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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 220449

Area Forecast Discussion
National Weather Service Charleston SC
1249 AM EDT Mon Jul 22 2019

The area will remain between an inland trough and Atlantic high
pressure through Tuesday. A cold front will move through the
region Tuesday night, then stall off the coast, perhaps
lingering into the weekend. 


Convection has ended for the night, and under clear or partly
cloudy skies temps will drop only another 2-4F through daybreak.
There are no fog concerns, and while most places will experience
SW winds less than 5 or 10 mph, winds over coastal sections of 
SC remain somewhat elevated and as high as 15 or 20 mph. 


Monday: The mid-levels in the morning will consist of high 
pressure in the western Atlantic and troughing stretching from 
the Great Lakes region southwards into the TN Valley. As time 
progresses, the high will get pushed away by the trough, which 
will strengthen/amplify along the Eastern third of the country. 
At the surface, our area will remain between an inland trough 
and Atlantic high pressure. Moisture is forecasted to increase, 
with PWATs starting ~1.75" in the morning, then exceeding 2" 
late at night". The general thinking is the morning hours should
be dry, with convection firing up along the sea breeze in the 
afternoon. Remnant convection could persist inland into the 
evening, followed by additional convection developing over/along
the coast during the overnight hours. We tried to show this the
best that we can with the hourly POPs, but in simple terms, 
expect chance POPs. Temperatures should remain a few degrees 
above normal during both the day and at night.

Tuesday: The mid-levels will consist of a persistently strong 
trough along the Eastern third of the country. In fact, NEAFS 
shows the 500 mb anomaly 2-3 standard deviations below normal. 
At the surface, our area will remain between an inland trough 
and Atlantic high pressure early. The high will get pushed away 
by a cold front, which will approach from the west and northwest
during the day. The front is expected to move through the 
region during the evening hours, then stall just off our coast 
overnight. A deep plume of moisture is expected with the trough,
with PWATs perhaps reaching 2.25". This is above normal per SPC
climatology. Additionally, NAEFS has above normal PWATs and 
Integrated WV Transport values just to our north and it's 
perfectly reasonable these values will creep into our area with 
time. The end result will be increasing showers during the day 
followed by widespread showers during the evening and overnight 
hours. Despite good lift from the front and plenty of moisture, 
instability is somewhat tame. The GFS shows Showalter values 
barely negative and MLCAPEs struggling to reach 1,000 J/kg. Even
the shear and DCAPEs with the front aren't very impressive. We 
backed off on the thunderstorm threat, lowering it to the chance
category. The risk of widespread severe storms is very low. The
rainfall will be the biggest impact, with some locations easily
exceeding 1". Temperatures should be near normal. Though, it's 
possible our highs may need to be lowered several degrees if the
sky cover is thicker than expected and the precipitation starts
sooner than expected.

Wednesday: The aforementioned mid-level trough persists in 
place while the front stalls off our coast. A deep plume of 
moisture will continue to brush along the coast with PWATs 
exceeding 2". Given the good model agreement, we have likely to 
categorical POPs over a good chunk of our area during the day. 
Similar to Tuesday, the severe thunderstorm threat will be low. 
Given the abundant cloud cover and precipitation, temperatures 
will be below normal. It's not out of the question some spots 
may have a hard time reaching the 80 degree mark. But we're not 
ready to go that cool yet.


Slightly improved model agreement today that the front will 
hang up offshore around the Gulf Stream Thursday and Friday. A 
broad upper trough will linger over the region with a wedge-like
surface pattern and multiple waves of low pressure riding along
the stalled front offshore. This pattern appears to be wetter 
for South GA with higher progged PWATs and a moisture low-level 
feed of Atlantic air. Our POP gradients from 50-60 percent GA to
40 percent SC may not be tight enough but we have moderate 
confidence in at least the gradient. Next weekend, model 
uncertainty at this point and we ran with mainly climo POPs. 
Temps are expected to run a bit below climo through late week, 
perhaps warming a bit next weekend.


VFR conditions are expected at both CHS and SAV terminals
through 06Z Tuesday. Isolated to scattered SHRA/TSRA will
develop from the sea breeze Monday afternoon and evening.
Should any direct impacts occur, then temporary flight 
restrictions can occur from 19Z to 01Z, mainly at KSAV. For now 
those chances are too minimal to include with the latest TAF 

Extended Aviation Outlook: Flight restrictions are expected as 
a cold front approaches the region on Tuesday, moving through 
Tuesday night, then stalling off the coast and maybe lingering 
into at least Friday.


Rest of tonight: The coastal waters will remain situated 
between high pressure across the western Atlantic and a trough 
of low pressure inland. The pattern will result in a fair 
pressure gradient across the local waters with SW winds topping
out in the 15-20 kt range for much of the night, including in 
the CHS Harbor.While some occasional gusts near 25 kt will occur,
no advisory is necessary. Wind speeds should slowly decrease to
the 10-15 kt range late tonight into daybreak hours. Seas will 
range between 2-3 ft.

Extended Marine: The coastal waters will remain between an 
inland trough and Atlantic high pressure Monday and Tuesday. 
Winds should be from the S to SW at that time, with higher gusts
in the afternoon/evening. A cold front will move through the 
region Tuesday night, which will allow winds to ease and turn in
direction. The front is forecasted to stall off the coast and 
linger into at least the weekend. Otherwise, Small Craft 
Advisory criteria is not expected to be reached.