Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
1241 am EDT Monday Oct 20 2014
high pressure over the region tonight will shift into the
Atlantic and will support a warming trend Monday and Tuesday. A
cold front will move through the region Tuesday night...with high
pressure building in its wake through late in the week.
Near term /until 6 am this morning/...
few changes were needed for the early morning update.
Stratocumulus is increasing over the coastal waters were weak
isentropic assent is beginning. These clouds will make a run for
the coast through sunrise...likely spreading through at least the
coastal counties overnight. Fog is possible...especially
inland...but its formation is somewhat uncertain. Will continue to
monitor trends and update if necessary to include mention.
Short term /6 am this morning through Wednesday/...
Monday and Monday night...a progressive and highly amplified pattern
will prevail aloft as a deepening trough drops east and southeast through
the Great Lakes and Ohio Valley. This prominent feature will send a
polar cold front through the Midwest Monday and through the Ohio and
tenn valleys at night. Closer to home...surface high pressure over
eastern NC at daybreak Monday will be forced into the Atlantic in
advance of the upstream trough and cold front. An ill-defined
inverted trough will attempt to develop over the nearby coastal
waters...but stays offshore and eventually pulls further away. Weak
isentropic upglide above the coastal trough and some return flow
around the ridge will generate an noted increase in stratocumulus
and cumulus cloudiness. While we can/T rule out a few low-topped
showers either developing or advecting ashore...probabilities have
been limited to a silent 10 percent since there is still a decent
subsidence cap that holds in place around 850 mb. Warm advection
will help in negating the greater cloud cover...and after a
cool/chilly start to the day will boost Monday afternoon temperatures into
the middle and upper 70s. Might even hit 80 near the Altamaha river if
insolation is in greater supply than now forecast. The beaches will
be a few degrees cooler with onshore synoptic flow to occur. Lows
Monday night will be some 6-10 degrees warmer than tonight with
insulating effects of more clouds and a modified air mass. Might see
a little ground fog late...but nothing substantial.
Tuesday and Tuesday night...the well pronounced trough in the NE
will close off as it drops southeast from the eastern Great Lakes to the
Delaware-Maryland-Virginia through the forecast period. This sends a moderating polar
cold front toward the local area by late Tuesday...before it pushes
through early Tuesday night. West and northwest flow throughout the entire
troposphere in advance of the cold front and its associated
compressional heating and plentiful sunshine will support maximum temperatures
near or above 80 just about everywhere...before a resultant sea
breeze might form along the barrier island communities. The best
dynamics and moisture stays well north of the region...and that
along with the lack of forcing suggests that the front will come
through dry between about 00z and 04z Wednesday. Cold advection will
usher in much cooler air in wake of the front...and we/ll see a
swing in temperatures of as much as 25-30 degrees colder by Wednesday
morning from what day time highs were Tuesday.
Wednesday...the closed low spins off the Delaware-Maryland-Virginia to the south of
Long Island...pulling behind it rising pressures from out of
southern and southeast Canada that heads into the southern and southeast United
States. Downward motion and a deep west/northwest flow will keep the area
void of any rain...with at most a little developing flat cumulus
and/or cool weather stratocumulus with cross-sections showing some
moisture trapped around 4-5k feet. 850 hpa temperatures are as cool as 6c
north to 9c south...which will prevent temperatures from achieving much
higher than 70 far north to 75 far south.
Long term /Wednesday night through Sunday/...
a deep/closed upper low remains over New England through Friday with
a shallow deep layer ridge centered just west of the area. This will
provide generally low level northerly flow with temperatures
slightly below normal through Thursday...then a gradual moderation
to near normal Friday through Sunday.
Forecast uncertainty increases by around Friday as models differ in
the next upper trough evolution behind the departing closed low over
New England. The GFS keeps a more open and progressive upper trough
that moves into the southeast U.S. By Friday and Saturday...while the European model (ecmwf)
drops a closed low over the western Gulf. The GFS kicks a strong
surface low /likely tropical in nature/ out of the Yucatan region
northeastward across southern Florida early Saturday...then well offshore
of our coast Sunday. The European model (ecmwf) keeps a weak/broad surface low in the
Gulf through the period. Although both solutions are quite
different...the official forecast from wpc is for a scenario
somewhere between these two extremes...or close to what the UKMET
suggests. For now this keeps precipitation/moisture out of our region
through the period. Thus...kept mention of any precipitation out of this
forecast for now.
Aviation /06z Monday through Friday/...
VFR. Low probability of MVFR ceilings at ksav/kchs early this morning as low-
level moisture pushes onshore.
Extended aviation outlook...low end risk of MVFR conditions Monday
night into early Tuesday.
overnight...a weak pressure gradient will translate to winds
mainly 10 knots or less. Wind direction will depend on the orientation
of surface high pressure and on the development/position of a weak
surface trough. Variable/offshore winds over SC waters and NE
winds over Georgia waters should transition to a more uniform east/NE flow
over all waters...but given the weak pressure gradient forecast
confidence regarding wind direction remains less than ideal. Seas
will range from 2 to 4 feet...dominated by a 12-14 second period
swell from the east/NE and highest beyond 20 nm.
Monday and Monday night...a 1021 mb high centered over the Outer
Banks of NC early in the forecast cycle will slide east into the
Atlantic in advance of the next cold front organizing upstream that
sweeps east/southeast and approaches the Appalachians by daybreak Tuesday.
On the SW periphery of the ridge as it shifts into the Atlantic a
very subtle coastal trough will form in or near the local waters by
late Monday into Monday night as it gradually lifts north and NE. NE
and east winds early Monday will veer around to east and southeast through
the day...clocking around to the west and northwest by late Monday night.
Speeds will hold near or below 10 knots...with seas at no higher than 2
or 3 feet.
Tuesday through Wednesday...a light west or SW flow will prevail out
ahead of the next cold front Tuesday...before the front pushes
through Tuesday night. This is followed by a northerly surge
overnight into Wednesday morning...in conjunction with cold
advection...small but steady isallobaric pressure rises and better
mixing profiles. With geostrophic winds at 1000 mb /or what is
considered a somewhat frictionless layer similar to the ocean
surface/ no more than 20 or 25 knots we feel that conditions will stay
just below any small craft advisories behind the cold front.
Wednesday night through Friday...Continental high pressure will
remain in control of the local waters...with conditions expected to
stay below any advisory criteria.
Rip currents...any slight deviation from the latest forecast for
Monday might bump US into the moderate range of rip currents. For
now we/ll ride with a low risk since we/re not sure on how much
swell energy will remain.