892
FXUS62 KCHS 272010
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
410 PM EDT Mon May 27 2024

...SEVERE THUNDERSTORM WATCH 334 IN EFFECT UNTIL 10 PM EDT THIS
EVENING...

.SYNOPSIS...
A cold front will move into the region tonight, then stall
nearby through the middle of the week. High pressure will
return for the second half of this week.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
A broad trough is found aloft over the eastern half of the
country, with several embedded short waves in the fast moving
westerly flow. One such short wave is to our north, with a
second over the northern Gulf Coast region that moves through
late evening and overnight. At the surface, a cold front is
shifting southeast as it approaches the Appalachians, while a
pre-frontal trough/lee side trough is just upstream from the
forecast counties. That trough eventually gets overtaken by the
cold front during the night, with the actual front to not move
in until several hours after midnight.

Height falls are rather minimal, but with a split jet structure
over and near the local counties and the resulting divergence
aloft, plus the short wave and cold front that get closer,
forcing for ascent is not a problem.

With temperatures well into the 80s, dew points that are in the
upper 60s and lower 70s, we`re seeing MLCAPE reaching near 2500
J/kg. Normalized CAPE has climbed to at least 0.20 units, mid
level lapse rates are at least 7.5-8C/km, and PWat is at least
1.5 inches. So the necessary ingredients are there for
convection. It`s just taken so long given previously existing
CINH and a decent cap that is finally eroding.

The 0-6 km Bulk Shear is 30-40 k, and with a 30 kt low level
jet topped by a 50 kt jet at 500 mb, the resulting hodographs
will be somewhat elongated, but also rather straight. These
conditions support initially discrete cells or maybe some
multicells over Georgia. But with time there will be more
clustering that transpires due to cold pooling. We show PoPs
increasing from west to east through the first part of the
night, with PoPs peaking at 70%. Nothing severe looks to reach
Charleston, Beaufort, and Savannah until after 8 pm.

The convective parameters for any severe weather, which is most
likely between 6 pm and 12 am, are quite impressive, despite
the nocturnal environment. DCAPE is at least 800-1200 J/kg and
WINDEX from the modified sounding is 65 mph. These imply that
damaging wind gusts are possible. Wet Bulb Zero is between 10
and 11 thousand feet, and the Hail CAPE is 600-900 J/kg. Both
would imply the potential for large hail. Given the shear, there
is a low end risk for an isolated tornado where boundaries
interact.

QPF on average looks to be between 1/2 and 1 inch due to the
somewhat progressive nature of storms. But given that storms in
some areas could last long, and rainfall rates could be as much
as 2 or 3 inches/hour, locally higher amounts are plausible. If
recent runs of the HRRR come to fruition, there might be some
heavier rains ongoing in downtown Charleston at or near the high
tide near midnight. That would enhance the risk for flooding.

We made adjustments to the hourly temperature curve, showing
temperatures dropping to the lower 70s wherever we have likely
PoPs. Actually lows won`t be much different, generally down to
the upper 60s and lower 70s, with decreasing PoPs from west to
east after midnight.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
Tuesday: The mid-levels will consist of a longwave trough over the
eastern half of the U.S. At the surface, a cold front will be
located along our coast early in the morning. It`s forecasted to
transition into a stationary front shortly thereafter and meander
along our coast through the early evening. By late evening, it`ll
transition back into a cold front and move further offshore, and
away from our area. Broad High pressure will be centered over the
Northern Plains overnight, with its periphery building into our
region. The highest moisture values will remain just offshore, on
the other side of the front. With less moisture in place, there
won`t be much instability in place across our area, despite
temperatures peaking in the lower 90s away from the beaches. The
synoptic models and long-range CAMs hint at maybe an isolated shower
or thunderstorm, mainly near are coastal SC counties in the
afternoon. Further south, more dry air and capping should limit any
convection from developing. Anything that does manage to develop
over our SC counties should be weak, dissipating by sunset. The
evening and overnight will be dry everywhere. Lows will range from
the lower 60s far inland to around 70 degrees at the beaches.

Wednesday and Thursday: The mid-levels will consist of a longwave
trough over the eastern half of the U.S. Wednesday. The trough will
become more amplified along the East Coast Wednesday night into
Thursday, in response to ridging approaching from the west. At the
surface, broad High pressure centered over the Northern MS Valley
Wednesday morning will shift to the Great Lakes region by late
Thursday. The periphery of this High will reach all the way into the
Southeast U.S. both days, bringing dry conditions. Afternoon dew
points may fall into the 50s away from the beaches, which will make
it feel very comfortable. Temperatures on Wednesday should peak
around 90 degrees, and the lower to mid 80s on Thursday. No
convection is expected either day, but fair weather cumulus is
expected both afternoons. Lows Wednesday night will range from the
lower 60s far inland to around 70 degrees at the beaches.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
A trough over the East Coast Thursday night will shift offshore by
later Friday, followed by a ridge building on Saturday, and possibly
prevailing into Monday. High pressure will be the dominant surface
feature during the long term. POPs will be around 20% each afternoon
with temperatures a few degrees below normal.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
KCHS/KJZI/KSAV: Upstream showers will likely fizzle before
reaching KSAV this afternoon. But we included VCSH just in case
they do survive this far east. Otherwise, SW winds are expected
to become a little gusty ahead of pre-frontal trough and cold
front, peaking at least as high as 15 or 20 kt.

As the pre-frontal trough and front draw closer, a better
chance of SHRA/TSRA exists during tonight at all terminals. The
most favorable timing is from 00Z to 07-08Z, which is covered
by VCTS. However, direct impacts currently look to occur between
01Z to 05Z, when we show gusty winds, TSRA and flight
restrictions. Adjustments certainly could be refined pending
latest trends.

VFR will return late tonight and Tuesday in wake of the cold
front.

Extended Aviation Outlook: VFR.

&&

.MARINE...
Sea breeze influences will help in the enhancement of winds into
this evening, with S and SW winds as high as 15 or 20 kt and
gusty. There will be a few gusts near 25 kt at times in
Charleston Harbor and near the coast of Charleston County. But
not enough to warrant a Small Craft Advisories. Winds will veer
more SW or WSW at less than 15 through the night, altered
temporarily by convection that moves through during the mid or
late evening into the early post-midnight hours as an upstream
cold front draws closer. Outside of convection, seas will
average 2 to 3 feet.

That convection will generate a risk to mariners of severe
t-storms capable of producing gust in excess of 35 or 40 kt,
hail, cloud to water lightning strikes, and reduced visibilities
in heavy rains. There is also a non-zero risk for a waterspout.
Special Marine Warnings will likely be required for some of
these storms.

Extended Marine: Rather tranquil conditions are expected. A cold
front will be located along our coast early Tuesday morning, moving
further offshore, and away from our area Tuesday night. High
pressure will then gradually move towards our area midweek and
prevail through the weekend. Even the sea/land breezes don`t appear
to be too strong during this time period. Wind speeds should average
15 kts or less. Seas will be 4 ft or less.

&&

.CLIMATE...
Record High Minimum Temperatures:

May 27:
KCHS: 76/1991
KCXM: 79/2019
KSAV: 77/1878

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$

NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...