National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KCHS 030908
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
408 AM EST Wed Mar 3 2021

.SYNOPSIS...
Low pressure will move across the area this morning and push
farther offshore later today. High pressure will then prevail
with another storm system likely brushing the area this
weekend. High pressure will return again early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Low pressure is starting to deepen off the southeast Georgia
coast as it moves across the warmer Gulf Stream waters. The low
will intensify a bit this morning, before it pulls further away
to the east this afternoon, as higher pressure starts to build
in from the west late in the day. The low is being driven by a
strong mid level short wave that will pass through this
morning, and into the ocean for the afternoon.

Upper level divergence/difluence within the right entrance
region of the jet aloft, plus strong frontogenesis through about
15Z will support widespread rainfall this morning. PWat as
great as 1.25 to 1.75 inches, plus a deep, warm cloud layer will
support efficient rainfall rates, with as much as another 1.00
to 1.75 inches of rainfall to occur. However, there will be
locally higher amounts, especially given the probabilities from
the HREF which depict as much as a 30-50% chance of amounts
exceeding 1 inch in 3 hours times early this morning. There is
even a 10% chance of more than 3 inches in a 3 hour block
through 12Z, centered in the Charleston region. These expected
rain amounts will generate minor flooding concerns, mainly for
the already swollen rivers, as well as the typical flooding in
urban and poor drainage locations.

Elevated instability will produce isolated thunderstorms over
mainly the coastal counties early this morning.

Rain chances are a solid 100% prior to daybreak, with coverage
to steadily decrease from the west-southwest through the
morning as the short moves through. Probabilities will be down
to just 20/30% by 17-18Z, then rainfree thereafter.

Temperatures are certainly a challenge, dependent upon how much
clearing sets in for the afternoon. It`s somewhat surprising
that all guidances seems to support max temperatures reaching
within a couple of degrees either side of 60F. Since that
meshes well with our surrounding WFO`s, we went with that in the
forecast. But given that much of the energy from the insolation
that develops will be utilized to evaporate the standing
rainwater, we wouldn`t be surprised if these values are too
warm.

Conditions will be breezy to windy along the coastal counties
this morning, when the best pressure falls followed by pressure
rises will occur in response to the proximity of the surface
low. North and northeast wind gusts will reach as high as 25 or
30 mph at times, especially along the coast.

Lake Winds: Conditions will be close to advisory criteria this
morning. But given only weak cold advection and a more stable
mixing profile, we capped winds at 15-20 kt. We`ll continue to
monitor for a Lake Wind Advisory.

Tonight: An expansive and consolidating region of high pressure
from southern Canada to the Gulf of Mexico will build into the
area, leading to clear skies and a mostly decoupled atmosphere.
This looks to produce good radiational cooling, maybe offset a
little by the wet grounds. Still we are showing minimum
temperatures in the upper 30s and lower 40s inland, with mid-
upper 40s along the coast and in downtown Charleston.

One concern that is not shown in the forecast will be the
possibility of some fog. Given that the grounds will still be
wet, we will have to watch for its formation during the late
evening and overnight.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
Moderate to high confidence through Friday night due to decent model
agreement, then low confidence Saturday as models diverge a bit with
respect to the impact of an upper trough moving through. High
pressure is expected to begin the period but a relatively weak
backdoor cold front looks to push through Thursday night with
stronger and chillier high pressure returning Friday into Saturday.
At this point it looks to dry so the front should come through rain-
free. Later in the day Friday we should start to see higher clouds
increase from the west ahead of an upper trough. Deeper moisture and
better upper forcing are expected Friday night into the first part
of Saturday so there could be some showers, mainly in GA, although
confidence in coverage/amounts remains low.

Temperatures should generally be near to below normal, mainly
50s/60s for highs and 30s/40s for lows.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
High confidence this period as models are in decent agreement
keeping high pressure in control. The main forecast challenge will
revolve around temperatures, which look to be near or below normal
until Wednesday. Still possible to see some frost and/or freezing
temperatures inland each morning through Tuesday.

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
Flight restrictions will persist overnight into Wednesday
afternoon, as low pressure passes off the nearby coast. Ceilings
will be down in the IFR/LIFR range, while visibilities will
vary from VFR to MVFR, maybe even occasionally IFR in light to
moderate rains. While TSRA can`t be ruled out, it will be
limited enough in areal coverage to not include with the 06Z
TAFs at KCHS. However, recent radar and lightning trends within
an area of elevated instability, supports the inclusion of VCTS
at KSAV early this morning.

As the rains taper off during the late morning at KSAV and by
early afternoon at KCHS, conditions will quickly approve as the
atmosphere dries out from the top, down. VFR is currently
forecast again by 21Z at KCHS and KSAV.

NE winds into early Wednesday will gradually back around to the
NW during the afternoon. Depending upon the exact track and
strength of the surface low, winds will be gusty at times.

Extended Aviation Outlook: High confidence in VFR through Friday and
again starting Sunday. However, flight restrictions are possible
Friday night into Saturday night, mainly at KSAV, as a storm system
possibly impacts the area.

&&

.MARINE...
Today: Forecast soundings, guidance from the HRRR, RAP and NBM,
plus 70-90% probabilities of greater than 30 kt per the HREF
supports the issuance of a short duration gale Warning for the
Charleston County Atlantic waters and the outermost Georgia
waters beyond 20 nm out. These conditions occur during the
morning, when low pressure off the southeast Georgia first
deepens, followed by increasing pressure rises as it pulls
further out by afternoon. The highest speeds look to be around
7-11 AM on the AMZ350 waters, and from 9 AM to 12 PM over the
AMZ374 waters. The other marine zones will be close to Gales at
times, but have opted for solid Small Craft Advisories on the
other waters. Seas will be as high as 5 to 8 ft within 20 nm of
the shore, but up to 7 to 9 ft further out.

Conditions will gradually improve this afternoon as pressure
climbs slow down, the low pulls further into the ocean, and more
of an anticyclonic pressure pattern develops. Once the Gales
come to an end early in the afternoon, they`ll need to be
replaced with Small Craft Advisories.

Mariners should also be alert for isolated thunderstorms this
morning, the combination of the nearby low and also energy in
the mid and upper levels of the atmosphere passing through.

Tonight: Continental high pressure will continue to slowly
build in behind the surface low that will pass out near Bermuda.
Winds will diminish to less than 15 or 15-20 kt, but the
elevated seas will take much longer to drop to 5 ft or less;
probably not until around 10 PM to 12 AM within 20 nm, and not
until after 2 or 3 AM on the outer Georgia waters.

Thursday through Monday: Moderate to high confidence that conditions
will be below Small Craft Advisory levels most of the period except
Friday night into Sunday when a storm system may track close enough
to the area to push winds to at least 25 kt and seas to at least 6
ft, especially in the offshore GA waters.

&&

.TIDES/COASTAL FLOODING...
Tidal departures continue to increase along the coast, and
dependent upon how quickly winds back more northerly will be key
in determining levels with the late morning high tide. We
currently anticipate that it won`t reach Coastal Flood Advisory
criteria, but given tidal anomalies of 1.8 to 2.1 ft MLLW early
this morning, conditions will be close. We will address this
concern in the Hazardous Weather Outlook.

&&

.CLIMATE...
Record rainfall for March 3:
KCHS: 1.81 inches/2012
KCXM: 1.73 inches/2012
KSAV: 2.70 inches/1948

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 1 PM EST this afternoon for AMZ330.
     Small Craft Advisory until 7 PM EST this evening for AMZ352-
     354.
     Gale Warning until 1 PM EST this afternoon for AMZ350-374.

&&

$$

NEAR TERM...
SHORT TERM...RJB
LONG TERM...RJB
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...
CLIMATE...