
strong cold front will across the area on Wednesday and reach the
coast on Thursday.


As of 345 PM Monday...

Latest surface analysis shows a decaying stationary front extending
from coastal SC west into GA with weak surface high pressure off
the Mid-Atlantic coast. A light east to southeast flow has developed
at the surface and 925 hPa across central NC which contributed to an
upward surge in dew points this morning before afternoon mixing
knocked them back into the upper 60s to lower 70s. A shear
axis/convergence zone across the western Piedmont resulted in the
development of a few rogue storms across the Triad early this
afternoon (unfortunately around eclipse time) with other scattered
storms developing in convergence across the higher terrain. Mainly
clear skies with no precipitation is noted across the remainder of
the RAH CWA.

The threat of isolated convection will persist through most of the
afternoon across the Triad and western Piedmont in a region of weak
to moderate instability (MLCAPE values around 1000-1800 J/kg) and
modest mid level lapse rates. An even more limited threat of
convection will continue across the far southern Coastal Plain for
the next few hours in southern Wayne, Sampson and Cumberland
Counties. Any convection should quickly dissipate by around sunset.
Mainly clear skies this evening will give way to increasing low
clouds and fog across the Coastal Plain and Sandhills late tonight
toward daybreak. Fair weather with mostly clear skies are expected
across most of the Piedmont. Overnight lows will range between 71
and 75 degrees. -Blaes


As of 400 PM Monday...

Mid-level heights at 500 hPa fall as a shearing northern stream
short wave trough approaches the southern Appalachians Tuesday
afternoon and moves into the Mountains Tuesday night. A lee
trough develops across the western Piedmont of Virginia and
the Carolinas. High resolution convection allowing models seem
uninspired for convective coverage on Tuesday afternoon and
evening. Looks like the greatest coverage will again be across
the higher terrain in a region of local convergence and more
generally across the coastal region of the Carolinas. Given
limited forcing for ascent and warm mid- levels this scenario
looks reasonable. Fairly uniform highs in the lower 90s are
expected with lows in the 71 to 76 degree range.

As of 400 PM Monday...

A high chance to likely probability of showers and storms late Wed-
Wed night will linger over the sern half of the CWFA on Thu,
followed by mainly dry conditions and temperatures near to about 5
degrees below average Fri-Mon.

The models are in relatively good agreement that a lead mid level
trough/shear axis, including some localized vorticity maxima likely
related to earlier upstream convection, will be draped at 12Z Wed
from the New England coast swwd across the central Appalachians and
Mid-South. This feature and associated forcing for ascent will drift
across central NC Wed and early Wed night and then reach the coast
by Thu morning. A related surface cold front will overtake a
preceding Appalachian-lee trough early Wed, then drift slowly sewd
into central NC late Wed afternoon and Wed evening, before settling
through sern NC on Thu. A trailing mid-upper level trough axis and
upper jet right entrance region will pivot across the Middle
Atlantic states on Thu, followed by lingering broad troughing aloft,
and underlying surface ridging, over the ern US through the end of
the period.

Weak to moderate warm-sector instability on Wed, fueled by early day
sun and temperatures in the upper 80s to lower to perhaps middle
90s, and ~25 kts of mid level wly flow, will likely support a few
generally ewd-propagating clusters capable of producing strong-
severe wind gusts Wed aft-eve. There may be a lull in precipitation
after midnight, when guidance indicates a relative minimum in deep
layer omega, and when diabatic cooling will have reduced
instability. However, the approach of the trailing mid-upper trough
axis/jet, in conjunction with lingering frontal forcing over sern
NC, should result in convective redevelopment in the vicinity of the
front from the srn piedmont and Sandhills to the ern piedmont and
Coastal Plain. It will otherwise be cloudy early, becoming partly to
mostly sunny, with high temperatures in the upper 70s to middle 80s.

Cool weather will continue through the weekend owing to the presence
of the surface ridge axis and associated nely low level flow.


As of 755 PM Monday...

24 Hour TAF period: VFR conditions will prevail through much of the
evening but short term models do show fog and low stratus taking
over at least the eastern terminal with the potential for some MVFR
visibilities at KRDU. All sites should return to VFR after 15z or so
on Tuesday. A few showers or a brief thunderstorm can`t be ruled out
on Tuesday afternoon but only a slight chance at best.

Looking beyond the 24 hour TAF period: A cold front Wednesday will
bring adverse conditions to the area through Thursday when the front
settles along the coast. High pressure will bring a return to VFR
conditions Friday through the weekend.




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