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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
000
FXUS62 KCHS 170806
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
406 AM EDT Wed Jul 17 2019

.SYNOPSIS...
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. 

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The forecast region will lie between an Atlantic ridge
with its axis roughly east-west along 30N, while a broad lee
side trough develops to the west-northwest. One interesting
feature that is worth noting is the remnants of T.C. Barry that
is now an open wave, that will move through Ohio and toward 
lake Erie.

Meanwhile, the mid and upper levels will feature a strong
5930-5940 meter anticyclone centered over southern Georgia and
northern Florida, that gets nudged a little south in response to
a tail of vorticity that one could argue is associated with the
remains of Barny that reaches the central and southern
Appalachians late. While this could have a minor effect on the 
formation of convection this afternoon, it looks like better
forcing will occur due to the sea breeze and the proximity to
the inland trough. The overall thermodynamic environment is
somewhat impressive, with the most noticeable feature the 
MUCAPE as great as 3000-4000 J/kg. That along with PWat in
excess of 2 inches will lead to scattered t-storms developing
near and inland from the US-17 corridor starting around 1-2 pm
as we reach our convective temps. DCAPE is forecast up near
1200-1500 J/kg which will support the possibility of strong or
marginally severe downbursts of wind in isolated storms. Also
with a slow and erratic storm motion (mainly to the east- 
southeast around 5 mph), there is also a heavy rain potential 
with localized flooding concerns.

It'll be another hot day with H8 temps of 19-20C, or around 1-2
standard deviations above normal. This will produce max temps in
the mid and upper 90s most places inland from the intra-coastal
waters, with even some locations reaching the century mark from
Allendale to Evans, Tattnall and Candler County where the lower
rain chances will occur. Dew points will mix out some far 
inland with a deep westerly flow, but will pool in the mid and 
upper 70s along the coastal corridor. This will equate to max 
heat indices reaching as high as 108-111F, and highest along the
coastal counties. For now we feel that with scattered storms 
during the time of peak heating, advisory level heat indices of
110F will be too short-lived and cover too small of an area to
hoist a Heat Advisory. Should convection be delayed and/or
coverage end up less than forecast, than we could issue an
advisory later this morning. 

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.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
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 
KCHS/KCXM.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
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.

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
Due to the wet grounds from convection on Tuesday, there could 
be some ground fog around daybreak at KCHS. Otherwise, VFR much
of the time through 06Z Thursday at KCHS and KSAV. The exception
would be for possible temporary flight restrictions in TSRA at 
both sites from about 19-23Z as activity develops along the sea 
breeze. 

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

&&

.MARINE...
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. Seas of 1-2 ft this morning will climb to 2-3 ft this
afternoon with the higher winds.

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
foot.

Waterspouts: Light winds and favorable moisture and instability
this morning concerns us regarding the formation of 
waterspouts. But so far there have not been any convergent lines
over the nearby Atlantic and the SPC Non-Supercell Tornado 
Parameter is showing values less than 1 unit, which are both 
favorable for the waterspout formation. We continue to monitor.

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. 

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...