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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
000
FXUS62 KCHS 171414
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1014 AM EDT Wed Apr 17 2019

.SYNOPSIS...
High pressure will persist through Thursday. A strong cold
front will sweep through the region Friday. High pressure will
return late weekend, then prevail into early next week. 

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Quiet and dry weather expected today as the local area remains 
under the influence of deep layer high pressure. Low level 
thickness values off of the 12z KCHS RAOB support highs in the 
low to mid 80s inland from the beaches. Temperatures at the 
coast will be cooler with the sea breeze circulation setting up.
Cirrus clouds will cross the area today, could be thick at 
times, however it is not expected to have a big impact on 
temperatures.

Very few changes were made with the late morning update. Mainly
just minor tweaks to the hourly temperatures, dew points, and
sky cover.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
Tonight: The synoptic pattern will change little across the
Southeast U.S. overnight. The surface high is forecast to shift
slightly east, which may result in a slightly tighter pressure 
gradient across the forecast area between it and the developing
surface low across the Southern Plains. Forecast soundings
suggest that the nocturnal inversion will remain rather strong,
so still expect a fairly light surface flow of only 2-3 kt to
prevail for much of the overnight period. It will be a slightly
warmer night, but opted to trend overnight lows to the cooler 
side of guidance as the cirrus aloft is forecast thin with 
time, which will enhance the degree of radiational cooling 
within the decoupled boundary layer. Lows will range upper 50s 
inland to the mid 60s at the beaches and Downtown Charleston 
with a few mid 50s possible across the Francis Marion National 
Forecast and the upper reaches of Dorchester and Berkeley 
Counties away from the Santee-Cooper Lakes.

Thursday: Dry and slightly warmer conditions will prevail during the 
day as the area remains along the western edge of Atlantic high 
pressure and corresponding mid/upper lvl ridge aloft. In general, 
highs should peak in the low/mid 80s, warmest across Southeast 
Georgia away from the coast. By Thursday night, the mid/upper lvl 
ridge axis will slide further east and well offshore in advance of a 
large trough of low pressure shifting over the Central United 
States. Latest guidance suggests isolated to scattered showers 
during the night as well as a few thunderstorms drifting onshore 
after midnight. Greatest chances should remain along and east of the 
I-95 corridor in Southeast South Carolina. Overnight lows should 
also be mild, ranging in the upper 60s to around 70 degrees.

Friday: An active day of weather is becoming more likely to start 
off the weekend. A strong low pressure system is expected to track 
over the Midwest and Great Lakes region with a southward extending 
cold front that sweeps through the Southeast United States. Aloft, a 
high amplitude trough of low pressure will help drive the cold front 
quickly towards the Southeast coast, most likely sometime Friday 
afternoon. Timing of the front, along with potentially ongoing 
showers/thunderstorms shifting onshore during the morning will be 
crucial in regards to how much instability develops across the area 
by Friday afternoon. Latest guidance suggests SBCAPE around 1000 
J/kg along with PWATs approaching 1.50-1.75 inches and sfc dewpts in 
the mid 60s. This amount of instability/moisture combined with the 
arrival of strong shear associated with a 50-60 kt low-lvl jet, 
suggests widespread thunderstorms, some being strong and/or severe 
as they pass through the area, particularly along and east of the I-
95 corridor as sfc temps peak near 80 degrees ahead of the front. 

Strong shear and enhanced forcing will be more than sufficient to 
support strong/severe thunderstorms Friday afternoon into early 
evening, but the potential will still remain highly dependent on how 
much instability develops across the region before the onset of 
thunderstorm activity along/near the passing front. Latest model 
soundings along with 850mb-500mb lvl crossover winds suggest mainly 
unidirectional wind fields during the most favorable time of 
convection, meaning the largest threat with thunderstorms being in 
the form of damaging wind gusts within bowing segments. However, 
there is still some directional shear component, suggesting the 
potential for isolated tornadoes along a squall line of 
thunderstorms quickly advancing toward the coast. Although not out 
of the question, marginal mid-lvl lapse rates suggest any hail to be 
small. Once fropa occurs, the severe weather threat will come to an 
end.

Strong wind fields associated with this system will likely support 
gusty southerly winds across the Southeast United States, even 
outside shower/thunderstorm activity. A Wind Advisory will be 
possible over the area, especially near the coast in Southeast South 
Carolina. Strong cold air advection behind the front will likely 
support gusty winds at least through the evening. A few showers 
could persist behind the departing front Friday night. Lows will be 
much cooler than the previous night as high pressure begins to push 
into the area late. In general, lows should dip into the lower 50s 
away from the coast. 

Saturday: A large mid/upper lvl low will be slow to shift off the 
Eastern Seaboard, suggesting the potential for a few showers across 
the area as some moisture wraps around its southern periphery. 
However, the bulk of deep moisture and precip activity should be 
offshore Saturday afternoon. Temps will be noticeably cooler as high 
pressure spreads across the area from the west/southwest. High temps 
should only reach the mid/upper 60s across most areas. Temps could 
touch 70 degrees near the Southeast Georgia coast.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
Temps will remain noticeably cooler Saturday night as high pressure 
continues to build across the area. In general low temps should dip 
into the mid/upper 40s inland to low/mid 50s near the coast. Dry sfc 
high pressure will then prevail across the region Sunday through 
Tuesday with temps warming each day as a mid/upper lvl ridge axis 
shifts over the Southeast United States. In general, high temps in 
the mid 70s on Sunday should warm into the low 80s Monday, then 
low/mid 80s on Tuesday. Overnight lows should range in the low mid 
50s away from the coast Sunday night, then mid/upper 50s inland to 
lower 60s near the coast Monday night.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
VFR.

Extended Aviation Outlook: VFR expected through Thursday. Flight
restrictions are then likely Friday with showers/thunderstorms 
shifting over the terminals, potentially from activity shifting 
onshore early, followed by potentially strong showers/tstms with
a passing cold front Friday afternoon into the evening. Gusty 
southerly winds are also likely Friday and Friday night outside 
of showers and thunderstorms. Flight restrictions could persist 
into early Saturday before improving to VFR Saturday afternoon.

&&

.MARINE...
Through Tonight: A broad, southerly flow regime will prevail as
the region remains along the western flanks of subtropical high
pressure centered well offshore. Winds will generally remain 10
kt or less through the period, but may become locally enhanced
at the coast and Charleston Harbor in the afternoon where the
sea breeze circulation will modulate the wind field. Seas will
average 1-2 ft nearshore waters and 2-3 ft over the Georgia
offshore waters.

Thursday through Sunday: Conditions will remain below Small Craft 
Advisory levels Wednesday into early Thursday as high pressure 
becomes centered western Atlantic. In general, south/southeast winds 
will remain at or below 15 kts while seas range between 2-3 ft with 
occasional 4 ft seas across offshore Georgia waters by Thursday 
afternoon. Conditions will then deteriorate Thursday night as a 
strong pressure gradient develops in advance of a cold front that is 
expected to shift off the Southeast coast Friday afternoon. Small 
Craft Advisories are likely for all coastal waters starting Thursday 
night, followed by a period of gale force wind gusts Friday morning 
into Friday evening. Seas should also build up to 5-8 ft across GA 
nearshore waters and 9-12 ft across offshore Georgia waters and 
beyond 10 nm from the coast in nearshore SC waters. Thunderstorms 
could be strong and/or severe Friday afternoon into early Friday 
night along/near the passing cold front. Damaging winds and isolated 
waterspouts will be the primary threat. Conditions should then 
improve Saturday, but Small Craft Advisories will likely persist 
behind the departing front as cooler high pressure builds over the 
waters. By daybreak Sunday, high pressure should become centered 
along the Southeast coast, supporting winds/seas that remain below 
Small Craft Advisory level conditions.

&&

.TIDES/COASTAL FLOODING...
Elevated tide levels are expected late week with the approach of the 
full moon, especially with the evening high tide cycles. Shallow 
coastal flooding is possible, primarily along the South Carolina 
coast, and Coastal Flood Advisories could be needed.

&&

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

&&

$$
NEAR TERM...ETM
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...