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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 171759

Area Forecast Discussion
National Weather Service Charleston SC
159 PM EDT Wed Jul 17 2019

The area will remain situated between inland low pressure and
offshore high pressure into early next week. A stronger storm
system may affect the area toward the middle of next week. 


Afternoon Update: Radar shows thunderstorms rapidly developing 
along the sea breeze this afternoon. SPC Mesoscale Analysis 
shows MLCAPEs approaching 3,500 J/kg in some spots with DCAPEs 
approaching 1,200 J/kg. This could lead to some marginally
severe pulse storms with damaging winds. Additionally, weak 
steering winds aloft will lead to slow storm movements. Given 
PWATs around 2", very heavy rainfall is a concern with local 
flooding possible in low-lying and poorly drained areas. Further
inland, temperatures are running above normal with dew points 
in the lower to mid 70s. This is keeping heat indices lower than
110 degrees, and the convection moving inland will only help to
lower the heat indices further. Hence, no Heat Advisories are 
expected. CAMs continue to be in good agreement showing the 
thunderstorms moving inland with the sea breeze this afternoon, 
then diminishing around sunset.

Tonight: Convection will rapidly fade this evening and along
there are indications of some late night showers in the ocean
where the best convergence and instability will occur, we have
the bulk of the night free of showers and t-storms. Depending 
upon how much convection occurs at the climate sites today, we 
could be close to our record high minimums for KCHS, KCXM and 
KSAV for both July 17 and 18.


Moderate confidence this period. At the surface inland troughing 
will prevail mainly north and west of the area while ridging 
prevails to the southeast. The pattern will favor a few showers 
and/or storms mainly over the Atlantic each morning with scattered 
showers and thunderstorms mainly inland during the 
afternoon/evening. No significant risk for severe storms is expected 
due to the mostly pulse nature of the convection but certainly can't 
completely rule out isolated severe storms each day because of 
mesoscale boundaries like the sea breeze. The main weather story 
this period will be the heat as temperatures remain well above 
normal, mainly mid-upper 90s inland each afternoon with heat indices 
peaking close to 110 degrees away from the immediate coast. Heat 
Advisories will be possible if confidence increases in greater 
coverage/duration of 110+ heat indices. Overnight lows could also be 
near record high levels in the upper 70s to lower 80s, especially at 


Moderate confidence this period. The low-level lee-side trough looks 
to persist through the period while upper ridging gives way to more 
troughing, especially toward the middle of next week. For now we 
have more or less maintained a climatology/persistence forecast with 
rain chances mostly peaking around 30-40 percent each afternoon. 
Temperatures should remain above normal through Monday night at 
least. Heat indices should mostly peak in the 105-110 degree range 
each afternoon through at least Monday, so the risk for Heat 
Advisories appears low at this time.


18Z TAFs: Thunderstorms will persist through this evening.
Probabilities for flight restrictions are reasonably high at 
both TAF sites this afternoon, so TEMPO groups are in place to
cover the most likely time ranges, mainly for IFR visibilities.
Conditions will improve this evening as the thunderstorms
diminish, allowing VFR to prevail.

Extended Aviation Outlook: Low risk of flight restrictions from
mainly afternoon/evening showers and thunderstorms through 
early next week.


Today: The coastal waters will be located under the W-NW 
portions of strong ridging in the ocean, while a well defined 
and broad trough forms inland. The gradient does tighten a
little this afternoon and as sea breeze circulations develop and
feed into storms inland, SW winds of 10 kt or less will back to
the S and increase to at least 10-15 the second half of the 
day. Expect seas of 2-3 ft.

Tonight: There is decent pinching that occurs between the inland
trough and Atlantic ridge to generate S-SW winds as high as
15-20 kt. Some of the soundings show the potential for higher
speeds, perhaps even approaching 25 kt at times. But since
geostrophic winds at 1000 mb are only 20-25 kt, this should keep
us shy of any advisory levels, even though seas build another

Thursday through Monday: Inland low pressure and offshore high 
pressure will lead to a fairly typical summertime pattern with 
mainly south/southwest winds through the period, generally highest 
near the coast during the afternoon/evening from the sea breeze. 
However, not expecting any Small Craft Advisories as significant 
wave heights will be 4 feet or less.