textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated. Added a key message for coastal flooding potential.

KEY MESSAGES

- 1) Hot and humid conditions with periods of showers and thunderstorms across SE GA/SC today. Locally heavy downpours are possible.

- 2) Localized flash flooding is possible mainly across portions of Southeast Georgia Friday, then across much of the area including the South Carolina Lowcountry Saturday.

- 3) High tide could bring minor inundation of very low lying areas along the southern South Carolina coast early Friday evening.

DISCUSSION

KEY MESSAGE 1: Hot and humid conditions with periods of showers and thunderstorms across SE GA/SC today. Locally heavy downpours are possible.

A mid level trough will extend across the mid-Atlantic states tonight, while ridging gets suppressed to the south. At the surface, a cold front will sink into the region, likely settling near the Savannah River by sunrise. Atmosphere remains quite moist with PWats near or just over 2 inches, which is above the 95th percentile of climo.

Radar early this afternoon has become more active, with increasing coverage of showers and thunderstorms. Convection should largely remain sub-severe, though there is potential for locally heavy downpours. 12z HREF continues to indicate a 10-30% chance of 3"/3 hr with a 50-70% of 1"/3 hr, highest over southeast Georgia. A Flood Advisory, or perhaps even a Flash Flood Warning, cannot be ruled out.

With the front in the vicinity, showers could linger at times throughout the night, possibly becoming more focused over southeast Georgia with time as drier tries to move in from the north.

KEY MESSAGE 2: Localized flash flooding is possible mainly across portions of Southeast Georgia Friday, then across much of the area, including the South Carolina Lowcountry, Saturday.

On Friday, a cold front is forecast to slow as it pushes over the Savannah River, expected to become stationary over SE GA Friday afternoon. At H5, a broad weaknesses will develop over the Southeast U.S. Deep moisture convergence will likely develop along the NW to SE oriented front across SE GA and high resolution guidance indicates that showers and thunderstorms will rapidly develop across SE GA from late morning through the afternoon hours. PWATs south of the front approach 2.25 in, in the 98th percentile with moderate CAPE in the 1000-1500 J/kg range. Additionally, given the aforementioned weakness aloft, storm motions will be very slow, likely around 10 kt or less, especially near the front. The greatest heavy rainfall threat exists south and west of Savannah, where probs for greater than 1 inch of rainfall at any given location are around 20-30%. Further north, especially across the SC Lowcountry, moisture content will be much more limited and rainfall coverage will be much less.

A relative lull in precip is expected through the overnight hours Friday night before coverage and intensity increases early Saturday morning as a shortwave approaches the CWA. With the boundary lifting a bit into eastern SC, the primary moisture band will be more effectively cover the area from the Charleston Metro to the Savannah Metro. With modest storm motions parallel to the front, PWATs still greater than the 96th percentile, and the increased upper forcing, there should be widespread convective coverage with some locations receiving prolonged heavy rainfall. Still a bit far out in time for the convective allowing models to really resolve the potential magnitude of the localized precip, but at least a localized flash flooding threat will exist in proximity to where the front eventually sets up early Saturday morning through the afternoon.

Precip coverage wanes later Saturday in the wake of the wave aloft and the surface ridge builds across the area on Sunday, resulting in widespread isentropic lift through the day, but effectively bringing an end to the flooding threat.

KEY MESSAGE 3: High tide could bring minor inundation of very low lying areas along the southern South Carolina coast early Friday evening.

A period of moderate to breezy northeast winds Friday in combination with the approaching full moon and apogee will bring elevated high tide levels Friday, possibly persisting into Saturday. Minor inundation of very low lying areas of Charleston and Colleton Counties is possible early Friday evening, with a lesser threat early Saturday evening.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Prevailing VFR is expected through the 18z TAF cycle, however there could be brief restrictions in showers and thunderstorms this afternoon into this evening. A cold front is expected to drop into the area overnight, which could spur additional convection. Probabilities of significant direct impacts are too low so kept mention out of the TAF at this time. As similar to previous nights, could not rule out some patchy ground fog overnight.

Shower and thunderstorm coverage is expected to increase again after daybreak Friday, especially at KSAV. Used a PROB30 to address this potential. At KCHS/KJZI, latest guidance indicates the front would be south of the terminals with drier conditions expected so no mention of additional convection at this time.

Extended Aviation Outlook: More periodic flight restrictions are possible Friday into the weekend as the potential for widespread showers/thunderstorms increases. In addition, patchy overnight fog and/or stratus cannot be ruled out after recent rains.

MARINE

Through tonight: Southwest winds will persist over the coastal waters the remainder of the day into the evening with speeds less than 15 knots. A cold front will drop into the region from the north tonight towards early Friday morning and with winds turning to the northeast in its wake. Speeds will begin to increase with some gusts around 20 kt over the Charleston county waters by around daybreak. Seas will average 2-3 feet.

Friday through Monday: Breezy NE winds Friday morning along the South Carolina coast will trend more moderate through the day, while south of the Savannah River, closer to the quasi-stationary boundary, winds will be lighter. Another period of breezy northeast winds arrives in the wake of a cold front pushing south Sunday morning, with SCAs likely needed into Sunday night. Seas also peak later Sunday as windswell builds seas to 6-8 ft, greatest across the offshore SC waters. Marine conditions improve early next week.

Rip Currents: An elevated rip current risk will remain through Friday with a modest easterly swell around 3 to 4 feet every 8 seconds and sea breeze influences. A moderate risk of rip currents is in place for all area beaches through Friday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...None.