Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 181027
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
630 AM EDT Fri Aug 18 2017

.SYNOPSIS...
A weak surface trough will hold over eastern North Carolina today,
while a second stronger trough sits over western North Carolina. An
upper level disturbance and accompanying weak surface cold front
will cross the region from west to east late this afternoon through
tonight. The front will stall out over the coastal Carolinas on
Saturday, as weak high pressure builds over the western and central
Carolinas through the weekend.

&&

.NEAR TERM /Today through tonight/...
As of 330 AM Friday...

Latest model output and observed trends support increased odds of
organized storms later today through this evening, particularly over
the northern Piedmont. But confidence in convection coverage through
the first half of the day is less certain.

The current surface map shows a subtle boundary sitting along
eastern NC just inland from the coast, and a second more distinct
trough along the eastern edge of the Foothills. An area of drier air
noted on water vapor imagery is currently moving across central NC,
mainly along and south of highway 64, limiting clouds and any
potential for elevated convection early this morning. But this will
soon give way to increasing moisture and dynamic forcing for ascent
as a shortwave trough extending from PA through E KY/E TN moves in
from the west this afternoon. Models depict fairly early
destabilization today given the rapid mixing with heating and very
warm 925 mb temps. MUCAPE is expected to rise to 1500-2000 J/kg in
the east by midday, increasing further to 2000-3000 J/kg late. Deep
layer bulk shear, as was the case yesterday, remains fairly low at
15-20 kts, but is projected to rise slightly late today across the N
and NW as the trough approaches.

Areas of stratus and patchy fog this morning should mix out and
disperse by late morning, and the heating along with high dewpoints,
especially in the east, should prompt convection by midday. But
given the dry air preceding the trough, the coverage may be limited
and focused near the eastern trough where weak low level mass
convergence will reside, and in far NE sections where moisture will
be a little better. Following the latest HRRR runs, which depict
this scenario fairly well, will have small chance pops over the far
NE (VA border) and along the eastern Coastal Plain through early
afternoon. Areas well east of I-95 will likely see a bit better
coverage given the mid level lapse rates well above 6 C/km.

Later today, with the arrival of the better moisture, steep lapse
rates through a deep layer (7-7.5 C/km in the low levels and 6-6.5
C/km in mid levels), and dynamic forcing for ascent with both mid
level DPVA and upper divergence, expect scattered to numerous
showers and strong storms, with the greatest coverage across the
northern Piedmont and northern Coastal Plain very late afternoon
through evening. A few severe storms are possible, mainly north of
Highway 64, if the deep layer shear can surpass 20-25 kts. Damaging
winds would be the greatest threat, with a secondary hail risk.

With strong heating and mixing in most places today, low level
thicknesses 15-20 m above normal, 925 mb temps perhaps exceeding
27C, and dewpoints holding in the mid 70s if not higher, heat index
values will likely approach or exceed 105F in many areas outside of
the Triad and VA border counties. Will have a heat advisory in place
for the Triangle region and points south and east today from noon to
6 pm. Expect highs of 91-96.

The incoming front should jump into the Piedmont trough and progress
east and southeast overnight, with storms pushing into the SE CWA
and decreasing in coverage late. Expect lows from 71 NW to 76 SE.
May see some light fog in the SE late. -GIH

&&

.SHORT TERM /Saturday through Saturday night/...
As of 345 AM Friday...

Much quieter day with the surface frontal zone and mid level shear
axis pushing to our SE and weak surface high pressure building in
from the west. A drier column with warm and subsiding mid levels
will result in mostly sunny skies NW and partly sunny SE, where
enough low level moisture will remain to foster flat convective
cumulus, topped with lingering mid level moisture streaming over
eastern NC. Will retain low chance pops only in the SE sections,
with dry weather elsewhere. Thicknesses dip slightly but remain near
or just above normal, so expect highs of 89-94. Fair Sat night with
no pops. Lows 69-73. -GIH

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
As of 310 AM Friday...

Sunday will begin with a frontal zone along the NC coastline and
continental high pressure moving into the area from the northwest.
This should give us very low precipitation chances for Sunday and
keep flow very stagnant over the area with very little forcing aloft
or at the surface. A similar pattern in place for Monday but the
offshore surface high becomes a bit more dominant, meaning southerly
return flow increases and potentially some more clouds and shower
activity but lagging continental high to the north should keep us
relatively dry and hopefully keep cloud cover suitable enough for
eclipse viewing.

The Piedmont Trough should be more active on Tuesday and coupled
with a few shortwaves moving in from the southwest, will result in a
slight increase in precip chances on Tuesday afternoon but still
only lower end of the chance range. On Wednesday a stronger surface
low in the northern stream will bring a cold front to central NC and
with it, our best chances for rain for the week. This front will
linger over the eastern part of the state through Thursday, keeping
precip chances higher in the east. Expect temperatures in the low
90s through the period until Thursday when the front will drop temps
back into the mid 80s.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
As of 630 AM Friday...

VFR conditions are expected for much of today through tonight at
most central NC terminals, with a few exceptions. At RWI, periods of
IFR conditions, primarily IFR cigs with MVFR to IFR vsbys, are
expected early this morning, through around 13z-14z, after which
time these low cigs will quickly mix and disperse to VFR. Scattered
storms are possible near FAY/RWI early today, starting as early as
15z with the potential lasting through the afternoon, although they
will remain scattered and not a prevailing condition. More numerous
showers and storms are expected starting after 19z at INT/GSO,
spreading eastward to RDU then to RWI/FAY during the 21z-05z time
frame. MVFR to IFR conditions with very heavy rain and strong gusty
and erratic winds are possible with any of these showers and storms
later today into early tonight. Any lingering showers will be
diminishing soon after 06z as a front passes to our east and weak
high pressure builds in from the west.

Looking beyond 12z Sat, MVFR vsbys in fog are possible early Sat
morning at RDU/RWI/FAY and early Sun morning at RWI/FAY. But
otherwise, VFR conditions and dry weather are expected Sat through
much of Mon with only a small chance of afternoon showers near FAY.
The chance of late-day storms and early-morning fog returns late Mon
into Tue, as the air mass over NC becomes increasingly hot, moist,
and unstable. -GIH

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon today to 6 PM EDT this evening for
NCZ025>028-040>043-073>078-083>086-088-089.

&&

$$
SYNOPSIS...Hartfield
NEAR TERM...Hartfield
SHORT TERM...Hartfield
LONG TERM...Ellis
AVIATION...Hartfield



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