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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 221429

Area Forecast Discussion
National Weather Service Charleston SC
1029 AM EDT Mon Jul 22 2019

The area will remain between an inland trough and Atlantic high
pressure today. A cold front will approach Tuesday and move
through the area Tuesday night, then stall off the coast
through late week. High pressure will return over the weekend. 


Late this morning: Today looks as though it will be quite
similar to the last two days, though with marginally better
chances for afternoon convection. Aloft, the upper ridge will
start to split apart with one center near the Four Corners
region and the other over the subtropical Atlantic. The weakness
in between will gradually fill with the longwave trough
spreading across eastern North America. The weakening of the
ridge and arrival of the trough should make atmosphere a bit
more conducive to convective development. Still, coverage should
remain in the isolated to scattered range with the best coverage
expected across southeast Georgia late this afternoon. DCAPE
values won't be quite as impressive as the last few days, but
should still be around 1000 J/kg this afternoon. So, it's not
out of the realm of possibility to see one or two strong to
marginally severe storms. Temperatures are almost a persistence
forecast, and we still expect mid 90s everywhere away from the
immediate coast. Should be a breezy day along the coast with
frequent gusts into the 20-25 mph range this afternoon.

Tonight: The cold front shifts closer to the area and reaches 
near the southern Appalachian Mountains by daybreak on Tuesday. 
This is in response to an amplified and broad mid and upper 
trough that develops over the eastern half of the nation. Since 
there isn't much shear, convection will quickly fade this 
evening with the loss of insolation and increasing CINH, with 
showers and t-storms done with by 10 or 11 pm. There will be 
some showers and t-storms developing over the ocean after 
midnight, but for the most part these look to stay offshore 
since the synoptic flow veers to the southwest. 

It'll be a warm night with a well mixed atmosphere in advance 
of the upstream cold front. plus an increase in high level cloud
cover. This will prevent temps from getting any lower than the 
73-77F inland from US-17, upper 70s to near 80F along the coast.


The focus of the short term period will be the wetter and 
relatively cooler weather. Mid level heights will fall on 
Tuesday as an unseasonably strong trough sets up over the 
eastern U.S. Meanwhile at the surface, a cold front will push 
into the forecast area Tuesday afternoon and slide offshore 
Tuesday night. The front will then stall off the coast and 
linger there through Thursday. 

This pattern will promote a fairly active couple of days given 
the forcing and juicy airmass in place, noted by PWats over 2 
inches. A good portion of Tuesday is expected to remain dry, 
then coverage will become more widespread late in the day and 
overnight as the front gets into the area. Pockets of heavy 
rainfall will be likely. Given the limited instability, the 
overall severe threat appears to be low; however there will be 
shear to work with, so it would not be out of the question. The 
Storm Prediction Center currently has the SC zones outlined in a
Marginal Risk for severe weather. The main threat within any 
stronger storm would be damaging wind gusts.

Models show varying degrees of precipitation coverage on 
Wednesday. NAM keeps the area mostly dry, while the Euro and GFS
show a wetter solution. Opted to lean on the wetter side as a 
wave of low pressure rides up along the stalled boundary. Have 
maintained the gradient of lower PoPs inland and higher at the 
coast, but did trend it down a touch. Thursday, again, there 
will be some gradient but think it will lie closer to the 
land/ocean interface. 

Near normal temperatures on Tuesday will be noticeably cooler 
Wednesday and Thursday with highs topping out in the mid 80s. 
Lows overnight will fall near or slightly below normal.


Surface high pressure will return late week into the weekend, 
however a stalled front will linger offshore before becoming 
more diffuse with time. The highest rain chances will be across 
coastal areas and our Georgia zones Friday into Saturday in 
association with the best moisture and forcing. Temperatures 
through the period will generally be at or slightly below 


KCHS: Although isolated SHRA/TSRA will develop on the sea 
breeze near the terminal this afternoon, any direct impacts are 
too low to mention in the 12Z TAF. Thus we have VFR weather 
through 12Z Tuesday.

KSAV: Scattered SHRA/TSRA will form along the sea breeze 
boundary this afternoon, and some of this activity could impact 
the terminal. Until trends are more evident we have VCTS from 
19-23Z, although sub-VFR weather is possible. Prior to 19Z and 
after 23Z, VFR conditions will dominate.

Extended Aviation Outlook: Flight restrictions are expected 
late Tuesday into Wednesday within showers/thunderstorms 
associated with a cold front. Additional restrictions are 
possible at times through late week as the front lingers 
offshore and keeps rain chances in the forecast.


Today and tonight: It looks like a persistence forecast is the 
way to go, with a somewhat pinched gradient between high 
pressure just to the E-SE and an inland trough to the W-NW. 
Given sea breeze influences this afternoon and evening, followed
by nocturnal low level jet activity, S-SW winds will be as high
as 15-20 kt. Some 25 kt gusts will occur at times, especially 
over the open ocean tonight. But the duration and coverage of 
such winds does not justify a Small Craft Advisory. Seas through
the period will average 2-4 ft. 

Tuesday through Saturday: A cold front will approach the waters
on Tuesday with wind speeds increasing as a result. At this 
time, conditions look to stay below Small Craft Advisory 
criteria, however there may be a few hours around 00z Wednesday 
where gusts get close. The front will push offshore Tuesday 
night then linger in the vicinity through much of the period. 
Winds will largely be from a northeast or east direction post- 
frontal passage, although it could get a bit variable at times 
in proximity to the stalled front. No additional marine concerns
are expected through Saturday.