Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 270846
AFDRAH

AREA FORECAST DISCUSSION
National Weather Service Raleigh NC
445 AM EDT Tue Jul 27 2021

.SYNOPSIS...
A surface trough will will waver across the region through
Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 345 AM Tuesday...

On the ern periphery of a sub-tropical ridge forecast to migrate
from the cntl Rockies to the cntl Plains, a low amplitude shortwave
trough over the upr MS Valley this morning will amplify across the
Great Lakes today and the middle Atlantic and Northeast tonight,
then offshore through early Wed. The approach of the shortwave and
tightening height gradient will result in the intensification of an
upr-level, wswly jet streak over the middle Atlantic, to 75-90 kt at
200-250 mb by 00Z Wed. The divergent, right entrance region of the
jet, as well as deep layer (700-250 mb) Q-vector convergence and mid
level height falls, are forecast to overspread and maximize over
cntl NC between 00-06Z Wed. Preceding the trough, precipitable water
values are expected to remain between 2" and 2.25" over the sern
half of NC (+1 to 2 sigma), ranging to 1.5-1.75" over the nw NC
Piedmont.

At the surface, an outflow boundary that has maintained a now-
weakening convective cluster over the cntl and srn NC Coastal Plain,
extended at 08Z from near ILM nwwd to FAY and GSO. In the absence of
renewed deep convective development along that boundary this
morning, it will likely remain quasi-stationary and serve as an
initial focus for convective initiation with diurnal heating by
midday. A synoptic-scale surface trough and convergence axis, one
that extends upward to around 850 mb from cntl VA swwd across the
wrn NC Piedmont and Foothills per regional VWP data, will also
likely serve as a focus for scattered to locally numerous showers
and storms this afternoon. With the presence and intersection of
those convergence axes, coverage of associated multi-cells, amidst
weak, generally wly lwr to mid-level flow, will likely be greatest
along and south of I-40 today. While diabatic/outflow and nocturnal
cooling should result in a gradual decrease in convective coverage
and intensity through the late afternoon and evening, showers and
isolated storms are apt to continue to percolate over cntl and ern
NC overnight, as the aforementioned synoptic forcing for ascent
overspreads the region and interacts with lingering weak,
conditional instability.

Temperatures are expected to be a couple of degrees less hot than
Mon owing to the aforementioned convective development expected to
occur directly over cntl NC, as opposed to developing upstream and
moving into the forecast area later in the day, followed by
persistence low temperatures in the upr 60s-lwr 70s.

&&

.SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/...
As of 430 AM Tuesday...

A mid to upr-level trough axis will move off the middle Atlantic
coast early Wed, with following nwly flow and height rises aloft
expected to prevail across cntl NC through Wed night. Mainly
diurnally-driven convection will be possible once again along a
lingering lee trough forecast to bisect cntl NC, but background
subsidence or neutral vertical motion, and also a drier, ~1.5"
precipitable water air mass behind the trough axis, should limit
coverage to isolated or widely scattered. It will otherwise be hot,
with temperatures mostly in the lwr to mid 90s, and with heat index
values around 100 F where surface dewpoints remain at or above 70 F
over the ern half of cntl NC. Lows again around 70.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 410 AM Tuesday...

The large scale pattern through the medium range will continue to
feature strong ridging over the west-central US and troughing over
the eastern US.  Dry NW flow aloft and a lee trough over central NC
will result in highs Thu/Fri several degree above normal in the mid
90s (and perhaps some sites across the southern Piedmont/Sandhills
pushing 100 on Thursday as 850mb temps reach 22-23)C. A shortwave
moving through the upper trough will bring a cold front and
increasingly unsettled weather into the mid-Atlantic states late
Friday through through the weekend, eventually stalling across the
Carolinas.  Models then suggest multiple disturbances swinging
through the trough as it deepens into early next week, setting the
stage for continued unsettled weather, particularly across eastern
NC, and slightly below normal temps across interior NC.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
As of 155 AM Tuesday...

An outflow boundary from rain-cooled air, and associated lift, has
resulted in the development and maintenance of small cluster of
showers/storms from near HRJ to JNX to GWW this morning; and some of
these may affect RWI and FAY before dissipating in the next few
hours. That rain-cooled and associated high relative humidity air
will otherwise support areas of IFR-MVFR ceilings until diurnal
heating causes the moist layer to lift and scatter by 14-15Z.
Additional scattered showers and storms are expected to develop this
afternoon primarily across the srn half of NC, including FAY, RWI,
and RDU.

Outlook:  Early-morning sub-VFR fog and low clouds will remain
possible through Fri morning at all sites. While isolated showers
and storms may develop each afternoon, they should be few in number.
A better chance of showers and storms will arrive Fri afternoon with
the arrival of a cold front. Drier VFR conditions are expected to
return Sat as this front settles to our S.

&&

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

&&

$$
SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...MWS
LONG TERM...Smith
AVIATION...MWS/Hartfield


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