Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
659 am EST Monday Nov 24 2014
..isolated strong to severe thunderstorms possible today...
..Dense sea fog to linger off the coast...
a slow moving cold front will shift toward the coast and offshore
today and tonight. The front will then meander off the southeast
coast Tuesday...before an offshore low system develops on the
front and pulls the feature further into the Atlantic on
Wednesday. High pressure will then prevail for the Holiday
Near term /until 6 PM this evening/...
a plume of tropical moisture featuring precipitable waters of 1.5-2.0 inches
will remain in place across the southeast U.S. Today as a large
upper trough remains positioned well to the west. An anomalously
moist and warm low-level flow from off the Gulf of Mexico will
result in the continuation of warm and humid conditions today with
the 70 degree isodrosotherm forecast to push into parts of the
forecast area this afternoon--highly unusual for late November.
Despite the expected cloud cover...temperatures are forecast to
soar well into the upper 70s to near 80 degrees...which will be
approaching record territory for both the Savannah and Charleston
airports. Instability is forecast to increase as temperatures and
dewpoints increase with sbcapes approaching 1500 j/kg by early
afternoon. This coupled with some uvm associated with weak
perturbations moving rapidly northeast in the fast upper flow
aloft will support a redevelopment of scattered showers and thunderstorms
later today. The best coverage is expected to occur across the
western zones early this afternoon...with activity forecast to
spread slowly east into the early evening hours. Most of the
mesoscale models appear to capture this trend well...so probability of precipitation will
be increased to 40-50 percent across the western half of the
forecast area with probability of precipitation 30 percent at the coast.
There will be a risk for isolated strong to severe thunderstorms this
afternoon with models showing the area remaining embedded in a
large ribbon of 50-55 knots of 0-6 km bulk shear. The 850 hpa low-
level jet is forecast to weaken ever so slightly by late
morning...but speeds are still prognosticated to be 40-50 knots during peak
heating as middle-level dry air promotes dcape values over 1200 j/kg.
A few of the thunderstorms therefore will have the potential to produce
damaging wind gusts...possibly exhibiting occasional supercelluar
characteristics at times given the strength of the shear. There is
a potential for a qlcs type feature to develop by mid-afternoon...
which the latest h3r has been showing a squall line signal for the
past several runs...but confidence in this scenario remains low
at this time. This will have to be monitored throughout the day...however.
Sea fog remains a concern at the coast today as high Theta air
advects over the cold Atlantic shelf waters. With southerly
trajectories in place...any sea fog that forms over the Atlantic
will be drawn to the beaches and intracoastal waters.
Short term /6 PM this evening through Thursday/...
tonight...a weak cold front will approach from the west this
evening and be positioned roughly along the I-95 corridor by
sunrise Tuesday. A risk for showers/thunderstorms will linger through the
night with the ribbon of tropical moisture holding in place...
although activity will tend to diminish by middle-evening with the
loss of insolation. Fog/stratus will be possible through the night
with stratus building a possibility. Lows will range from the
lower 50s across far interior southeast Georgia behind the front
to the upper 50s/lower 60s east of the I-95 corridor.
Tuesday and Tuesday night...strong shortwave energy will sweep from
the Southern Plains states across the lower Mississippi
Valley...driven east by a notable 150 knots upper level jet. This
significant large scale support will assist in cyclogenesis along
the stalled cold front lingering off the southeast coast. A
deepening wave of low pressure will thus lift from the Gulf of
Mexico up toward the Carolina coast late Tuesday night/early
Wednesday. Much of the sensible weather on Tuesday will be highly
dependent on the track of the coastal low and the location of the
wavering axis of deep moisture. In general...expect rain coverage to
increase from east to west as the day progresses...peaking during
the late night time frame when both the core of the shortwave and
the coastal low make their closest approach to the forecast area.
Numerous to widespread rain...thick cloud cover...and winds steadily
veering north on the back side of the system suggest that high and
low temperatures will be significantly cooler than the previous day.
Expect highs to peak in the low to middle 60s and overnight lows to
fall into the middle to upper 40s.
Wednesday and Wednesday night...the coastal low will rapidly lift
north away from the region...as dampening shortwave energy moves
toward and across the East Coast. Rain coverage will gradually
decrease from southwest to northeast during the daytime hours
behind the departing system...with precipitation chances falling
below 15 percent by the evening period. Dry high pressure will
build from the southwest in the wake of the coastal low...while
cold advection supports even cooler conditions. High temperatures
will be suppressed in the low to middle 50s...more than 10 degrees
below seasonal averages. Clearing skies and light winds overnight
will allow temperatures to fall into the middle to upper 30s.
Thursday...a stronger area of surface high pressure will build
south/southeast from the northern plains/upper Mississippi
Valley. Expect temperatures to moderate slightly on the periphery
of the surface high...reaching the upper 50s to lower 60s under
Long term /Thursday night through Sunday/...
high pressure will continue to build into the region from the
north late Thursday...descending into the southeast states by
Friday...before gradually shifting toward the coastline and nearby
Atlantic waters over the weekend. Expect temperatures to slowly
moderate as a result of the track of the high pressure center.
High temperatures will only reach the upper 50s Friday...while
peaking in the middle to upper 60s by early next week. Low
temperatures will also reflect this warming trend...ranging from
the low to middle 30s Thursday and Friday nights...to the low to middle
40s by Sunday night. Conditions appear dry through the extended
period...and will keep rain out of the forecast.
Aviation /12z Monday through Friday/...
fog has so far been held at Bay with 300 feet winds holding 20-25
knots. Ceilings near alternate minimums will linger through about 14z
with levels rising to high-end IFR or low-end MVFR. VFR should
return by early afternoon as mixing heights increase in response
to temperatures rising well into the 70s. Scattered showers and thunderstorms
will likely approach the terminals from the west after 20z...but
it is unclear this far out whether this activity will survive
before reaching the terminals. Will leave mention of rain showers/thunderstorms and rain out
for now. Another round of low ceilings are likely again tonight.
Limited conditions to low-end MVFR for now...although IFR or lower
is certainly a reasonable possibility.
Extended aviation outlook...a better chance for MVFR or lower
conditions in rain late Tuesday or Tuesday night into early
Wednesday...as an area of low pressure develops on a cold front
stalled off the southeast coast. Solid VFR conditions will return
for the remainder of the period. Breezy west/northwest winds
webcams at Tybee Island indicate widespread sea fog has pushed
south into the Georgia nearshore waters. Its unclear how far south
the fog extends...but have gone ahead and expanded the marine
dense fog advisory to include the Georgia nearshore waters through
Today...elevated winds and seas will remain in place across the
waters today with conditions remaining well into Small Craft
Advisory thresholds. Flags will therefore be maintained with seas
diminishing to 4-7 feet nearshore waters and 6-8 feet offshore waters
this afternoon. There remains a risk for sea fog development today
as high dewpoint air flows over the relatively cool Atlantic shelf
waters. It is unclear exactly how widespread any fog will become...
but the Charleston Harbor pilot boat recently reported widespread
dense sea fog just offshore of the Charleston Harbor with visibilities
measured in terms of feet. Hilton Head also dropped to 1 mile in
fog with a ceiling of 100 feet...so suspect the fog extends down to
near the Savannah River entrance. A marine dense fog advisory is
in effect for all nearshore zones through 11 am.
Tonight...winds will veer to the southwest ahead of an approaching
cold front. Seas are expected to drop enough that small craft
advisories will likely be discontinued. The risk for at least
patchy sea fog will linger through the night.
Tuesday through Saturday...the cold front will stall near the
Gulf Stream Tuesday...before low pressure develops along the
lingering boundary and rapidly lifts north Wednesday. Small craft
advisories...as well as a small chance for gales in the outer
Georgia zones...are possible Tuesday night into Wednesday. The
timing and duration of surging west flow along and west of the
tightened pressure gradient will be highly dependent on the exact
track and location of the deepening low system. Winds and seas
will fall below advisory thresholds by Thursday...yet could remain
somewhat elevated as the gradient remains tight on the periphery
of building high pressure inland. Conditions will improve by late
week into the weekend...as the surface high shifts toward the
waters and the gradient relaxes.
record highs for 24 November...
kchs...82 set in 1986 and previous.
Kcxm...81 set in 1998.
Ksav...83 set in 1967.
Record high minimums for 24 November...
kchs...63 set in 2001 and previous.
Kcxm...66 set in 2001.
Ksav...65 set in 1979.
Marine...dense fog advisory until 11 am EST this morning for amz350-352-
Small Craft Advisory until 11 PM EST this evening for amz352.
Small Craft Advisory until 5 am EST Tuesday for amz350-374.
Small Craft Advisory until 5 PM EST this afternoon for amz354.