Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
705 am EST Monday Mar 2 2015
weak high pressure will persist today. A backdoor cold front will
drop south through the region tonight...then stall just south of
the area early Tuesday. The front will lift back north as a warm
front late Tuesday into Wednesday. A cold front will approach from
the northwest Wednesday night...passing through the region Thursday.
Colder high pressure will build over the area into early Saturday...
with a coastal trough to develop offshore into Sunday. Another cold
front could impact the area early next week.
Near term /through tonight/...
nearing-dawn...15c-16c air at 1500 feet above ground level this morning topping
off a huge inversion as surface temperatures were stable in the middle 40s
just about everywhere. The atmosphere through and atop the
inversion layer was saturated and widespread stratus and fog the
result across the forecast area overnight. Dense fog advisories
at daybreak with visibilities variable at times but most areas somewhere
either side of 1/2 mile.
A cloudy and cool...foggy and damp start to the day with a rather
nebulous surface pattern across the region. Given the strength of
the inversion and fog locked in this morning...we think it could
be a slow process to scour out the low clouds and fog with the
advisory in place until middle morning. A west flow in the lower
levels and an increasing downslope component ahead of a cold front
moving into the mountains is expected to finally mix-down some of
the warmer air off the deck this afternoon. 1000-850 mean relative humidity forecasts
continue to show plenty of moisture and while a lot of clearing
is unlikely...forecast soundings do support a much warmer day
across the region. Maintained general persistence with high temperatures
65 to 70 this afternoon with potential to go higher if some spots
see clouds eroding more than expected and vice-versa to the lower
side if fog and low clouds are more tenacious into the midday or
just beyond hours.
Early this evening...a backdoor cold front will advance south
into our South Carolina zones initially...then move through the
region early tonight. High pressure will wedge in the wake of the
front overnight. A weak vorticity lobe embedded within the weak
west/northwest flow aloft will interact with the front and
associated baroclinic zone/overrunning regime to produce another
round of light rains...accompanied by lowering ceilings/some
fog...especially across northern counties later tonight into
Tuesday morning. The surface front will eventually stall over or
just south of southern counties of the forecast area Tuesday
Short term /Tuesday through Thursday/...
Tuesday...another high pressure wedge will yet again be found inland
as a stationary front is situated just south of the forecast zones.
Given the lack of large scale forcing for ascent we look to
isentropic upglide on mainly the 290k and 295k surfaces to generate
chance and likely probability of precipitation...mainly to the north of I-16 in southeast Georgia.
Coverage will wane during the afternoon as the stationary front
begins to drift back north as it starts its transition into a slow
moving warm front. Depending upon when and where the front aligns
itself will produce a huge north to south thermal gradient...which
also has the risk for a large bust in regards to temperatures. For now we
have coolest conditions in the middle and upper 50s far north...lower
and middle 60s central zones...and 70-74 in many communities south
of I-16. Sounding profiles support at least patchy fog in the
morning...with even some light fog to linger over the coastal waters
for the afternoon as higher dew points advect in.
Tuesday night...the models might be a little too rapid in shifting
the warm front back north of the local district...and we have
trended toward a slightly slower transition. Even so...a
favorable southerly trajectory will spread across the cooler
nearby coastal waters...allowing for the development of sea fog to
advect sections east of I-95. This will blend with fog and stratus
build-down further west as the warmer and very humid air mass
moves atop the cooler grounds. Additional dense fog advisories
seem likely as a result. Temperatures will drop some in the
evening...then with increasing warm advection will hold steady or
even rise through the overnight period.
Wednesday...an extensive sub-tropical ridge will expand across the
region and this brings with it impressive warm advection within a
deep SW flow. If we went entirely with the pure low level thickness
values it supports lower 80s everywhere. But a resultant sea breeze
forms in this springlike pattern and will hold temperatures down east of I-
95. Even so we/re still forecasting upper 70s or even a few lower
80s west of I-95...with lower and middle 70s to the east. For now
we/re forecasting temperatures some 6-8f below any records. The barrier
islands will be considerably cooler given trajectories across sea
surface temperatures in the upper 40s and lower 50s. This will support a
range of some 18-20f if not more from the beaches to the warmest
interior locales. Morning fog will burn off/lift by noontime...but
will lurk near or just off the coast in the afternoon. Differential
heating boundaries and the sea breeze could spur a few afternoon
showers...but most of the day and most of US will be rainfree.
Wednesday night and Thursday...our next cold front located upstream
Wednesday night will make headway toward the region overnight...then
shifts into and through the local district Thursday. There are some
timing uncertainties as to when showers will arrive...but
probabilities are still in the high end chance to likely range
Thursday. Right now there doesn/T appear to be any instability and
lapse rates are paltry. Thus no mention of any T-storms. Patchy to
areas of fog will be prevalent again Wednesday night into Thursday
morning...a mixture of sea fog and land-based radiation fog. A
limited diurnal range of temperatures is anticipated...far above normal
Wednesday night and close to seasonal levels Thursday.
Long term /Thursday night through Sunday/...
the cold front slides into the Atlantic Thursday night with colder
Canadian high pressure arrives behind the front into Saturday. The
high then progresses into the Atlantic as a coastal trough develops
out near the western wall of the Gulf Stream. The trough may close
off Sunday...but it does lift off to the NE and east...before
another cold front looks to approach early next week. Conditions
will be colder into the weekend...before some moderating temperatures could
develop by early next week. Return flow of moisture off the Atlantic
will produce a slowly increasing chance for showers Friday into the
weekend. Unless this progression is faster than now anticipated
there doesn/T appear to be any p-type issues.
Aviation /12z Monday through Friday/...
dense fog at both terminals this morning should begin to
dissipate in the 14z-16z time frame this morning. Ceilings should
begin to lift by middle morning with IFR ceilings possibly until 16z-17z.
Slightly drier air will arrive from the west/northwest this afternoon
and as the inversion lifts there will be an improvement back into
the MVFR range...perhaps even VFR for a short time. This evening
and prior to 06z Tuesday...low clouds in the wake of a backdoor
cold front will redevelop around kchs and may impinge on ksav
sometime after middle evening. IFR ceilings are possible after midnight
at both terminals with a few showers passing through the coastal
corridor. At this time...it appears to be too much wind for a
build down stratus/fog event at kchs. If it occurs at ksav...it
would probably be closer to daybreak on Tuesday.
Extended aviation outlook...flight restrictions much of the time
through Thursday. Potential for some VFR conditions Wednesday and
again Thursday night and Friday.
build down stratus clouds have limited visibilities along the immediate
coast...sounds and Harbor overnight. We have not received many
reports other than the chs Harbor closure earlier. Enough evidence
however from web cams to go with a marine dense fog advisory for
all near shore waters into late morning. The fog may linger past
the midday hours under the marine inversion right along the coast
but confidence in how it plays out with fog of this nature is
low. Of somewhat higher confidence is additional fog potential
tonight as higher low level dew points build into the Georgia
shelf waters. Sea fog development is possible along the Georgia
coast but the earlier arrival of the backdoor cold front off the
Charleston coast should preclude sea fog...will just have to
monitor for more stratus build-down.
Winds should be fairly light today as they back to to west or northwest.
Wind shift NE and onshore from north to S tonight as the front moves
in. Models suggest a tight pressure gradient will develop across
Charleston waters and a Small Craft Advisory was issued for the
time period after midnight and especially toward dawn on Tuesday. Seas
will re-build to 4 to 6 feet with NE winds gusting to 25 knots at
times. The gradient looks weaker to the south.
Tuesday...another inland wedge of high pressure will be the
prominent feature behind a stationary front just south of the local
waters. There is enough pinching between these two systems to
produce a Small Craft Advisory for at least amz350 into midday. In
addition...late night and morning fog from both the water and land
sources could linger into the afternoon over the cooler shelf waters.
Tuesday night and Wednesday...the stationary front will have
transitioned into a northward moving warm front...putting the area
into the so-called warm sector...as a sub-tropical Atlantic ridge
fills in behind the front. Winds and seas will be below any advisory
criteria during this time as they veer around to the south and SW.
However...this favorable flow will overspreads the considerably
cooler Continental shelf waters...leading to at least areas of sea
fog. Dense fog advisories appear a good bet...and the ports of
Charleston and Savannah certainly could be closed for an extended
length of time.
Wednesday night and Thursday...the Atlantic ridging gives way to a
cold front that approaches from the northwest and moves into the waters
Thursday. Fog will continue to be a concern in advance of the cold
front...before the arrival of stronger winds and at least scattered
showers. For now we look to remain below any advisory conditions.
Thursday night through Saturday...a strong Continental high will
build from the northwest behind the cold front...and the combination of
cold advection and a tight pressure pattern as a coastal trough
eventually forms late in the period should lead to at least small
craft advisories. There is even a low risk for gales for especially
the outer Georgia waters Thursday night into Friday if the cold advection
is strong enough.
Georgia...dense fog advisory until 10 am EST this morning for gaz087-088-
SC...dense fog advisory until 10 am EST this morning for scz040-
Marine...dense fog advisory until 11 am EST this morning for amz330-350-
Small Craft Advisory from 1 am to 1 PM EST Tuesday for amz350.