textproduct: Charleston

WHAT HAS CHANGED

Aviation section has been updated to reflect the current 00Z forecast.

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) 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. There have been some changes to the original forecast and where the focus for storms are. The overall pattern has drier northwesterly starting to move into the area as a broad upper trough digs into the Mid-Atlantic. A subtle shortwave currently over the Ohio Valley will eject this afternoon and evening over North Carolina and round the base of the upper trough. A rather stable airmass is in place across the area as outflow from yesterdays storms has laid down a boundary that extends from around Beaufort SC westward towards Atlanta. This boundary will likely play a role in where the best chance for both severe weather and heavy rainfall will setup. Storms have already begun to develop along the warm side of this boundary across eastern GA and are expected to spread throughout the afternoon and into the early evening across eastern GA potentially sneaking into far southern South Carolina. This will be the first round of storms capable of heavy rainfall and isolated damaging winds. Once these storms depart the area we will begin to focus on the next round to our northwest. This round of storms will be associated with the more prominent shortwave trough and approaching cold front. Storms have already begun to develop across the higher terrain and into the Midlands this afternoon and will steadily surge southeast towards the coast through the evening and overnight hours. This round will likely be our severe weather threat with localized damaging winds possible especially across areas that do not see storms during the afternoon across interior South Carolina. There are some questions on whether or not storms even maintain severity as they approach given the more stable airmass across SC, loss of diurnal heating overnight and the fact that we will become increasingly removed from the parent forcing.

Severe Weather: Looking at the severe potential of these storms, the environment ahead of any thunderstorms this afternoon should be conducive for localized damaging winds up to 60 mph. Temperatures have warmed into the low to 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 especially across eastern GA, supporting the potential for damaging wind gusts. There should even be ~20 kts of deep layer shear across SC which could be enough to support some organization in the form of multi-cell clusters. Hi-res guidance has become a little more consistent on the evolution this afternoon and evening with two main rounds. The first being more ordinary cells across eastern GA and then the main round coming out of the SC Midlands this evening. The storms across Georgia will pose a rather marginal severe threat; however, if the storms across the Midlands can survive the trek down and persist into the area they could be the ones to pose a little more punch. However, they will likely be arriving during the late evening into the overnight hours leading to lower confidence on if storms will be able to maintain their intensity into the Lowlands.

Heavy Rain: Precipitable water values across eastern GA will be in the 2-2.25" range with PWAT values across SC in the 1.8 to 1.9 range. Given the location of the aforementioned boundary and this split in PWATs we have trended our heavy rain threat towards coastal GA vs the SC lowlands. However, thunderstorms will be very efficient rainfall producers through the afternoon and evening. 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 07/12Z HREF shows 30-50% probabilities of 3 inches of rain in a 3 hour period across chunk of the area. 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 and this is backed by observations and where the best PWATS and low level convergence is located. However, parts of South Carolina could get in on the action this evening as storms move into the area from the Midlands. 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 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 /00Z MONDAY THROUGH FRIDAY/

A mix of MVFR and VFR conditions will prevail for most of the area as clusters of thunderstorms have developed across most of South Carolina and in the vicinity of KCHS and KJZI. Storms across GA have moved out of the area leaving MVFR conditions in their place. MVFR conditions and continued storms are expected near CHS and JZI through around 05Z before clearing. MVFR ceiling are expected to develop around 10z to 12Z and persist through most of the day tomorrow. Behind these storms, MVFR ceilings could persist through most of the day tomorrow. Another round of scattered showers and thunderstorms is expected to develop around 18Z to 19Z in the vicinity of KSAV and have added a prob30 group for this potential.

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

Tonight: Light onshore flow this afternoon around 10 to 15 knots will quickly turn northeasterly 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 3-5 feet across much of the nearshore waters and as high as 6-7 feet across the outer waters peaking Tuesday. Conditions will begin to improve quickly Tuesday night into Wednesday as the gradient eventually 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 and Tuesday. Depending on trends, a HIGH risk may be needed for some area beaches on Tuesday.

CHS WATCHES/WARNINGS/ADVISORIES

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