-- Discussion containing changed information from previous version are highlighted. --
506
FXUS62 KRAH 081820
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
220 PM EDT Wed May 8 2024

.SYNOPSIS...
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.

&&

.NEAR TERM /TODAY THROUGH TONIGHT/...
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.

&&

.SHORT TERM /THURSDAY AND THURSDAY NIGHT/...
-- Changed Discussion --
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/MCV, 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. -CBL

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). -Danco
-- End Changed Discussion --

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
As of 215 PM Wednesday...

Sat begins with broad troughing over generally the Northeast CONUS
characterized mainly by two embedded shortwaves, one of which is
shifting out over the western Atlantic while the second dives ESE
out of Upper Great Lakes and into the Ohio Valley by Sat afternoon.
The former will have shifted the axis of deep, rich moisture (PWAT
~1.6 inches) well offshore and usher in PWAT values around 0.5 of an
inch (near the 10th percentile for GSO, RNK, MHX RAOB) driven by
briefly strong subsidence and downsloping flow WNW flow. The latter
shortwave will pivot across Mid-Atlantic Sat afternoon and evening,
but central NC may only see glancing synoptic forcing from this
feature as the better DPVA remains north of the VA border. Point
soundings suggest shallow afternoon cumulus development to the base
of the subsidence inversion with better 700mb moisture (deepening
the moisture layer) moving into the area late Sat afternoon into the
evening hours. This may be enough to squeeze out some sprinkles over
central NC with better chance for very light rain closer to the
NC/VA border.

Surface high pressure and a secondary area of subsidence on the
backside of the departing trough will lead to dry conditions Sun
through Mon with temperatures in the 70s to low 80s. The next system
will move towards the area Mon night into Wed as a closed low near
the Four Corners Region Sun morning slowly drifts eastward. Closed
lows situated in between a split-flow regime often have timing
issues, and this one is no exception. Latest forecast gradually
brings in precipitation early Tues morning with chances lingering
into Wed with greatest chances diurnally favored during the
afternoon/evening. It would not be surprising to see the quicker
GEFS trend towards the slower EPS and precipitation onset becoming
12 hours later than is currently forecast. Temperatures will trend
back towards above normal mid-level height rises and the 850mb anti-
cyclone shifts offshore.
-- End Changed Discussion --

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
As of 140 PM Wednesday...

While VFR conditions will be dominant through this evening across
the area, important details hinge on the path and timing of showers
and strong thunderstorms expected to cross the area, affecting
mostly W and S areas, passing near or over INT/GSO between 20z and
00z, RDU between 22z and 02z, and RWI/FAY 23z to 03z. Sub-VFR
conditions with high/erratic winds, strong gusts, heavy rain, in-
cloud and cloud-to-ground lightning, and hail will be possible in
and near these storms. These storms are likely to be a solid line
and not easily circumnavigable. Behind this line, a few hours of
stratiform rain with embedded isolated lightning may occur, and a
trend toward prevailing MVFR cigs/vsbys is expected overnight, with
lowest confidence at RWI where the occurrence of rain late tonight
is less certain. Late tonight toward sunrise, widespread showers and
isolated storms are possible, keeping cigs/vsbys mostly MVFR at all
terminals with an increase in surface winds from the SW at 10-15 kts
with periodic 15-25 kt gusts.

Looking beyond 18z Thu, high uncertainty in the details of showers
and storms and their impacts lingers through Thu, although fairly
high coverage is expected to persist through the mid to late
afternoon. A lull in shower/storm chances is expected Thu evening
through Fri morning, but the risk of sub-VFR cigs/vsbys with fog is
high 07z-15z Fri. Another chance of showers/storms Fri afternoon and
evening will be focused across the east, all ahead of a cold front.
VFR conditions are expected to prevail Fri night through Mon with
low rain chances and drier weather as high pressure arrives. -GIH

&&

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

&&

$$

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