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Area Forecast Discussion
339 FXUS62 KCHS 290018 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 818 PM EDT SUN AUG 28 2016 .SYNOPSIS... High pressure centered to the north will prevail into Monday. Then through mid week, Tropical Depression Nine is expected to track northward through the Gulf of Mexico. By late week, the system is expected to move across Florida and off the Southeast coast as a Tropical Storm. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... This evening: Showers and thunderstorms have diminished considerably in coverage within the prevailing northeast flow. Any lingering activity has become concentrated across the southeast Georgia coast. This activity seems to be mainly supported by a weak upper level disturbance that is situated near the lower Georgia coast. With this disturbance around, and other noted upstream, we will likely see isolated showers and thunderstorms continue to pop up through the night mainly along the coast and over the coastal waters. Models also show some subtle low level convergence along the coast where inland flow will be more northeasterly and flow over the waters will be more east- northeasterly. This could account for the coastal redevelopment of showers and storms that some of the models show late tonight. The only real impact will be brief periods of moderate to heavy rain. Clouds will likely dissipate for a time before increasing in coverage toward day break. Look for lows in the low to mid 70s inland, increasing to mid to upper 70s at the coast. && .SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/... Tropical Depression 8 is forecast to near the North Carolina Outer Banks Tuesday before ejecting out to sea in response to the development of weakness in the upper ridge. No direct impacts are expected this far south. Upper shear axis/upper low will meander along the Southeast U.S. coast through the period. This will promote scattered showers and possibly a few tstms each afternoon with the low-level flow regime supporting a risk for isolated to perhaps scattered nocturnal showers impacting coastal locations. The convective pattern should be similar each day; initiating along the coast then moving inland as the afternoon progresses. Pops will be capped at 30-40% each afternoon as confidence on the placement and timing of the greater rain chances is just not high enough to higher pops attm. The 28/12z GFS spins up weak low pressure off the Georgia coast Wednesday along a weak surface trough. This weak cyclogenesis scenario is not really supported by the ECMWF nor most of the GFS ensemble members and will be discounted, for now. Highs will range from the lower 90s inland to the mid-upper 80s at the coast with overnight lows ranging from the mid 70s inland to around 80 at the beaches. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... NHC indicates that what is currently Tropical Depression Nine will move northward through the Gulf of Mexico and strengthen into a Tropical Storm. The forecast then calls for the Tropical Storm to cross north Florida and move northeastward off the Southeast coast. Have had make some adjustments to much of the extended period to take into account the expected track of the low per the NHC forecast. There is still much uncertainty on the track and intensity of this system, so confidence is low on any direct impacts. Additional changes are likely over the coming days. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... VFR conditions are likely to prevail through the TAF period, with the main issue to consider being the possible development of MVFR ceilings late tonight. Overall confidence is low but the forecast does reflect a low end VFR ceiling at both sites closer to sunrise. There will also likely be showers and storms developing along the coast late tonight within a subtle convergence zone. Models show the bulk of the near sunrise activity occurring along the Georgia coast, so have added a VCSH at KSAV. Then for Monday, another round of scattered showers and storms for the area. Confidence is low in the probability of direct impacts, so no restrictions or thunderstorms have been introduced into the forecast. Extended Aviation Outlook: Primarily VFR. && .MARINE... This Evening and Tonight: East winds will prevail over most waters this evening and early tonight while an inverted trough is in place over coastal waters. Winds should then tip back to northeast late tonight over northern SC waters. Outside convective activity, conditions are expected to remain below Small Craft Advisory levels with wind speeds no higher than 15 kt and seas between 2-4 ft, mainly due to long period swell from the east. Monday through Wednesday: Tropical Depression 8 will approach the Outer Banks Tuesday and stay well northeast of the local marine area. An east/northeast flow will persist through the period as a weak trough remains just off the coast. Winds will remain 15 kt or less with combined seas holding 2-4 ft nearshore and 4-5 ft offshore. Long period swells that were generated by Hurricane Gaston will continue to produce rough conditions at the entrances to rivers, bays, harbors and inlets along the coast. Thursday and Friday: Recently formed Tropical Depression Nine is expected to become a Tropical Storm as it moves northward through the Gulf of Mexico through mid week. By late week, it is expected to cross north Florida and then track northeastward off the Southeast coast based on the current forecast. There is considerable uncertainty surrounding the track and intensity of this system during this time so confidence in exact impacts is low. Certainly based on the current forecast, strong gusty winds and increasing seas will be possible. Rip Currents: Given the reports received of rip currents today and Saturday, it seems another high rip current risk is warranted for all beaches Monday. An enhanced rip current risk will linger through at least mid-week, possibly longer depending on the evolution of Tropical Depression Nine across central/eastern Gulf of Mexico. && .TIDES/COASTAL FLOODING... Tides remain elevated due to ongoing east/northeast winds, long period swell energy and influences from the approaching new moon. The latest extratropical surge guidance suggests afternoon high tides through mid-week will be very close to levels which will produce shallow coastal flooding, especially along the lower South Carolina coast. Coastal Flood Advisories may eventually be needed. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...High Risk for Rip Currents through Monday evening for GAZ117- 119-139-141. SC...High Risk for Rip Currents through Monday evening for SCZ048>051. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM...DPB/ST LONG TERM...ST/BSH AVIATION...DPB/ST MARINE...DPB/ST/BSH TIDES/COASTAL FLOODING...