textproduct: Charleston

WHAT HAS CHANGED

The aviation section has been updated for the 12Z TAFs.

KEY MESSAGES

- 1) A cold front will stall over southeast Georgia today. Widespread showers and thunderstorms will develop along and south of the boundary. These storms will produce torrential downpours, 50% chance of rainfall rates of 3"/3hr or greater expected this afternoon into early this evening. Localized flash flooding is possible.

- 2) Localized flash flooding possible across portions of Southeast Georgia and the South Carolina Lowcountry on Saturday.

- 3) Evening high tides could bring minor inundation of very low lying areas along the Charleston and Colleton County coasts this evening through this weekend.

DISCUSSION

KEY MESSAGE 1: A cold front will stall over southeast Georgia today. Widespread showers and thunderstorms will develop along and south of the boundary. These storms will produce torrential downpours, 50% chance of rainfall rates of 3"/3hr or greater expected this afternoon into early this evening. Localized flash flooding is possible.

A backdoor cold front, observed on satellite and sfc observations, will push south across the SC Lowcountry this morning. The front is expected to slow as it crosses the Savannah River late this morning, then stalling over SE GA. High resolution guidance shows a sea breeze developing early this afternoon, pushing inland through the rest of the afternoon.

Satellite water vapor shows a plume of deep moisture across SE GA. SPC mesoanalysis indicated a tight gradient of PW across the forecast area, ranging from around 1 inches across the Santee River to 2.1 across extreme SE GA. Based on blend of MOS, high temperatures may range from the mid to upper 80s across the SC Lowcountry to the mid 80s across SE GA. Sfc dewpoints along and south of the stationary front will remain in the mid 70s. The warm and humid conditions should yield a large field of SBCAPE between 1500-1800 J/kg across SE GA. As the environment destabilizes, showers and thunderstorms should develop near the stalled front and along the sea breeze, generally along and south of the Savannah. Interestingly, near term guidance indicate an amplifying H5 shortwave will lift across the region during the heat of the afternoon. The short wave may trigger a band of thunderstorms over the Deep South, tracking NE, reaching SE GA late this afternoon and evening. SE GA may see collisions of multiple boundaries, primarily the sea breeze, stationary front, and outflow from Deep South storms. These boundaries will likely allow for back-building and slow moving thunderstorms, with storm motions less than 15 mph. Given PW in excess of 2 inches and warm cloud layer of 12 kft, these storms should tap into the tropical moisture and produce torrential downpours. 0Z run of the HREF highlights SE GA with a 50% chance of rainfall rates of 3"/3hr or greater expected this afternoon into early this evening. Localized flash flooding is possible. WPC has highlighted this region with a Slight Risk in the Excessive Rainfall Outlook. Flood Advisories and Flash Flood Warnings may be needed for portions of SE GA later today.

The passage of the shortwave will likely allow showers to drift NE tonight. However, north of the stationary front, the environment will become dryer with less instability. Deep convection should gradually dissipate as it moves northward.

KEY MESSAGE 2: Localized flash flooding possible across portions of Southeast Georgia and the South Carolina Lowcountry on Saturday.

A localized flash-flood threat is expected on Saturday as the stalled frontal boundary across southeast Georgia slowly lifts northwards across eastern South Carolina. With the boundary lifting northward, a mid-lvl shortwave will approach the region from the west yielding an increasingly favorable corridor of low-lvl convergence from the Savannah Metro towards the Charleston Metro and portions of the South Carolina Lowcountry. This forcing will overlap with a very moist airmass characterized by anomalously high PWAT values, generally around, or above 2 inches. This will support efficient warm-rain processes and high rainfall rates.

The main forecast challenge remains the exact placement of the front and the associated convergence axis. A farther south solution would focus the heavier rainfall across southeast Georgia and the Savannah Metro, however a farther north solution would bring a greater flooding concern into the Charleston Metro and the broader South Carolina Lowcountry.

Overall, the environment appears more favorable for a heavy rainfall/flooding threat rather than a organized severe weather threat. Instability should be sufficient enough for some embedded thunderstorms, but the primary threat will be localized flooding especially where training develops. Localized rainfall totals of 2 to 3 inches could be possible, especially from the Savannah Metro northeast towards Charleston Metro depending on the where the front sets up. It's important to note that the Weather Prediction Center (WPC) has highlighted the region under a Marginal Risk (Level 1/4) for Excessive Rainfall on Saturday. Showers and thunderstorms will likely begin to wane late Saturday night as a surface ridge starts to build across the region into Sunday.

KEY MESSAGE 3: Evening high tides could bring minor inundation of very low lying areas along the Charleston and Colleton County coasts this evening through this weekend.

A period of moderate to breezy northeast winds today in combination with the approaching full moon and apogee will bring elevated high tide levels this evening. Elevated tidal departures may persist through this weekend. Minor inundation of very low lying areas of Charleston and Colleton Counties is possible early this evening, with a slightly lesser threat Saturday and Sunday evenings. However, showers and thunderstorms may track across the City of Charleston during high tide Saturday evening. The combination of moderate to heavy downpours coinciding with high tide may increase the potential for flooding.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

12Z TAFs: Periods of IFR fog and ceilings are possible across the terminals through 14Z. A cold front is timed to reach KCHS and KJZI by 14Z, turning winds from the northeast and lifting cloud bases to VFR. The cold front will slow as it approaches the Savannah River this morning, likely stalling over SE GA this afternoon. High resolution guidance indicates that a sea breeze and coverage along the front will trigger numerous to widespread thunderstorms across SE GA this afternoon. Some of this activity may track over KSAV today, highlighted with a PROB30 from 16-20Z, with -TSRA from 20-23z. KCHS and KJZI should remain too stable for deep convection. The passage of the sea breeze should turn winds from the SE by 19Z, some gusts near 20 kts possible. Showers with periods of IFR ceilings area possible overnight, primarily after 7Z.

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

MARINE

Today, ENE winds may surge as the cold front pushes across the SC waters, with gusts approaching 25 kts. It is expected that gusts will remain just below Small Craft Advisory criteria. However, wave heights should build through the day, reaching 3-4 ft by this afternoon. The pressure gradient may lessen on tonight into Saturday as a frontal wave develops over the marine zones.

Saturday through Tuesday: Moderate east southeasterly winds will be apparent on Saturday in the wake of a cold front, and then veer out of the northeast overnight into Sunday. Additionally, expect winds to become quite breezy as this cold front pushes southward on Sunday morning with speeds ranging from 20 to 25 kt with gusts up to 30 kt possible into Sunday afternoon. Seas will also increase on Sunday as a short-lived swell builds across the local Atlantic waters. Expect the nearshore and offshore southeast South Carolina waters to peak to 6-8 ft, while the nearshore and offshore waters southeast Georgia waters to peak 4-6 ft. Therefore, Small Craft Advisories will likely be needed on Sunday. Expect marine conditions should improve on Monday and Tuesday.

Rip Currents: A Moderate Risk of rip currents is expected for today across all beaches as modest easterly swell around 3 to 4 ft every 8 seconds persists.

CHS WATCHES/WARNINGS/ADVISORIES

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