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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
327 
FXUS62 KCHS 171125
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
725 AM EDT Tue Jul 17 2018

.SYNOPSIS...
A cold front will approach the region tonight, then stall over
or nearby our area through Thursday. The front will lift north
on Friday. Another cold front will approach from the northwest
over the weekend. 

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The setup is ripe for another active afternoon today. 
Scattered to numerous showers and tstms are forecast to develop 
in a very moist and unstable environment, enhanced by upper 
troughing and a steady dose of moisture transport buoyed by a 
stronger low level southwest jet today. Forecast soundings show 
a weak cap in the wake of last night's mid level short wave to 
start the day, but additional weaker short waves moving in from 
central GA and upstate SC should result in some convection 
developing to the west of I-95 by early afternoon, perhaps 
closer to the CSRA/Midlands. Temps could be tempered somewhat by
morning cloud cover in some areas but we made little change to 
forecast high temps around 90 degrees most areas. Any areas that
see greater insolation prior to convective initiation could see
temps rise into the lower 90s.

Convection: The main concern this afternoon will be heavy rain 
potential closer to the SC resultant sea breeze later this afternoon
given the strong 850 MB moisture transport progs above this low
level convergence zone embedded within a larger scale surface
trough. PWATs were forecast to exceed 2.30 inches in this region.
There could be some multicell convection which results in localized
bands of 2-3 inch rains in eastern SC. There should also be good
coverage of convection in southeast GA again this afternoon as 
well, locally heavy downpours possible. Despite increased deep 
layered wind fields today, the deep moisture profiles precludes 
much of a severe weather risk. Overall, pulse/multicell parameters
not looking overly impressive at mid/late afternoon. We think 
there will be enough convective coverage to warrant likely POPs 
across the entire area.

Tonight: Scattered convective rains will persist into the evening
and even the overnight in some areas ahead of a cold front easing
into north GA and the western Carolinas late. Coastal areas appear
to have the better late night chances. We think by late night, the
land breeze pushing offshore will focus better shower and storm
potential off the coast.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
The mid levels will consist of an amplifying trough hugging the East 
Coast on Wednesday. Thursday the trough will get shoved offshore as 
ridging builds to our north, generally leaving our area with nearly 
zonal flow. By Friday the ridging will be pushed offshore as a 
trough builds from the Great Lakes Region down into the Deep South. 
At the surface, a stationary front will slowly meander southward 
across our area Wednesday. This front should roughly be located 
along our southernmost counties on Thursday. By Friday, the front 
will transition into a warm front and lift northward. However, the 
exact location of it each day could lead to a much different weather 
forecast. Part of this revolves around the moisture. Moisture will 
be plentiful. Models show PWATs exceeding 2.25" across our entire 
area on Wednesday. SPC climatology indicates these values are near 
record for CHS for this time of year. These high values are pushed 
slightly to the south and offshore Thursday by the front.
However, they creep back into our area on Friday as the front lifts 
northward. We continue to forecast likely showers each afternoon, 
but POPs will probably need to be adjusted further based on the 
location of the front. The severe threat is low due to the limited 
instability. Wednesday appears to be the best day. GFS BLCAPEs 
approaching 1,500-2,000 J/kg along with pockets of negative 4 
Showalter values. Additionally, there are hints of DCAPEs 
approaching 1,000 J/kg. But this is highly dependent on the exact 
location of the front. Thursday the instability lowers as the front 
moves southward. Friday the instability ticks upward as the front 
starts moving northward. Again, this instability is highly dependent 
on the location of the front. The result is a chance of 
thunderstorms with a low severe threat. The main concern continues 
to be the rainfall potential. The deep moisture and slow storm 
motions resulting from light steering winds higher up should allow 
for periods of heavy rain. Local flooding is possible.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
A warm front will be north of our area Friday night. A trough is 
expected to develop inland over the weekend while another cold front 
slowly approaches. The end result will be a rainy weather
pattern.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
VFR this morning with lower cigs remaining well inland prior to
mid morning dissipation. Showers and tstms are likely this 
afternoon with little change in timing from previous thinking
as latest CAMs suggesting outflow from inland storms colliding
with the sea breeze to produce additional storms from 18Z-20Z 
to around sunset. Locally heavy downpours are possible at the 
terminals which would result in mainly brief flight 
restrictions. 

Tonight: Convective rains should tend to decrease in the evening
with a chance some lower clouds move into the coastal corridor 
late in a moist southwest flow. Chances are good that both
terminals remain VFR however.

Extended Aviation Outlook: A front will bring unsettled weather in 
the form of showers and thunderstorms through Friday. More 
convection is possible on Saturday. Periodic flight restrictions are 
possible, mainly during the afternoon and evening hours each
day.

&&

.MARINE...
A tightening pressure gradient between an approaching cold front
to the west and strong Atlantic high pressure should increase 
winds this afternoon into tonight to 15-20 knots with some gusts
in excess of 20 knots. This will result in seas building to 
generally 3 to 4 feet with some 5 foot seas just beyond 20 NM 
off the Charleston County coast. Showers and tstms will 
periodically impact local waters with reduced vsbys in heavy 
rains, locally gusty winds and frequent lightning, especially 
tonight.

Extended Marine: A stationary front will meander over or nearby our 
area Wednesday and Thursday. This front will transition into a warm 
front and lift northward on Friday. A trough will develop inland on 
Saturday while another front slowly approaches from the northwest. 
Despite all of this activity, no Small Craft Advisories are
expected.

&&

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

&&

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