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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 170530

Area Forecast Discussion
National Weather Service Charleston SC
1230 AM EST Thu Jan 17 2019

Dry high pressure will build from the west and prevail into
Thursday. A weak upper level disturbance will move through
Thursday night. A stronger storm system will then impact the
area Saturday night and Sunday. 


As of 1225 AM: Near term guidance indicates that sfc high pressure 
will surge southward across the CHS Tri-county around dawn this
morning. Moisture convergence is expected to increase and will 
support a period of patchy fog.

Previous Discussion:
Very few changes required again with the late evening update. 
We did tweak a few inland temps down another 1-2F based on 
recent and expected trends, now showing the lowest values of 
30-32F far inland, 33-36F most elsewhere near and west of US-17,
with 38-43F on the barrier islands, in downtown Charleston and 
near the shores of Lake Moultrie. Some fog is possible closer to
daybreak, especially along rivers and marshes. 

Previous discussion...
For the early evening update we have expanded the range of
minimum temps a little bit more, based on excellent radiational
cooling inland and just enough mixing along the coast to occur.
This will produce some lows at or below freezing along parts of
the coldest rural locations of our northwest tier. Otherwise
very few changes required at this time.


Surface high pressure over the eastern United States will
gradually shift to the east through Saturday. A weak upper level
disturbance will bring a cold front through the area Thursday
night. Moisture return ahead of this wave will be quite limited
and the quasi-zonal flow aloft will limit upward vertical
motion. Therefore we only show isolated showers moving through
far northern SC zones Thursday night. Strengthening southerly
flow Thursday into Friday will bring warming temperatures with
highs reaching the middle 60s by Friday.

An upper trough will move out of the central United States on
Saturday with a strong cold front approaching the area late in
the day Saturday. Most of the associated precipitation will
remain west of the area through sunset. However, despite
extensive cloud cover, continued warm advection will result in
high temps in the low to middle 70s.


Moderate forecast confidence most of the period. There is more model 
agreement now that a cold front will likely move through from the 
west Saturday night into early Sunday, with much colder high 
pressure building in thereafter through early next week. By the end 
of the period a coastal trough and a possible low pressure system 
could bring some rain back into the forecast but confidence is much 
lower toward the middle of next week. Most locales should see a half 
inch to maybe 1 inch of rain through the period, with most of the 
rain coming over the weekend. Although deep layer shear is strong we 
don't see a lot of instability so not overly concerned about the 
severe thunderstorm threat at this point and in fact we don't even 
have any mention of thunderstorms. It will also be breezy Sunday 
into early Monday and a Lake Wind Advisory may be needed for Lake 

Most, if not all, locales should be below freezing Monday morning 
with inland areas in the mid to upper 20s and wind chills near 20 
degrees. It's not out of the question that coastal areas reach Wind 
Chill Advisory levels of 15 degrees. Highs Monday should only range 
from around 40 near the Santee River to 50 near the Altamaha River. 
Temperatures Monday night are a bit tricky as much depends on how 
much a coastal trough and high clouds come into play but inland 
areas should get back below freezing again. Thereafter temperatures 
will modify into mid week to near or above normal levels.


Near term guidance indicates that sfc high pressure will surge
southward across KCHS around dawn this morning. Moisture
convergence is expected to increase and will support a period of
patchy fog. I will include a TEMPO between 10Z to 13Z for FG.
KSAV should remain VFR. During the daylight hours, south winds
will develop within the warm sector of low pressure tracking
across the Ohio River Valley. Winds across the terminals will
become light to calm during the evening hours.

Extended Aviation Outlook: Restrictions expected Saturday night
into early Sunday, likely IFR or worse at times. Also, 
breezy/gusty conditions are expected later Saturday through at 
least Sunday night.


Tonight: High pressure over the region will support varying winds
around 5 kt. Seas will be no more than 1 or 2 ft.

Extended Marine: No significant concerns through Saturday but 
conditions will be deteriorating significantly Saturday night 
and Sunday as a strong cold front affects the area with some 
heavy showers and possible thunderstorms (mainly near the Gulf 
Stream). Winds could even gust near gale force across the GA 
waters beyond 20 nm. Otherwise, Small Craft Advisories are 
likely for the entire area. Should see improving conditions 
Monday as winds turn offshore and diminish with the weakening 
cold air advection.


Coastal Flooding: Astronomical tide levels will be high early next
week and strong north/northeast winds will help contribute to even
higher levels which could lead to minor saltwater flooding 
around high tide. Thus, Coastal Flood Advisories will be