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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 230240

Area Forecast Discussion
National Weather Service Charleston SC
1040 PM EDT Sun Apr 22 2018

High pressure will remain northeast of the forecast area
tonight as a low pressure system approaches from the west. The
low will stay west of the area on Monday, then move up the east
coast through mid week. Weak high pressure will prevail before
a cold front crosses the area late week. 


Biggest update was again adjustment of PoPs for this evening and
overnight. Radar and surface observations indicating scattered
light rain/showers moving across the area from south to north,
but the more widespread rain/showers is still west/southwest of
the area. Precipitation chances will increase steadily 
overnight in line with the previous forecast. Maintained 
categorical PoPs over the south and west zones toward sunrise, 
and likely rest of the area. 

Latest observations, especially around Charleston and across
immediate coastal locations, have been showing winds increasing
with gusts of 20-25 knots fairly common. Most models were
holding off this increase until after midnight with the
increasing pressure gradient from the approaching low center
form the southwest. However, if these trends continue, will 
need to bump winds up even more in the grids for tonight and
early Monday morning.

As mentioned in the previous discussion, not expecting 
temperatures to fall much. Most areas are still in the mid to 
upper 60s. Kept lower 60s north and mid 60s south for mins, but 
would not be surprised if they come in a little warmer given the
sustained southeast winds and thick cloud cover. 


Monday: Aloft, the deep upper low will sit nearly stationary 
across west Tennessee through the day, with plenty of vort 
energy passing over the forecast area. Similarly, the attending 
surface low will be nearly stationary over west Tennessee with a
secondary weak low developing over central Georgia in the 
afternoon. High pressure will continue to extend into the region
with a warm front progged to lift northward across the forecast
area in the afternoon. The last few model runs have started to 
show the upper low and surface low making less eastward 
progress, and that has implications for the heavy rain 
potential. Rain chances start out around 100 for the southern 
half of the area around sunrise, then the 100 percent steadily 
spreads across the rest of the forecast area through the day. 

Heavy rain potential: The trend in the models the last several 
runs has been to focus the best rain potential further northeast
and seems to now be settling on an area including the Tri-
County. We will have a very moist airmass with precipitable 
waters values near the climatological high for 4/23, with 
increasing forcing for ascent through the day. The forecast now 
features 2-3 inches across southeast South Carolina, with 
highest amounts focused around Colleton County and the Tri-
County region. For southeast Georgia, amounts are more in the 
1-2 inch range. Differences remain in the global/regional models
and the hi-res models, so we will have to see how the bands of 
precipitation take shape overnight. Fortunately, since we are 
4-7 inches below normal across the region, the flooding threat 
is relatively low. We will have to watch the afternoon high tide
cycle as heavy rain could be ongoing at the same time causing 
issues along the coast. 

Severe thunderstorms: The overall thunderstorm and severe 
weather potential is more interesting than in previous days. Low
level wind profiles should exhibit impressive veering with 
elevated storm relative helicity values and very low LCL's. In 
fact, winds at 1km could be in the 40-50 knot range. However, 
instability is virtually non-existent. However, there could be 
enough mechanical lift to overcome the lack of thermodynamics, 
and if any convective elements are able to become surface-based 
it will certainly bear watching. The main feature will be the 
northward lifting warm front, where any storms that are able to 
develop could experience enhanced helicity in the vicinity of 
the front. There will be the potential damaging wind gusts, and 
even a tornado due to the strong low level flow, veering, and 
low LCL's. 

Gradient winds/lake winds: Strong winds will funnel into the 
Charleston Tri-County region by late morning and continue into 
the afternoon. The current forecast has wind gusts topping out 
in the 30-35 mph range, but there is a low end chance a Wind 
Advisory could be needed (mainly for Charleston County). 
Regardless, wind could impact travel on area bridges tomorrow. 
Also, a Lake Wind Advisory could be needed for Lake Moultrie.

Monday night: Deep moisture will begin to strip out and rain 
chances steadily diminish through the night. Any lingering heavy
rain and isolated thunderstorms will focus up through the 
Charleston area in the evening. 

Tuesday through Wednesday: The upper low will finally lift 
northeastward and eventually phase with a northern stream trough
by Wednesday, with a more zonal flow setting up across the 
area. At the surface, the low will linger on Tuesday and with a 
cold pocket within the trough aloft, we could certainly see 
isolated to scattered showers and possibly a thunderstorm. 
Models have minimal precipitation response, thus the low rain 
chances. The forecast is then dry Tuesday night and Wednesday as
weak high pressure builds in. Temperatures will actually be on 
the mild side with upper 70s progged for highs.


Forecast confidence is fairly low in the extended period due to
large differences in model solutions. A weak cold front is 
progged to move across the area early in the period, then a 
rather ill-defined pressure pattern will be in place through 
early Friday. Models seem to diverge thereafter, but in general 
it looks like another front could cross through early in the 
weekend. Main rain chances will be on Thursday as shortwave 
energy traverses overhead. Temperatures will be near normal.


00Z TAFs:
KCHS: Gusty winds this evening are still expected to ease some,
before coming back more consistently after midnight as the
pressure gradient increases. Expect VFR to hold this evening,
then decreasing to MVFR ceilings with increasing chances for
showers after midnight. As the showers become heavy after
daybreak Monday, IFR seem favorable. Due to some uncertainty
with timing, continue to keep prevailing conditions just above
for now. Any chance for thunder will likely hold off until
Monday afternoon. Gusty winds will develop late in the TAF, 
with gusts ~30 kt.

KSAV: Periods of gusty conditions through tonight, strongest 
during the daylight hours on Monday, with gusts of 25 knots,
possibly to 30 knots at times. VFR should hold until about 
midnight, then falling into MVFR as shower chances rapidly 
increase. As the showers become heavy around daybreak Monday, 
IFR conditions will become more likely, especially in any 
isolated thunderstorms.

Extended Aviation Outlook: Flight restrictions in reduced 
ceilings and visibilities are expected into Monday night, mainly
impacting KCHS. Gusty winds expected on Monday, with the 
highest winds at KCHS.


Rest of tonight: No significant changes were made with the
previous forecast. Winds have been increasing to near Small
Craft levels, especially near the coast. Expect winds to
gradually increase later tonight as the pressure gradient
increases with a low center approaching from the southwest.
Rainfall chances will also increase from south to north through
the night, with scattered to numerous showers, and possibly
isolated thunderstorms near the Gulf Stream, by late tonight and
early Monday morning.  

Monday through Friday: Strong southeast winds will setup on 
Monday, bringing high end Small Craft Advisory conditions across
all zones. The strongest winds will be in the Charleston County
waters where a Gale Watch remains in effect. Wasn't confident 
enough at this point to upgrade to a Gale Warning, and the 
threat remains. Briefly contemplated extending the Gale Watch 
into the Charleston Harbor, but still think gusts will top out 
around 30 knots there. Seas will increase, and could top out as 
high around 10-11 feet out near 20 nm in the Charleston County 
waters. Seas will be lower elsewhere, but still topping out 
around 7-8 feet in the southern South Carolina waters and the 
outer Georgia waters, and 6-7 feet in the nearshore Georgia 
waters. Conditions will improve Monday night with the relaxing 
of the gradient, but lingering seas will keep Small Craft 
Advisories going. All waters should be advisory-free by 
Wednesday morning. No additional marine concerns are expected 

High Surf: With strong southeasterly winds setting up on 
Monday, seas will increase significantly in the Charleston 
County waters. Breakers in the surf zone could reach 6 feet for 
a period of time, and a High Surf Advisory might be needed.

Rip Currents: Monday will bring a strong surge of onshore winds
and increased surf along the entire coast. Winds will be 
strongest for the Charleston County beaches, and after 
collaboration with NWS Wilmington, NC we decided on a High Risk 
of rip currents. A Moderate Risk of rip currents is in place 
everywhere else.


SC...High Rip Current Risk from 8 AM EDT Monday through Monday 
     evening for SCZ050.
MARINE...Small Craft Advisory until 2 PM EDT Tuesday for AMZ352-354.
     Gale Watch from Monday morning through late Monday night for 
     Small Craft Advisory until 11 AM EDT Monday for AMZ350.
     Small Craft Advisory until 8 AM EDT Wednesday for AMZ374.
     Small Craft Advisory until 11 PM EDT Monday for AMZ330.