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684
FXUS62 KRAH 081340
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
940 AM EDT Wed May 8 2024

.SYNOPSIS...
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A series of upper level disturbances will interact with an
unseasonably moist airmass over North Carolina through early Friday,
bringing unsettled weather. A cold front will move across the region
on Friday, ushering in drier and less humid air for the weekend.
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&&

.NEAR TERM /TODAY THROUGH TONIGHT/...
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As of 940 AM Wednesday...

Few changes this morning. Areas of mid clouds persist over the
area this morning, mostly convective debris from upstream convection
along with patchy altostratus and altocu castellanus. Isolated
showers are not growing upscale despite elevated moderate CAPE
(noted well on the 12z GSO sounding), given lingering SB CINH and a
relative lull in larger scale forcing for ascent as a low-mid level
perturbation that passed through the region earlier this morning is
pushing to our ENE, leaving behind a brief period of weak low-mid
level shortwave ridging. The latest convection-allowing models still
vary in details and timing, but most latch onto the organized
convection spanning the KY/TN border region currently and take it,
or some portion thereof or outflow from it, E of the NC mountains by
17z and track organized convection through our area 20z-02z,
focusing on areas from RDU to the W and S with lesser or lighter
coverage across our far NE, which will be followed by other rounds
of convection stretching back across our SW through the overnight
hours, amidst lingering CAPE and increasing low and mid level winds
to 25-35 kt/40-45 kt respectively. The earlier forecast is close to
this scenario, with just minor tweaks needed. Strong buoyancy is
expected, with SBCAPE exceeding 3000 J/kg this afternoon, surface
dewpoints in the mid 60s to lower 70s, an eff deep layer shear of 30-
35 kts, and 925-700 mb lapse rates reaching their peak mid afternoon
through mid evening. A period of a decently curved hodograph with
this buoyancy profile suggests that all hazards are possible,
particularly large hail and straight line wind damage, with a lower
but non-zero tornado threat as well with 100-200 m2/s2 of 0-1 km SRH
on the RAP. Still expect highs in the upper 80s to lower 90s, over
10 deg above normal and well into the 90th percentile with high
humidity, leading to an elevated risk of heat-related illness. -GIH


Earlier discussion from 330 AM:

*  Slight/Level 2 threat for multiple rounds of severe storms from
   with the primary threat during the late afternoon/evening,
   followed by a secondary threat late tonight/early Thursday.
*  Summerlike heat and humidity is expected with highs in the upper
   80s to lower 90s and heat index values in the lower to mid 90s in
   many locations.

Upper ridge centered over the SE US this morning will shift offshore
early in the day, leaving the Carolinas under the influence of
perturbed quasi-zonal flow aloft until the arrival of the strong
Northern Plains/Upper Midwest trough into the region on Friday. At
the surface, a lee sfc trough will sharpen over western NC.

A complex and multi-episode severe threat exists today through
Thursday. Model guidance continues to exhibit considerable spread
and high run-to-run variability wrt convective evolution
(timing/location). A majority of this uncertainty can be attributed
to the fact that the forcing mechanisms are upstream convective
artifacts that currently developing or have yet to develop. As such
confidence is lower than normal. However, there is high confidence
that the near-storm environment over central NC will be very
conducive for severe storms that starts first with the summer-like
heat and humidity.

Under the influence of SWLY flow, low-level thicknesses will be more
comparable to late June than early May. Afternoon highs are expected
to warm into the upper 80s across the north to lower 90s over the
remainder of the forecast area. When coupled with the humid BL
dewpoints of 65-70, heat indices over the central and eastern
portions will be in the mid 90s. The resultant low-level lapse rates
will foster  moderate to strong destabilization with MLCAPE of 2500-
3500 forecast across the area. Convective initiation could be
triggered by storms or associated outflow moving off the higher
terrain and/or just develop along the lee side trough or
differential heating boundary. This first round of convection should
propagate east through the afternoon and early evening. Effective
shear of 30-40kts and another high DCAPE environment of 1000-1500
Joules will support a threat for supercells with damaging winds and
large hail, given the robust/fat CAPE and initially straight
hodographs.

An evening/early night lull in convection/rain chances is possible
before a potential upstream convective complex crosses the mtns and
moves  into the area during the overnight and morning hours. While a
nocturnal curtailment of instability will occur, moderate
instability will likely survive with hodographs becoming strongly
cyclonically curved, resulting in added threat for an isolated
tornado as well. Lows in the 65-70.
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&&

.SHORT TERM /THURSDAY AND THURSDAY NIGHT/...
As of 330 AM Wednesday...

* Slight/Level 2 Threat continues into Thursday...

Lower than normal forecast confidence carries over into Thursday
with looming uncertainty as to whether or not nocturnal organized
convection will be ongoing across central NC during the first half
of the diurnal cycle and the pending negative consequences of
associated widespread clouds, rain, and convective overturning that
would greatly limit destablization. It`s quite possible that the
severe threat may peak during the morning and early afternoon,
decreasing thereafter as the convection moves south and east of the
area.

Alternatively, if the morning/early day convection is less organized
or is not realized across the area, additional upstream convectively
enhanced disturbances/MCC, with an attendant effective front moving
through the area, could potentially spur another round of strong to
severe storms Thursday afternoon and evening.  Weak to moderate
instability with deep-layer shear progged to increase will pose a
continuing threat for damaging winds and hail. Severe threat should
wind down with the passage of the effective front late Thursday
evening/night.

Not as warm given the clouds and rain chances. Highs ranging from
lower/mid 80s north to near 90 SE. Lows near 60 north to upper 60s
SE.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 400 AM Wednesday...

A pair of shortwaves will move across central NC on Friday, one
around midday and the other in the late afternoon/early evening. At
the surface, the cold front will begin to cross the area as a wave
of low pressure rides along it, but exactly how far it gets is a bit
uncertain. Regardless, guidance is in good agreement that moisture
and instability will be much less impressive than today and
Thursday, and the highest will be focused across the south and east.
CAPE looks to be more in the 500-1000 J/kg range, so severe weather
looks like less of a threat, but isolated storms still can`t be
ruled out. After the deterministic 12z ECMWF was largely dry, the
00z run came in wetter and more in line with most of its ensembles
as well as the GFS/GEFS and NAM. So keep likely POPs across the SE
and chance elsewhere. Forecast highs range from mid-70s far north to
lower-80s far south. Any precipitation will come to an end Friday
night as the cold front sweeps off the coast and lows turn slightly
below normal (upper-40s to mid-50s).

NW flow will dominate this weekend as surface high pressure and
associated mid-level ridging approach from the west. A mid-level
trough will move across the OH Valley on Saturday and the northern
Mid-Atlantic and New England on Saturday night/Sunday. This could
bring glancing weak forcing for ascent on Saturday afternoon and
evening, but considering how dry the airmass will be from
downsloping flow, any precipitation would be light and confidence is
low we will see anything at all. The forecast only has slight chance
POPs across the far north. Otherwise it will be a very pleasant
weekend with temperatures remaining slightly below normal (highs
lower-70s to upper-70s on Saturday and mid-70s to 80 on Sunday). Dew
points will only be in the 40s.

The next shortwave will move into the southern Plains on Monday, but
models differ on how quickly it will move east with the GFS much
faster and the ECMWF and Canadian holding it back. This affects when
we would begin to see any precipitation from this system. For now
have gradually increasing POPs early next week, with enough wet
ensemble members to justify chance areawide by Tuesday. The surface
high will move offshore and turn the flow southerly, which should
help increase temperatures a bit, but they will also depend on the
timing of the aforementioned system which is still highly uncertain.

&&

.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...
As of 635 AM Wednesday...

Patchy/isolated pockets of fog across the area this morning will
quickly dissipate. Some minor gustiness will develop in the
afternoon, with SWLY gusts of 15 to 20 kts possible during the peak
afternoon heating/mixing.

As upper level disturbances move through the region, a moist/humid
and very unstable airmass will support the potential for multiple
rounds of showers and storms as early as this morning and continuing
into the afternoon and evening. Forecast spread in the exact timing
and location is high. Thus, given the low confidence, will handle
with a PROB30 group during the climatological peaked diurnal hours.
Though confidence is low on the exactly timing and location, there
is the potential for another area of showers and storms to move east
into the region during the overnight and early morning hours with
some associated sub-VFR restrictions possible.

Outlook: Scattered to possibly numerous showers and storms are
expected through Thursday with associated CIG and VSBY restrictions.
Some storms may be strong to severe, especially on Thursday.
Somewhat more limited convective coverage is expected on Friday and
generally restricted to eastern portions of the forecast area. Much
quieter conditions with generally VFR conditions expected into the
weekend.

&&

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

&&

$$

SYNOPSIS...Hartfield
NEAR TERM...Hartfield/CBL
SHORT TERM...CBL
LONG TERM...Danco
AVIATION...CBL/Blaes