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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 212347

Area Forecast Discussion
National Weather Service Charleston SC
747 PM EDT Sun Jul 21 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. 


This Evening and Tonight: The area will remain situated between high 
pressure over the western Atlantic and an inland trough of low 
pressure. Most areas are expected to remain dry through the evening 
hours, but a few showers and/or isolated thunderstorm can not be 
ruled out along a slowly inland moving seabreeze for the next couple 
hours. Further south, h5 shortwave activity will ripple west-northwest
across northern Florida/southern Georgia along the southern periphery
of the ridge over the Atlantic, helping promote showers and 
thunderstorms near the Altamaha River and into Southeast Georgia
where sufficient lifting associated with the shortwave interacts
with the slowly inland moving seabreeze. Ample instability characterized
by MLCAPE around 2000 J/kg, PWATs near 1.75 inches and DCAPE around
1100 J/kg could support a stronger thunderstorm or two, but convection
should be limited in coverage and should eventually display a 
weakening trend heading into sunset given a lack of significant 
shear and loss of diurnal heating. Any convection remaining at 
sunset is expected to dissipate within 1-2 hours. The rest of 
the overnight is expected to be quiet/dry. A light southwest 
flow will prevail as the forecast area continues to sit between 
the inland trough and the high across the western Atlantic. Lows
are forecast to fall into the mid 70s in most areas, with upper
70s along the coast.


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 00Z Tuesday. An isolated shower/thunderstorm can not be
ruled out at either terminal (especially at SAV) late 
afternoon/early evening Monday, but confidence is too low to 
include in the latest 00Z taf issuance.

Extended Aviation Outlook: Brief flight restrictions are 
possible in afternoon/evening showers/thunderstorms on Monday. 
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.


This Evening and 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 this evening with south to 
southwest winds topping out in the 15-20 kt range, including in the 
CHS Harbor. Wind speeds should slowly decrease to the 10-15 kt range 
late tonight and toward daybreak hours. Seas will range between
1-3 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.