542
FXUS62 KRAH 180136
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
935 PM EDT Fri May 17 2024

.SYNOPSIS...
The region will remain in a warm, humid, and unsettled weather
pattern through early Sunday. Cool high pressure will build in from
the north Sunday through Monday, then shift off the Mid Atlantic
coast late Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 935 PM Friday...

Will maintain high pops over much of the area through the overnight
hours, although the better rain chances will be from midnight
through about 4 AM. The latest surface analysis still shows a weak
frontal zone with weak surface confluence oriented from the Triad
across the Triangle region and to the SE where it becomes even more
diffuse. A cluster of showers and storms that developed quickly over
the Triad late this afternoon expanded further E along the I-40
corridor across the rest of the N Piedmont, growing upscale into
several embedded storms, with one particular shower/storm cluster
dropping a few inches of steady moderate rain over W and N portions
of the Triangle region and prompting flood advisories. The
thermodynamic environment remains somewhat marginal, with SBCAPE
having dipped to just around 500 J/kg with a downturn in deep layer
effective bulk shear from around 40-45 kts earlier to around 30 kts
according to the latest RAP-based mesoanalysis. The most intriguing
cell so far is now just N of Wake county and exhibiting weak shallow
rotation, not unexpected given the KRAX hodograph and the 00z GSO
sounding which show notable low level curving and a longer straight
profile atop, resulting in sfc-1 km SRH of 100-200 m2/s2. Any
updraft along this confluent zone will feed on the low level
vorticity along it, although our weakening CAPE will limit this
threat. Regardless of the spin-up threat, with lingering weak CAPE,
very slow-to-develop CINH, incoming minor mid level perturbations,
and PWs in the 90th percentile, the threat of showers with perhaps
an isolated storm will continue overnight. We`ll also see areas of
low stratus continue to fill in within weak surface winds and moist
low levels. Still expect lows in the 60s to around 70. -GIH

Earlier discussion from 300 PM: A shortwave over OK/TX will move
east to the lower MS Valley through tonight, as mid/upper ridging
remains just off the Southeast US coast, with central NC under the
influence of WSW flow aloft in between these two features. The
latest surface analysis shows a weak boundary/convergence axis
stretching from the NE Piedmont into the central Coastal Plain,
accompanied by a wind shift from E/NE to S. Meanwhile a quasi-
stationary frontal zone extends across the northern Gulf Coast,
which is being overtaken by an MCS across southern MS/AL/GA. This
MCS will continue to move east and off the coast by this evening,
weakening as it does so. Weak perturbations in the WSW flow aloft
will help generate scattered showers and maybe a few storms this
afternoon and evening. A few showers with limited lightning have
already begun to develop in the Coastal Plain along the convergence
axis, and more showers are observed across western NC and eastern
TN. The MCS to our south seems to be choking off some of our
moisture feed, though we are still in a soupy airmass with dew
points in the mid-to-upper-60s and PW values around 1.5 inches.
Today`s mid-level lapse rates are very unimpressive as temperatures
aloft are fairly warm. There are also widespread mid and high clouds
across the area which will continue through the afternoon. These
factors will limit instability, as current SPC analysis shows MLCAPE
around 500 J/kg and it is only expected to reach 500-1000 J/kg at
most. Thus not expecting storms today to be widespread, and not
concerned about anything severe. POPs are mostly slight to chance
today, highest across the north and west. As another perturbation
aloft approaches and the MCS to our south moves away, shower
coverage may actually increase in the evening and overnight hours,
still highest in the north and west where POPs are likely. However,
with the nocturnal timing any instability looks limited and
elevated, and only carry a slight chance of thunder overnight. High
temperatures today will be in the mid-70s to lower-80s. Lows tonight
will be mild (in the 60s) with the overcast skies and precipitation.

&&

.SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
As of 240 PM Friday...

Overall showers and thunderstorms are expected in all locations
tomorrow, although confidence is low in overall potential for severe
weather. Showers/storms should be ongoing across the northeast
around sunrise, then there should be a relative lull across the area
during the morning hours. By the afternoon, coverage of storms
should increase from north to south, with greatest coverage likely
to occur during the late afternoon. Showers will continue through
the evening, with rain possibilities dropping to chance after
midnight.

Confidence is low in where severe weather could occur because of the
uncertainty of today`s upstream MCSs moving into the region and how
those boundaries will affect development of storms tomorrow along
with moisture transport from the south. However, instability has the
potential to range between 1000 and 2000 J/kg along with 30-40 kt
effective bulk shear. The afternoon Day 2 update from SPC had
minimal changes in the convective risk from the overnight issuance,
with half a dozen southern counties still under a slight (level 2 of
5) risk and the rest of the forecast area under a marginal (level 1
of 5) risk. Damaging winds will be the primary threat for severe
weather, with large hail the secondary threat. In addition, the
entire forecast area is under a marginal (level 1 of 4) risk for
excessive rainfall.

Temperatures appear to be cool near the VA/NC border with highs in
the lower 70s with the rest of the area in the upper 70s and lower
80s. Overnight lows should range from the mid 50s to the lower 60s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
As of 255 PM Friday...

Sunday and Monday: Aloft, the mid/upper s/w will be over the mid-
Atlantic and Carolinas early Sun, drifting slowly sewd through the
area through Sun night. By Mon morning, the trough should be
offshore, with the sub-tropical ridge building from the southern
plains to the Northeast US. Both the trough and the ridge will shift
slightly ewd through Mon night, with the ridge over the Appalachians
by 12Z Tue. At the surface, the surface low will be off the Carolina
coast, with a backdoor cold front through the area and cool high
pressure ridging into the area by early Sun. The ridge will
strengthen over the area Sun and generally remain in place through
Mon night, while the low sits/drifts slowly eastward offshore. There
is some continued forecast uncertainty, especially wrt temperatures
Sun and Mon. The model solutions have trended drier for Sun and Mon,
with Sun still potentially wet. However, chances for storms will be
limited to the far south or southeast. While Mon should be largely
dry, there is still the possibility for a brief period of lingering
light early.

Temperatures: Tricky, especially Sun as there is some bust potential
with the wedge airmass and possible light rain, but for now expect
highs ranging from mid/upper 60s NE to low/mid 70s SW. Lows Sun
night generally in the low to mid 50s. For Mon, some continued
uncertainty in highs, but for now expect low/mid 70s east to
mid/upper 70s west. Lows again in the low to mid 50s.

Tuesday onward: The ridge axis will generally remain over the area
through at least Wed night. The ridge will be suppressed sewd
Thu/Fri as a s/w tracks from the mid-MS Valley, enewd into the
northern mid-Atlantic/Northeast US. This wave may amplify the larger
scale trough over the Northeast and mid-Atlantic Thu night/Fri. The
next potential for showers/storms across the area will be with this
disturbance Thu/Thu night. Expect temperatures to moderate through
mid-week.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
As of 735 PM Friday...

A few rounds of showers and isolated storms will track toward the
NNE over central NC terminals tonight, bringing periods of MVFR to
IFR cigs, with overcast skies. While a shower can`t be ruled out at
any given location during this time, the focus for potential thunder
and the greater chance of rain will be 03z-08z. IFR cigs and MVFR to
IFR vsbys are expected to spread across the area tonight, mainly
after 06z, with high confidence that IFR conditions will persist at
all central NC terminals from through at least 15z (and perhaps
lasting until 18z at RWI). Cigs will lift to MVFR by 15z-19z then to
VFR for the rest of the day, although the chance of thunderstorms
will increase starting around 20z Sat. Some of these storms could be
quite strong. Outside of any storms, surface winds will be light and
variable, but becoming from the S and SW after 15z at 5-10 kts.

Looking beyond 00z Sun, we`ll stay in an active weather pattern with
periods of wet weather and sporadic sub-VFR conditions likely
through early Sun, as a slow-moving upper level trough drifts over
the region. The greatest chance for sub-VFR cigs/vsbys will be late
Sat night, which are likely to last through Sun morning, although
mostly dry weather is expected midday Sun through Mon morning. VFR
conditions should return for midday Mon through Wed. -GIH

&&

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

&&

$$

SYNOPSIS...Hartfield
NEAR TERM...Hartfield/Danco
SHORT TERM...Green
LONG TERM...KC
AVIATION...Hartfield