textproduct: Charleston

WHAT HAS CHANGED

All sections updated.

KEY MESSAGES

- 1) Numerous to widespread thunderstorms are expected to impact southeast Georgia and southeast South Carolina this afternoon and evening. These thunderstorms will bring a risk for damaging wind gusts as well as locally heavy rainfall and flooding.

- 2) Hot and humid conditions expected again today.

- 3) The potential for coastal flooding will increase beginning Monday and persist through the middle of the week.

DISCUSSION

KEY MESSAGE 1: Numerous to widespread thunderstorms are expected to impact southeast Georgia and southeast South Carolina this afternoon and evening. These thunderstorms will bring a risk for damaging wind gusts as well as locally heavy rainfall and flooding.

The forecast continues to focus on the potential for widespread thunderstorms to produce severe weather and heavy rainfall this afternoon and evening. Overall, the setup is unchanged from previous forecast cycles as thunderstorms are expected to develop just to our north across the SC Midlands and then progress southward ahead of an upstream cold front.

Severe Weather: The environment ahead of these thunderstorms should be conducive to maintaining the storms and perhaps even strengthening. Temperatures will warm into the mid 90s today and we expected to see MLCAPE values in excess of 2,000 J/kg. Model soundings also suggest DCAPE values upwards of 1,000 J/kg, supporting the potential for damaging wind gusts. There should even be ~20 kts of deep layer shear which could be enough to support some organization in the form of multi-cell clusters. Hi-res guidance is still mixed on exactly where the biggest severe weather threat will be, and will likely be highly dependent on exactly where convective initiation takes place upstream. Once storms do get going and propagate to the south, the progression and interaction of outflow boundaries will play a notable role in the areas at the biggest risk of seeing severe thunderstorms.

Heavy Rain: Precipitable water values across the area will be in the 2-2.25" range and thunderstorms will be very efficient rainfall producers. We should see hourly rainfall rates of 2+ inches within the strongest storms which will heighten the locally heavy rainfall and flash flood risk. The 06/00z HREF shows 30-50% probabilities of 3 inches of rain in a 3 hour period. So, needless to say, the ingredients for locally heavy rainfall are present and the question is how quickly do these storms move through and is there potential for either training or persistent development. There remains some question as to what areas are at the biggest risk of seeing the heaviest rainfall. Overall, model guidance has generally shifted the focus from around the Charleston metro area to be further south to be more towards the Beaufort and Savannah areas. But, much will be determined based on where convective initiation takes place upstream. The overall risk period will run from the mid to late afternoon through the late evening hours, depending on how quickly storms progress.

KEY MESSAGE 2: Hot and humid conditions expected again today.

Today will again bring high temperatures peaking in the mid 90s across the entire forecast area and even a few upper 90s possible across portions of southeast GA. Dewpoints are expected to be in the mid to upper 70s along the coast with low to mid 70s elsewhere. This setup should yield heat indices peaking in the 105-110 degree range. However, current thinking is that we will not meet the duration component of Heat Advisory criteria (108 for 2 hours or more) and overall heat indices will be quite marginal. Afternoon convection could also be robust and disrupt peak heating and peak heat indices. Therefore, we will not issue a Heat Advisory. Fortunately, Sunday will be the last day with Heat Advisory concerns for the week.

KEY MESSAGE 3: The potential for coastal flooding will increase beginning Monday and persist through the middle of the week.

Astronomical tides will continue to rise through the middle of the week thanks to the lunar perigee which peaks today and the upcoming new moon (9/11). These factors will push astronomical tides as high as 6.55 ft MLLW at Charleston Wednesday evening, setting the stage for coastal flooding potential. A cold front will push through Sunday night and usher in an extended period of elevated northeast flow through Tuesday which will drive increased tidal departures. Coastal flooding will be favored during the evening high tide cycles at Charleston and the Monday evening high tide looks like the first one that could exceed the minor coastal flooding threshold of 7 ft MLLW. While minor coastal flooding is the most likely scenario, the potential for moderate coastal flooding (7.5 ft MLLW) will increase with the Tuesday and Wednesday evening high tides as tidal departures build within the persistent northeast flow. Coastal Flood Advisories will likely be needed for Charleston and coastal Colleton counties.

For the rest of the from Beaufort County south including the southeast GA coast, tidal departures would need to exceed 1 ft to reach the minor coastal flood threshold (9.5 ft MLLW) at Fort Pulaski. Therefore, coastal flooding is less likely along this part of the coast but still can't be ruled out especially if the northeast flow behind the passing cold front turns out a bit stronger than currently forecast.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

The 06z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. The main forecast issue for this TAF period concerns thunderstorm potential this afternoon and evening. Storms are expected to develop north of the area and drop southward toward the TAF sites through the late afternoon and into the evening. We have timed thunderstorms into KCHS first around 21-22z, followed by KJZI around 22-23z, and then KSAV last around 23-00z. There could be isolated storm development ahead of the later line, but confidence isn't high enough to account for this explicitly. These storms will be capable of producing strong wind gusts, so we have bumped up winds in the PROB30 groups to include gusts to 30 knots. This could be stronger, but will depend on if the terminals take a direct hit or not. These storms should bring at least MVFR conditions with reduced visibilities due to heavy rain, but at least brief IFR conditions will be possible as well.

Extended Aviation Outlook: Showers and thunderstorms could again impact the terminals on Monday and Tuesday before coverage diminishes mid to late week. Breezy northeast winds will also set up along the coast Monday and Tuesday.

MARINE

Today through tonight: Winds will turn light and variable this morning, followed by onshore southeasterly flow this afternoon. Wind speeds should peak in the 10-15 knot range. The main change across the local waters will come late tonight when a cold front drops through from the north. As sunrise Monday approaches, the surge of northeast flow behind the front will begin to spread in across the SC waters. Seas should remain in the 2-3 ft range through the period.

Also of note, thunderstorms are expected to develop inland this afternoon and evening. These storms could impact the coast and the coastal waters with strong wind gusts and frequent lightning.

Monday through Thursday: The surge of northeast flow behind the cold front will ramp up Monday morning and spread across all waters through the afternoon. Sustained wind speeds are forecast to be near 20 knots with gusts around 25 knots across all waters. These conditions should persist through Tuesday as well. Small Craft Advisories will almost certainly be needed for all waters, including Charleston Harbor at times. Seas will ramp up in response to the northeast surge, with 4-6 feet across much of the nearshore waters and as high as 6-7 feet across the outer waters peaking Tuesday. Overall continues will begin to improve quickly Tuesday night into Wednesday as the gradient weakens.

Rip Currents: Persistent and gusty northeast winds and building seas will support a Moderate Risk of rip currents at all beaches along the southeast SC and southeast GA coast on Monday.

CHS WATCHES/WARNINGS/ADVISORIES

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