textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated. KEY MESSAGE 3 has been added for coastal flooding potential next week.

KEY MESSAGES

- 1) Hot and humid conditions expected this weekend.

- 2) Unsettled weather will continue through Labor Day weekend, with increasing chances of rain each day. Increased risk for flooding Sunday.

- 3) Coastal flooding potential increases behind a cold front early next week.

DISCUSSION

KEY MESSAGE 1: Hot and humid conditions expected into the upcoming weekend.

Aloft, a ridge of high pressure will extend from the Central United States to the Southeast United States this weekend, although could slightly break down across the local area as a broad trough develops across the Northeast and Mid-Atlantic regions. Even though the setup suggests conditions less favorable for hot conditions this weekend, a strong day of sfc heating and a compressional heating component along/ahead of an approaching cold front should still support hottest conditions Saturday afternoon, when temperatures push into the upper 80s/lower 90s near the beaches to mid 90s inland. Sfc dewpts will likely mix out into the low-mid 70s inland, but should still pool into the upper 70s/near 80 across portions of the coastal corridor along/behind a sea breeze circulation. This will push heat index values into the 108-110 range, especially across the lower South Carolina coast. Given the placement of high pressure and downward trend in precip chances during peak heating hours, a Heat Advisory has been issued from Berkeley and Charleston County north to Effingham/Chatham GA Counties south from 11 AM to 8 PM Saturday. Further south and well inland, conditions appear more marginal with heat index values in the 102-107 range, short of Heat Advisory criteria. However, conditions will need to be monitored for possible inclusion of remaining areas into the advisory should temperatures or dewpts trend warmer/higher than currently anticipated. There is also some risk (although quite low) of isolated convection during peak heating hours Saturday afternoon, which could provide some relief from the heat. A backdoor cold front and numerous/widespread convective coverage will likely keep conditions below Heat Advisory thresholds Sunday.

KEY MESSAGE 2...Unsettled weather will continue through Labor Day weekend, with increasing chances of rain each day. Increased risk for flooding Sunday.

Saturday into Saturday night: A rather unsettled pattern will begin to setup on Saturday as our upper ridge begins to breakdown and a large upper low will dig into the northeast. This will begin to push a "cold" front into the area by late Saturday night into Sunday. For Saturday, conditions should remain rather dry throughout the day on Saturday as the ridge still has some hold on the area; however, as we head into the late afternoon and evening we should begin to see some isolated to scattered showers and storms start to develop with the influences of the upper trough and increasing moisture build into the area. Rain chances look to increase in to the overnight hours on Saturday as the slow moving front pushes down from North Carolina. A cluster of thunderstorms will likely move towards the Tri-county area. These storms could be strong to severe with a few instances of damaging winds. 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.

Sunday: The front will enter our area sometime Sunday morning and widespread showers and thunderstorms will likely develop along and around the front. PWATS are expected to climb 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 and 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 with time. The faster the front the less concern for heavy rainfall; however, if the front slows or stalls then some locations closer to the coast could easily start to see the rain totals pile up and localized flash flooding could be possible. Rainfall totals of 1 to 3 inches look to be possible Sunday afternoon; however, there is a avenue where localized rainfall totals might push 3 to 5 inches in the heaviest bullseyes. This certainly raises some concerns across the Charleston Metro 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. While this scenario really does not scream severe; there's enough of a potential to worth a mention and a gee whiz strong to severe storm. 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 behind a cold front early next week.

Elevated high tides return Sunday afternoon during the lunar Perigee (09/06), and linger throughout the week until after the new moon (09/11). With astronomical high tides ranging from 6.30-6.55 ft MLLW in the Charleston Harbor throughout the week, tidal departures of 0.45-0.70 ft would result in minor coastal flooding. A high pressure wedge could also set up along he East Coast next week, favoring larger tidal departures and coastal flooding, especially along the Charleston and Colleton County coasts. Latest trends suggest minor coastal flooding could start as early as the Sunday afternoon high tide, but even more likely with the Monday afternoon/evening high tide cycle with departures up to 1 ft.

For the rest of the coast from Beaufort County, SC, south to McIntosh County, GA, tidal departures would need to be larger to reach minor coastal flooding issues, which appears unlikely at this time. However, should the gradient pinch a bit more than expected with the inland high pressure wedge, then the risk for coastal flooding could increase.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

0Z TAFs: Recent KCLX images indicated that the sea breeze has pushed well inland of the terminals this evening. The rest of the night should feature dry conditions with light southwest winds. On Saturday, the day should begin with west-southwest winds between 5 to 10 kts. The sea breeze is timed to push across KJZI by 17Z and KCHS and KSAV by 19Z, followed by a south wind. Recent runs of the HRRR and HREF indicates that terminals will likely remain dry through the TAF period.

Extended Aviation Outlook: Chances for flight restrictions with showers/thunderstorms increase at all terminals Saturday night into Sunday as a cold front passes through the region. Periodic flight restrictions are possible through early next week with scattered to numerous showers/thunderstorms and a wedge of high pressure building north and inland. Breezy conditions could also develop at all terminals as a wedge attempts to take shape inland.

MARINE

Today and Tonight: High pressure will remain the dominant weather feature across local waters while h5 vort energy continues to shift onshore, favoring marine conditions that remain well below Small Craft Advisory levels through the night. In general, southerly winds will gust up to 15-20 kt, strongest along coastal areas this afternoon, then turn more southwest-west overnight tonight. Seas will range between 2-3 ft.

Saturday through Wednesday: The main concern centers on a likely tightening of the pressure gradient associated with wedge of high pressure that will develop Sunday night into Monday in the wake of backdoor cold front. 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 3-5 ft, but could end up as high as 6 ft at times, especially across the South Santee- Edisto Beach nearshore and offshore legs. A Small Craft Advisory could be needed for some waters for Monday into Monday night, possibly extending into Tuesday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from 11 AM to 8 PM EDT Saturday for GAZ101-218- 219. SC...Heat Advisory from 11 AM to 8 PM EDT Saturday for SCZ042>045- 147>152. MARINE...None.