Scientific Forecaster Discussion
fxus62 kchs 180517
Area forecast discussion
National Weather Service Charleston SC
1217 am EST Mon Dec 18 2017
a warm front will move into the area tonight and Monday,
lingering nearby into Tuesday. Low pressure will pass over or
close to the region late Wednesday, followed by an inland high
pressure wedge Thursday and Friday, before the high shifts
offshore next weekend.
Near term /until 6 am this morning/...
no major changes were made for the early morning update.
Maintained slight chance pops across the far interior, although
measurable rainfall is not likely prior to daybreak.
Short term /6 am this morning through Wednesday/...
monday: the forecast counties lie to the north of abnormally strong
mid level anticyclone centered near Andros Island and Cuba, with 500 mb
heights of 5940-5950 meters that is some 2 to 3 Standard deviations
above normal. This along with 800 mb temps of 14-154c or more than 1
Standard deviation above normal will support an unusually warm mid-
December day within a deep westerly flow throughout the vertical.
However, low cloud cover will dominate with considerable low level
moisture in place. This will prevent temps from not getting as warm
as the "pure" low level thickness forecast, but still upper 60s and
lower 70s most places inland from the coast.
At the surface a warm front will lie roughly east/NE to west/SW across the
area, while pwat climbs near or above 1.5 inches in most places with
moisture surging across from the Gulf of Mexico on the west-
northwest periphery of the strong Mid Ridge aloft. Lift will be
provided by the front and channeled vorticity far in advance of a
mid level cyclone over southern Arizona, southern New Mexico and
Mexico. However, there is concerns over the timing of the showers,
so we have capped coverage in the scattered range, with quantitative precipitation forecast
generally less than 1/10 inch.
Monday night: the warm front lingers in the vicinity, as does the
channeled vorticity aloft, but deep moisture trends downward during
the late evening and overnight. So too does the risk of showers. The
build-down of stratus will lead to the formation of at least patchy
to areas of fog, with the potential for some of the fog to become
dense. It'll turn out to be the warmest night of the month or at
least since the first week of December due to considerable cloud
cover and the lack of any influx of cool air.
Tuesday and Tuesday night: a large and strong anticyclone will
encompass the area from the SW Atlantic across the Bahamas, Cuba,
Florida and into the Gulf of Mexico, as a strong short wave trough
passing into Texas Tuesday dampens as it lifts toward the lower and
mid Mississippi Valley at night. An ill-defined warm front nearby
Tuesday will redevelop into a more pronounced warm front to our north-northwest
Tuesday night, as surface low pressure deepens from the arklatex to
the western Tennessee Valley Tuesday night. Other than lift provided
by the warm front(s) there isn't much forcing, so we have a silent
10% pop Tuesday, with 20-30% chances at night due to the large
cyclonic circulation around the surface low and mid level short wave
off to the distant W-NW. 800 mb temps are again around 14-15c and there
should be at least breaks of sunshine to boost temps to a good 10-
15f above normal. Not quite at record levels, except maybe close at
ksav where the record for December 19th is 78f set in 2008 and
previous. Kchs and kcxm have records in the lower 80s that day. The
build-down of stratus and the formation of fog could again be a
concern at night.
Wednesday: the strong mid level ridge across Cuba and the northwest
Caribbean flattens as a deamplifying short wave passes eastward from
the Tennessee Valley to the Carolina's. This occurs in tandem with a
surface low that treks over the southern Appalachians in the
afternoon and approaches the County warning forecast area by 00z Thursday. A trailing cold
front extends southwest from the low into the Gulf, while the warm
front lies to our north. This puts US solidly within the so-called
"warm sector", supportive of yet another abnormally warm day. There
is certainly enough moisture and dynamics available, but timing
discrepancies between some of the computer models prevents US from
showing anything more than scattered to numerous coverage of showers
(greatest in the afternoon). Due to this uncertainty we are not yet
adding mention of T-storms to the forecast in this moderate
shear/Low Cape environment.
Long term /Wednesday night through Sunday/...
the low will move offshore Wednesday night, with showers diminishing
thereafter. Thursday and Friday should be dry with an inland wedge
to develop behind the departing low. A cold front could slowly
approach from the west next weekend, as high pressure moves offshore
and extends across the local region. Thursday will be the coolest
day, with temps not far from normal, then a warming trend again
Friday into the weekend.
Aviation /05z Monday through Friday/...
* cigs and late night fog
* rain chances
Low confidence forecast for the next 24 hours as a warm, soupy
airmass will be in place. Guidance is all over the place with
respect to cigs/vsbys through daybreak and again overnight
leading to lower than normal confidence. Statistical guidance
shows LIFR cigs developing prior to daybreak, while sref
probabilities show a less than 5% chance of occurrence. Narre-
tl output favors the sref so will not include prevailing IFR
cigs at this time for any of the next 24 hours, although
amendments will likely be needed as shorter term trends become
more apparent. Will limit cigs to MVFR for now, mainly from
sunrise on. A band of scattered showers will likely move across
the region later this morning with the potential for steadier
rains late this afternoon. Coverage/timing is a bit uncertain
with high resolution guidance showing slightly differing
positions on the locations of the primary rain axis. The risk
for late night fog will increase as dewpoints rise and scattered
rains end. Better fog chances will hold off until after 06z, so
no mention will be included at either kchs or ksav attm.
Extended aviation outlook: flight restrictions will be likely much
of the time Monday through Tuesday morning, and again Tuesday night
into Wednesday, before VFR weather returns by Thursday.
overnight: south/southeast winds will veer to southwest/west as
high pressure weakens across the area, mainly staying 10 kt or
less. Seas will average 1 ft across the nearshore waters and 2
ft over the offshore waters.
Monday and Monday night: a warm front will lie inland over the
southeast states, with high pressure from the Atlantic extending
west across Florida. While there are no wind nor sea concerns, there
is the possible the development of fog during the night, either
arriving from the south/southwest and/or moving offshore from the
land. Dense fog could certainly be a problem in both Charleston
Harbor and the nearshore waters.
Tuesday and Tuesday night: the waters will be situated between an
inland warm front and high pressure from the Atlantic into the Gulf
of Mexico. Once again winds and seas will stay well below any
advisory conditions, but sea fog could again be a problem with the
warm and humid air atop the "cooler" shelf waters.
Wednesday through friday: low pressure will travel offshore late
Wednesday, probably over or just north of our northern waters,
followed by a cold front later at night. An inland high pressure
wedge then develops Thursday into Friday. Conditions could be close
to advisory thresholds for at least a part of this time. Sea fog is
still possible into Wednesday morning, followed by at least a low-
end potential for T-storms late Wednesday into early Wednesday