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Area Forecast Discussion

492
FXUS62 KCHS 280748
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
348 AM EDT TUE JUN 28 2016

.SYNOPSIS...
A cold front will approach from the northwest today, becoming nearly
stationary and lingering nearby Wednesday and Thursday. The front
will lift back north on Friday as Atlantic high pressure builds
across the region. Another cold front will approach the area early
next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The overall pattern today will favor a more active afternoon
and evening with increased coverage of showers and thunderstorms.
Aloft, the persistent mid/upper level ridge will break down and
dissipate as shortwave energy approaches from the northwest. This
shortwave energy is on the southern periphery of a sharp upper
trough that will move through the Great Lakes region today. As the
larger scale flow becomes more cyclonic and the shortwave energy
approaches, it will help to push a cold front into the area from the
northwest this afternoon and evening. Moisture will pool along and
ahead of this front, and when combined with the aforementioned upper
support and low level convergence along the boundary will result in
at least numerous coverage of showers and thunderstorms this
afternoon and evening. The models seem to be in pretty good
agreement that initiation will take place to the north and northwest
across the Midlands, and will then propagate southward into the
area. The main time period for thunderstorms will be from roughly
around 2 pm for inland locations to around 8-9 pm along the coast.
Confidence is high that most, if not all, locations will see
measurable rainfall today. But pops have been capped at the 70
percent range due to uncertainties with regard to exactly when the
storms develop and how quickly the proceed southward. POP
adjustments will likely need to be made through the day as radar
trends become evident.

Regarding impacts, the main threat for today appears to be locally
heavy rainfall. Precipitable water values surge to in excess of 2
inches and the presence of embedded heavy thunderstorms could result
in rainfall totals of 1-2 inches. It appears that these storms will
have enough forward speed to preclude a significant flooding threat,
but we could still see isolated areas with locally excessive
rainfall that produces minor flooding issues. Given the expected
movement, a mention in the Hazardous Weather Outlook is not needed
at this time. Finally, regarding the severe potential, the forecast
area is in a general thunder from the Storm Prediction Center. Model
soundings do not reveal a particularly conducive atmosphere given
the deep moisture and lack of steep mid level lapse rates. However,
one thing to keep an eye on is if these storms can get any
organization as the form across the Midlands and put out much
leading outflow. The severe threat is certainly on the low side
given the marginal thermodynamics and modest CAPE values of 1000-
1500 J/kg, but we could still see an isolated strong to marginally
severe storm or two.

Tonight: In the evening the bulk of the ongoing activity is expected
to focus along the coast and be on a diminishing trend with the loss
of diurnal heating. Still, with the front in the vicinity, shortwave
energy aloft, and deep moisture, isolated to scattered showers and
storms could continue for much of the overnight. POPs feature a
diminishing trend but we do maintain mid to high range chances for
many areas through the period. Lows will fall into the low to mid
70s.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
Wednesday: An amplified and broad large scale pattern will exist
aloft across the country, featuring a strong anticyclone near the
Four Corners in the west, and the eastern trough that stretches South
into the Gulf of Mexico. At the surface we find an undulating cold
front near or over our north and northwest zones. An abundance of
moisture will prevail within a deep southwest feed of sub-tropical
air evidenced by Pwat that is near or in excess of 2 inches. Throw
in some weak upper divergence and subtle mid level perturbations,
convergence and large scale forcing for ascent from the proximity to
the cold front and the typical sea breeze boundary, and an elevated
convective rain chance will be prevalent. Our gridded forecast
depicts 40% chances of showers/t-storms north and north to 50% south
and southeast. The overall thermodynamics are modest at best, but
with some 20-25 kt of bulk shear in the 0-6km layer and MLCAPE
upwards of 1500 J/kg, there could be a low end risk for isolated
marginally severe storms. Probably a bigger concern will be the
potential for locally excessive rains with weak storm motion and the
high Pwat. Maximum temps will be held somewhat in check by the
higher rain chances, generally in the upper 80s or near 90.

Thursday: A broad and large scale trough will persist in east and
southeast, while a Piedmont trough and a rather weak cold front
wavers nearby. The proximity to the front and trough, the sea breeze
boundary, plus forcing for ascent due to subtle impulses aloft and
some divergence in the upper atmosphere will support yet another day
with higher than typical convective rain chances. We again have 40-
50 Pops across the entire CWFA, and locally heavy rains are again a
problem since storm motion is less than 10 kt and Pwat is up near
the 90th percentile for this time of year. There may also be a
slightly greater risk for isolated severe storms with better
instability and DCAPE on the order of 1200-1600 J/kg. With the high
moisture content this may allow for a few marginally severe storms
with wet downbursts. Temps will be near or a tad above normal.

Friday: The flow aloft starts to flatten and becomes quasi-zonal
with small height rises to occur locally by late in the day. The
cold front will have either dissipated or lifted far to the north,
allowing for sub-tropical high pressure to build from the Atlantic.
While there is the lack of any significant forcing, given the sea
breeze and the Piedmont trough still in place and still plenty of
deep moisture, scattered Pops are in order. With the higher heights
and an expansion of low level thickness, temps will reach at least
into the lower 90s before storms develop.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
The weekend will feature more typical summer weather, with the
Bermuda-Azores High and Piedmont trough to encompass the synoptic
pattern. Zonal flow aloft and mid level heights up near 5920-5940
meters will limit the amount of convection both Saturday and Sunday
to generally 30 or 40 percent. Low level thickness and 850 mb
temps are a little above normal, so max temps will peak around 93-
95F. With dew points in the lower and middle 70s will generate heat
indices as great as 103-105F each afternoon, but well below our July
1st Heat Advisory criteria of 110F or greater heat index.

An upstream cold front will attempt to approach early next week, but
the flow aloft stays fairly zonal, which indicates that the front
will struggle to get this far south. For that reason we won`t go any
higher than 30-40 Pops for Independence Day and next Tuesday.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
The main forecast problem for the next 24 hours revolves around
the potential for direct impacts from thunderstorms this afternoon
and evening. A cold front will approach the area and showers and
thunderstorms are expected to develop to the north. These showers
and storms will then move southward and near the terminals by the
afternoon. Confidence is relatively high that both sites will see
a thunderstorm at some point. The time period for KCHS appears to
be 20z through 00z and the time period at KSAV appears to be 22z
through 02z. Have introduced a TEMPO group for TSRA with just MVFR
conditions for now. At least brief IFR conditions will be possible
as will some low end wind gusts. Once the storms push south and
dissipate overnight, there will be the potential for fog and/or
stratus development. The best chances for fog and/or stratus
appear to be beyond the current TAF window.

Extended Aviation Outlook: Flight restrictions are possible at least
periodically in SHRA/TSRA Wednesday through Thursday as a cold
front lingers nearby. While there are less chances for direct impacts
Friday and Saturday, there still does remain the potential for
SHRA/TSRA.

&&

.MARINE...
Today and tonight: A cold front will approach from the north and
northwest today and then stall in the vicinity of the coast
overnight. Winds will primarily be south to southwest through the
period, generally 15 knots or less. Seas will range 1-3 feet,
highest from 20 nm and beyond. We could see thunderstorms approach
the coast this evening and even push out into the waters tonight. As
such, mariners should be prepared for possible thunderstorms and
lightning and gusty winds.

Wednesday through Thursday: An oscillating cold front will be
situated near or just north of the marine community during the
middle of the week, with the Atlantic ridge suppressed across
Florida. A fairly light pressure pattern will prevail, although with
occasional small surges due to the sea breeze circulation and
nocturnal jetting, winds will be up near 15 kt at times. Seas for
the most part will average 2 or 3 ft through the period.

Given a rather slackened wind field both Wednesday and Thursday
morning, and the close by cold front and the expectation of
convergent cloud lines, there might be an enhanced risk for
waterspouts. There is also the potential for greater coverage of
showers and t-storms than what is typical.

Friday through Sunday: the Atlantic ridge will become the dominant
system, although inland a Piedmont trough will prevail. A general
southwest synoptic flow will prevail at or below 15 kt, with seas 3
ft or less.

&&

.TIDES/COASTAL FLOODING...
The upcoming Perigee occurs early on Friday, and the New Moon
occurs on Independence Day. This could allow for another bout of
"King Tides" and we`ll see if enough departure will occur to
generate shallow coastal flooding late this week into early next
week.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$

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



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