Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
354 am EST Tuesday Jan 27 2015
strong low pressure southeast of New England will lift northward
through tonight. A reinforcing cold front will push through this
afternoon and evening...followed by high pressure from the northwest
later tonight through middle week. A cold front will push across the area
early Friday...with a return of high pressure on Saturday. A coastal
trough will develop by late weekend...then lift north ahead of a
storm system that passes over the area late Sunday into Monday. High
pressure should then build over the region into next Tuesday.
Near term /through tonight/...
today...the powerful surface cyclone to the east and southeast of
Long Island at daybreak will occlude as it lifts northward through
the day. In wake of this pronounced system we find digging short
waves from the upper Great Lakes to the southeast...which maintains the
highly amplified large scale long wave trough aloft over the
Atlantic Seaboard. One of these middle level impulses will arrive
during the afternoon from the northwest and with moisture trapped near the
inversion around 4-6k feet it will result in the formation of
scattered to perhaps broken stratocumulus clouds.
We start off with clouds surrounding the County warning forecast area...but latest consensus
is for the clouds seen on satellite in northern Georgia to advect in for
the late morning and afternoon. We/ll begin the day sunny...but
skies will be partly to perhaps even mostly cloudy at times this
afternoon as short wave energy coincides with the passage of a
reinforcing but fairly nebulous cold front. The lack of
moisture...the absence of convergence...plus plenty of subsidence
will negate the need for any showers. This along with a continued
feed of cold and dry air from the northwest and the 850 mb 0c isotherm
penetrating to near the Georgia/Florida border...will limit maximum temperatures to
only the lower and middle 50s.
Deep mixing will allow for breezy conditions at times...greatest
over southeast Georgia...but less than yesterday. On average northwest winds will be
10-20 miles per hour and gusty until sunset approaches.
Lake winds...there is still optimal mixing on Lake Moultrie to
maintain the lake Wind Advisory until late morning. At that time the
cold advection fades and isallobaric pressure rises come to an end
and improving conditions will take hold.
Tonight...a transitional pattern develops as the northwest Atlantic low
pulls toward the Canadian Maritimes and allows for colder Canadian
high pressure to build from the Great Lakes and Ohio Valley. Any
lingering clouds in the evening will give way to clear skies as
the deep layered large scale trough shifts into the Atlantic and
dry northwest and north trajectories prevail throughout the vertical.
There/S probably a little too much mixing to occur given that
ridge axis is west of the Appalachians. But despite the lack of
good radiational cooling we look for ample cold advection to
occur...sending temperatures down to their coldest levels in more than 2
weeks. Freezing temperatures should reach near the intra-coastal in SC
and close to I-95 in Georgia. Associated wind chills will be as low as
the lower and middle 20s by daybreak Wednesday.
Short term /Wednesday through Friday/...
Wednesday...cool and dry weather conditions are expected as surface high
pressure builds from the northwest along the western edge of a
middle/upper level trough positioned over the eastern Seaboard. In
general...expect afternoon temperatures to only peak in the low 50s and
overnight temperatures to dip into the upper 20s to lower 30s away from the
Thursday...a large middle/upper level trough will shift off the eastern
Seaboard by daybreak while surface high pressure becomes centered over
the area early...then slides offshore ahead of a fast tracking low
pressure system passing over the Midwest. The pattern will favor
warmer temperatures under middle level ridging and a southerly flow that
develops between the western edge of surface high pressure sliding
offshore and the low pressure system tracking well to the north. In
general...high temperatures should peak in the upper 50s in South Carolina
to lower 60s in southeast Georgia.
Thursday night into Friday...a low pressure system will continue to
advance quickly over the northeastern United States with a trailing
cold front that pushes through the southeastern United States
Thursday night into early Friday. Deep moisture will be lacking
during the time of frontal passage...but should be sufficient to produce a few
showers before daybreak Friday...thus have maintained a slight
chance of showers. Dry high pressure will then move into the
area behind the front while winds quickly become northwest. Temperatures
should be a few degrees cooler Post frontal passage...but a downsloping wind
component along with a full day of insolation should still result in
maximum temperatures near the upper 50s over most locations Friday afternoon.
Long term /Friday night through Monday/...
fairly dry and below normal temperatures are anticipated on Saturday as surface
high pressure builds from the north. A coastal trough is then
expected to develop off the southeast coast by Sunday...producing
chances of showers that shift onshore as it lifts north/northwest
ahead of a low pressure system tracking from the south central
United States to the Ohio River valley. Temperatures will be slightly more
mild as the coastal trough makes a shift onshore...with overall
temperatures peaking in the middle 50s north to low 60s south. Chances of
showers will remain in the forecast through Monday along with cloudy
conditions as moisture is drawn from the southeast and isentropic
lift occurs ahead of an approaching cold front. Conditions should
result in below normal temperatures Monday afternoon through Tuesday as
high pressure builds into the area Post frontal passage. A developing low pressure
center over the Gulf of Mexico could result in a few showers
drifting back north and into extreme southeast Georgia by Tuesday
Aviation /09z Tuesday through Saturday/...
VFR conditions to prevail at both airfields through 06z Wednesday.
West/northwest winds will be gusty at times until sunset Tuesday...and
strongest at ksav during the late morning/afternoon on Tuesday.
Although not depicted in the 06z tafs...there will likely be several
hours of broken ceilings around 6k feet at ksav this afternoon and
early evening as a cold front moves through in conjunction with
Extended aviation outlook...VFR conditions are expected at both chs
and sav terminals through early weekend...but a few showers are
possible Thursday night with a passing cold front.
early this morning...a pinched cyclonic gradient prevails around
intense low pressure southeast of New England. That along with
considerable mixing within the cold advection...will hold west and
northwest winds up as high as 20-30 knots in Charleston Harbor and the 0-20 nm
waters...and 25-35 knots on the outer Georgia waters. As a result we still
have a Gale Warning for amz374 until 7 am...and small craft
For the balance of today...cold advection wanes by late morning and
more so this afternoon...and that along with a more relaxed and a
slightly more anticyclonic gradient will allow for some improvement
in winds and seas. We should be able to drop the small craft
advisories by late morning in Charleston Harbor...and by 1 PM for
the 0-20 nm waters. Once the gales come down on the outer Georgia waters
it will need to be replaced by a Small Craft Advisory...at least
until late in the day.
Tonight...as the low off New England pulls further away and starts
to fill...the large cyclonic circulation across the nearby waters
will give way to Canadian high pressure building from the Great
Lakes and Ohio Valley. What improvement we experienced from earlier
in the day will be short-lived as larger isallobaric pressure rises
return and cold advection redevelops. We/ll be very close to
requiring another Small Craft Advisory on amz350 and amz374 as winds
and seas climb again.
Wednesday through Monday...high pressure will build over the waters
behind a departing cold front...with much quieter conditions over
the coastal waters through Thursday. A cold front will then shift
over the area Thursday night into early Friday...enhancing and
changing a southerly wind to northerly as high pressure builds in
behind it. Winds/seas are expected to remain just below Small Craft
Advisory levels behind the front...but there is a low chance of
conditions touching Small Craft Advisory levels in offshore Georgia
waters and northern SC waters. A coastal trough will develop just
off the southeast coast on Saturday...producing showers that
eventually shift over nearshore waters. The coastal trough should
then lift north and onshore on Sunday ahead of another low pressure
system that strengthens while tracking well north of the area into
Monday. A southward extending cold front from this low pressure
system is expected to sweep through the southeastern United
States...then offshore by Monday night...with winds/seas possibly
reaching Small Craft Advisory levels over all nearshore waters early
SC...lake Wind Advisory until 11 am EST this morning for scz045.
Marine...Small Craft Advisory until 1 PM EST this afternoon for amz350-
Gale Warning until 7 am EST this morning for amz374.
Small Craft Advisory until 11 am EST this morning for amz330.