Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
739 am EDT Monday Sep 22 2014
a cold front will advance through the region today and tonight
before stalling south of the area. High pressure will build in from
the north through next weekend while a trough of low pressure
strengthens near the coast.
Near term /through tonight/...
nearing-dawn...a strong short wave trough for this time of year was
digging south-southeast early this morning from the Missouri Valley region
toward the Tennessee Valley and upper Gulf Coast areas. A cold
front extended from north Georgia into the western Carolinas.
Warm air advection middle level clouds preceding the front into
the csra and midlands...some of these clouds seen over our inland
areas at 11z...mainly western and northern zones.
By midday the cold front could reach into our northwest tier of zones
north of I-16 and just west of the coastal corridor. A few showers
may push into our forecast area this morning northwest of Charleston and
Savannah but overall forecast soundings indicate a strong cap in
place during this time period.
This afternoon...the sharp upper trough with a slight positive tilt
will induce good middle level height falls across central and southwest
Georgia. Once deep layer forcing for ascent erases the capping
inversion this afternoon...models remain quite aggressive with
regards to rapid development and coverage of convective rains by
late afternoon. Model solutions have slightly better agreement
regarding the degree of instability which will pool ahead of the
front in space and time.
Our area of concern looks to be mainly along and south of I-16 in
southeast Georgia into southern far South Carolina perhaps into
coastal Beaufort and southern Charleston counties. The 00z NSSL arw
portrays a risk for a complex mesoscale convective complex or even
a short qlcs with an inherent threat for severe weather. The
window for timing appears to be in the 3 PM to 9 PM range today
with damaging thunderstorm wind gusts the main threat. We continue to
highlight at least an isolated severe weather potential within the
hazardous weather outlook this morning.
We made some changes to tonight/S forecast with the upper short
wave trough slowing and cutting off. We increased overnight rain
chances many areas after likely evening probability of precipitation over eastern and
southern zones. Lighter rains may linger through the night with
cool NE winds at the surface. Lows in the 60-65 range.
Short term /Tuesday through Thursday/...
the upper shortwave is expected to close off and stall over the
Carolinas Tuesday and Wednesday while a coastal trough strengthens
along the southeast coast. Although the GFS has come into better
agreement with the European model (ecmwf) regarding the upper low cutting off...the
European model (ecmwf) remains quite a bit wetter. The Euro appears a bit too bullish
with bringing the coastal trough inland by Wednesday morning. We
stuck closer to the GFS for this package...keeping fairly high rain
chances over the coastal waters with rapidly diminishing probability of precipitation as one
moves inland. We show a bit more inland coverage on Thursday to
account for the trough potentially shifting inland.
Cold advection with high pressure building in from the north and
extensive cloudiness due to isentropic ascent will yield a cooling
trend during the period. Highs on Tuesday should reach the lower 80s
but by Thursday readings will be in the middle 70s during the day.
Long term /Thursday night through Sunday/...
perhaps one of the longest high pressure wedge patterns is in store
for the medium to long range periods. The expansive surface high
over the northeast states looks to remain in place through Saturday
night while a coastal trough maintains considerable cloudiness...
better rain chances and cool temperatures. Sunday into Monday there are
large model differences regarding the potential for a deep longwave
upper trough moving into the eastern United States. A Gulf low
pressure system may lift northeast late in the weekend or early next
Aviation /12z Monday through Friday/...
VFR today...however the risk for showers and thunderstorms will increase
after 18z at both terminals. Models suggest MVFR or lower ceilings
are possible late day and this evening as stronger upper lift
moves in and convection becoming more numerous. MVFR or lower ceilings
possible tonight with areas of lighter rainfall although thunder
is possible during the evening with elevated instability on the
Extended aviation outlook...brief flight restrictions possible in
scattered showers Tuesday through Friday. Chances for MVFR ceilings
increase Thursday into Friday as low pressure strengthens off the
warm air advection SW flow enhanced into the 15 knots range overnight
with seas 2 to 4 feet. This morning...winds should lay down as the
surface frontal trough nears. Strong thunderstorms are possible later
today or this evening across the marine waters...especially south
of Charleston. If some of the higher resolution models are
correct...the threat from thunderstorm winds could be significant if they
The synoptic flow will shift north and then northeast tonight with
surging building the flow close to 20 knots over much of the marine
area by dawn on Tuesday. Seas will build tonight to 3 to 5 feet.
A prolonged northeast flow will exist from Tuesday into next weekend
as broad high pressure builds from the north and a coastal trough
develops. The best gradient and surge is anticipated Tuesday into
Wednesday during which small craft advisories are likely for most if
not all of the coastal waters. Late in the week into the weekend we
may still be flirting with some 6 feet seas and 25 knots wind gusts
though the overall gradient looks a bit weaker.
increasing northeast flow Tuesday through next weekend will result
in moderate chances for shallow coastal flooding along the coast
especially the SC coast including downtown Charleston around the
times of high tide.