Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service Charleston SC
415 am EDT Wednesday Apr 16 2014

Synopsis...
high pressure will build north of the region through Thursday night.
An area of low pressure may affect the region Friday and Saturday
as it passes by to the east over the Atlantic. High pressure will
then return to the area early next week.

&&

Near term /through tonight/...
pre-dawn...the last of the stratocumulus fields exiting far southern
South Carolina around Beaufort and the immediate Georgia coast. Clear
skies with a few thin cirrus otherwise as surface high pressure builds
in from the northwest in the wake of the strong cold front which swept offshore
last evening. Steady cold air advection ongoing as the 0c degree
925 mb isotherm forecast to reach Charleston by middle morning. Lows
from the upper 30s inland to the middle-upper 40s at the coast. North
winds will produce wind chills in the lower to middle 30s many areas.

Surface high pressure building into the New England and middle Atlantic regions
will wedge south into Georgia and South Carolina today and tonight
as the baroclinic region eventually stalls across the southern half
of Florida and the SW Atlantic. Sunshine with breezy weather on tap
today. Low level geostrophic flow tends to decrease with time
after midday...thus wind gusts should become less frequent toward
the middle to late afternoon except beaches and barrier islands. Cool
low level thickness values and northerly flow becoming northeast
suggest high temperatures only in the 60-65 degree range all inland areas.

Tonight...will have to watch for any significant decoupling potential
well inland to the north of I-16 as low temperatures dip toward the middle
30s at some of the normally colder rural locations from Allendale
to Summerville. We continue to show just enough gradient/wind to
omit any mentions for patchy frost especially since the gradient
tightens before dawn on Thursday and there is a weak moist flux
coming in from the Atlantic in the boundary layer. More of an onshore
component across southeast Georgia should keep readings a bit warmer
along and S of I-16 overnight. Some increase in stratocu coastal
areas is likely late with weak isentropic lift developing.

Lake winds...a tight pressure gradient and decent mixing across Lake
Moultrie supporting a lake Wind Advisory into the first part of
this afternoon. The advisory continues through the morning hours.

&&

Short term /Thursday through Saturday/...
Thursday and Thursday night...aloft the region will be in the
southwest flow associated with the broad longwave that encompasses
much of the eastern half of the Continental U.S.. the strong damming 1043 mb
high over the northeast will persist though it will begin to retreat
even further northeastward with time. This will leave the area in
the strong ridging east of the Appalachians with cool northeast flow
prevailing. Model time heights and soundings show all of the
moisture in the column confined to the lowest 5 kft with
considerable dry air in the middle and upper levels. The models
continue to show potential for shower development over the coastal
waters and perhaps along the Georgia coast on Thursday. By Thursday
night rain chances begin to expand northward and further inland as
an area of low pressure begins to organize over the northeast Gulf.
Thursday probability of precipitation have been limited to slight chances and Thursday night
have been capped at chance for now. Highs will be on the cool side
with upper 60s in most locations. Lows will be in the upper 40s
inland and north...with middle to upper 50s further south.

Friday through Saturday...the forecast becomes more interesting and
inherently less certain for the late week and early weekend. In
general...the models show a southern stream disturbance in the
middle/upper levels swinging through the northern Gulf and kicking off
surface low development. As this low develops...moisture will spread
northward along and just off the southeast coast with the potential
for increasing rainfall chances. Specific model solutions truly run
the spectrum. For instance the 00z/16 European model (ecmwf) allows the upper
disturbance to deepen significantly and form a closed low over the
Florida Panhandle Friday night. This results in a slower developing
low with tremendous moisture and deep forcing to spread into
southeast Georgia and southeast South Carolina. Such a solution
would result in the potential for widespread heavy rainfall through
the day on Saturday. On the other hand...the GFS is much weaker with
the upper disturbance and resulting low and keeps the precipitation axis
lighter and over North Florida and the Atlantic. Given the
uncertainty and existing model disagreement...I have not made any
significant changes to the forecast. Probability of precipitation feature a tight gradient
with highest values over the waters. I have introduced some
categorical probability of precipitation over the waters and to the south...but maintained
the slight chance to chance probability of precipitation for inland areas. Obviously all of
the forecast parameters will ultimately be determined by the
development and track of the low but for now temperatures feature another
cool day on Friday followed by near normal values for Saturday.

&&

Long term /Saturday night through Tuesday/...
early in the period there is still considerable uncertainty
regarding the surface low off the Florida coast. The deeper model
solutions keep the low lingering around with rain still ongoing
along the southeast coast. By Monday or Monday night...the low is
expected to pull away and high pressure will return to the region. A
cold front may attempt to sag in from the north late in the period
with some modest increase in rain chances. Overall...temperatures
are expected to be right around climatology...especially by the middle of
next week.

&&

Aviation /08z Wednesday through Sunday/...
VFR conditions at both ksav and kchs through 06z Thursday. The
main issue today with be north to NE surface winds producing prevailing
gusts in the 20-25 knots range from middle morning through middle-late
afternoon.

Low chances for periodic sub-VFR conditions beginning Friday as
marine stratocumulus attempt to move onshore. Better chances for
flight restrictions will occur by Saturday as low pressure moves out
of the Gulf of Mexico and off the East Coast of Florida.

&&

Marine...
we have continued the Gale Warning for the outer waters early this
morning as latest surface analysis shows a impressive gradient
seaward from Grays Reef and the low level lapse rates were only
steepening with time.

Otherwise through tonight....high pressure wedging into Georgia and
the Carolinas from the north will maintain tight thermal and
moisture gradients over the Atlantic waters from Charleston to south
of Savannah. Winds will veer NE by later this morning with the
morning surge ebbing and then ramping up again after midnight
tonight. Small craft advisories will continue all waters and
the Charleston Harbor through tonight with seas building to 5
to 7 feet near shore and remaining 7-9 feet closer to the Gulf
Stream.

Thursday through Sunday...a strong ridge of high pressure will
remain across inland areas with a tight northeast gradient in place
across the local waters. The gradient will help support solid Small
Craft Advisory winds/seas through the rest of the week and into the
upcoming weekend. Forecast confidence then lessens early in the
weekend as an area of low pressure is expected to cross the Florida
Peninsula and emerge over the Atlantic. The eventual track of this
low and the resulting gradient on its north side will determine
marine conditions through the end of the forecast period. Winds and
seas could increase if the low moves closer to the area during this
time...but there is not enough confidence in the forecast to say at
this Point.

Rip currents...moderate risk for rip currents all beaches today as
the low-level flow tips northeast and combine with elevated seas
and lingering astronomical influences. Strong northeast winds and
lingering astronomical influences will result in an enhanced risk
of rip currents through at least Friday.

&&

Chs watches/warnings/advisories...
Georgia...none.
SC...lake Wind Advisory until 2 PM EDT this afternoon for scz045.
Marine...Small Craft Advisory until noon EDT Saturday for amz350-352-
354.
Gale Warning until 8 am EDT this morning for amz374.
Small Craft Advisory until 5 PM EDT Thursday for amz330.

&&

$$

Synopsis...
near term...
short term...bsh
long term...bsh
aviation...
marine...bsh