Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
106 am EST Tuesday Mar 3 2015
a backdoor cold front will drop south through the area 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 /until 6 am this morning/...
a backdoor cold front was sagging south of the Savannah River
entrance overnight. We have trimmed probability of precipitation back to slight chance
north of I-16 late given regional radar trends. A strengthening
low level inversion within the developing wedge will result in
extensive low stratus formation. Meanwhile...south of the cold
front abundant low level moisture and a warmer airmass will
support the formation of areas of fog tonight...mainly south of
the I-16 corridor...which could become locally dense at times
closer to the Altamaha.
Lows are forecast to range from the middle to upper 40s in the
Charleston tri-County area...to the lower and middle 50s farther
Short term /6 am this morning through Thursday/...
Tuesday...the backdoor front will be south of the Altamaha river in
southeast Georgia by daybreak...with a well-defined wedge of high
pressure building down the Lee of the Appalachians in its wake. This
pattern will support a strong inversion that will trap low level
moisture for much of the day and allow thick clouds to persist.
Patchy fog in the early morning hours will dissipate over
land...while possibly lingering over the coastal waters through the
afternoon. Light rain/drizzle will hold The Wedge in place through
at least early afternoon...especially across far inland and northern
locations. Much of the sensible weather impacts for the
afternoon/early evening will be highly dependent on where the warm
front/southern periphery of The Wedge sets up...before the feature
eventually lifts north overnight. As is typical in an inland wedge
pattern...have indicated a rather notable high temperature gradient
from northwest to southeast. Temperatures will likely be suppressed
to a range from the middle 50s near the Santee river to the lower 70s
near the Altamaha river. However any change in the location of the
southern periphery of The Wedge could greatly impact temperatures.
Tuesday night... The Wedge is expected to gradually erode
overnight...allowing a solid southerly flow to set up in its wake.
Any light precipitation chances will end from south to north by the
evening hours. The advection of higher dewpoint air and the build-
down of stratus will support fog development overnight...with dense
fog advisories quite likely. Low temperatures will range from the
middle 50s to around 60 degrees.
Wednesday...the southeast will be within solid southerly flow
between Atlantic high pressure and a cold front far upstream. The
combined effects of warm advection and deep layer ridging from the
Atlantic suggests a considerably warmer day than seen in quite some
time for the forecast area. Inland temperatures will peak in the
upper 70s north to lower 80s south. The coastal areas will be
notably cooler...a result of both the proximity of cooler marine air
and possibly some intrusion of sea fog from the nearshore waters.
Although a few forecast models indicate some light precipitation
inland...prefer to keep rain chances below 15 percent considering
deep layer ridging and negative vorticity advection across the area.
Wednesday night...the cold front will be approaching from the
west...yet remain just upstream by daybreak Thursday. Associated
rain will also remain outside of the forecast area overnight...and
have kept rain chances below the mentionable 15 percent accordingly.
Fog is possible again...however increasing low level flow could
prevent fog from becoming as dense as Tuesday night. Sea fog could
still persist and slip locally onshore.
Thursday...latest GFS and European model (ecmwf) model runs support the cold front
being just to our north and west at noon on Thursday and then
quickly moving through the area during the afternoon. This would
support slightly higher temperatures and raised highs a little...but
I am still several degrees below guidance and the ensemble means.
12z NAM is even slower with the front and if this is correct and I
think it is overdone...but would result in highs well into the 70s.
For now have highs mainly in the middle to upper 60s. If the model
trends continue though...later forecasts will need to raise highs on
Thursday further. Cannot rule out a few showers Thursday
morning...but best chances will be during the afternoon. Still think
that upper end chance levels look reasonable...but based on the best
lift and moisture shift by to the north and west of the area...would
not be surprised if the threat for showers would need to be lowered
a little in future forecasts...especially if the front is slower.
Long term /Thursday night through Monday/...
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 /06z Tuesday through Saturday/...
kchs...a back door cold front has advanced south of the terminal
this morning. Lower ceilings are likely later tonight through Tuesday
morning as moisture convenes under a wedge pattern inversion. There
is probably a little too much wind for IFR conditions...but sub-1000
feet ceilings and mist do remain a definite concern for late tonight
into Tuesday if winds slacken enough. In addition...spotty light
rains are possible later this morning. Extremely complex forecast
this afternoon and evening as a warm front progresses north. There
could be a brief period of VFR conditions before stratus and
perhaps fog develop after sunset or by middle evening at the latest.
Ksav...VFR conditions early tonight will give way to the
development of lower stratocumulus/stratus overnight as a back
door cold front approaches and moves through from the north and NE.
This will result in MVFR or IFR conditions that will persist
through early afternoon Tuesday as considerable moisture remains
trapped underneath a decent inversion. Models have trended
quicker with a warm front this afternoon and we have a faster
wind shift to southeast to S today and a period of VFR conditions.
IFR or lower ceilings and perhaps some fog will develop this
evening as higher dew points stream into the region.
Extended aviation outlook...flight restrictions likely much of
the time through Thursday. Potential for some VFR conditions
Wednesday and again Thursday night and Friday.
overnight...a backdoor cold front will drop into the Georgia waters
overnight. Winds will shift to the NE/east and surging in its
wake. Winds appear strongest across the Charleston County waters
where a Small Craft Advisory remains valid for 15-20 knots winds and
gusts to 25 knots. Conditions could also approach Small Craft
Advisory criteria in Charleston Harbor but confidence is too low
to issue any advisories at this time. Across the remainder of the
waters...winds should peak upwards of 15 knots. Seas will build
within strengthening winds and a favorable fetch...reaching 4-6 feet
across the Charleston County waters and 3-5 feet elsewhere.
Sea fog remains a concern for the waters tonight. Given middle and
upper 50s dew points atop the colder shelf waters where temperatures are
in the upper 40s and lower 50s...conditions are still favorable
for sea fog to redevelop tonight in advance of the southward
moving cold front...most especially on the southernmost SC waters
out 20 nm and the Georgia waters out 20 nm. Dense fog advisories may be
needed once again for these areas.
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 afternoon and evening. Fog will continue to be a concern in
advance of the cold front...before the arrival of stronger winds and
at least scattered showers. Winds reaching or exceeding 20 knots are
likely by late in the day across the northern waters. As a
result...a Small Craft Advisory may be needed for at least amz350
by late Thursday.
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.
Marine...Small Craft Advisory until 1 PM EST this afternoon for amz350.