Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 220058

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

Weak surface high pressure near the Mid-Atlantic coast will move
offshore tonight and Tuesday. A weak surface trough is forecast over
the Piedmont Tuesday afternoon and evening. Finally, a strong
cold front will push SE across the area Wednesday afternoon and
Wednesday night. Much cooler and drier conditions are expected
late week into the weekend.


As of 900 PM Monday...

Scattered mid and high level clouds were all that remained of the
earlier convection over the NW Piedmont, and across the southern
Sandhills and Southern Coastal Plain. Otherwise, the skies were
clear. Weather parameters indicated convective inhibition
with sunset; however, some weak moisture convergence was noted over
the western Piedmont. Other than a 10-15 percent chance of a shower
in this region, POP will be nearly NIL overnight.

The main forecast problem resides around fog development again later
tonight. Most guidance is suggesting that areas from Raleigh south
and east through the Coastal Plain have a good chance of fog
development toward daybreak. High pressure over the region along
with calm conditions and high dew points suggest areas of dense fog
are possible again between 500 and 800 AM.


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.




NEAR TERM...Badgett
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