Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 241512
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1112 AM EDT Wed May 24 2017

.SYNOPSIS...
Unsettled weather will persist across the Southeast through
today. A cold front will push offshore tonight into Thursday
followed by high pressure prevailing into the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1110 AM: I will update to add mention of Tornado Watch 263
to the forecast products. Watch in effect until 6 PM.

As of 10 AM: Visible satellite showed a large area of mostly to
partly sunny conditions across the SC coastal counties early
this morning. Satellite trends show debris cirrus sliding over
the CWA over the next hour or two. Temperatures have already warmed
into the upper 70s to near 80 across the areas of thin cloud
cover. Upstream: A large area of strong to severe thunderstorms
was across northern FL and southern GA. Latest W/V showed a deep
mid level trough sweeping across the deep south. GFS1deg H5
suggests that deep forcing associated with moderate Q-vector
convergence will develop ahead of the trough. Regional radar
already indicates that thunderstorms have started to develop
across southern GA, north of the convection lifting from
northern FL.

Likely scenario: New convection over southern GA will grow
upscale during the morning hours, while tracking NE. CAPE values
across the forecast area will increase to 2000-3000 J/kg by the
early afternoon hours. Convection should easily develop and
increase in coverage across SE GA by mid day, aided by synoptic
scale forcing, widespread instability, and shear. Large clusters
of storms are expected to pass over the southern Savannah River
valley around 1 PM, pushing across the SC Lowcountry through the
rest of the afternoon into the early evening. Embedded
supercells will be expected as the storms push over SE GA/SC. In
addition, DCAPE values are expected to increase above 1000 J/kg
across the region this afternoon and early evening. Severe
weather will consisted of tornadoes, damaging winds with
organized convective features, and large hail. The potential for
tornadoes appears the highest across the coastal areas across
the lower Savannah River Valley. Damaging winds and hail will be
possible across SE GA/SC, highest potential across the SC
Lowcountry.

Previous forecast:
Today: The forecast scenario is very complex and confidence
isn`t very high regarding the timing and exact placement of
convection. A well defined mid/upper level low will begin the
day over Missouri and is progged to gradually sink south and
eastward to a position near west Tennessee by the evening. As
it does, the trough on its southeast periphery will take on a
more neutral/negative tilt with notable shortwave impulses
poised to impact the Southeast region this afternoon and
evening. As the upper low translates through the day, an area of
low pressure at the surface will develop and steadily deepen as
it lifts into the Ohio Valley. Overall, the atmosphere today
will be quite similar to Tuesday. The forecast area will be
within an uninhibited warm sector with precipitable water values
approaching 2 inches by the afternoon. The main forecast
question for today will be what areas will be favored for
convection and will this convection be severe. Regarding favored
areas, that is pretty hard to ascertain. The atmosphere is
fairly uniform across the forecast area and a
shower/thunderstorm will be capable of developing pretty much
anywhere at any time. The best dynamics and forcing for ascent
will spread in from the southwest in the afternoon, so the PoP
scheme shows values increasing in southeast Georgia first.
However, 80-90 percent chances are in place everywhere.

Regarding the severe potential, it certainly looks like a good
setup with an uncapped warm/moist atmosphere in place,
coincident with increasing deep layer shear and mid/upper level
dynamics. As the aforementioned shortwave energy spreads in this
afternoon, a north/south oriented 300 mb jet will strengthen to
around 120 knots with a strong divergence signal in its right
entrance region. This upper level forcing should help make up
for any lack of destabilization we experience from widespread
cloud cover and minimal insolation. Model soundings show the
wind field increasing nicely in the afternoon as well and it
appears the best severe threat will come as the 70+ knot 500 mb
jet noses in from the southwest in the mid/late afternoon hours.
Around this time, model soundings and plan view feature CAPE
values of 1500-2000 J/kg and 0-6 km shear values around 50
knots. The wind field is essentially unidirectional, but the
combination of the strengthening magnitude and impressive
indices will result in a scenario similar to Tuesday. The near
storm environment should favor multicells and some broken
convective bands, with the potential for some embedded supercell
type structure. Damaging wind gusts are the main threat, but
given the magnitude of the shear we will once again see the
potential for a few isolated tornadoes. Thunderstorms will be
possible at any time, but the main time period for severe
weather will be from roughly 3-9 pm. There are certainly some
caveats regarding the threat, mainly the impact of convective
overturning from Tuesday and the impact of early morning
convection. However, the environment should be able to
sufficiently recover, especially with a solid replenishment of
warm/moist air. Locally heavy rain is again expected today, but
we don`t think widespread flash flooding will be an issue.

Tonight: Convection will likely reach its peak coverage and
intensity in the evening, then become increasingly concentrated
along the coast in the first portion of tonight. Current
thinking for timing is that the by midnight the bulk of the
convection will be off the coast and the severe threat will be
greatly diminished. PoP`s steadily decrease from the west
through the night, such that by sunrise Thursday only slight
chance PoP`s are in place. Temperatures will cool off late and
we could see some low 60s well inland for lows.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
Thursday: The upper level trough axis will cross the forecast
area mid-day as stacked low pressure continues its trek
northeast into the mid-Atlantic region. Surface high pressure
will begin to spill back in from the south and Gulf of Mexico
behind the cold front as weak upper-level ridging slides slowly
east. This should result in primarily dry conditions Thursday,
but a shower or thunderstorm cannot be entirely ruled out. A
tight pressure gradient will exist across the region, leading to
elevated and gusty surface wind speeds. Temperatures look to be
slightly below normal, with highs just under 80 degrees most
locales and low temperatures averaging around 60 inland and
around 65 at the coast.

Friday: Surface high pressure will overspread the region under
weak ridging aloft, continuing a period of calm weather with
almost zero probability for precipitation and mostly sunny skies
anticipated. Near-normal temperatures are anticipated with
highs reaching the lower to mid-80s before cooling into the mid
to upper 60s overnight away from the coast.

Saturday: Strengthening trough aloft to the west will induce an
area of low pressure at the surface over the central US. The
associated cold front will begin a slow trek southeast towards
the forecast area. High pressure at the surface will persist
locally under a ridge aloft, and another dry day looks highly
probable under this scenario. Temperatures climb back above
normal, aided by subsidence and some compression in advance of
the front. Expect high temperatures to reach the lower 90s away
from the coast and upper 80s at the beaches, with low
temperatures only dropping to around 70.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
A ridge over the East Coast combined with surface high pressure
will result in mostly dry weather through the remainder of the
weekend. The surface cold front will approach the area through
the first half of the week as the trough aloft slides east,
bringing the potential for another period of unsettled weather
Monday through Wednesday.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
Shower and thunderstorm activity is very limited this morning
and it appears that the bulk of development will take place
later this morning and into the afternoon. No change in thinking
regarding late day thunderstorms. The forecast still features a
TEMPO at both sites for low end MVFR conditions in thunderstorms
for the late afternoon/early evening time period. Given that the
environment will support development at almost any time, we
could see impacts outside of the TEMPO, but that is the most
likely time. Then overnight, any lingering showers and
thunderstorms will push offshore and VFR conditions will return.


Extended Aviation Outlook: Expect VFR conditions to prevail
into the weekend.

&&

.MARINE...
Today and tonight: Poor conditions will continue across the
local waters through tonight. As a surface low strengthens
inland across Tennessee, the pressure gradient will tighten and
south to southwesterly winds will remain elevated. Most of the
waters will be in the 20-25 knot range through the day thanks to
the gradient. In fact winds look sufficiently strong today to
support expanding the Small Craft Advisory to include Charleston
Harbor through midnight. Seas will steadily increase thanks to
the persistent winds. By late today, seas will become generally
4-6 feet out to 20 nm and 6-8 feet beyond. By late tonight, seas
could be up to 8 feet near 20 nm and up to 10 feet in the outer
waters. One thing to keep an eye on is this evening there could
be a window of time where gales occur across the Charleston
County waters and the outer Georgia waters. Confidence is too
low to introduce a Gale Warning however. Also, another round of
strong thunderstorms are expected to develop and impact the
waters beginning late today. Wind gusts in excess of 35 knots
and waterspouts will be possible. The main time period for
strong thunderstorms will be from around 4 pm through midnight,
however thunderstorm development will be possible at any time.

Thursday through Sunday: Tight pressure gradient behind the
surface cold front will lead to elevated and gusty SW to WSW
winds and increasing seas. A Small Craft Advisory has been
hoisted for all zones Thursday, with conditions gradually
improving Friday into Saturday under high pressure.

Rip currents: Strong southerly to southwesterly winds combined
with increasing astronomical influences will result in a
moderate risk of rip currents at all beaches.

&&

.TIDES/COASTAL FLOODING...
Evening high tide levels will trend higher each day as we
approach the new moon and perigee this Friday, though we expect
tides to remain just below Coastal Flood Advisory stage. The
bigger issue is if heavy rainfall occurs around the time of high
tide this evening, in which case more significant street
flooding would be possible due mainly to freshwater.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until midnight EDT tonight for AMZ330.
     Small Craft Advisory until 8 PM EDT Thursday for AMZ352-354.
     Small Craft Advisory until midnight EDT Thursday night for
     AMZ350-374.

&&

$$
NEAR TERM...NED
SHORT TERM...JMC
LONG TERM...JMC
AVIATION...BSH/JMC
MARINE...BSH/JMC
TIDES/COASTAL FLOODING...


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