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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 171433

Area Forecast Discussion
National Weather Service Charleston SC
1033 AM EDT Mon Jun 17 2019

Atlantic high pressure will prevail into Tuesday. An inland
surface trough of low pressure is expected to develop by mid
week, then persist over the Southeast into next weekend. 


Update: Just did the typical plugging of the observations into
the grids and blending into the afternoon. Otherwise, the 
previous forecast thinking is fine.

For today: A short wave trough will lift through the southern 
plains to the lower and middle Mississippi Valley, as it's 
deflected by a deep layered anticyclone over the Atlantic and 
Southeast. This will result in a deep southerly flow across the 
local area, as PWat climbs to around 1.75 to 2.00 inches, and 
it'll be noticeable with dew points generally in the lower and 
middle 70s. 

The cap of recent days has broken down, and that along with the
increase in moisture and MUCAPE of 1500-2500 J/kg, will allow 
for convection to develop on the sea breeze, mainly in the 
afternoon. Coverage though won't be anything more than isolated
or scattered, and with weak updrafts and only so-so lapse 
rates, there doesn't appear to be any risk of severe storms. 

Sky coverage will vary throughout the day, with early patches 
of low stratus and stratocumulus to lift into a scattered to 
broken cumulus layer, most prevalent near and in advance of the 
sea breeze. Where there is convection skies will occasionally 
become mostly cloudy/cloudy, while skies are mostly sunny to the
east of the sea breeze.

Utilizing the temps at H85 and the low level thickness scheme,
we arrive at max temps today 90-92F most places inland from the
intra-coastal, with lower and middle 80s in places such as
downtown Charleston, Hunting Island and Tybee Island.

Tonight: Convection far inland will quickly fade during the
early and mid evening hours, leaving us with mostly clear or
partly cloudy skies into the overnight hours. However, as the
upstream short wave shifts through the Tennessee Valley and
northern Gulf of Mexico, it will draw deeper moisture from north
of the Bahamas to off our coasts late. That along with enough
low level convergence should allow for some late night
convection that develops near the Gulf Stream to drift onshore
of the coastal counties after 5 am. There doesn't appear to be
too many signals just yet for fog, but where it does rain there
could be a little fog where skies remain void of cloud cover. 
The deep southerly flow will prevent temps from getting any 
lower than the lower and middle 70s.


Rain chances are expected to increase through this period. 

Tuesday: The region will be bracketed by two short waves and 
associated weak surface lows. One to the east over the Atlantic 
waters, the other passing to the north over the Mid Atlantic and NC. 
Much of the precipitation with the Atlantic system should stay 
mainly offshore. However, the shortwave passing to the north should 
combine with sufficient deep layer moisture to produce scattered 
showers and thunderstorms, mainly during the afternoon and evening. 
With southerly low level flow, the best concentration for convection 
should be inland. Showers/thunderstorms are expected to become more 
isolated overnight Tuesday night.  

Wednesday: Zonal to southwest upper flow will build over the region. 
Increasing southwesterly low level flow with again scattered mainly 
afternoon/evening showers thunderstorms. Models indicating the 
slightly higher deep layer moisture will reside south of the 
Savannah river, and have kept highest chance PoPs in our GA zones. 

Thursday: This looks to be the period with the highest chances for 
rainfall. A strong upper short wave trough is expected to move 
across the OH River valley region, with the trough axis tailing 
southward into our northern SC zones. The associated surface 
trough/front will likely stay just west of the area through the day, 
which will help produce scattered to numerous afternoon and evening 
showers and thunderstorms. It is possible some storms could be on 
the strong to severe side, but current probabilities too low to 
mention in the forecast. West-southwest low level flow will help to 
push scattered showers/thunderstorms from west to east across the 

Temperatures are expected to be slightly above normal, with highest 
in around 90 to the lower 90s and minds in the lower to mid 70s.


A weak surface cold front is expected to move through the area 
Thursday night. Mid and upper level ridging then rebuilds from the 
west, with a low level/surface inland trough of low pressure Friday 
through Monday. Lack of any significant upper level forcing along 
with marginal deep layer moisture, is expected to translate to 
generally isolated to scattered diurnally forced convection each 
day. The GFS is hinting at the potential for convection for form up-
stream, then propagating southeast with the northwest upper level 
flow, especially Saturday through Monday. This type of pattern is 
very difficult to predict the details of how the convection will 
form and progress each day due to the interaction of upstream 
convective cold pools/boundaries that spark new convection 
downstream over our region. 

Temperatures will continue above normal through the period. Highs in 
the lower to mid 90s, especially later in the period. Lows in the 
lower to mid 70s, warmest closer to the coast. Humidity will be up 
with afternoon surface dewpoint temperatures in the lower 70s each 


The 12Z TAF set has prevailing VFR. However, as the sea breeze 
shifts inland, broken ceilings around 3000-4000 ft will occur, 
along with isolated to scattered SHRA/TSRA. But any direct 
impacts are too minimal to include at this time.

Extended Aviation Outlook: Tuesday through Thursday: Brief flight 
restrictions possible within scattered mainly afternoon and 
evening showers and thunderstorms through Wednesday. Showers and
thunderstorms becoming more numerous on Thursday. 

Friday and Saturday: Showers and thunderstorms expected to become 
isolated with brief flight restrictions possible in the


Today: The Sub-tropical High will extend across our waters,
generating SW winds that back to the SE and S with sea breeze
influences this afternoon. On average speeds will be at or below
15 kt, with seas 2 ft within 20 nm and 3 ft further out. 

Tonight: Atlantic ridging will again prevail, keeping any
frontal systems off to the distant north and west. There is
enough of a gradient and nocturnal mixing to cause S winds to
hold around 10 or 15 kt through the night, as seas remain around
2 or 3 ft.

Tuesday and Wednesday: No highlights are expected. Winds generally 
15 knots or less and seas 3 feet or less. 

Wednesday night and Thursday: An increasing pressure gradient is 
expected as a strong low pressure center passed across the OH River 
valley and the northeast states. Winds increasing from the southwest 
at 15 to 20 knots, with gusts up to 25 knots possible. Seas may 
build to around 5 feet beyond 20 nm. Marginal Small Craft Advisory 
conditions may be reached during this period, especially north of 
the Savannah River. 

Friday and Saturday: Winds and seas decrease behind a weak cool
front. Winds 15 knots or less and seas 3 feet or less.