Area Forecast Discussion
Issued by NWS Charleston, SC
FXUS62 KCHS 300053
Area Forecast Discussion
National Weather Service Charleston SC
853 PM EDT Thu Sep 29 2016
A cold front will stall along or just off the coast tonight. The
front will linger near the southeast coast through the weekend and
into early next week. High pressure will build into the region
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
A large and deep gyre spinning over Kentucky and covering most of
the nation east of the Mississippi River tonight is driving plenty
of dry air across the Deep South and Southeast states, currently
well depicted on water vapor imagery. While this drier air is
penetrating into most sections west of I-95 in and near our CWFA,
there is still a SW flow both surface and aloft east of there
which continues to advect just enough moisture into the region to
produce a chance of showers and t-storms. Given that a surface
cold front, still located just inland from our forecast zones,
that will be slow in moving into the area given that it is
aligned somewhat parallel to the flow aloft, we are still showing
20-30% PoP in many sections, with even some 40-50% PoP from
Hampton County south to Bryan and inland Liberty County prior to
midnight. This is due to interaction of lingering meso-scale
boundaries, plus a little shear and where the best MUCAPE is
found. Coverage will diminish overnight, but we still have 20%
PoP near and east of I-95 in SC in proximity to the approaching
front, and subtle isentropic lift in conjunction with the best
low level moisture.
Temps are the most problematic aspect of the forecast tonight, as
a S-SW flow east of I-95 will persist until after midnight, even
to as late as 5 or 6 am in Charleston County, while winds veer
around to the W-NW further inland. This will generate a large
thermal gradient ranging from the lower and middle 60s west of
I-95, with even the risk for a few upper 50s far interior SE GA,
to as high as the upper 60s along the GA coast and lower 70s
across the Charleston barrier islands.
.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
Friday and Friday night: The mid and upper level pattern will
continue to feature a large and deep closed low over the Ohio
Valley. The low will sit nearly stationary through the day and then
begin to meander back northward overnight. At the surface, weak
secondary low pressure will sit over the forecast area as the main
surface low spins just south of the Great Lakes. The residual front
will continue to reside just offshore, as will the deep moisture and
convergence along it. Several models show a few showers brushing the
far eastern portion of Charleston County in the morning with the
rest of the day being dry. In fact, much drier air is expected to
work into portions of the forecast area, with dewpoints across some
locations in southeast Georgia falling into the low 50s. It will be
a few degrees cooler than Thursday, with highs generally in the mid
80s. Overnight, more isolated showers could brush coastal areas of
Charleston County but otherwise the forecast is dry. Expect another
tight low temperature gradient with mid to upper 50s possible inland
ranging to 70 at the beaches.
Saturday through Sunday: For the weekend, the upper pattern will
continue to feature cyclonic southwesterly flow as the upper low
drifts north into the Great Lakes and eventually east. The surface
pattern will remain somewhat disorganized, with just weak low
pressure sitting over the area. Similar to Friday, the remnants of
the front will be offshore, though it may try to wobble back toward
the coast at times. The forecast features low PoP`s all right along
the coast. Inland areas will be dry. Temperatures will begin to warm
back up into the mid and upper 80s by Sunday.
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Very uncertain long term forecast with no truly dominant feature
through the period. The front near the coast should essentially
deteriorate overhead as high pressure over eastern Canada gradually
veers the flow to Northeast and then onshore as it moves closer to
the Maritime Provinces through the first half of the week. The
track and speed of Matthew is still highly uncertain, but the
effects of the storm, mainly the gradient increasing along the coast
and building swell filling in along the beaches could begin to be
felt locally by mid-week. Refer to the National Hurricane Center for
the latest information on TC Matthew.
.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
We`ve had to make adjustments to KSAV to account for scattered
SHRA/TSRA that will impact the terminal this evening. We`ll then
need to see if the activity survives to impact KCHS overnight. Low
end chance of a little ground fog and/or stratus around 10-13Z
Friday. Otherwise VFR at both airfields.
Extended Aviation Outlook: VFR conditions will prevail.
This Evening and Tonight: A cold front will slowly approach the
waters late this evening, then begin to shift over parts of
nearshore waters overnight. We could see a brief enhancement of
winds/seas as the fropa nears, but conditions are expected to
remain well below Small Craft Advisory levels. In general, south
winds will peak between 12-18 kt and gusty through early tonight,
then shift to the southwest as the front nears the coastal waters.
Seas will range between 2-3 ft with 20 nm and 3-4 ft on the outer
Friday through Tuesday: Conditions across the local waters will
generally be quiet as the remnants of a cold front remain situated
over or near the area. Winds will generally be 15 knots or less with
seas 2 to 3 feet.
Winds may begin to strengthen early next week as high pressure
builds inland, but details are still uncertain at this point as
there is still considerable disagreement amongst the models with the
handling of TC Matthew, which could begin to bring elevated seas and
contribute to an enhanced pressure gradient locally by the middle of
next week. Refer to the National Hurricane Center for the latest
information on TC Matthew.