Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
657 PM EST Friday Nov 21 2014
high pressure will prevail through Saturday...before a warm front
moves north through the area Sunday. A cold front will move through
the area Monday night...followed by high pressure into the middle of
Near term /until 6 am Saturday morning/...
Friday evening...lowered hourly temperatures through the evening
to account for ongoing/expected trends. Otherwise...ongoing
forecasts remain on track and required only minor adjustments to
Strong high pressure will continue to expand into the region
from the north tonight. The airmass will initially remain very
dry with clear skies. Eventually...the over-water fetch will
and a developing coastal trough will support stratocumulus
developing over coastal waters...perhaps spreading onshore
especially into southeast Georgia late. There could also be a few
showers late but we think the best chances will remain well
offshore. Decent radiational cooling inland and a persistent east/NE
wind near the coast should produce a decent temperature
gradient...with low temperatures ranging from near freezing inland
to 45-50f along lower SC/northern Georgia coast...where temperatures
could even begin to rise late especially if clouds develop and
Short term /6 am Saturday morning through Monday/...
Saturday...flat ridging will prevail aloft in sync with a prominent
core of surface high pressure over the middle-Atlantic region that east
in the ocean late in the day. While the return flow around this
feature will coincide with isentropic ascent on the 290-295k
surfaces...there is still a well defined subsidence inversion in
place. That along with the absence of moisture above the lowest 6-7k
feet will support one more day of rainfree conditions. An onshore
synoptic flow...the formation of maritime-induced cumulus and/or
stratocumulus and cirriform clouds from the south and west late will
hold temperatures down to the lower and middle 60s away from the coast.
Saturday night...as the surface high pulls further offshore a warm
front will develop across the Gulf of Mexico in response to a
southern stream short wave moving steadily through Texas toward the
northwest Gulf of Mexico. Middle level perturbations out ahead of the more
pronounced short wave will arrive from the west/SW and causes a
subtle decrease in height falls aloft. This also opens the door for
an influx of deeper moisture from off the Atlantic and the Gulf of
Mexico. This along with a further increase in isentropic upglide and
the forcing from the approaching warm front will produce a lowering
and thickening of cloud cover...which eventually leads to an
increasing risk of rains arriving from the SW. The models often are
a little slow in regards over-running rains arriving...so we have
attempted to make reasonable adjustments. The evening will be
dry...but rains will reach near the Savannah River by around
06z...and overspreading most if not all of the forecast area by 12z.
Categorical probability of precipitation are anticipated south of I-16 by late in the valid
period...trending to chance probability of precipitation far north. This will be the warmest
night in over a week.
Sunday and Sunday night...the fairly potent southern stream short
wave takes on a negative tilt as it lifts east/NE through the lower
MS valley and starts to weakens as it heads through the Tennessee Valley
late Sunday and then pulls further away as it approaches the NE
states late Sunday night. Simultaneously at the surface rapidly
intensifying area of low pressure strengthens in response to a
northern stream short wave that drops through the Great Plains. This
will lift the warm front originally south of the area at daybreak
Sunday...into southern Georgia by late Sunday and then north of the
entire region by early Sunday night. It certainly looks to be a
soaker of a day...with a deep and rich feed of sub-tropical/tropical
moisture out of the Gulf and from the Atlantic. Precipitable waters climb to 1.8
or 1.9 inches...which is greater than the 99th percentile for this
time of year. With considerable forcing for ascent and a fully
saturated column we will carry near 100 probability of precipitation areawide along with a
mention of rain heavy at times. At the present time rain amounts
look to be around 1.5 to 2 inches...locally higher. This is below
latest flash flood guidance...so no Flash Flood Watch at this stage.
However...tide levels will be elevated since we/re in a high
astronomical cycle from the new moon and onshore fetch. Should heavy
rains coincide with high tide around 8-9 am and 8-9 PM then there
could be an enhanced flooding risk...especially near/along the coast
and in downtown Savannah/Charleston. It/S Worth noting that there
are signs of some frontogenesis taking shape in the middle levels in
the morning and early afternoon...which could lead to some banded
and heavier precipitation rates. This bears watching in future forecasts.
There remains a slight risk for severe weather given the highly
sheared environment...right turning hodographs and the resulting
strong differential advection from the negatively tilted trough.
However...lapse rates are poor...sherbs3 values are below a unit of
1 and there is a constant onshore flow across the stabilizing
waters. These are certainly big negating factors for severe
weather...but not enough to preclude mention in the hazardous
weather outlook. Damaging winds and maybe an isolated tornado are
the main hazards.
Temperatures are dependent upon the timing of the warm fronts northward
progression...but we should reach near seasonal norms with upper 60s
north and lower 70s south for highs Sunday. A small diurnal range
Sunday night as the flow turns southerly and warm advection
maintains its strength...preventing temperatures from dropping any lower
than the upper 50s/lower 60s.
Rain chances will diminish with the passage of the warm front Sunday
night...and in it/S place there are some hints of either sea fog
and/or stratus build-down. My current thoughts are that there is still too
much wind off the deck for this...but should marine-layering effects
occur given the cooler shelf waters with temperatures down in the middle-upper
50s and the widespread earlier rains we have added mention of patchy
fog to the entire forecast area...with the exception of the outer Georgia
Monday...we/re solidly in the warm sector...awaiting the next
upstream cold front which is still found to our northwest through the day.
A band of deep moisture...marginal instability...slightly better
lapse rates and forcing from the upper jet will support chance probability of precipitation.
Compressional heating out ahead of the cold front...pre-frontal warm
advection and a warmer start to temperatures initially will allow for maximum
temperatures to peak in the middle or upper 70s.
Long term /Monday night through Friday/...
the cold front is expected to move through the region Monday
night...then linger just off the coast into the middle of next
week while the axis of the upper trough remains just to the west.
Scattered showers could continue to affect portions of the
forecast area as a result...and will thus maintain a chance of
rain through Tuesday afternoon. The axis of the upper trough will
finally sweep toward the coast Wednesday...pushing the offshore
system farther away and allowing drier high pressure to build in
from the west. A cooling trend is expected through the
week...initially driven by passage of the cold front...then
reinforced by the progression of the trough axis middle week. High
temperatures will reach the low to middle 60s Tuesday
afternoon...before becoming suppressed in the upper 50s to lower
60s for the remainder of the week. Low temperatures will fall into
the middle to upper 30s for Wednesday and Thursday nights.
Aviation /00z Saturday through Wednesday/...
the probability for MVFR ceilings will increase late tonight
through Saturday. However...guidance including MOS/model soundings
remains inconclusive...thus opted to maintain VFR conditions
through Saturday with only a mention of scattered MVFR level
Extended aviation outlook...conditions will deteriorate beginning
late Saturday night at ksav and at kchs early Sunday as a warm front
lifts north into the area. This will result in MVFR conditions in
rain showers...isolated thunderstorms and rain and locally heavy rains. The warm front will
then lift north of the area by Sunday night...and although rain
chances will decrease there is an elevated potential for IFR or
lower conditions into early Monday. Periodic flight restrictions are
still possible early next week until a cold front pulls further
offshore by middle week. Breezy conditions Sunday into Monday...with a
risk of a period of wind shear Sunday night.
tonight...high pressure sinking southeast into North Carolina and
likely development of a trough near the northern Florida/southern Georgia
coast will lead to an tightening pressure gradient. Thus...expect
northeast winds to increase and this will in turn lead to building
seas. Conditions should be poor enough to warrant small craft
advisories beyond 20 nm and likely pushing into the nearshore
waters south of Edisto Beach by daybreak.
Saturday and Saturday night...a 1036 mb high centered over NC early
in the period will slide east into the Atlantic as it becomes
elongated east-west. It/S associated ridge axis will lie north of
the local waters as this transpires...while a warm front organizes
over the Gulf of Mexico and lifts north/NE. There is a decent
pinching of the gradient which supports a continuation of the small
craft advisories for our central and southern waters...which may
need to be expanded to our Charleston County waters in upcoming
forecast cycles. Any gale potential is too low to consider at this
juncture. Rains will overspread the coastal waters from south/SW to
north/NE during Saturday night.
Sunday and Sunday night...as the Atlantic high pressure system
weakens and moves further out into the Atlantic it allows for the
warm front to lift into and through the coastal waters. Although
warm advection and the resulting marine-layering effects could limit
the amount of mixing...wind fields just off the surface still look
strong enough where additional small craft advisories will become
necessary...with a low end risk for marginal gales across
amz350-374. Numerous to widespread showers and scattered T-storms
will impact the marine area...a few of which may result in special
marine warnings for damaging winds and isolated tornadic
waterspouts. Since the shelf waters have cooled considerably into
the 50s...as the warm and saturated air mass overspreads the area
there could be a concern for low stratus build-down and/or sea fog.
It/S certainly not a high probability...but enough to cause US to
add patchy fog the gridded forecasts Sunday night.
Monday through Wednesday...a slow moving cold front over the southeast
states on Monday will limp into and through the waters by
Tuesday...before low pressure forms on the front outside our marine
area and pulls the front away by Wednesday. Depending upon exactly
how this pattern unfolds we might need additional small craft
advisories during this time.
Marine...Small Craft Advisory from midnight tonight to 11 am EST
Saturday for amz352.
Small Craft Advisory until 5 am EST Tuesday for amz374.
Small Craft Advisory from midnight tonight to 7 am EST Monday