Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
347 am EDT Tuesday Sep 16 2014
..one last day of heavy rain and possible flash flooding...
a strong cold front will approach from the northwest today and
push through the region late Wednesday. An inland wedge of high
pressure will then prevail in the wake of the front into the
weekend. Another cold front will approach from the west early next
Near term /through today/...
through sunrise...stratus build down has resulted in areas of
dense fog across mainly the Charleston tri-County area early this
morning. A dense fog advisory will be issued for this area until 9
am for visibilities 1/4 nm or less. There is a chance the fog may lift
or visibilities improve above 1/4 mile prior to 9 am as convection moving
over the midlands could brush portions of the tri-County area...
but its unclear whether this will happen. Areas farther to the
south are also being monitored for possible dense fog...especially
over areas that saw significant rains Monday. An expansion of the
dense fog advisory may be needed over the coming hours.
Today...one more day for heavy rain and possible flash flooding.
Weak shortwave energy moving across the Central Plains this
morning is forecast to move quickly east/southeast today...
approaching the southeast U.S. This afternoon. Models show broad
difluence developing ahead of this feature while the right
entrance region of yet another 250 hpa jet streak passes overhead
during peak heating. A northwest flow aloft will result in the
sea breeze circulation being pinned to the coast for much of the
day before slowly spreading inland late. It appears convection
will likely initiate then concentrate along the pinned sea breeze
circulation by middle-afternoon and spread very slowly inland
coincident with the sea breeze itself. Several high resolution
models show a distinct signature that an area of concentrated
convection will cluster in the Savannah...Darien...Springfield...
Glennville area by middle-late afternoon. This area of convection
will pose a risk for flash flooding. See the hydrology section
below for additional information on the threat for heavy rain and
Probability of precipitation will be increased slightly across the coastal zones this
afternoon...ranging from 60 percent across the Charleston metropolitan
area to 70 percent from Beaufort and Savannah south...including
areas south of I-16. Father inland...more scattered convection is
expected...but will likely be a bit more loosely organized. Probability of precipitation
of 30-50 percent will be maintained in this area. Highs will top
out in the upper 80s inland with middle 80s at the beaches.
Short term /tonight through Friday/...
tonight...convection will quickly wane this evening with the loss
of insolation. The risk for rainfall will gradually shift offshore
closer to the western wall of the Gulf Stream as a cold front
quickly approaches from the north. The front looks to remain north
and northwest of the area through sunrise Wednesday. Some clearing
will likely take hold early Wednesday...especially across the
western zones. Will have to watch for possible low stratus and fog
once again given the wet grounds and the light wind fields that
will be in place.
Wednesday...the surface trough will have shifted off the southeast
coastline by daybreak...just downstream of a series of shortwaves
embedded within the base of a sharpening upper trough axis.
Progression of a strong cold front through the forecast area
will likely be delayed until the evening or nighttime
period...when a more distinct air mass change is evident along the
leading edge of strong high pressure building from the
west/northwest. The ribbon of deep layer moisture responsible for
several days of heavy rain earlier in the week will have moved to
the east/southeast with the surface trough. However...there
appears sufficient residual moisture to support scattered
thunderstorm development during the afternoon time
frame...especially as shortwave energy tracks through the region
and seabreeze development provides some forcing/convergence.
Isolated showers/thunderstorms could accompany the front toward
the coastline through midnight...and then expect best rain chances
to shift over the marine zones during the pre-dawn hours. High
temperatures will once again rise above seasonal normals within
downslope flow and pre-frontal compression...peaking in the upper
80s away from the locally cooler coastline. Low temperatures will
still be a degree or two above normal...yet trend cooler than
previous nights by falling into the middle to upper 60s.
Thursday and Friday...northeast flow developing in the wake of the
front and within building high pressure from the north will
support a notable cooling trend late in the week. High
temperatures on Thursday will only peak in the low to middle
80s...while only rising to the lower 80s for Friday afternoon.
Nighttime low temperatures will also reflect the air mass
change...as lower dewpoints/drier air allow temperatures to fall
into the low to middle 60s. As the center of the surface high tracks
from the Great Lakes region Thursday and into the New England
states Friday...the inland wedge will amplify down the
Appalachians and a coastal trough will take shape on the southern
coastline. This pattern could allow a distinct gradient in
precipitation potential between the drier inland areas and coastal
counties near convergence along the trough. Rain coverage will
still be considerably lower than earlier in the week for all
locations...and will advertise just isolated showers/thunderstorms
inland and scattered along the coast...mainly during the afternoon
Long term /Friday night through Monday/...
a wedge of surface high pressure inland and a coastal trough will
control the pattern through Saturday...before breaking down Sunday
ahead of another strong cold front approaching early next week.
Expect best rain chances to be found along the coast and over the
marine zones through the weekend...as these locations will be
closest to moisture and convergence associated with the coastal
trough. High temperatures will once again be suppressed in the
lower 80s for Saturday afternoon...before conditions begin to
moderate Sunday as the inland wedge pattern weakens. Medium range
models are suggesting that the next cold front could move through
the region by Tuesday...yet are not indicating a substantial
amount of precipitation associated with its progression. Will thus
remain rather conservative with rain chances toward the end of the
forecast period...capping probability of precipitation in the slight chance to low chance
range for Monday and Tuesday.
Aviation /06z Tuesday through Saturday/...
low stratus and fog is the primary concern through sunrise.
Kchs...cigs/vsbys have already dropped at kchs...but conditions
have been bouncing around. IFR conditions appear likely to persist
through sunrise and may drop to prevailing LIFR. Maintained tempo
LIFR conditions for now. VFR expected to return 13-15z. Chances
for thunderstorms this afternoon appear to low right now to justify a
Ksav...VFR for now...but conditions are expected to lower to
prevailing IFR after 09z. There is a chance for LIFR
conditions...but its unclear at this point whether that will
occur. Amendments may be needed. VFR by 13-15z with thunderstorm impacts
possible 19-22z. Limited thunderstorm impacts to MVFR for now.
Extended aviation outlook...periodic flight restrictions possible
in occasional rain showers and/or thunderstorms and rain during the afternoon and early
night time periods...and in late night/morning low stratus and/or
today...near term concerns centers on the potential for dense fog
moving in the Charleston Harbor. Harbor webcams suggest ceilings are
slowly lowering...but it remains unclear whether visibilities will drop
enough prior to sunrise for a marine dense fog advisory. Will
maintain a mention of areas of fog for now and continue to monitor
trends for a possible marine dense fog advisory. Otherwise...light
winds will persist again today. Light west/southwest winds will
turn more southerly this afternoon as the resultant sea breeze
takes hold. Speeds will remain less than 10 knots with seas 1-2 feet.
Tonight...long period swells from Hurricane Edouard will begin to
affect the waters with periods of 11-14 seconds. Seas of 1-2 feet
will build to 2-4 feet nearshore waters and 4-5 feet offshore waters.
At this time it appears seas will remain below Small Craft Advisory
thresholds...although it will be close for the Georgia offshore
Wednesday and Thursday...a cold front will progress into and
through the waters during the middle week period. Light winds in
advance of the front Wednesday will become north/northeast and
increase in its wake by Thursday. Seas will be comprised mainly of
long period swells from distant tropical cyclone Edouard...around
2 to 4 feet within 20 nm and 3 to 5 feet farther offshore.
Friday through Sunday...the pressure gradient will become quite
pinched as the cold front stalls far offshore and remains fairly
stationary along the periphery of an inland wedge of high
pressure. Small Craft Advisory conditions are possible for
portions of the marine zones Friday and Saturday...before
conditions moderate within a transitioning pattern by Sunday.
Rip currents...long period swell from Hurricane Edouard will
arrive at the beaches late this afternoon and continue into
Thursday. There does not appear to be much more than about 1 feet
swell that makes it to the beaches by late afternoon...so the rip
current risk will remain in the low category for today. Larger
swells around 3 feet will occur Wednesday into Thursday and this
will lead to an enhanced risk of rip currents. For Friday through
Sunday swell from Edouard will diminish...but stronger NE winds
will occur and this may allow for an elevated risk of rip currents
to again prevail at area beaches.
one more day for heavy rain and possible flash flooding. High
resolution models are showing a distinct signal that an area of
heavy rain will setup in the area from Savannah south to Darien
and over to Springfield and Glennville for several hours this
afternoon along a pinned sea breeze circulation. Although storm
motions will be faster than previous days...the potential for
numerous mesoscale boundary interactions...strong upper divergence
and high precipitable water values near the 99th percentile suggest there is a
high potential for excessive hourly rainfall rates exceeding 2-3
in/hr...which is higher than the latest 1-hour flash flood guidance.
In addition...some areas have already seen in excess of 6 inches
over the past few days.
A Flash Flood Watch will be issued for areas south of I-16 in
southeast Georgia and the coastal zones of southern South Carolina
through 8 PM EDT as these areas have the highest potential for
flash flooding today. The coastal South Carolina zones will be
included in the watch mainly to account for possible heavy rain
falling around high tide this afternoon. Similar to the past few
days...isolated rain amounts in excess of 5 inches will be
Georgia...Flash Flood Watch through this evening for gaz114>119-137>141.
SC...Flash Flood Watch through this evening for scz048>052.
Dense fog advisory until 9 am EDT this morning for scz044-045-