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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
679 
FXUS62 KCHS 220945
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
545 AM EDT Sat Sep 22 2018

.SYNOPSIS...
High pressure will prevail into Sunday. A weak wave approaching
from the east is forecasted to bring some impacts to the area
Monday into Tuesday. A cold front is expected to bring more
impacts to the area towards the end of next week. 

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Surface moisture convergence is really beginning to increase
along the upper Georgia coast and latest RAP and H3R expand
light rain from off the Atlantic into the Georgia coastal 
counties over the next few hours. Increased pops to 30% and 
introduced a more pessimistic sky forecast through late morning 
in this area. Temperatures are a few degrees cooler than 
expected as sunrise approaches, so have adjusted hourly 
temperatures to match going trends. The rest of the forecast 
looks good.

The region will remain positioned between strong a H5 
subtropical anticyclone centered offshore of the North Carolina 
Outer Banks and a TUTT low moving across the eastern Gulf of 
Mexico. Elongated surface high pressure extending from central 
Georgia into the Atlantic north of Bermuda will remain quasi-
stationary today as low pressure organizes about 150 miles 
southwest of Bermuda. The orientation of the surface high over 
the region will keep onshore flow conditions in place with 
northeast winds becoming more solidly east this afternoon as a 
robust sea breeze circulation takes hold. 

High resolution guidance is similar in showing isolated marine-
based showers moving onshore along the Georgia coast this 
morning into the early afternoon where low-level moisture is 
deepest. Similar to yesterday, this activity should dissipate by 
mid-late afternoon as the boundary layer wind environment over 
the beaches and coastal waters becomes increasing divergent as 
the sea breeze pushes inland. Slight chance pops will be 
maintained roughly along/south of Ludowici-Hinesville-Ossabaw 
Island line with rain-free conditions elsewhere across Southeast
South Carolina and Southeast Georgia. RAP soundings show a very
strong H7-H5 capping inversion holding in place so tstms are 
not expected. Highs will range from the lower 90s well inland to
the mid 80s at the beaches.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY/...
Tonight: Surface high pressure will change little overnight as
low pressure southwest of Bermuda begins to move west. Boundary
layer winds will gradually back along the coast with time with
some semblance of a coastal trough developing along/just offshore
of the beaches. Still looks like the best chances for isolated 
marine-based showers to move inland will be along the Georgia 
coast and slight chance pops will be maintained there. There are
some signals that a more concentrated area of showers could 
develop along the Georgia coast where low-level convergence 
maximizes in the vicinity of the coastal trough, but it is 
unclear how strong this feature will be. Higher pops may 
eventually be needed pending further analysis later today. Lows 
will range from the mid 60s well inland to the mid-upper 70s at 
the beaches and Downtown Charleston.

Sunday: The mid-levels will consist of high pressure off the 
Southeast coast in the morning, with it's western periphery 
stretching into our area. The high will slowly weaken and shift to 
the east through the day and into the first part of the night, 
causing heights to lower. But it strengthens slightly late at night, 
causing heights to rise back to where they were at the beginning of 
the day. At the surface, high pressure will be offshore in the 
morning while a stationary front is spread across NC. The highs 
influence will dissipate across our area as the day progresses. At 
the same time, the front may shift southward, especially overnight. 
But it will stay north our area. A sharp PWAT gradient will be in 
place during the morning, with values ranging from ~1.25 over 
Berkeley County to almost 2" across McIntosh County. The higher 
values will be ushered to the north, so most locations may be 
approaching 2" by the overnight hours. POPs are in the slight chance 
and chance categories, highest in our GA counties. There is some 
instability in this locations, so we have a slight chance of 
thunderstorms. Temperatures will be a few degrees above normal.

Monday: The mid-levels will consist of high pressure off the 
Southeast coast in the morning, with it's western periphery barely 
stretching into our area. Additionally, a ridge will be over the 
Great Lakes Region. The ridge will move eastward through the day and 
into the night. This will help to strengthen the high off the coast 
and allow heights to rise over our area. At the surface, a weak wave 
is forecasted to slowly approach from the east. Meanwhile, a 
stationary front will remain spread across NC, trending southward as 
the day progresses and dissipating into the night.
PWATs will be over 2". SPC sounding climatology indicates this is 
above the 90% moving average for CHS for this time of year. With the 
approach of the wave and the front, and the moisture in place, 
models show good precipitation coverage, so we have POPs in the 
chance category. Instability is a little bit higher than previous 
days, but not overly impressive. Regardless, a chance of 
thunderstorms seems reasonable. Steering winds are not too strong, 
so some storms could produce locally heavy rainfall if they are slow 
to move. The precipitation will dissipate in the evening. More 
showers are expected to develop over the coastal waters overnight 
and move towards the coast within easterly flow. The precipitation 
will also allow temperatures to lower to near normal for a change.

Tuesday: The mid-levels will consist of high pressure off the 
Southeast coast. At the surface, the weak wave is forecasted to 
approach from from the east. However, it will weaken as it 
approaches during the day and perhaps get absorbed into the 
dissipating front that will be over or near our area. PWATs appear 
to trend downward. But they may remain slightly above normal for 
this time of year. Models have the best rainfall potential offshore 
and right along the immediate coast. We have the highest POPs at the 
coast, tapering as one heads inland. Instability is highest near the 
coast, then quickly lowering inland. Again, it's not overly 
impressive. Regardless, a chance of thunderstorms seems reasonable. 
Temperatures will be within a few degrees of normal.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
An active weather pattern combined with plenty of moisture will lead 
to an above normal chance of showers. Tuesday night the coastal wave 
is forecasted to weaken while it moves overhead or just to our 
north. At that time, a cold front will slowly approach our area from 
the far west. Wednesday the cold front will continue to slowly 
approach. Thursday the cold front is forecasted to start moving into 
our area, then possibly stall over or nearby our area into Friday.

&&

.AVIATION /10Z SATURDAY THROUGH WEDNESDAY/...
VFR will prevail at both KCHS and KSAV for much of the 22/12z 
TAF period. Showers will be near the KSAV terminal by mid- 
morning, but latest data suggest these should remain east and 
south of the terminal. Will carry VCSH along with a cig of
BKN035 to account for this. No concerns are KCHS.

Extended Aviation Outlook: Multiple atmospheric disturbances will 
bring the potential for convection, leading to brief flight 
restrictions.

&&

.MARINE...
Today: No concerns as northeast winds veer to the east this
afternoon with the development of the afternoon sea breeze.
Speeds will average 10 kt with seas 1-2 ft nearshore waters and
2-3 ft offshore waters.

Tonight: East winds 10 kt will back to the northeast 5-10 kt near
the coast after midnight as a weak coastal through tries to 
develop near the beaches. Seas will average 1-2 ft nearshore
waters and 2-4 ft offshore waters.

Sunday through Wednesday: High pressure will be dissipating 
Sunday while a stationary front spreads across NC. A weak 
atmospheric wave will approach from the east on Monday. The 
pressure gradient ahead of this feature will increase, causing 
winds/seas to trend upwards. However, no Small Craft Advisories 
are expected. This feature is forecasted to weaken while it 
moves overhead or just to our north on Tuesday. At that time, 
winds/seas are forecasted to trend downward. A cold front will 
slowly approach from the west on Wednesday.

&&

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

&&

$$