textproduct: Charleston

WHAT HAS CHANGED

Updated to remove Severe Thunderstorm Watch 658.

KEY MESSAGES

- 1) Hot and humid conditions expected this weekend.

- 2) Numerous to widespread thunderstorms are expected to develop across the region Sunday afternoon into Sunday night. These thunderstorms are expected to bring a risk for damaging wind gusts as well as locally heavy rainfall and flooding.

- 3) Coastal flooding potential increases early next week and persists through the middle of the week.

DISCUSSION

KEY MESSAGE 1: Hot and humid conditions expected this weekend.

Heading into Sunday, another hot and humid day is expected ahead of a cold front quickly approaching the area from the north. High temps are still forecast to peak in the low-mid 90s with sfc dewpts reaching the mid-upper 70s ahead of the cold front, with even the possibility of slightly warmer conditions across Southeast Georgia prior to cold fropa late afternoon/early evening. Heat index values could very well peak in the 105-110 degree range with marginal Heat Advisory conditions along the entire SC/GA coast as well as some areas across inland Georgia. However, the duration and likelihood of this event is highly dependent on shower/thunderstorm development prior to and during cold fropa, which starts/occurs by peak diurnal heating hours. Given low confidence in the duration of the hot/humid setup, and likelihood of "cooling" rains keeping a shorter duration event to early afternoon hours at best, a Heat Advisory will not be issued at this time. Convective trend/guidance will need to be monitored for possible changes to the forecast as the event approaches.

KEY MESSAGE 2: Numerous to widespread thunderstorms are expected to develop across the region Sunday afternoon into Sunday night. These thunderstorms are expected to bring a risk for damaging wind gusts as well as locally heavy rainfall and flooding.

Saturday night: A rather unsettled pattern will begin this evening as the first upper shortwave rounds the base a much large upper trough centered over the Northeast. A "cold" front will begin to push into the area by late Saturday night into Sunday. Conditions should remain rather dry throughout the afternoon but our attention is to our northwest as convection has begun along the advancing front across western NC and western SC. This activity is expected to expand across the Carolinas. This activity should put down a rather robust outflow boundary that should approach our area late tonight around 10pm to midnight. Rain chances will likely increase as the boundary and the railing cold front pushes down. A strong storm cannot be ruled out tonight as the outflow boundary moves into the Tri-county area; however, by the time they likely reach the area we will be approaching midnight and instability should be limited which would limit any severe potential. Some heavy rainfall could be possible but with storms weakening the bigger threat likely comes Sunday.

Sunday: The front will enter our area sometime early Sunday morning and widespread showers and thunderstorms will likely develop along and around the front. PWATS are expected to climb well above 2 inches along and ahead of the slowly advancing boundary and given deep layer wind profiles parallel to the boundary and slow storm motions could result in storms lingering and building along the front. Couple this with likely heavy rain rates of 1 to 3 inches per hour you can see where some concern for locally heavy rainfall will be possible. The question is how fast does the front progress and is there potential for storms outflow to help modify and push the front further south. Some early morning guidance has hinted at the potential for the outflow that arrives tonight to modify the environment shifting the focus for heavier rain further south from Charleston to closer towards Savannah. We will have to monitor the trends overnight as this outflow enters the area to have a better idea of what areas could see the heaviest rainfall threat. If the boundary pushes south of Charleston then the metro may be spared from some of the heavier totals. On the other hand if this boundary slows and hangs up near the Charleston Metro then things could get dicey during the mid to late afternoon. Nonetheless, rainfall totals of 1 to 3 inches look to be possible for a large portion of the area Sunday afternoon and there is a avenue where localized rainfall totals might push 4 to 6 inches in the heaviest bullseyes. Where this bullseye happens will be highly dependent on the caveats mentioned above. This certainly raises some concerns across the Charleston and Savannah Metros on Sunday afternoon for some flooding potential lining up with the peak high tide approaching 7 feet MLLW (more on this later in the discussion).

On-top of the heavy rainfall risk, a few storms could get strong to severe given rather strong instability approaching 2000-2500 J/kg with deep moist profiles capable of wet microbursts if storms can cluster up enough. Once again the overnight progression of the remnant outflow boundary may dictate where and how widespread the severe potential is. Slower the outflow, more of the area destabilizes and likely more areas could see a strong to severe storm. Faster progression of the outflow and the severe risk could be confined more to eastern Georgia. While this scenario really does not scream any organized severe potential; there's enough of a potential to worth a mention and given the likely higher coverage of storms there will likely be a few strong wind gusts. Storms should slowly progress southward through the evening and weaken during the overnight hours as instability weakens.

Monday into Monday night: By early Monday morning the front will have pushed through most of the area and the best moisture will likely be limited to parts of southeastern GA maybe just along the boarder of GA/SC. Once again scattered to numerous showers and storms should develop along the boundary and push south with time. PWATS will still be in the 2 inch range south of the front and some localized chances for heavy rainfall will still exist across Georgia including the Savannah metro on Monday afternoon. However, coverage may be a little less than Saturday leading to a slightly lower risk. Drier conditions should improve by Tuesday as the front fully clears the area and dry northwesterly flow begins to build over the area.

KEY MESSAGE 3: Coastal flooding potential increases early next week and persists through the middle of the week.

Astronomical tides will continue to rise through the middle of next week with the approach of lunar perigee (9/6) and the upcoming new moon (9/11). These factors will push astronomical tides as high as 6.55 ft MLLW at Charleston with the Wednesday evening high tide, setting the stage for the coastal flooding as tidal departures increase with northeast flow behind a passing cold front and building wedge pattern early week. Coastal flooding will be favored during the evening high tide cycles at Charleston and could begin as early as the Monday evening high tide. While minor coastal flooding is the most likely scenario, the potential for moderate coastal flooding will increase with the Tuesday and Wednesday evening high tides. Coastal Flood Advisories will likely be needed for Charleston and coastal Colleton counties.

For the rest of the coast, from Beaufort County, SC to McIntosh County, GA, tidal departures would need to exceed 1 ft to reach the minor coastal flood threshold (9.5 ft MLLW) at Fort Pulaski. Coastal flooding is less likely in this area, 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 SUNDAY THROUGH THURSDAY/

Prior to the 0Z TAFs, KCLX detected a band of thunderstorms reaching northern Berkeley and Dorchester counties, pushing southeast at around 25 mph. This convection may reach or new convection may develop near KCHS between 0-4 Z, highlighted with a TEMPO. The convection should gradually weaken by late this evening as a outflow boundary over takes the area and conditions cool after sunset. Convection appears a bit active again Sunday afternoon across the terminals. High resolution guidance shows convection expanding across the region once the sea breeze develop and begins to push inland. HREF shows the greatest coverage near KSAV, but scattered coverage is expected near KCHS and KJZI. The window of greatest thunderstorm potential was highlighted a KSAV with a TEMPO between 18-22Z and PROB30 at KCHS/KJZI between 19-24Z.

Extended Aviation Outlook: Numerous showers/thunderstorms are forecast to impact the area with a passing cold front Sunday afternoon. At least TEMPO flight restrictions are likely at all terminals between the 18-24Z timeframe Sunday. A few thunderstorms could also be strong with gusty winds and heavy downpours while passing over the terminals. Outside shower/thunderstorm activity, expect breezy winds to develop as a wedge of high pressure takes shape inland. Additional TEMPO flight restrictions are possible at all terminals with afternoon thunderstorms through the middle of next week.

MARINE

This Afternoon and Tonight: High pressure across the western Atlantic will slowly retreat east and south of local waters late tonight with the approach of a cold front from the north. Although marine conditions are expected to remain below Small Craft Advisory levels through the night, there should be a slight uptick in southerly flow this afternoon that eventually turns more southwest- west tonight, peaking no higher than 15-20 kt. Seas will range between 2-3 ft.

Sunday through Wednesday: The main concern continues to be the potential for small craft advisory conditions during the early part of next week. The overall pressure gradient should tighten behind the advancing cold front as high pressure wedges into the Carolinas. Expect northeast winds as high as 20-25 kt to occur, especially across the South Carolina coastal waters and 15-20 kt with gusts to 25 kt across the Georgia waters as well as the Charleston Harbor. Seas could reach as high as 4-6 ft, but could end up as high as 7 ft at times, especially across the South Santee-Edisto Beach nearshore and offshore legs. A Small Craft Advisory will likely be needed for some waters for Monday through Tuesday night. Along the front, there will be the potential for strong thunderstorms across the local waters Sunday and Sunday night. These storms will bring the potential for strong wind gusts in excess of 35 knots as well as frequent lightning.

Rip Currents: Gusty northeast winds and building seas developing behind a passing cold front early week will likely support a Moderate Risk for rip currents along the entire Southeast South Carolina and Southeast Georgia beaches on Labor Day.

CHS WATCHES/WARNINGS/ADVISORIES

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