Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
1055 am EST Thursday Nov 26 2015
strong high pressure will prevail inland through the weekend...as
a weak coastal trough lingers nearby late this week. The area of
high pressure will weaken somewhat next week as a cold front passes
by to the north.
Near term /through tonight/...
for today...a monstrous and extremely robust 1050 mb high off
Newfoundland will encompass much of the eastern third of the
nation...western Atlantic and Gulf of Mexico...as it wedges SW to
the Lee of the southern Appalachians. Upstairs a building ridge
from the Gulf will expand atop the southeast states...while a middle/upper
cut off low spins to the NE of the Bahamas.
Although there is sufficient moisture within the first mile or so
of the troposphere given a strong easterly low level wind and the
nearby proximity to a subtle coastal trough near the Gulf
Stream...the atmosphere above has considerable dry air with mean
relative humidity no more than 20-30 percent. With no more than some hints of
weak isentropic ascent and coastal convergence our prospects for
rain are limited with large scale subsidence. Activity over the
Gulf Stream has increased in both spatial coverage and vertical
depth. But overall it continues to dwindle as it moves into the
cooler shelf waters...and this trend will continue into this
afternoon. However...during the warmer part of the afternoon a
few showers could certainly survive or even form over our coastal
counties...where we are showing 20 probability of precipitation. Even so any quantitative precipitation forecast will be
Warm advection and rising heights aloft should support maximum temperatures
close to the pure low level thickness forecast in the lower 70s.
Shoreline communities with the onshore fetch will be held in the
60s...along with breezy conditions as NE winds climb as high as
15-25 miles per hour and gusty.
Tonight...cut off low pressure spirals to the NE of the Bahamas
as the amplified Gulf of Mexico ridge remains overhead. Meanwhile
at the surface the poorly defined coastal trough lingers
offshore...while the inland ridge forms a secondary high center
near the Virginia/NC border. Sinking motion under the ridging
aloft and only limited low level moisture suggests that rain
chances are slim to none. We/re carrying 20 probability of precipitation near the
immediate coast...but similar to recent nights the bulk of the
showers will stay in the Atlantic. There will likely be some
stratus formation overnight as moisture becomes trapped beneath a
strengthening nocturnal inversion. That along with condensation
pressure deficits falling under 10 or 20 mb could support a little
fog. But geostrophic winds are some 15 or 20 knots...so no need to
add fog to the grids at this time.
A continued feed to moisture from off the Atlantic and the mixing
will prevent temperatures from falling any lower than lower or middle
50s inland...upper 50s to near 60 on the barrier islands where
breezy conditions will persist.
Short term /Friday through Sunday/...
the strong surface wedge should begin to slowly weaken on Friday
and slowly move further into the North Atlantic as a shortwave
moves into eastern Canada. However...it will leave behind a modest
high pressure center over the Middle Atlantic States just north of
the area. This will keep winds generally out of the
northeast...although the weaker flow will keep the low clouds and
moisture a little further offshore and not threaten the shoreline
with any showers. Plentiful sunshine and warmer than normal
boundary layer temperatures should allow maxes to get into the lower to
middle 70s away from the immediate coast...which is several degrees
above normal for the last week in November.
Saturday and Sunday...the pattern changes little as a weak cold
front stalls well to our north. Its possible there could be a little
patchy fog in the low spots each morning around sunrise...but not
enough coverage expected to warrant any mention in the grids at this
time. Temperatures will remain warmer than normal...although it may be a
few degrees cooler Sunday afternoon due to a few more clouds.
Long term /Sunday night through Wednesday/...
Sunday night and Monday the northeasterly low level flow will
become reinforced as a bubble of high pressure builds in over New
England and wedges down the east side of the Appalachians. We
could see a few more clouds and a few showers around...but still
not much change to the temperatures.
On Tuesday and beyond...the 00z European model (ecmwf) and GFS start to go their
separate ways. The European model (ecmwf) develops a strong surface cyclone over the
western Great Lakes and move the system eastward across the
U.S./Canadian border...eventually forcing a cold front through the
area and allowing high pressure to build in behind the front for
Wednesday and Thursday. However...the GFS is weaker with the Great
Lakes system and splits the energy to emphasize a southern stream
disturbance that develops a low over the western Gulf states on
Wednesday and brings it through the southeastern states on Thursday
before heading off the coast of the eastern Seaboard. With this
being so far out in the forecast time...I will go more with a blend
of the two solutions and see how the models handle the situation
Aviation /16z Thursday through Monday/...
generally VFR through 16z Friday. While not in the latest
tafs...there is a risk for some late night MVFR/IFR
ceilings...especially at ksav. Also of note...NE breezes will
reach 15-20 knots at times during the late morning into the afternoon
at both airfields.
Extended aviation outlook...generally VFR conditions are expected
through the period.
today...a tight packing of the isobars or the so-called pinching
will persist...as much as a 3-4 mb spread from north to south
across the local waters around a 1050 mb high off Newfoundland.
This will easily support a continuation of the small craft
advisories for all Atlantic waters for NE winds of 20-30 knots and
seas 5-8 feet within 20 nm of the coast and 8-10 feet across the outer
Tonight...as the core of the strong northwest Atlantic high pulls further
into the ocean...a portion of its associated ridge breaks off and
forms as a 1036 mb high near the Virginia/NC border as a subtle
trough lingers near the western wall of the Gulf Stream. Pinching
remains tight and there is little decrease in both winds and
seas...as the ongoing small craft advisories persist.
Friday through Tuesday...not much change in thinking here. Small Craft Advisory/S
will continue to be needed for the next several days due to a
combination of northeasterly winds and seas. An extended fetch
across the North Atlantic into the coastal waters will cause seas
to get up as high as 10 feet in the offshore Georgia waters and as
high as 7 feet in the nearshore waters beyond 10 miles through
Saturday morning before slowly subsiding. Winds will come down
slowly by Saturday as well. Despite the improving conditions seas
will still be high enough to warrant headlines through the
weekend. Conditions will gradually improve early next
week...although seas may still be high enough to continue scas
over the outer Georgia waters.
the risk of coastal flooding will continue through Saturday. This
is the result of lingering astronomical influences...but more
importantly also from moderate to strong NE winds and seas that
are as high as 5-8 feet across the nearby coastal waters. These
conditions will cause tides to rise above the highest astronomical
tide at both fort Pulaski and Charleston. Coastal flood advisories
may be needed for the high tide cycles both Friday and Saturday
We considered a high surf advisory...but given that the
trajectories are more parallel to the shoreline rather than
onshore...we feel that 5 foot breakers in the surf zone will be
tough to come by and thus no high surf advisory at this time.
Marine...Small Craft Advisory until 6 PM EST Sunday for amz350-352-354.
Small Craft Advisory until 6 am EST Tuesday for amz374.