Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 191443

Area Forecast Discussion
National Weather Service Charleston SC
1043 AM EDT Sat Aug 19 2017

A weak cold front is forecast to slowly slide over eastern
South Carolina and Georgia this weekend, likely becoming
stationary over the region on Sunday. Broad low pressure is
expected to move inland from the western Atlantic on Monday.
Summertime weather should result from ridging high pressure
Tuesday and Wednesday. A stronger cold front is timed to pass
over the region on Thursday, likely followed by a slightly
cooler and drier airmass through the rest of the week.


No major changes with this update as the forecast remains on

Today: An interesting pattern aloft features a TUTT low moving
W-NW through the SE Gulf, an embedded short wave to pass through
the OH valley within the westerlies just to our N, and
anticyclones in the SW Atlantic and over the southern Great
Plains. At the surface the sub-tropical ridge is suppressed to
the S-SE, as the aforementioned short wave bumps a weak cold
front near or just upstream from the NW zones.

While the mid levels remain warm and there is still some
evidence of a subsidence cap, forcing from the nearby front and
most especially the sea breeze will allow for better coverage of
showers and t-storms than yesterday. Isolated activity will form
due to the sea breeze, near the US-17 corridor, as we reach our
convective temps around 90F by 12-1 pm. Coverage than increases
further into the scattered range during the course of peak
heating through mid afternoon. This occurs mainly E of I-95 in
SC and areas just inland from the GA beaches to E of a line from
about Springfield to Hinesville and Ludowici.

Probabilities are capped at 30-40%, but with 15-20 kt of 0-6 km
bulk shear, there is a risk of a few multi-cellular clusters
developing. The overall risk for severe weather is quite low,
but with DCAPE 800-1200 J/kg there is some potential for strong
downbursts of wind where boundary collisions and/or mergers
occur. There is also the chance for locally heavy downpours
given PWat of at least 2 inches and weak storm motion.

Low level thickness and both 925 mb and 850 mb temps are similar
to or a bit less than Friday, and that along with more cloud
cover and higher convective rain chances will keep most places
from getting quite as hot as recent days. Still, lower and
middle 90s are forecast, and the hourly temp curve will need
some adjusting later in the day due to rain-cooling storms.

Far interior SE GA and places west of I-95 will experience a
mixing out of dew points this afternoon, but pooling of dew
points in the mid and upper 70s elsewhere will generate maximum
heat indices of 106-108F. This is shy of the requirement of a
Heat Advisory, but still at dangerous levels.

Tonight: Convection will fade quickly during the early and mid
evening, and the bulk of the night will be rainfree. However, a
light southerly flow from off the ocean and the resulting
convergence could allow for widely scattered late night showers
and t-storms moving onshore from the Atlantic. Some of the
guidance suggests late night fog, and with the previous expected
rainfall and some places reaching their cross-over temps this is
possible. But I think there will be just too much cloud cover
and condensation pressure deficits look a little too high. Thus
no mention of fog at this time. Temps will again remain far
above normal, but a little "cooler" than the past few nights
with more of a decoupled atmosphere.


Sunday: Short term guidance indicated that broad high pressure
centered over Bermuda will remain just off the SC/GA coast. Weak
flow under the H5 ridge should result in the weak cold front to
become nearly stationary near the coast on Sunday. In fact, llvl
flow is forecast to back, possibly resulting in the weak boundary to
drift inland as a sea breeze during the afternoon hours. This
pattern will likely favor highs in the mid to upper 90s across the
inland counties to the low to near 90 near the beaches. Forecast
soundings show a subsidence inversion around H725, favoring low PoPs
across the region. I will forecast generally isolated convection on
Sunday, favoring areas along the I-95 corridor.

Monday: Both the GFS and ECMWF indicate that the center of the H5
595 dm shifting west just off the NC coast by the afternoon. Short
range guidance shows a band of moisture associated with a tropical
wave rippling west over the forecast area during the daylight hours.
Forecast soundings show PW values rising above 2.2 inches by Monday
afternoon. In addition, soundings indicate multiple layers of clouds
during the afternoon hours, especially during the afternoon hours.
However, sfc flow across the high pressure should support winds from
the SE, favoring a sea breeze developing during the mid day.
Forecast soundings for points near the beaches indicate thinner
decks of cloud cover. It appears that areas east of the potential
sea breeze, near the beaches, will see the greatest potential for
viewing the solar eclipse. Sct thunderstorms will likely develop
over the land zones during the afternoon, with isolated coverage
expected over the marine zones. High temperatures are forecast to
range a few degrees below values reached on Sunday.

Tuesday: Typical summertime conditions are expected as the H5 ridge
shifts over the deep south and weakens slightly. I will forecast
high temperatures within a degree or two of normals. Thunderstorms
should generally follow a diurnal trend with sct coverage across SC
and GA.


Wednesday through Saturday: Broad high pressure will gradually
weaken across the region on Wednesday as a cold front slides across
the southern Appalachians by the afternoon. A cold front should
push across the forecast area on Thursday, likely supporting a band
of thunderstorms. High pressure sourced from Canada will remain
centered well north of the region through the rest of the week.
However, the forecast area should see slightly cooler and drier
conditions Friday night into the weekend. In fact, guidance supports
afternoon dewpoints in the upper 60s inland by Saturday afternoon.


VFR with no concerns through about 16-18Z, before isolated to
scattered SHRA/TSRA develops on the sea breeze. This boundary
will be near both KCHS and KSAV from around 19-23Z, and it is
during this time that brief flight restrictions can occur. For
now VCTS with CB clouds will suffice at both terminals during
the mid afternoon through early evening.

Extended Aviation Outlook: Thunderstorms may result in short periods
of flight restrictions, greatest potential during the afternoon and
early evening.


Today: The waters will be sandwiched between a weak cold front
inland over the SE that nudges a little closer and sub-tropical
ridging just to the S-SE. WSW winds will be 10-15 kt,
continuing that way through midday. Then, sea breeze
circulations will back winds around to the S or SSW and speeds
ramp up several knots again. Seas will average 2-3 ft.

Tonight: Once sea breeze influences fade this evening, the
pressure gradient slackens as the cold front attempts to nudge a
little closer. S and SW winds will hold at or below 15 kt (quite
a bit lower than recent nights), and seas remain in a steady
state around 2 or 3 ft.

Sunday through Thursday: Western Atlantic high pressure combined
with a broad inland trough will yield a weak pressure gradient over
the coastal waters through mid week. A cold front should sweep
across the forecast area on Thursday. Sfc winds are forecast to
remain light from the SE through Tuesday, then veering from the
south-southwest Wednesday and Thursday. Wave heights are expected to
generally range between 2-3 feet Sunday through Thursday.


Astronomical influences will lead to a round of elevated tides
into early next week. Only small tidal departures are necessary
and we could approach shallow coastal flooding levels with the
evening high tides through Tuesday along parts of the SC coast.


Record High minimums for 19 August...
KCHS 79/2010
KCXM 81/2009
KSAV 79/1878


The temperature and dew point sensors at the Downtown Charleston
observation site (KCXM) could periodically fail. Technicians
plan on fixing the problem.




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