Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 230600
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
100 AM EST Thu Nov 23 2017

.SYNOPSIS...
High pressure will persist through this evening. A series of
low pressure systems will track from the northern Gulf of
Mexico up the Southeast coast tonight through Friday night. A
cold front will shift through the area Saturday night, followed
by dry high pressure through the middle of next week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
As of 1 AM: Sfc low pressure is expected to develop near the
Big Bend of Florida early this morning, as high pressure
remains centered over the DelMarVa region. The axis of a mid
level trough is forecast to slide east through the day, reaching
the SC/GA coast late this evening. As the trough pushes east,
low pressure is expected to track northeast off the Southeast
coast. It appears that a compact rain shield will exist
northwest of the low center, with the greatest rainfall rates
remaining off the coast. I will update the forecast to adjust
sky, PoPs, and temps.

Previous Discussion:
No major changes were made for the late evening update. Did
nudge lows up a few degrees across the far south where a
developing/strengthening onshore flow and thickening cloud cover
will limit potential cooling sooner than other places.

Isentropic assent ahead of weak low pressure developing across
the northeast Gulf of Mexico will be slow to increase across the
region tonight. Best chances of showers prior to daybreak will
be across the far southern zones, roughly along and south of a
line from Reidsville to Savannah and Hilton Head. Pops in this
area will range from 20-40%, highest around the Darien area. Did
opt to delayed the onset of rain by a few hours based on the
latest output from the various high resolution models. Other
than adjusting hourly pops slightly, adjustments were made to
hourly temperatures and dewpoints based on 22/22z observations.
Also increase coastal winds just a tad, especially in the more
exposed Tybee Island area where breezy conditions will develop
early Thursday.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH SATURDAY/...
A large mid-lvl trough of low pressure will slowly advance east over
the East Conus Thursday with an embedded h5 shortwave near its
southern extent over the Gulf of Mexico. This shortwave will help
spawn an area of low pressure over the western Gulf of Mexico by
Thursday evening, before it tracks northeast and gradually deepens
under the right rear quadrant of an upper lvl jet off the Atlantic
coast. The amount of precip coverage to the area will be highly
dependent on how close the area of low pressure passes along/near
the Southeast Coast Thursday night into Friday. At this time, most
guidance suggests a slightly slower and more offshore track than
previously indicated. Given this trend, greatest precip coverage
should remain over coastal counties Thursday evening and night when
isentropic lift peaks and PWATs approach 1.25 to 1.5 inches. Precip
should gradually shift off the coast Friday as the h5 shortwave
reaches the Southeast coast and the low pressure system continues
advancing to the northeast. Conditions will then remain dry on
Saturday as high pressure builds over the area ahead of a dry cold
front approaching from the north late.

High temps will struggle to reach 60 degrees on Thursday due to
increasing cloud cover, showers and a northerly flow. On Friday,
temps should rebound into the lower 60s once precip clears the area
from the southwest to northeast. By Saturday, temps will be
noticeably warmer. Saturday afternoon highs should approach the
upper 60s to around 70 degrees in advance of the cold front arriving
late Saturday night. Overnight lows will generally range in the
lower/middle 40s inland to upper 40s/lower 50s near the coast each
night.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Dry high pressure will prevail Saturday night through the middle
of next week. Cooler temps will be the rule Sunday into Monday
due to cold air advection, but a quick warm-up occurs mid-week
as the surface high shifts offshore.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
Sfc low pressure is expected to develop near the Big Bend of
Florida early this morning, as high pressure remains centered
over the DelMarVa region. The axis of a mid level trough is
forecast to slide east through the day, reaching the SC/GA coast
late this evening. As the trough pushes east, low pressure is
expected to track northeast off the Southeast coast. It appears
that a compact rain shield will exist northwest of the low
center, with the greatest rainfall rates remaining off the
coast. However, MVFR ceilings and rainfall should pass over the
terminals during the daylight hours. KSAV is expected to see
MVFR ceiling for much of this afternoon. The lower ceilings and
rainfall will track away from the region this evening, returning
conditions to VFR. Steady NE winds are expected through the 6Z
TAF period.


Extended Aviation Outlook: An extended period of MVFR/IFR conditions
is expected late Thursday night into Friday night due to showers and
low clouds associated with low pressure tracking along/near the
Southeast coast. VFR conditions should prevail at both terminals
Saturday through Monday.

&&

.MARINE...
Tonight: High pressure to our north will persist through this
evening. At the same time, a system to our south will gradually
approach the region. The interaction between these features will
increase the pressure gradient, allowing gusts to approach 25
kt. As a result, seas will build. Seas are forecasted to be 4-5
feet within 20 nm after midnight. The wave models hint that the
extreme northeastern portion of the Charleston waters could
have 6 foot seas. Given the small areal coverage and low
confidence in this occurring, we opted to not issue a headline
for this zone at this time. But for the GA waters beyond 20 nm,
waves of 5-6 feet are expected, so we issued a Small Craft
Advisory.

A northeast flow will persist Thursday and Friday between high
pressure inland and a low pressure system tracking along/near the
Southeast coast. A tightened pressure gradient over the area favors
the continuation of a Small Craft Advisory for offshore Georgia
waters into Friday night with winds gusting around 25 kts and seas
building as high as 5-8 ft. Conditions could also approach marginal
Small Craft Advisory criteria in nearshore waters Thursday or Friday
depending on how close the low pressure system tracks along/near the
coast. High pressure will then build over the coastal waters this
weekend, favoring northeast winds to become primarily offshore. Wind
speeds and seas are expected to improve and remain below Small Craft
Advisory levels through the weekend.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 3 AM EST Saturday for AMZ374.

&&

$$
NEAR TERM...NED
SHORT TERM...
LONG TERM...
AVIATION...NED
MARINE...



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