Scientific Forecaster Discussion
fxus62 kchs 220814
Area forecast discussion
National Weather Service Charleston SC
414 am EDT Tue Aug 22 2017
high pressure will extend across the area through Wednesday
with a weak low eventually developing inland. A cold front is
expected to pass over the region on Thursday. A wave of low
pressure could form along the front offshore Friday. Slightly
cooler and drier conditions are then expected into early next
Near term /through tonight/...
calm winds, the wet grounds and near 100% relative humidity will allow for fog
and low stratus in some areas early this morning, with perhaps
the fog becoming locally dense in spots where the higher clouds
remain thin enough. Temps will begin the day in the lower and
Today: a piece of short wave energy remains in place locally within
a weakness of a fairly prominent ridge aloft that begins to
shift a little south as a long wave trough shifts through the Ohio
Valley to the Appalachians. Concurrently at the surface we find
the formation of a weak Piedmont trough with the western part of
sub-tropical ridging across the local area. Considerable deep
moisture from both the Gulf and Atlantic remains anchored in
place within a a deep S-SW flow throughout the vertical, with
pwat's well in excess of 2" or greater than 130% of normal. Lift
provided by both the sea breeze, nearby inland trough and the short
wave aloft, plus at least modest instability will act to enhance
Isolated to scattered activity will prevail this morning, mainly
over the eastern third of the County warning forecast area where the best convergence
and isentropic ascent occur in tandem with higher MLCAPE around
1000 j/kg. Then as the sea breeze pushes inland this afternoon
rain chances will ramp up at least into the high-end scattered
category, but once radar trends are better defined we expect
likely pop's or greater to occur for parts of the region. There
isn't any severe risk, but heavy rains will definitely be a
concern given weak storm motion and the abundance of moisture.
Some places can pick up a quick 1-2 inches in an hour or less.
The flash flooding risk is low, but given many places getting
rain yesterday it won't take much to cause at least minor to
moderate flooding. Lightning will also be prevalent in all
Considerable cloud cover and the expected rains will keep temps
from getting any warmer than upper 80s or around 90f, about
where they should be for August 22. We attempted to alter the
hourly temp curve to account for rain-cooled conditions this
afternoon, so expect many places to drop into the 70s where the
steadier convective rains develop.
Tonight: with a stronger short wave trough dropping through the
broad long wave trough upstairs, it shunts the ridge to the
south, while at the surface the periphery of the Atlantic ridge
holds on. Convection will have faded quickly inland this
evening, leaving US with several hours of rain-free conditions.
But there is still some isentropic ascent on the 300k surfaces
with with some convergence within the S-SW planetary boundary
layer flow. This can cause isolated or scattered convection
from the Atlantic to move onshore after midnight, mainly
impacting the coastal counties with 20-30 pop. There is likely
too much cloud cover for any serious fog concerns, and little to
no indications looking at soundings, MOS guidance and/or hi-res
model visibility forecasts. Temps generally down into the mid
and upper 70s for the coastal corridor, 73-74f far inland.
Short term /Wednesday through Friday/...
Wednesday, the center of the 500 mb ridge should push south near the
Panhandle of Florida as a l/west trough swings over the Appalachians during
the afternoon. Forecast soundings indicate that the inversion will
lift to around h75 and inhibition should fall below 10 j/kg. Flow
should shift from the S to south-southwest during the daylight hours Wednesday.
A slow moving sea breeze will likely serve as a focus for moisture
convergence, supporting at least sct thunderstorms over the forecast
area during the day. High temperatures are forecast to range from
near 90 over the beaches to the mid 90s within the mid Savannah
Thursday: guidance has been consistent with the timing of a cold
front across the County Warning Area during the daylight hours Thursday. The axis of
the 500 mb trough is expected to slide over the region during the
afternoon. The arrival of the trough over the slowing front will
likely support a frontal wave to develop, the GFS and European model (ecmwf) indicate
the wave developing over the coastal plain of SC/GA. Given the
passage of the mid level trough and sfc front, precipitable water near 2 inches, level of free convection
less than 4 kft, and widespread weak to moderate instability, I will
maintain likely pops. 850 mb cold air advection is not expected until Thursday night,
highs should range around 90 degrees for most areas Thursday
afternoon, possibly upper 80s north of the edisto river basin.
Friday: building high pressure centered to the north will result in
sfc winds to shift from the NE. Guidance indicates that the cold
front will likely stall near the coast, possibly kinked west across
southeast Georgia. Given lingering moisture and instability, coverage of showers
and thunderstorms should expand during the daylight hours on Friday,
greatest over southeast Georgia and adjacent waters. High temperatures should
range in the mid to upper 80s.
Long term /Friday night through Monday/...
high pressure sourced from Canada will remain centered well
north of the region through this week. Confidence in the day 5
through 7 forecast remains very low. Medium range guidance
indicates that the sfc high center will shift over New England,
ridging SW across the Carolinas into Georgia. In addition, both the
GFS and European model (ecmwf) show a coastal low developing off the Georgia/SC coast
by late this week, with run to run trends shifting east and
south. In addition, guidance suggests that low pressure
associated with the remnants of Harvey will take a curved track
from south Texas Friday night to MS valley by Tuesday. With a low
to the east and west and ridging over the Carolinas, it appears
at least cloudy and possibly a wet period. Perhaps one area
where confidence is medium, the complex pressure pattern should
support prolonged NE winds across the County Warning Area. NE flow should yield
afternoon dewpoints in the upper 60s inland by this weekend. I
will favor the marine and near shore zones for chc to schc pops
through much of the day 4 through 6 period. By Tuesday,
strengthening llvl flow and deeper moisture may provide greater
coverage of convection, I will forecast chc pops across the County Warning Area.
Daytime temps should see limited warming given the cloud cover
and NE winds, mid 80s should be common through the medium range.
Aviation /08z Tuesday through Saturday/...
a few showers and a little fog/stratus will impact ksav and
perhaps kchs this morning, but any flight restrictions would be
brief and marginal. Looks like another good shot at rain showers/thunderstorms and rain
with flight restrictions along the sea breeze, aided by energy
aloft during the afternoon from about 16-22z. Temporary flight
restrictions are forecast from 17-21z at both terminals, mainly
into the MVFR range. But a period of IFR conditions is possible
especially since rain could be heavy at times. Current
indications are that VFR conditions will return this evening,
although isolated- scattered convection could redevelop beyond
Extended aviation outlook: thunderstorms may result in short
periods of flight restrictions, greatest potential during the
afternoon and early evening. Numerous thunderstorms should
develop along and ahead of a cold front during the daylight
today and tonight: the coastal waters will lie under the
influence of high pressure extending onshore from the Atlantic,
while a Lee side trough is found far Inland. Sea breeze
circulations will cause southerly winds as high as 10 or 15 kt
this afternoon and evening (unless altered by convection), then
veering around to the SW with land breeze influences overnight.
Seas will average 2-4 ft, highest across the outer Georgia waters.
Waterspouts: we're keeping a sharp eye on the possibility of a
convergent boundary forming near the coast this morning, and if
so then there could be an elevated risk of rip currents. No
mention in any supplemental product at this time.
Wednesday through thursday: weak high pressure on Wednesday will
give way to a cold front on Thursday. Through this period, winds
will generally range from southeast to SW with speeds 5 to 15 knots, with
some gusts to near 20 knots. Seas will range 2 to 4 feet.
Friday through sunday: the pressure pattern is expected to feature
low pressure off the Carolina/Georgia coast with another low
pressure system expected over East Texas/la by late this weekend. Between
the two lows, high pressure is expected to ridge SW from a New
England centered high. Gusty NE winds maybe common across the marine
zones by this weekend, with gusts between 20 to 25 kts. Wave heights
may increase to 3 to 6 feet within 20 nm, with 8 to 9 feet beyond 30
Rip currents...there remains around a 2 ft swell every 10
seconds that reaches the surf zone today, and that along with
onshore winds developing and lingering astronomical influences
will support a moderate risk of rip current at area beaches.
tide levels could flirt with 7.0 ft MLLW in Charleston Harbor
with the high tide this evening, and if so a coastal flood
advisory could be required for Charleston, coastal Colleton and
Beaufort counties. This information will be included in the
hazardous weather outflow. Conditions will likely stay below
advisory levels for Jasper County south to McIntosh County.
the kclx radar continues to be down due to lighting strikes.
Parts are on order but the radar is not expected to return to
service until Tuesday afternoon at the earliest.
The temperature and dew point sensors at the downtown
Charleston observation site (kcxm) could periodically fail.
Technicians plan on fixing the problem once parts arrive.