Heat Advisory 7, NWS Raleigh, NC

Area Forecast Discussion
National Weather Service Raleigh NC
1105 AM EDT Thu Aug 17 2017

.SYNOPSIS...
A surface low pressure trough will extend from western North
Carolina through South Carolina today through tonight. This trough
will shift northeastward late tonight, hold over the Piedmont of
North Carolina Friday, then shift east to the I-95 corridor on
Saturday.

&&

.NEAR TERM /Today through tonight/...
As of 1105 AM Thursday...

The latest surface analysis shows a weak surface trough extending
north to south across the western Piedmont of the Carolina's into
central Georgia with a weak low center on the trough near the NC/SC
border. The backdoor cold front across the Mid Atlantic marking the
leading edge of an air mass with dew points in the the 50s across
parts of PA and NY has begun lifting north as a warm front. A low-
level southerly flow has developed across the area allowing both
surface and deeper layer moisture to surge north. Dew points at 10
range from 82 at KMEB, 81 at KGSB, and 80 at CTZ to 77 at KRDU and
KRWI, and 76 at KGSO. Analyzed precipitable water values have
rebounded into 1.9 to 2.2 range this morning.

Expect today to be more convectively active than yesterday with a
couple of potential bouts and clusters of storms. Once area of
showers moving across southern VA and far northern NC near the lakes
appears to be driven in a region of isentropic lift/moisture
advection atop the 500 hPa short wave ridge across the area.
More importantly, additional scattered showers and storms will
develop early this afternoon across the eastern Piedmont and
Sandhills in a region instability is expected to be maximized and
coincident with moisture convection. Additional convection may move
into central NC from the west late this afternoon and early this
evening as mid-level troughing approaches from the west.

Low level thickness values today are a handful of meters greater
than yesterday which would support highs of 89-95 this afternoon.
Crazy high dew points will push heat index values into the 97 to 103
range today with values of 100-103 over the Sandhills. Warm and
continued very muggy lows tonight of 71-77 under partly cloudy skies
with patchy fog. -Blaes

&&

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

Weak mid level height falls will continue into Fri as a shortwave
trough pivots from Michigan/SE Ontario through the St Lawrence
Valley, with a weaker/broad reflection down through the southern
Appalachians. The Piedmont trough will strengthen and shift slightly
eastward through Fri night, with plentiful low level moisture
remaining in place over central NC (especially along and east of the
surface trough), PW over 2.0", and passage of weak perturbations
within the gently cyclonic southwesterly mid level flow. Under these
conditions, convection should start early, in the morning (as soon
as the surface-based inversion mixes out), over the Coastal Plain
and eastern Piedmont/Sandhills. Additional convection is likely by
afternoon just to our W and NW, over the Appalachians, and this
should slide eastward later in the day, bringing a second peak in
shower and storm coverage over central NC during the evening and
early overnight hours. Expect plenty of clouds near and soon after
daybreak, perhaps briefly breaking up with mixing, mainly over the
southwest, before broken to overcast skies redevelop quickly with
heating. Highs 89-95 with partly to mostly cloudy skies. Expect a
good chance of showers/storms in the evening, decreasing and
shifting southeast overnight as the surface trough kicks east toward
the coast with a weak surface high building into W NC from the west.
Lows from 70 NW to 76 SE. -GIH

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 335 AM Thursday...

A strong low pressure system moving north of the Great Lakes will
move eastward and drag a cold front across the Carolinas on Friday
and Friday night before stalling out near the Carolina Coast. This
will keep precip chances in the forecast for Friday and but should
confine things to the extreme southeastern portions of the area by
Friday night. With the leftovers hanging around for much of the
weekend expect eastern areas to maintain higher precip chances
through much of the weekend. As the weekend comes to a close,
continental high pressure will move in from the northwest which
could help to lower precip chances by Sunday afternoon/evening.

Interesting forecast for the eclipse on Monday with models starting
to come into agreement with zonal flow aloft and surface high
pressure sitting right off of the Carolina Coast. The positioning of
the high pressure will most likely yield moist return flow around
the back side of the high which unfortunately would most likely lead
to at least partial cloud cover for Monday afternoon. That being
said, the GFS solution is interesting that it lags the continental
high back to the west a little bit and if that happens it could
potentially suppress cloud cover to the south. Still way too early
tell but at least a partly cloudy forecast looks likely.

For midweek return flow around the high will continue, bringing
daily precip chances with another surface low well to the north that
may bring another front through the area by later on Wednesday.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
As of 640 AM Thursday...

IFR/LIFR conditions will persist for another couple of hours over
the northeast portions of the forecast area, including RWI, where
cigs and vsbys will slowly lift to VFR through 15z. Elsewhere,
isolated pockets of MVFR fog will linger until around 13z. After
late morning, VFR conditions will then dominate through the day,
although scattered showers and storms generating a period of sub-VFR
conditions and erratic winds will be possible at each site, mainly
18z-00z. VFR conditions should prevail this evening into tonight,
although MVFR fog may develop starting around 05z. Winds will be
light and variable this morning, becoming mostly from the south by
midday, at 6-10 kts this afternoon diminishing to 3-6 kts starting
this evening. There is a good chance of MVFR fog at all sites 06z-
12z.

Looking beyond 12z Fri, after the light fog lifts, VFR conditions
are expected after 13z. But scattered showers and storms are
expected, mainly mid afternoon through mid evening Fri, with the
greatest coverage impacting RDU/RWI/FAY. Additional isolated storms
remain possible through Fri night, with a chance for sub-VFR
fog/stratus. Best shower/storm chances shift to our SE for Sat into
Mon, with VFR conditions prevailing, although FAY may continue to
see a storm threat. -GIH

&&

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

&&

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

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