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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 221055

Area Forecast Discussion
National Weather Service Charleston SC
655 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. 


Today: While the surface pattern is basically the same as the
past few days, featuring the high pressure ridge just offshore
and a lee side trough, the mid and upper situation will undergo
changes. The persistent mid and upper ridge over the Southeast
since late last week will weaken as a short wave trough drops 
through the upper Midwest and upper Great Lakes. This begins the
process of amplifying the large scale pattern aloft that will
aide in sending a cold front found from Ohio to the Texas
panhandle early, that reaches from near Long Island to the
Arklatex late. 

Adequate moisture, theta-E ridging and weak-moderate  
instability/CAPE will exist over the area. But since the main 
focus for convection will come from the sea breeze, and there 
remains quite a bit of dry air above 700 mb, we won't go 
anything more than isolated to scattered coverage. Activity will
hold off until after 2 or 3 pm with as much as a 3-4C cap in 
place. Once it does form, the activity will initially form over
southeast Georgia and then spread north and west. The highest 
PoP will be 30-40%, located inland from I-95 in Georgia and 
across our southern counties of South Carolina. 

DCAPE is forecast to reach as much as 1000-1500 J/kg this
afternoon with the dry air aloft. That points to at least a low
end risk for a couple of strong or marginally severe storms 
with gusty to potential damaging winds possible.

The MOS guidance has done well the past few days, so we'll stay
close to that again today with the air mass basically the same
as those past few days. This supports max temps 94-95F inland 
from the intra-coastal, with upper 80s and lower 90s closer to 
the shoreline where a modest resultant breeze will develop and 
generates south- southeast winds as high as 15-25 mph.

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 normal.


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

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.