-- Discussion containing changed information from previous version are highlighted. --
819
FXUS62 KRAH 040843
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
443 AM EDT Thu Jul 4 2024

.SYNOPSIS...
A subtropical ridge will remain anomalously-strong over the Deep
South today, then gradually weaken ahead of a cold front that will
move into and stall over the southern Middle Atlantic and Carolinas
this weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
As of 430 AM Thursday...

...Hot, with scattered afternoon-evening convection that will drift
sewd and across the nrn NC Piedmont...

An anomalously-strong (+3 sigma) 500 mb subtropical ridge, centered
at 597 dam over the Deep South, will weaken a few decameters as a
mid-level cyclone develops and deepens from the nrn Plains to the
upr MS Valley. In the intervening wswly flow between the two, a
primary corridor of convectively-amplified disturbances will extend
from an ongoing MCS/MCV now over ern KS, enewd and across the nrn
Middle Atlantic and lwr Great Lakes. Another weaker MCV now over n-
cntl KY, and likely more influential for convective development over
the srn Middle Atlantic today-tonight, will move anticyclonically
around the foregoing subtropical ridge and ewd across the Virginias
through early-mid afternoon, then sewd to ssewd across ern NC
tonight. Weak mid-level height falls (10-20 meters/12 hr) and
adiabatic cooling will favor continued weakening and eventual
removal of a preceding strong subsidence inversion evident in
regional RAOB data the past few days. As such, initially weak mid-
level lapse rates, only 4-4.5 C/km in GSO and MHX RAOB data last
evening, are forecast to steepen to 5.5 to 6 C/km from west to east
over the srn Middle Atlantic and Carolinas through this evening.

At the surface, very warm-hot sswly flow will expand throughout the
Southeast, equatorward of a synoptic front that will extend from the
nrn Middle Atlantic to n-cntl TX by 12Z Fri. The synoptic front will
be preceded by composite outflow that will settle sewd and across
the Virginias and n-cntl NC through the same time. Meanwhile, an
Appalachian-lee trough will become established across the Foothills
and wrn Piedmont of the srn Middle Atlantic/Carolinas.

The influence of the subtropical ridge and sswly low-level flow will
allow heat to expand newd and across cntl NC once again, with
forecast high temperatures in the mid 90s to around 100 F and at
daily records at RDU and FAY. Mixed/afternoon surface dewpoints in
the upr 60s to around 70s will somewhat temper associated heat index
values that are expected to peak in the upr 90s to near 105 F,
highest from the Sandhills to the Triangle.

That strong diurnal heating and seasonable low-level moisture will
yield weak-moderate diurnal destabilization characterized by MLCAPE
to around 1500 J/kg maximized over the Piedmont by this afternoon.
Surface convergence along the lee trough will provide a focus for
isolated storm development by early-mid afternoon over the nw and
srn NC Piedmont, while scattered, multi-cell development will
probably result along the aforementioned outflow and into the nrn NC
Piedmont during late afternoon through evening. There has been a swd
trend in 00Z HREF guidance such that this convection may reach the
Triangle around peak July 4th festivities/fireworks times, before
dissipating through midnight. This swd trend is supported by a
similarly swd trend in the location of the aforementioned composite
outflow now stretching across cntl WV and KY, and also based on the
presence and position of the MCV poised to move from n-cntl KY ewd
and across the Virginias through peak diurnal heating. Despite a
lack of shear, isolated strong to damaging wind gust will be
possible with any afternoon-early evening cells, owing to strong
diabatic cooling potential in an unseasonably deep and strongly-
heated boundary layer characterized by depths of ~7-8 thousand ft
AGL, dry adiabatic lapse rates, and 25-30 degree dewpoint
depressions - and associated inverted-V thermodynamic profiles that
skew toward the hybrid spectrum of microbursts. Given that the
outflow will likely lose character and strength as it moves
(probably) inconsequentially sewd through cntl and perhaps sern NC
through Fri morning, it will remain very mild and muggy tonight,
with lows centered in the middle 70s.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 410 AM Thursday...

...Hot and humid conditions bring the risk for heat related
illnesses Friday and Saturday...

A mid-level subtropical ridge centered over the GOM states will
extend up through the southern Mid-Atlantic in between broad
troughing over the central CONUS and a closed low drifting by
Bermuda through Sat. At the surface, an occluded low over the Upper
Great Lakes will slide ENE into southern Quebec by Sat evening as a
compact shortwave lifts through the eastern edge of the longwave
troughing. Within the deep tropospheric southwesterly flow, low-
level thickness steadily rise to around 1445m (30m above normal) and
850mb temperatures soar to 21 to 23 C Fri afternoon. In the absence
of showers and convective outflow during the afternoon, this pattern
will support well above normal temperatures into the upper 90s and
low 100s with heat index values ranging from 100 to 107 degrees.
Little overnight recovery is expected Fri night as continued humid
airmass and lingering cloud cover will keep lows in the mid to
potentially upper 70s in spots. This would result in heat illnesses
likely for anyone without access to effective cooling and/or
adequate hydration. Some afternoon relief will be possible on Sat
especially over the western Piedmont mid/upper level clouds spread
in from the west ahead of trailing cold front. The cooler and drier
air behind the cold front will likely struggle to traverse the
mountains with another night of mid 70s expected.

As for convective potential, on Fri greatest coverage of
showers/storms will likely originate over the NC mountains in the
vicinity of the lee trough and slowly shift eastward through the
afternoon into the evening hours. The airmass within and ahead of
the storms will be characterized by around 2000 J/kg of MLCAPE and
20 kts of 0-6km shear. Little storm organization is expected given
the weak deep layer shear, but isolated strong to potentially severe
wet downbursts and isolated congealed clusters could certainly be
possible with above normal moisture and DCAPE around 1000 J/kg. By
Sat, the mid/upper trough will sag farther southeast and direct more
perturbed 500mb flow into the western Piedmont and combine with lee
trough and/or differential heating boundaries to develop more
widespread showers/storms. Deep layer shear will again be weak, but
closer to 15-25 kts (highest NW) and may result in some cold pool
organized clusters resulting in mostly a wet downburst threat as
well. Confidence on Sat is low given it will likely depend on the
prior day activity.

&&.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 410 AM Thursday...

Overall a stagnant pattern will exist over Southeast and the Mid-
Atlantic through the remainder of the forecast period. Central NC
will be tucked between broad reinforced troughing through the
central CONUS with the stationary sub-tropical high broadly draped
across the western Atlantic in the eastern GOM. At the surface a
diffuse pressure gradient will feature a lee-trough redeveloping
over the region each afternoon with unseasonably moist deep layer
moisture in place as PWAT values remain at-or-above 2 inches.

This pattern will favor continued above normal temperatures with
highs in the mid 90s in the urban areas and overnight lows only
reaching into the mid 70s. With continued heat related stresses heat
related illnesses will be possible especially among individuals
sensitive to heat and do not have access to effective cooling and/or
adequate hydration. Diurnally favored showers/storms and storms will
also be possible each afternoon into the evening, likely focused
along the lee-trough and any subsequent outflow boundaries
thereafter. Organized convection appears to be at a minimum with
deep layer shear remains weak around 10 to 20 kts, strongest across
the NW Piedmont, and result in mostly pulse summer-time
showers/storms. Greatest coverage could be Tues into Tues night with
proximity of mid/upper troughing swings through the Great Lakes
region.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
As of 200 AM Thursday...

VFR conditions, influenced by high pressure extending from just
offshore the Middle Atlantic coast southwestward into the Carolinas,
will continue through at least mid-afternoon. An Appalachian-lee
surface trough and/or outflow from upstream convection will provide
foci for shower/storm development that will probably affect INT/GSO
and surrounding vicinity across the nw NC Piedmont this afternoon-
evening. Some of that gusty outflow, and focus for additional,
isolated to widely scattered showers/storms, may drift to near RDU
and RWI through 00-03Z, before dissipating.

Outlook: A chance of showers/storms will exist Fri and especially
Sat through Mon, as a pre-frontal trough and surface front move into
and stall over cntl NC. Late night-early morning stratus and fog
will also be possible this weekend.

&&

.CLIMATE...
Record High Temperatures:

July 4:
KGSO: 98/1970
KRDU: 101/2012
KFAY: 98/2019

July 5:
KGSO: 98/2012
KRDU: 102/1999
KFAY: 101/2002

July 6:
KRDU: 102/2022
KFAY: 99/1990


Record High Minimum Temperatures:

July 4:
KGSO: 73/2018

July 5:
KGSO: 74/1999
KRDU: 78/1902
KFAY: 77/2005

July 6:
KGSO: 76/1999
KRDU: 79/1900
KFAY: 76/2017

July 7:
KGSO: 73/2019
KRDU: 77/1900
KFAY: 78/2017

&&

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

&&

$$
SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...Swiggett
LONG TERM...Swiggett
AVIATION...MWS
CLIMATE...RAH