Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
609 am EST sun Nov 23 2014
..heavy rains with a risk for severe weather through tonight...
..Record daily rainfall likely...
a warm front will move north through the area today. A cold front
will then shift toward the coast and offshore by Monday night. The
front will meander off the southeast coast into early Wednesday...
before an offshore low pressure pulls the front to the northeast and
allows high pressure to return into the Holiday weekend.
Near term /until 6 PM this evening/...
a powerful southern stream shortwave is trekking quickly across
Texas early this morning and will become negatively tilted this
afternoon as it moves across the deep south and eventually through
the southeast U.S. This evening. Copious amounts of moisture
featuring precipitable waters near 2 inches advecting north via 40-60 knots of 850
hpa flow coupled with an expanding corridor of upper difluence
forming out well ahead of the approaching shortwave is already
supporting widespread rains over much of South Carolina... Georgia
and northern Florida this morning. This large area of rain will
continue to expand in both area and intensity through the day as
the combination of anonymously high moisture and increasing upper
dynamics become aligned.
The window for the heaviest rains will gradually shift northward
across the forecast area through the day as a warm front currently
stretching across northern Florida then arching back into far
southeast Georgia to a position off the lower South Carolina
coast moves north. Models are similar in showing the inland wedge
steadily breaking down as the parent high moves farther offshore
of the North Carolina Outer Banks. In fact...modest pressure falls
are already occurring across the core of The Wedge...which suggest
weakening is already in progress. This will allow the warm front
and axis of heaviest rains to lift into the southern midlands and
Pee Dee by late in the afternoon...after dropping several inches
of rain across the lowcountry and coastal Empire. Probability of precipitation near 100
percent will be maintained for all areas today...although
diminishing probability of precipitation down into the 30-50 percent category will be
highlighted moving in from the south in the wake of the warm front
as the strongest warm air advection/isentropic assent and low-
level convergence all shift north with the front.
Pinning down highs will be tricky...especially inland along the
southern midlands...where the warm front will reach very late in
the day. There is a significant bust potential here... especially
over parts of Allendale-Jenkins-Screven counties where it is
possible The Wedge may not erode until after sunset. Will forecast
highs in the upper 60s in these three zones with Lower- Middle 70s
elsewhere. Further refinements will be needed as the day
progresses once mesoscale trends and warm frontal timing becomes
There will be two opportunities for severe weather today...one
with the warm front later this morning and afternoon with the
second associated with the core of the strongest dynamics tonight.
For today...its a typical Low Cape/high shear scenario. Surface
based instability looks fairly weak with modified soundings only
yielding 500-800 j/kg of SBCAPE with a lifted index of -2 to -4c.
Although instability is poor...kinematic profiles become quite
favorable for rotating updrafts with as much as 50-60 knots of bulk
shear and 0-1 km helicity values reaching as high as 500-700
m2/s2. The best chance for severe thunderstorms with damaging winds and/or
an isolated tornado will be along and just south of the warm front
where low- level vorticity and sherbs3 values in excess of 1 unit
will be maximized...although any convective cell in the warm
sector could easily produce damaging winds with an impressive a
50-60 knots low-level jet in place. An arguably better chance for
severe thunderstorms will occur this evening and will be addressed in the
short term section below.
Short term /6 PM this evening through Wednesday/...
tonight...models show the warm front displaced just to the north
across the southern midlands and Pee Dee as a powerful...
negatively tilted shortwave pivots over the southeast states.
Although only scattered convective activity will likely be in
progress at sunset...mesoscale models hint that at a somewhat
organized line of strong to severe thunderstorms...possibly in the form of
a qlcs/squall line...could be approaching the far western zones.
This activity appears to organize along the leading edge of a
corridor of intense upper forcing associated with the shortwave
itself as it encounters an increasingly unstable atmosphere across
south Georgia and southeast South Carolina in the wake of the warm
front. Nam12 instability forecasts show SBCAPE values reaching as high
as 800-1200 j/kg within the core of the warm sector with
impressive kinematic profiles featuring 0-6 km bulk shear values
of 50-55 knots aligned with a corridor of sherbs3 values of 1-1.25
units. Although instability is only weak to modest at
best...impressive upper dynamics could be enough to cover come
this to support a continuation or even a strengthening of the
convective line as it crosses the area this evening into the early
morning hours Monday. The primary hazard with this line looks to
be damaging winds...although there will be a risk for an isolated
tornado...especially across the northwest tier closer to the warm
front. The confidence in this scenario is only moderate at best at
this point in time and further refinement will be need throughout
Probability of precipitation of of 30-50 percent will be depicted tonight with gridded
probability of precipitation reaching as high as 60-70 percent aligned with the expected
placement of the convective line. It will be be a warm/humid night
with lows only dropping into the middle-upper 60s for most areas.
Despite impressive wind fields...guidance suggests widespread
stratus and eventually fog will fill in behind the squall line.
It is unclear exactly how this part of the forecast will
unfold given the amount of wind that will be in place...but its
certainly possible the fog could become dense at times. Will show
areas of fog in the grids...roughly timed with the passage of the
Monday...the southeast coast will become located between ridging
over the Bahamas and the axis of an upper trough across the central
United States. The progression of a cold front will be slow and
steady Monday and Monday night due to its nearly parallel
orientation to the upper flow. Considering much of the large scale
dynamics and forcing will be lifting out of the region...convection
appears limited to a narrow band of deep moisture ahead and along
the approaching boundary. Will thus keep rain chances capped in the
30 percent range...highest during the afternoon hours and closer to
the coast...where low level moisture could pool ahead of the front.
Temperatures will become quite warm Monday afternoon considering a
warmer start to the day and persistent solid south/southwest flow.
Expect highs to reach the middle to upper 70s...and given the
unseasonably warm conditions...have included a slight chance for
thunder. Shower potential will decrease from west to east during the
evening and nighttime hours...as the front moves offshore and drier
air approaches in its wake. Low temperatures will be nearly 10 to 15
degrees cooler than the previous night...falling into upper 40s far
inland to the low/middle 50s closer to the coast.
Tuesday...the cold front will stall just off the southeast
coast...as high pressure struggles to push eastward. Strong
southern stream shortwave energy will sweep through the Southern
Plains states and across the Gulf Coast through Tuesday
night...supporting the development of waves of low pressure along
the southern end of the stalled boundary. Rain chances will
continue due to the lack of a clean frontal progression...and
coverage will be highly dependent on the location of the
meandering front and associated convergence zone. Both the 00z
European model (ecmwf) and 00z GFS suggest that rain coverage will increase Tuesday
night...when the core of the shortwave energy swings toward the
area and the surface wave lifts from the Gulf of Mexico toward the
southeast coast. Have indicated probability of precipitation in the chance range all areas
during the daytime hours...increasing to likely rain for the
coastline and Charleston tri County area overnight. Temperatures
will be notably cooler on the back side of the cold front and low
pressure system...with highs only in the low 60s and nighttime
minimums in the low 40s.
Wednesday...low pressure just offshore will steadily lift north
up the coast...ahead of the shortwave energy and upper trough
axis moving into the southeast states by midday. The day will
begin with a rather notable gradient in rain coverage...with the
Charleston tri County area seeing numerous showers...and the
Georgia zones farther south toward the Altamaha river seeing
scattered coverage at best. Drier air will quickly wrap into the
region from the southwest behind the departing system...and expect
rain chances to fall below 15 percent across the forecast area by
the evening hours. The region will be located on the cold side of
the system...and cold advection combined with falling heights
aloft will support an unseasonably cold day...with high
temperatures suppressed in the low to middle 50s.
Long term /Wednesday night through Saturday/...
drier high pressure will build into the region from the
west/southwest late Wednesday into Thursday. A stronger area of
high pressure will then build in from the north late Thursday...
descending into the southeast states by Friday...before gradually
shifting toward the coastline and nearby Atlantic waters over the
weekend. Unseasonably cool conditions will prevail through the
period. High temperatures will only reach the upper 50s to lower
60s Thursday and Friday...with low temperatures falling into the
30s. A gradual warming trend will occur over the weekend as the
center of the surface high shifts toward the coastline...with
highs reaching the 60s by Sunday afternoon and lows moderating
into the upper 30s to low 40s by Sunday night. Conditions appear
dry through the extended period...and will keep rain out of the
Aviation /11z Sunday through Thursday/...
conditions will continue to deteriorate at the terminals this
morning. Heavy rains have reached ksav and will impact kchs by
sunrise. Prevailing IFR and MVFR conditions are likely this
morning with the steadier rains ending from south-north through
the day as a warm front moves north. The heavier rains are
expected to end at ksav by 16z and kchs by 19z. Until then...IFR
conditions will remain a possibility with an outside chance for
LIFR ceilings/visibilities at times during pockets of the most intense
rainfall. Winds will become gusty at both terminals by late
morning/early afternoon with the passage of the warm front. Gusts
to 25 knots are likely. Low-level wind shear will remain a concern at
ksav and possibly kchs given the strong 950-975 mb winds noted in
rap soundings and kclx VAD wind profile. There will be another
round of shower/thunderstorms possible this evening...possibly in the form
of a squall line. For now limited conditions to MVFR with a
mention for thunderstorms and rain. Low stratus and fog are possible tonight once
the squall line moves through. There remains some questions on
exactly how widespread/dense the stratus/fog will become...so
limited conditions to just above airfield alternate minimums at
both terminals for now.
Extended aviation outlook...flight restrictions will prevail at
both terminals into early Monday as a warm front lifts north of
the area. This will result in MVFR or lower conditions in moderate
to occasional rain...isolated thunderstorms...and gradually lowering
ceilings with possible fog/mist. Conditions will also be breezy
Sunday into Monday. Periodic flight restrictions are still
possible into early Wednesday...until a cold front pulls farther
offshore during middle week.
today...hazardous marine conditions will occur over the waters
today a strong low-level jet develops overhead and a coastal front
shifts inland. Mixing will be somewhat limited in the warm air
advection regime given the cold shelf waters in place. This should
limit winds to 20-25 knots with gusts to 30 knots. The only exception
will be over the eastern portions of the Georgia offshore waters
were water temperatures are in the Lower-Middle 70s per latest modis
SST imagery. This will support much deeper mixing profiles which a
risk for frequent gusts to 35-40 knots gales. Per collaboration with
the ocean prediction center...a Gale Warning will be posted for
the Georgia offshore waters through this evening. Small craft
advisories will remain for the nearshore waters and will be
expanded to include the Charleston Harbor. Seas will build to 5-8
feet nearshore waters...except 6-10 feet Charleston County waters and
7-12 feet over the Georgia offshore waters.
Tonight...hazardous marine conditions will persist over the waters
tonight with the area firmly embedded within the warm sector.
Winds will diminish some after midnight as the axis of the low-
level jet begins to move off to the east/northeast...but both
winds and seas will linger well above Small Craft Advisory
thresholds. Seas will diminish to 4-8 feet nearshore waters...highest
over the Charleston County waters...with 7-9 feet over the Georgia
Monday through Wednesday...a slow moving cold front over the
southeast states on Monday will move into and through the waters
by Tuesday. Low pressure will then form along the lingering front
just outside of the marine area and eventually pull the front away
from the region by Wednesday. Ongoing small craft advisories are
expected to end Monday. However...the location and movement of
the developing low pressure system could impact marine conditions
through middle week and may support additional advisories at a later
High surf...a high surf advisory will be issued for the southern
South Carolina coast for this afternoon into early Monday morning.
Seas are forecast to build rapidly into the 6-10 feet range
today...with the highest seas occurring over the waters off
Charleston County. Seas at buoy 41004 are forecast to rise to 12
feet...which will yield breaking wave heights of 5-7 feet along the
lower South Carolina coast...especially along east and southeast
facing beaches. Breakers of 4-5 feet are expected along the north
Georgia coast...but the orientation of the wave energy with respect
to the coast suggest the greatest risk for high surf will remain
along the lower South Carolina coast...especially along the
beaches of Charleston County.
anomalously high precipitable waters of 1.75-2 inches coupled with intense upper
dynamics will support widespread heavy rains with embedded thunderstorms
across southeast South Carolina and southeast Georgia during the
period. Storm total amounts of 2-4 inches with locally higher
amounts can be expected...with the bulk of the heaviest rains
falling through tonight. The risk for flash flooding will be
mainly confined to the coastal counties where the combination of
heavy rain and above normal high tides will support a higher risk
for flash flooding...especially in the Charleston...Beaufort and
and Savannah metropolitan areas. However...more localized/urban flooding
will be a possibility across inland areas.
record precipitation for 23 November...
kchs...0.70 inches set in 1961.
Kcxm...1.17 inches set in 1942.
Ksav...2.11 inches set in 1948.
SC...high surf advisory from noon today to 5 am EST Monday for
Marine...Small Craft Advisory until 5 PM EST Monday for amz352-354.
Small Craft Advisory until 1 am EST Tuesday for amz350.
Gale Warning until 11 PM EST this evening for amz374.
Small Craft Advisory until 11 PM EST this evening for amz330.