Scientific Forecaster Discussion
fxus62 kchs 251755
Area forecast discussion
National Weather Service Charleston SC
155 PM EDT Fri may 25 2018
a trough of low pressure will persist inland for the next few
days. Developing low pressure over the Gulf of Mexico will
slowly move north toward the central Gulf Coast this weekend,
possibly lingering through early next week. This pattern will
maintain higher than normal rain chances across the area.
Near term /through tonight/...
as of 150 pm: convection was located generally west of the
coastal counties and was tracking inland. Latest visible
satellite detected towering cumulus over the coast, but nothing more than
sprinkles has been reported. I will update the forecast to
mention isolated thunderstorms over both land and water.
As of 1020 am: visible satellite indicated that a sea breeze has
developed along the coastline, drifting east around 10 kts. Kclx
has detected a few showers along the sea breeze, especially over
the SC lowcountry. I will update the forecast to increase pops
along the sea breeze. In addition, based on little to no
normalized cape on the latest sfc analysis I will remove the
mention of thunder until after noon.
today: the stagnant mid level vorticity area across eastern Georgia
is forecast to lift north and weaken today while deep layered
ridging remains offshore. The overall pattern remains similar
with moisture fairly deep across the region. This is especially
true along and east of I-95 where our more substantial pops in
the 50-60 percent range were aligned along the lingering axis of
deeper moisture. Layered clouds around the region this morning
will make for a difficult forecast of convective initialization
and coverage into the early afternoon. We think chances for sea
breeze showers and storms will persist along all of the coastal
corridor. After mid afternoon, convection should be concentrated
further inland with good consensus from the latest cams and 00z
guidance. It appears that the departure of the embedded low-mid
level eddy bordering our inland zones will diminish focus for
convection and while there will be some pocket of heavier rains
in multicells, we think the potential for coverage of downpours
will be somewhat less than Thursday. Risk for strong to severe
storms looks low once again with poor environmental lapse rates
and less than stellar downdraft cape potential. Highs should
average out in the mid 80s with some upper 80s where we see more
Tonight: after evening convection wanes, most areas can expect
a dry night. Once exception may to the east of I-95 in SC where
a few showers may develop late as some weak instability tries
to advect onshore. Another round of stratus development is
likely inland but fog potential does not appear to be very
impressive at this point. Temps will only fall to the upper 60s
to lower 70s from inland to coast.
Short term /Saturday through Monday/...
Saturday will be the driest day of the period as weak mid/low level
ridging settles over the area. Isolated to scattered storms will be
confined to inland areas, where there will be just enough
instability to overcome the mid-level dry air and immediately along
the sea breeze closer to the coast. Mainly rain-free conditions and
pockets of sunshine will spell the warmest day of the period, with
highs in the upper 80s away from the beaches, with a few spots
As we transition into early next week, our eyes turn to the tropics,
where low pressure will strengthen in the northeastern Gulf of
Mexico, becoming the dominate weather feature for the southeast.
Though this system is expected to develop tropical characteristics,
the core of the storm will remain well to our southwest, and whether
or not it is deemed a tropical/subtropical cyclone has little
bearing on the forecast. The real story locally will be the
exceptionally moist airmass, sourced near the inter-tropical convergence zone well into the
southern Caribbean, that will fill in beginning early Sunday.
The first round of showers and thunderstorms is expected Sunday
morning along the initial surge of high Theta-E air, which will
progress northward through the area over the course of the day.
Modest diurnal instability could result in another active period of
showers and thunderstorms through the afternoon and early evening
hours Sunday, especially for southeast Georgia. Rainfall could be
heavy at times, but showers/thunderstorms should remain progressive
enough to limit the flooding potential to mainly just nuisance. The
exception is if rainfall persists into the evening hours Sunday,
when it would combine with elevated tides to result in an enhanced
flooding risk for low-lying areas near tidal waterways.
Global ensemble guidance shows the deepest moisture passing over the
area Sunday night and Monday morning, with pwats approaching 2.5 in,
or 3 Standard deviations above normal. Upper level difluence at the
same time should result in a rather wet period. If the area of
forcing moves slowly enough over the area, there could be flooding
concerns, but there is not enough agreement in the guidance to nail
down any details yet. Moisture advection will wane some beginning
Monday afternoon as the surface low shifts westward - though the
airmass will remain warm and humid enough to justify likely pops all
The severe risk is low Sunday and Monday, owing to limited
instability. Still, gusty winds are possibly in the strongest storms
as winds from a persistent southerly low level jet are mixed to the
Long term /Monday night through Thursday/...
unsettled weather will generally continue as the area remains
between low pressure near the Mississippi Valley and high pressure
over the sargasso sea. The expectation is for atmospheric moisture
content to trend downwards through the second half of the week,
though there is significant forecast uncertainty owing to
uncertainty in the position of the low as it progresses generally
northeastward. Temps will be near normal through the period.
Aviation /18z Friday through Wednesday/...
prior to the 18z tafs, convection was located generally west of
the terminals and was tracking inland. Latest visible satellite
detected towering cumulus near kchs and ksav, I will carry a vcsh through the
rest of this afternoon. Scattered stratcu is expected to develop
through the overnight hours, bases within MVFR range. Convection
should develop a few hours later on Saturday. I will indicate cu
building to broken coverage by mid to late morning. Winds will
remain light from the south through the 18z taf period.
Extended aviation outlook: VFR conditions will persist Saturday.
Increasing unsettled weather will ensue Sunday through the early
part of next week, and periods of flight restrictions in showers and
thunderstorms are possible.
an onshore mainly southeast flow will veer a bit more southerly
for most of the waters by tonight. Speeds most of the time will
be below 15 kt but a slight uptick will be noticed when the
gradient becomes just a bit tighter in the boundary layer prior
to 12z Saturday. Seas will average 2-3 ft near shore but some
increase to near 4 ft should occur later today and tonight over
Georgia waters from 20-60 nm off the coast.
This weekend will feature increasing southerly winds as low pressure
strengthens in the northeastern Gulf of Mexico. Seas will begin to
increase Saturday night reaching 4 to 6 ft, with higher values
beyond 40 nm, Sunday morning. Small craft advisories will likely be
needed for most coastal zones Sunday and Monday. Winds and seas will
moderate as we head into the middle of next week.