Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
1024 am EDT Friday Aug 28 2015
a stationary front will dissipate near the coast today. Atlantic
high pressure will remain in place into early next week. Tropical
cyclone Erika could impact the southeast United States by the
middle of next week. Refer to the latest advisory on Erika issued
by the National Hurricane Center.
Near term /until 6 PM this evening/...
low level stratus deck continues to hang on across much of
southeast Georgia well past sunrise. Updated sky cover and morning temperatures
to reflect this pattern. Still anticipate that we will break out
across the area within the next couple of hours and temperatures should
rebound quickly after that...so little chance in forecast high
temperatures. Otherwise...pop forecast is on track and morning showers
have remained focused along the stationary front so far this
Previous discussion...a decaying stationary front will extend
from the South Carolina coastal waters into the I-95 corridor of
southeast Georgia today as high pressure tries to nudge in from
the north. Models are similar in showing the better chances for
showers/thunderstorms focusing roughly along/east of a Glennville-
Savannah-Hilton Head line today where the southwest-northeast
oriented resultant sea breeze will intersect the decaying
stationary front. This enhancement to the low-level convergence
field combined with modest instability and weak uvm induced by
passing impulses of middle-level vorticity ejecting northeast ahead
ahead of a pronounced north-south shear axis over the northern
Gulf/southern Mississippi Valley will support isolated to
20-40 percent probability of precipitation look reasonable today given the synoptic
setup. It is very possible communities from Allendale to the
Charleston metropolitan area and points north will see little to no
convection as the atmosphere looks to become increasing capped in
response to lowering 700 hpa Theta-E associated with the southern
fringe of a shortwave ridge positioned over central/eastern North
Carolina and the Virginia Tidewater. Highs will range from upper
80s to around 90 inland to the middle 80s at the beaches.
Short term /6 PM this evening through Monday/...
tonight...convection will dissipate quickly this evening. Isolated
to perhaps scattered showers/thunderstorms could re-fire over the Atlantic
as low-level convergence increases along the western wall of the
Gulf Stream. With and east/northeast low-level flow in
place...some of this activity could make a run for the
beaches...especially from Hilton Head south to Darien and Sapelo
Island given the orientation of the coastline. 20-30 percent probability of precipitation
will be maintained overnight at the coast to account for this with
dry conditions inland. Lows will range from the lower 70s inland
with middle-upper 70s at the beaches.
Saturday into Saturday night...28/00z guidance suggests that drier
air will push into northern counties of the forecast area...while
the upper low centered west of the region will continue to push
moisture into southern counties of the forecast area. This scenario
could support a fairly significant north to south moisture gradient.
Accordingly...maximum probability of precipitation are graduated from slight chance north to
high chance south...and locally higher probability of precipitation could eventually be
required across the south. Precipitable waters around 2 inches and weak steering
flow could support locally heavy rain across the southern 1/2 of the
Sunday into Monday...the upper low should open and gradually eject
northeast as an open wave...perhaps breaking down into multiple
shortwave troughs which could cross the region. This scenario should
support scattered to numerous showers/thunderstorms. Sunday...the
ongoing north to south moisture gradient could persist...this probability of precipitation
are tapered from likely across the south to chance north.
Monday...the increasingly ill-defined moisture gradient shifts to a
west/northwest to east/southeast orientation...and probability of precipitation are
distributed accordingly with likely probability of precipitation across central/southern
coastal counties and adjacent coastal waters.
Near normal temperatures are expected through the period...although
locally cooler daytime high temperatures in the Lower/Middle 80s cannot be
ruled out where precipitation persists.
Long term /Monday night through Thursday/...
many forecast details depend on the highly uncertain track and
strength of Tropical Storm Erika next week. As of 5 am EDT...the
National Hurricane Center forecast shifted a weaker system farther
west...resulting in a landfall over southern Florida and nothing
stronger than a tropical storm tracking north over land through
early next Wednesday morning. Also of note...most invest tracks have
also shifted farther west...and of particular interest 28/00z
operational runs of the GFS and European depict an even weaker
system progressing farther W/NW...pushing into the Gulf of Mexico
then perhaps coming ashore along the northeast Gulf Coast. The
latest forecast still offers the potential for some impacts
including heavy rain, gusty winds, marine hazards, shallow coastal
flooding and even a threat for tornadoes around the middle of next
week. The GFS/Euro scenario could be accompanied by an influx of
enhanced tropical moisture from the south/southeast but otherwise
suggests no tropical cyclone impacts in our forecast area. Aside
from lowering wind speeds over land...significant changes were not
required for ongoing weather/probability of precipitation grids through this period.
Please continue to monitor the latest forecasts from the National
Hurricane Center and our office concerning Erika.
Aviation /14z Friday through Tuesday/...
low stratus at ksav has hung around through 14z but...based on
satellite observation...ceilings are expected to lift and VFR conditions
to return return very quickly this morning. Thunderstorms probabilities
look too low to justify a mention at either terminal
today...although probabilities are higher at ksav. Will not
mention thunderstorms and rain with the 12z taf cycle.
Extended aviation outlook...this weekend into early next week...
flight restrictions could occur with showers/thunderstorms and
in late night/early morning stratus/fog. Otherwise...mainly VFR.
Tuesday and beyond...conditions remain highly dependent on the
uncertain track and intensity of tropical cyclone Erika.
today...a bit of an east to northeast wind surge will work down
the coast today. The strength of surge is uncertain with the
latest NAM showing winds increasing to 15-20 knots for the South
Carolina coastal waters. This is higher than any guidance and is
likely due to the model/S depiction of a strengthening pressure
gradient induced by developing low pressure well offshore. It is
unclear whether this low will develop or not...but the NAM is
really the only model showing this signal. Will therefore discount
this solution and blend the various short term models to construct
winds through this afternoon. This yields speeds around 15 knots
Charleston County waters...10-15 knots Beaufort County waters with
around 10 knots for the Georgia waters. Seas will average 2-4 feet.
Tonight...the low-level wind field will relax a bit overnight with
speeds settling in around 10 knots for much of the night out of the
east or northeast. Seas will average 2-4 feet.
Saturday through Monday...on the periphery of Atlantic high pressure
east/NE winds during the late night/morning hours and east/southeast winds during
the afternoon/evening hours will average no more than 10-15 knots.
Seas of 2-4 feet Saturday will gradually build mainly due to
increasing long period swell from the east/southeast...and by late
Sunday through Monday 6-7 feet seas could push into amz374...with 4-5
feet seas spreading into nearshore waters.
Monday night through midweek...winds/seas will depend on the
uncertain track and intensity of Tropical Storm Erika. Based on the
latest forecast track pushing a weaker system inland over the
Florida Peninsula...lowered winds/seas over coastal waters during
this period. However...winds/seas could still become hazardous for
Please stay tuned to the latest forecasts from the National
Hurricane Center and our office concerning Erika.
Rip currents...slowly building east/southeast swell combined with
increasing coastal winds will support a moderate risk for rip
currents for all beaches today. A persistent long period swell...
onshore winds and astronomical influences will likely support an
elevated risk of rip currents this weekend. If tropical cyclone
Erika approaches the region next week...dangerous rip currents
could develop on area beaches.
this evening...expect positive tidal anomalies to increase today
as strengthening east to northeast winds help to pile water into
the South Carolina and Georgia coast. Early morning surge guidance
brings tide levels to shallow coastal flooding thresholds at both
the Charleston Harbor and fort Pulaski tide gages...but the
forecasted levels look a bit too high given previous tidal trends
and expected wind speeds. Even after compensating for this...tide
levels at the Charleston Harbor are still expected to peak 7.0-7.2
feet MLLW so will go ahead and hoist a coastal Flood Advisory for
all of the South Carolina coastal zones...valid from 6pm-10pm
tonight. Levels at fort Pulaski are expected to only peak 9.0-9.2
feet MLLW...which is at or just below coastal Flood Advisory
criteria for coastal Georgia. Confidence is therefore not high
enough to justify the issuance of a coastal Flood Advisory for any
portion of the Georgia coast with this forecast cycle. Tide trends
will be monitored carefully throughout the day.
Saturday through Monday...astronomical influences and onshore winds
will maintain elevated high tide levels. The evening high tides are
forecast to exceed shallow coastal flooding levels on Charleston
Harbor and should at least approach coastal Flood Advisory
thresholds farther south including the Georgia coast. On Charleston
Harbor...even the morning high tides are forecast to approach 7 feet
mean lower low water each day.
Tuesday through Wednesday/Thursday...astronomical influences will
wane. However...if the latest forecast from NHC for Erika holds...we
will have the potential for more significant rises in The Tides
along with some degree of coastal flooding.
With regards to potential conditions next week...please stay tuned
to the latest forecasts from the National Hurricane Center and our
office concerning Erika.
SC...coastal Flood Advisory from 6 PM to 10 PM EDT this evening for