Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 291124

Area Forecast Discussion
National Weather Service Charleston SC
724 AM EDT Fri May 29 2020

A trough of low pressure will linger inland through Saturday,
before a cold front crosses the area early Sunday. High
pressure will return early next week and prevail through the
middle of the week.


Today: The northern and southern branches aloft will come more
into phase as one short wave dropping through the western Great
Lakes merges with another short wave that shifts into the Ohio
and Tennessee Valley. This results in a deep southwest flow
across the local area, maintaining a steady supply of rich
tropical-like moisture over the Southeast where PWat is near or
above 2 inches or about 2 standard deviations above normal for
late May. At the surface we find a diffuse cold front or
surface trough just outside the region to the northwest, while
the subtropical Atlantic ridge extends across the Florida

Convection is already occurring in and near the forecast counties,
resulting from a confluence zone across the ocean and also in
close proximity to the inland front/trough. This will generate
isolated to scattered showers and t-storms between now and 11
am, with the best chances across northern Charleston County.

The afternoon looks to be quite active and unsettled, with mid
level impulses to ride through, coinciding with a 25-30 kt 700
mb speed max, lower heights aloft and various meso-scale
boundaries. Given MUCAPE of 1500-2500 J/kg and the HREF
depicting vigorous updraft potential, we are showing numerous to
widespread convection between about 1 pm and 5 pm. Heavy rain is
a certainty given the excessive moisture and low level inflow in
advance of the diffuse front. Given the antecedent wet grounds
there will be a concern for flooding rains. And with high tide
around 2 to 3 pm, the potential would be further aggravated
near and along the coast, plus in downtown Charleston. Flood
Advisories are likely over parts of the area, and this goes
along with the WPC thinking of a Slight Risk of excessive
rainfall this afternoon. The HREF guidance 6 hourly rainfall
amounts are on the order of 1/4 to 1/2 inch through 18Z, then
another 1/2 to 1 inch from 18-24Z. But undoubtedly locally
heavier rainfall will occur.

Since there is too much cloud cover, lapse rates are meager and
deep layer shear is no more than about 20 kt, the potential for
severe weather is very low. Perhaps an isolated wet microburst
is possible where boundary interactions occur, but that is more
the exception than the rule.

We`re off to an exceptionally warm start to the day, so we`ll
reach the middle 80s before the greater coverage of convection
breaks out. Our hourly grids for this afternoon show temps
dropping into the lower and middle 70s once the convection
becomes more numerous to widespread, but adjustments will likely
be required in this regard in later updates.

Tonight: Another cold front that is found pushing toward the
Appalachians early on, will approach the area late as it gets a
push from an elongated vort lobe that crosses the mountains
overnight. The atmosphere will have been so worked over from
previous daytime convection that due to the loss of heating and
instability, our shower and t-storm chances will diminish fairly
quickly this evening. However, given some QG forcing and the
approaching cold front, plus low level convergence from off the
Atlantic, we do hold chance PoP the balance of the night. Temps
will be several degrees above climo. Not seeing much chance of
fog, but given the risk of stratus and the wet grounds, fog
remains a low end probability.


A cold front approaching from the west on Saturday will get hung up
inland, before finally moving offshore early Sunday. Aloft, a mid
level trough axis will linger over the East Coast with shortwave
energy progged to pass through the area. Plenty of moisture
available with additional lift from the sea breeze will provide
another fairly active convective day. Models show moderate
instability developing, however lack of substantial shear will limit
storm organization. Certainly could see a stronger storm or two, but
widespread severe weather threat is not expected at this time. High
temperatures will be near normal. Low temperatures Saturday night
will stay pretty mild.

Mid levels will begin to dry out late Saturday night/early Sunday
and rain coverage is expected to decrease. Although, with the return
of solar insolation on Sunday and another weak wave of vort energy,
isolated to scattered showers and thunderstorms are possible on
Sunday, mainly along the coast. Temperatures will be near normal.

High pressure will build into the region on Monday. Despite plenty
of sunshine, temperatures will be a few degrees cooler than normal,
peaking in the low to mid 80s. Humidity levels will also be much
more pleasant than recent days.


High pressure will shift into the Atlantic on Tuesday and then
remain the dominant feature into Thursday, although troughing will
develop inland. Aloft, ridging will encompass much of the southern
half of the country. Models are in good agreement showing a quiet
period with minimal rain chances. Temperatures will gradually
warm as return flow sets up and heights build aloft.


Convection will get an early start during the middle and latter
part of this morning and then increase in coverage during the
afternoon, the combination of a nearby cold front, energy aloft,
considerable moisture and ample instability. Expect numerous to
widespread SHRA/TSRA to impact the area from about 16-17Z
through 22-23Z. Flight restrictions will occur much of that
time, with the latest forecast showing MVFR conditions. However,
IFR is certainly possible.

Convection will fade during the evening, but there could still
be some SHRA/TSRA around through the night.

Extended Aviation Outlook: Periods of low clouds, showers, and
thunderstorms will lead to occasional flight restrictions through
Saturday night.


Today and tonight: The coastal waters will be situated between
high pressure to the east and southeast, with lower pressure
over land to the west and northwest. Outside of scattered
convection, winds will average S or SW at or below 15 kt, while
seas for the most part are no more than 3 or 4 feet.

Saturday through Wednesday: South to southwest winds will prevail on
Saturday. A cold front will pass through early Sunday, with winds
veering to the northeast in the afternoon. Wind speeds and seas will
increase Sunday night into Monday as high pressure shifts into the
region, and Small Craft Advisories are possible, especially for the
outer Georgia waters. Conditions will improve Tuesday as the high
drifts into the Atlantic. Winds will increase again on Wednesday as
the pressure gradient becomes enhanced.




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