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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
000
FXUS62 KCHS 191736
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
136 PM EDT Tue Mar 19 2019

.SYNOPSIS...
High pressure will remain north of the area through tonight. A
trough is expected to develop offshore Wednesday, then move
northeast, away from the area. High pressure will then return
through the weekend, followed by a storm system next week. 

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 130 PM: Latest visible satellite indicates that thin high
clouds will continue to filter the sunlight through the rest of
the afternoon. Recent guidance indicates that temperatures will
remain nearly steady in the upper 50s to low 60s this afternoon.
I will update the forecast to adjust sky and temperature trends
through the rest of the afternoon.

As of 1125 AM: Recent visible satellite images indicate thinner
patches of cirrus have started to enter of the forecast area
from the southwest. I will update the forecast to adjust the sky
forecast through the rest of the daylight hours. Steady and
occasionally gusty NE winds are forecast to continue through the
rest of the day. I will update the high temperature forecast to
cool values by a degree. 

As of 915 AM: Visible satellite indicated a large area of high
clouds across the forecast area. Based on satellite trends,
cloud cover should continue through early to mid afternoon, 
then could thin from south to north late this afternoon. I will 
forecast temperatures remaining in the mid to upper 50s for much
of the day, peaking in the low 60s.

Previous Discussion:
Today: The synoptic pattern features expansive high pressure 
from New England/Mid Atlantic extending down into N GA and the 
western Carolinas. To the south, a persistent baroclinic area 
lies north of a stalled front across the Florida Straits. Deeper
moisture will remain well south of our region today and dry 
weather will prevail with the forecast highlighting breezy 
conditions at times into this afternoon as a tight low level 
pressure gradient will continue over our region. Temps will run 
cooler today as cold air advection will result in readings in 
the 50s much of the day with highs only in the lower 60s many 
areas. Strongest winds today will be right along the coast with 
northeast flow, the beaches and barrier islands will see gusts 
over 30 mph at times.

Tonight: The pattern will remain unchanged through the evening
with models suggesting an inverted trough developing offshore
late and this feature could result in some moisture advection 
and increase in clouds late night across parts of our northern 
SC zones. Inland from I-95, high clouds are forecast to thin out
with time and temps may fall into the mid 30s in zones bordering
the CSRA and Southeast Midland. We considering mentioning patchy
frost some spots, but there still is a bit of gradient and the 
advancement of the late night clouds from the east is uncertain.
Coastal areas and locations to the south of I-16 in GA likely 
to see readings in the lower to mid 40s overnight.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
The mid-levels Wednesday morning will consist semi-zonal flow over 
the Southeast. A low moving within the general flow will round the 
base of an amplifying trough along/over the East Coast on Thursday, 
causing lowering heights for us. The trough axis will shift offshore 
on Friday, allowing northwest flow and rising heights over our area. 
At the surface, high pressure will be inland and to the north 
Wednesday, while a weak trough develops just offshore. Models keep a 
majority of the moisture offshore. However, isolated to scattered 
light showers are possible for the Charleston Tri- County area. QPF 
should be minimal, if anything. High pressure will then push the 
trough away Wednesday night, prevailing Thursday into Friday. 
Wednesday highs will struggle to reach the lower 60s across our SC 
counties due to northerly winds, lowering heights/thickness values, 
and cold 850 mb temperatures. The mid 60s are expected in our GA 
counties. Lows will be in the 40s, assuming clouds don't clear out 
too quickly. Thursday and Friday highs will be near normal due to 
northwesterly winds with a bit of downslope component.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
Strong continental high pressure will prevail across the region 
through the weekend. Models then hint at a coastal trough forming 
Monday while a system approaches from the west. But there is a lot 
of uncertainty over the timing, strength, and the track of this 
system.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
The terminals are forecast to remain VFR through the 18Z TAF
period. NE winds may continue to gusts into the upper teens
through late this afternoon. Tonight into Wednesday morning, the
sfc pattern will feature an inverted sfc trough over eastern NC
with high pressure ridge over the PeeDee and Coastal Plain
regions of SC. This pattern should support a steady NNE wind
tonight through mid-day Wed. Forecast soundings support cloud
bases lowering to 070-150 kft on Wed. 

Extended Aviation: VFR.

&&

.MARINE...
A tight pressure gradient in place through tonight. This gradient
fueled by tight moisture and thermal gradients between cool/dry
high pressure north and deeper moisture and lower pressures to 
our south. We have Small Craft Advisories in place through 
tonight for all waters and even opted to issue a SCA for the 
Charleston Harbor for the gusty portions of the daytime period. 
There is a potential for pinched gradients and some marine 
locations could see periods of near 30 kt winds at times if this
is becomes the case. Seas were building this morning, topping 
out 5-7 ft near shore and 8-10 ft beyond 20 NM off the GA coast.

Extended Marine: High pressure will remain inland and to our north 
Wednesday while a coastal trough develops. The interaction between 
these two features will cause gusty northeast winds and wind-driven 
seas to continue through Wednesday afternoon. Conditions will 
improve Wednesday night as the trough moves away and the winds 
become offshore. The Small Craft Advisories will expire as seas 
subside in each zone, dropping off from the GA waters beyond 20 nm 
last. Another area of high pressure will bring lower winds and seas 
leading into the weekend.

&&

.TIDES/COASTAL FLOODING...
The combination of the perigee and full moon midweek along with
moderate to strong northeast winds through Wednesday will 
result in elevated tides. Coastal Flood Advisories for minor or 
maybe moderate flooding are likely with each high tide Tuesday, 
Wednesday, and maybe Thursday if the anomalies linger.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for AMZ330.
     Small Craft Advisory until 6 PM EDT Wednesday for AMZ352-354.
     Small Craft Advisory until midnight EDT Wednesday night for 
     AMZ350.
     Small Craft Advisory until 9 PM EDT Thursday for AMZ374.

&&

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