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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 202238

Area Forecast Discussion
National Weather Service Charleston SC
638 PM EDT Tue Aug 20 2019

Atlantic high pressure and an inland trough will prevail
through late week. A cold front push into the region this
weekend then will stall over or just south of the area through
early next week. 


Convection should percolate across the interior for a few more
hours before diminishing. A pronounced vorticity lobe associated
with the mid-upper low spinning over the South Carolina upstate
is progged to cross the region early Wednesday. Given the warm,
moist environment that is in place, there will likely be at 
least some form of convection associated with this feature. The 
best lift/upper divergence ahead of the vorticity lobe looks to 
remain over the eastern Midlands into the Pee Dee regions, but 
its tail end will likely brush the Charleston Tri-County area. 
Will maintain slight chance pops in this region during the very
early morning hours with rain-free conditions elsewhere. 

Convection associated with various convective outflows over the 
Gulf Stream should diminish over the next few hours. Some of 
this activity could approach the Colleton and Charleston County 
coasts through mid-evening. Overnight, the low-level steering 
flow will veer with time which should keep any noctural, 
marine-based convection confined to the coastal waters.

Lows will range from the mid 70s inland to around 80
at the beaches.


Wednesday through Friday: Expect typical conditions for the latter 
half of August, featuring scattered showers/thunderstorms, 
especially midday into early evening, and slightly above normal 
temperatures with highs mainly in the lower 90s and lows in the 
lower/middle 70s inland and in the upper 70s/near 80F on the 
beaches. As usual, maximum heat index values in the 100-105F range 
will be locally punctuated by the cooling influence of 

Of note, the southern periphery of low pressure/an associated pool 
of mid-level vorticity centered near the NC/VA coast will brush our 
region Wednesday, and this could enhance diurnal thunderstorm 
coverage to some degree, particularly across SC counties. Otherwise, 
thunderstorms will focus along the sea breeze and other mesoscale 
boundaries through late week, so precise placement of thunderstorms 
will remain elusive beyond the near-term forecast period. Also, a 
cold front will approach the region Friday, but latest guidance 
suggests that this boundary could remain too far to the north to 
influence thunderstorms coverage across our region, unless organized 
convection upstream manages to hold together to push into our area 
late afternoon or during the evening. Meanwhile, PWAT values 
exceeding 2 inches will support locally heavy rainfall, while 
diurnal instability will provide a daily potential for a 
brief/isolated severe thunderstorm. However, the potential for 
significant coverage of excessive rainfall or for significant severe 
weather will remain low through late week.


Low to moderate confidence this period. The pattern doesn't change 
much until a cold front likely pushes into the area this weekend, 
possibly moving through southeast GA as well early next week. We 
prefer the slower WPC/ECMWF guidance with regard to the frontal 
passage timing. Eventually a more fall-like high pressure wedge 
pattern should set up and feature cooler temperatures and above 
normal rain chances. Think the severe storm risk will be low with 
possibly a better risk for some flooding given the deep moisture and 
weak storm motions leading to some training.


VFR. Afternoon showers/tstms will once again develop near the
terminals Wednesday afternoon. Impact probabilities are too low
to justify a mention of TSRA at either terminal this far out. 

Extended Aviation Outlook: Occasional flight restrictions at 
KCHS/KSAV each day through Friday, mainly from afternoon/evening 
showers and thunderstorms. However, VFR most of the time during this 
period. More frequent/significant flight restrictions are likely this 
weekend due to increased moisture and a cold front moving south 
through the area.


Tonight: Southerly winds will prevail with speeds around 10 kt.
Seas of 1-2 ft will build to 2-3 ft overnight.

Wednesday through Sunday: Winds/seas are expected to remain below 
Small Craft Advisory levels. However, thunderstorms will produce 
locally hazardous conditions at any time. Through Saturday, 
south/southwest winds should average 10-20 knots, with strongest 
winds likely during the daily sea breeze and at night give typical 
nocturnal trends. A weakening cold front could push through the 
waters by Sunday, and winds could turn toward the east. Seas outside 
thunderstorms will average 1-3 feet through Saturday, highest beyond 
20 nm, then seas could build slightly to 2-4 feet Sunday.