Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 240253

Area Forecast Discussion
National Weather Service Charleston SC
953 PM EST Thu Nov 23 2017

Low pressure will track along the Southeast coast tonight into
Friday. A cold front will shift through the area late Saturday
night, followed by dry high pressure through the middle of next


Secondary cyclogenesis is ongoing off the Florida east coast
with weak low pressure (1009 hPa) centered about 30 NM north-
northeast of Cape Canaveral. The primary low (1007 hPa) is still
located just north of Tampa Bay, but this low should dissipate
over the next few hours as the secondary low becomes the
dominate one as it treks east/northeast off the Georgia and
South Carolina coast. Most of the measurable rainfall has
shifted offshore, but KCLX and surface observations still show
some patches of very light rain or drizzle hugging the coastal
counties. Lowered pops to 20% in this area for the next few
hours, but this activity should end soon as isentropic downglide
takes hold.

GOES-16 fog product shows low clouds filling in across central
Georgia and extending northeast into Reidsville and Statesboro.
These clouds should be fairly short-lived and erode from
northeast-southwest as the core of the strongest isentropic
descent punches southwest. While mostly clear to partly cloudy
skies could reach the far inland zones over the coming hours,
mostly cloudy to cloudy skies will linger across much of the
region through the overnight hours.

Northerly winds will help advect cooler temperatures into the
region tonight as low pressure passes offshore. Most of the high
resolution models are bringing a corridor of upper 30s with a
few mid 30s into the far interior zones prior to daybreak. Given
temperatures are already in the 30s across the South Carolina
Midlands, these numbers seem reasonable. Plan to lower overnight
temperatures by several degrees for most areas with lows
ranging from the upper 30s well inland to temperatures holding
steady in the lower 50s along the Georgia coast. Winds look to
remain too high to support scattered frost (except in highly
sheltered areas), but there will be about a 2-4 hour period for
patchy frost to develop. Will include "patchy frost" for areas
roughly along/north of a Thrift-Sylvania-Fairfax-Smoaks-Saint
George-Saint Stephen line, but away from Lakes Marion and
Moultrie. In fact, with northerly winds flow over the relatively
warmer waters of Lake Moultrie, Moncks Corner may very well end
up warmer than North Charleston tonight.

Interestingly, today`s synoptic pattern would have been a
classic heavy snow producer for the Lowcountry and Coastal
Empire if enough cold air had been in place. This is typically
the setup that is needed to get heavy snows this far south in
the winter with low pressure crossing the central Florida
peninsula. Thankfully, no cold air was in place for this
Thanksgiving Day!


Friday: An area of low pressure will quickly depart to the northeast
while an h5 shortwave shifts off the Southeast coast. The pattern
favors rain and/or showers to end over all areas by late morning as
a northeast wind turns to west and pushes the bulk of deep moisture
offshore. A gradual break or clearing in clouds from west to east is
anticipated during the afternoon, which should allow high temps to
rebound into the lower 60s for most areas and potentially middle 60s
south of I-16 in Southeast Georgia. Dry high pressure will then
build into the area Friday night. Friday night lows should range in
the lower 40s inland to mid/upper 40s closer to the coast.

Saturday and Sunday: High pressure will slide east over the area
this weekend with a west/southwest flow contributing to noticeably
warmer temps on Saturday ahead of a backdoor cold front approaching
from the north. In general, Saturday afternoon highs should peak in
the upper 60s for most areas. A large mid/upper lvl trough of low
pressure will expand over the East Conus Saturday night, helping
push a dry backdoor cold front south into the area late Saturday
night into early Sunday. Temps will be cooler post fropa as sfc
winds turn northerly and cold air advection takes place. In general,
Sunday afternoon high temps should peak in the lower 60s in
Southeast South Carolina and middle 60s in Southeast Georgia.


Dry high pressure will prevail Sunday night through Wednesday. A
cold front may approach late Wednesday or on Thursday,
potentially bringing scattered showers to the area. Sunday and
Monday will be slightly cooler due to the influence of cold high
pressure. Temps warm up during mid week as the high shifts east
and southerly flow develops.


Primary concerns:
* Timing of clouds lifting to VFR at KSAV

LIFR cigs at KSAV should lift to MVFR and eventually VFR this
evening as rain exits the region and drier air to the north
filters in. Timing is a bit uncertain, but most guidance points
to lifting above alternate minimums by 01z, if not sooner
conditions returning to VFR by mid-evening, roughly 03-04z.
Confidence is low given timing uncertainties. Amendments will
be issued as needed. At KCHS, cigs are already beginning to
respond with recently rising back into VFR thresholds. Could
still see a period of MVFR this evening, but no impacts are

Extended Aviation Outlook: VFR conditions should prevail at both
terminals Friday afternoon through Tuesday.


Tonight: Surface low pressure will track offshore while high
pressure is to our north. The interaction between these two
features will increase the surface pressure gradient across the
coastal waters, leading to gusty NE winds and hazardous seas.
Wave heights 6 ft and greater are forecasted beyond 10 nm, so
Small Craft Advisories remain in effect for all of the coastal

A northeast flow will persist into Friday between high pressure
inland and a low pressure system tracking along/near the Southeast
coast. A tightened pressure gradient over the area early favors the
continuation of Small Craft Advisories for nearshore waters into
late morning Friday, while high seas in offshore Georgia waters
favor a Small Craft Advisory into early Friday night. In general
northeast winds will gust around 25 kt while seas build as high as 4-
6 ft in nearshore waters and 6-8 ft in offshore Georgia waters. High
pressure will then build over the coastal waters this weekend,
favoring northeast winds to become primarily offshore. Wind speeds
and seas will likely remain below Small Craft Advisory levels
Saturday into early next week, but a backdoor cold front will
approach the waters from the north late Saturday into early Sunday,
causing winds to temporarily increase to 15-20 kt.


MARINE...Small Craft Advisory until noon EST Friday for AMZ350-352-354.
     Small Craft Advisory until 1 AM EST Saturday for AMZ374.



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