Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
642 PM EDT Sat Oct 22 2016
high pressure will prevail through Sunday. A dry cold front will
move through late Monday and be followed by strong high pressure
into the middle of next week.
Near term /through Sunday/...
high pressure will build from the west as an upper trough shifts
farther offshore. This will mean dry weather to continue with
chilly temperatures as conditions become more ideal for
radiational cooling as winds diminish. We lowered temperatures a
bit for tonight with lows ranging from around 40 inland to
lower/mid 50s at the coast. Some upper 30s are possible in the
normally colder rural spots and can't completely rule out some
patchy frost but confidence too low to mention at this time.
Either way, those with cold sensitive outdoor plants should
consider covering them or bringing them inside.
Short term /Sunday night through Tuesday/...
sunday: after starting the day with our coldest temperatures in
about 7 months, it'll be another fine mid-autumn day under the
influence of surface high pressure atop the area and a deep northwest flow
through the vertical, with a strong mid level anticyclone centered
south of Louisiana. Neutral to weak warm advection, full sunshine
and downslope flow will allow for a huge diurnal swing of temps,
more than 30f in many areas, as highs peak in the lower and middle
70s in most communities. An elongated region of high pressure along
the northern Gulf Coast states will support decent radiational
cooling in Georgia much of the night, with a little better mixing
in the boundary layer over South Carolina. This will prevent temps
from getting as cool as tonight despite the clear sky conditions.
Monday: the large scale features an amplified pattern, with the mean
trough position over the eastern states and ridging from the Gulf of
Mexico through the plains and rockies. Surface high pressure
retrogrades west along the Gulf Coast, eventually becoming absorbed
by a much stronger and larger high building S-southeast from southern
Canada and the Midwest. As this transpires it allows an east-west
cold front void of any moisture to approach from the north by late
in the day. The front gets a push from the building high from the
western Great Lakes, Midwest and Ohio Valley, reaching near the
Savannah River after midnight, clearing south of the Altamaha by
daybreak Tuesday. Warm advection in advance of the front and plenty
of sunshine, aside from few-scattered cirrus clouds, will support a
gigantic swing in temps again, as high reach 80f in many sites
inland from the intra-coastal. Cold advection lags a little behind
the cold front Monday night, but still allows for lows a bit below
normal for late October.
Tuesday: a modified Canadian air mass encompasses the region, with
high pressure covering much of the area from the plains east into
the Atlantic. Clockwise flow around the high from off the ocean and
the associated low level convergence as the synoptic flow veers to east-
NE will allow for a little marine-induced stratocumulus over coastal
Georgia, otherwise mostly sunny/sunny skies elsewhere. Staying close
to or a tad below the 1000-850 mb thickness scheme supports highs
very close to seasonal averages. A pinching of the gradient over the
coastal corridor will allow for breezy conditions as speeds climb as
high as 25 or 30 mph at times with deep mixing to occur.
Long term /Tuesday night through Saturday/...
high pressure to our north is forecasted to build towards our area
by Wednesday, bringing dry conditions and seasonal temperatures. As
the high shifts offshore Thursday the models have differing outcomes
for the forecast. The 00z European model (ecmwf) brings a surface low through the
Great Lakes region and an associated cold front stretching into
south. The front crosses through our area Thursday night into
Friday, bringing some showers. Meanwhile, the 00z GFS takes the same
low, links it up with an upper level low that it develops and takes
both of these features into the deep south where they weaken by
Friday. This outcome will give US some showers. Went with slight
chance showers for now, but this will need to be refined as future
model runs come into better agreement.
Aviation /00z Sunday through Thursday/...
Extended aviation outlook: VFR.
tonight: winds will slowly diminish as high pressure builds from
the west allowing the pressure gradient and cold advection to
wane. Speeds up to 20 kt at times early this evening, then mainly
15 kt or less the remainder of the period. Seas ranging from 1-2
ft near shore to 3-4 ft generally beyond 10 nm.
Sunday: conditions will turn out good for boating interests by
afternoon, as a 1025 mb high centered over the southeast quad of the
country realigns itself near the coast of the local area by late in
the day. N'erly winds are still a little frisky in the morning with
cold advection and steady pressure rises, but the gradient relaxes
considerably in the afternoon, enough so that a weak sea breeze will
form near the coast by mid-late day. Quiet weather will prevail
Sunday night, as the surface high remains overhead, keeping winds
at or below 10-12 kt and seas no more than 2 or perhaps 3 ft.
Monday: high pressure slides west as a cold front makes some
headway from the north to near our north waters by nightfall. The
gradient is relatively lax, with SW-west flow at or below 10-12 kt,
while seas are generally limited to only around 2 ft.
Monday night and tuesday: the cold front slides south through the
waters Monday night, pushing south of the marine zones by around
sunrise Tuesday. Steady pressure rises, cold advection and/or a
packing of the gradient will allow for increasing winds and seas,
enough so that small craft advisories appear likely over most if not
all waters around 12-15z Tuesday.
Tuesday night through thursday: the local waters will lie on the
southern portion of an robust and extensive region of high pressure
over the East Coast. There is certainly enough pinching of of the
gradient to keep winds and seas elevated, maybe enough for small
craft advisories for at least parts of the area.