textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Hot and humid conditions will continue through Saturday. Isolated to scattered thunderstorms are expected to develop this afternoon and evening, and again on Saturday. A few strong to severe storms will be possible, with damaging wind gusts being the primary threat.

- 2) Rain chances and thunderstorm coverage expected to increase Sunday through Tuesday.

- 3) Coastal flooding is possible with the evening high tide from Sunday through Wednesday.

DISCUSSION

KEY MESSAGE 1: Hot and humid conditions will continue through Saturday. Isolated to scattered thunderstorms are expected to develop this afternoon and evening, and again on Saturday. A few strong to severe storms will be possible, with damaging wind gusts being the primary threat.

This afternoon and evening: Overall, the setup and conditions are quite similar to Thursday. The modified 12z KCHS RAOB (using 98/73) MLCAPE on the order of 2,000-2,500 J/kg along with DCAPE in the 1,000-1,200 J/kg range. The freezing level is a bit lower than yesterday at ~15 kft and the -20C height is essentially unchanged around 26.5 kft. We are once again going to be dependent on surface heating and subtle gradients for convective initiation, followed by boundary interactions and the sea breeze. Surface winds are westerly, which will keep the sea breeze more pinned and less progressive. Overall coverage should be scattered at most, though locally higher where boundary interactions help to drive multi-cell convection. Damaging wind gusts will again be the primary threat, with some potential for locally heavy rainfall as storm motion will be on the order of 10-15 knots. Storms could begin to develop in the early afternoon, but the potential will continue into the evening after sunset depending on how boundaries evolve. However, by the late evening convection should come to an end and the rest of the overnight should be dry.

Saturday: Another very hot day with widespread highs into the upper 90s (and a few triple digits) is expected. Dewpoints inland of the coast should mix out a bit, potentially into the upper 60s for much of the area and even a few mid 60s for southeast GA. Once the sea breeze begins to move inland, dewpoints will surge along with heat indices. It does appear that the potential for heat indices of 108 or higher will become increasingly confined to the coastal corridor.

Regarding thunderstorms, the coverage and environment looks similar to the last few days. Moderate instability and high DCAPE will yield an environment supportive of damaging wind gusts in the strongest storms. Convection could start in the early afternoon, but the sea breeze will be pinned closer to the coast thanks to westerly surface flow. Once a few storms develop and produce outflow boundaries, placement and strength of storms will depend on the evolution of these boundaries. Storms could continue through much of the evening until dissipating and ending by midnight and thereafter.

KEY MESSAGE 2: Rain chances and thunderstorm coverage expected to increase Sunday through Tuesday.

As noted in the previous discussion, will see a weak sfc low pressure drift across Virginia/North Carolina before ejecting into the Atlantic on Sunday. Meanwhile, it's attendant sfc cold front is expected to gradually sag southeastward across the region, resulting in increased chances for showers and thunderstorms. While model consistency still remains rather poor, there are a few models that develop another sfc low along the front, moving it across our area Sunday night then pushing offshore on Monday. If this were to happen, could certainly see a bit more moisture pool across us, and thus result in more rain - something to keep an eye on in the coming shifts. In terms of severe weather, there will still be ample instability (1500-2000 J/kg) to work with despite having cooler afternoon highs. So, like days past, can't rule out seeing a few isolated strong to severe thunderstorms, capable of producing damaging winds, small hail, and frequent lightning. With astronomical high tide of 6.44 ft MLLW occurring shortly after 7 PM, any thunderstorm activity near the Charleston peninsula will also lead to increased chances for impactful flooding.

Expect the aforementioned cold front to meander south of the region heading into the new week, keeping periodic chances for rainfall in the forecast through Tuesday. Afternoon highs during this time should also be notably cooler, as temperatures only rise into the upper 80s to low 90s. Unfortunately, this will be short lived, as a building ridge causes highs to moderate back into the low to mid 90s by the end of the week.

KEY MESSAGE 3: Coastal flooding is possible with the evening high tide from Sunday through Wednesday.

Astronomical influences on tides will increase early next week, with a lunar perigee on Monday and a New Moon on Tuesday. Minor coastal flooding is possible across Charleston and coastal Colleton Counties Sunday evening. Coastal flooding may increase Monday through Wednesday with rising astro tides and onshore winds. In addition, rainfall may overlap with the evening high tide cycles, see Key Message 2 for additional details.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

The 18z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. The main concern for the next 6-8 hours is the potential for thunderstorms. Current thinking is that the best chance of thunderstorm impacts will be at KCHS and KJZI based on current shower and thunderstorm development and the look of the cumulus field on satellite. Thunderstorms could be an issue around KCHS and KJZI through about 00z. Brief MVFR or IFR conditions will be possible, mainly due to visibility reductions in moderate to heavy rain. Gusty winds will also be possible. KSAV could also have some thunderstorm concerns, but we will wait to see how storms evolve over the next few hours and handle with amendments. Thunderstorm activity should dissipate by the late evening hours and the rest of the overnight should be quiet. Another round of storms will be possible Saturday, but the best chances will be beyond the 18z TAF period.

Extended Aviation Outlook: Shower and thunderstorm coverage, along with risks of associated restrictions, may increase Sunday through Tuesday.

MARINE

Through tonight: Much like the last couple of days, winds will strengthen this afternoon with the onset of the sea breeze. This will primarily impact the SC waters and Charleston Harbor, where gusts into the 25-30 knot range are expected through the evening. Therefore, a Small Craft Advisory has been issued for Charleston Harbor through 10 pm and for the Charleston County waters through 2 am. Winds will remain elevated through the overnight, but should diminish enough to allow these advisories to end. Seas will mostly average 2-4 feet, but could be up to 5 ft in the Charleston County waters near the 20 nm line. Also, thunderstorms are expected to develop inland along the coast. These storms could move out into the coastal waters and produce frequent lightning and strong wind gusts this evening.

Saturday through Saturday night: Another round of elevated southwest flow, with enhancement in the afternoon and evening with the sea breeze. Winds should peak in the 15-20 knot range, with some 25 knot gusts possible in the SC waters. Conditions could become marginally supportive of a Small Craft Advisory in the Charleston County waters and possibly even Charleston Harbor. Seas should average 2-4 feet. Thunderstorms are again expected to develop inland in the afternoon and evening hours. These storms could impact the coast and the coastal waters through the evening with frequent lightning and strong wind gusts.

Sunday through Wednesday: A weak cold front to our north will slowly sag across the region Sunday into Monday, causing winds to become notably weaker. While winds look to start out westerly Sunday, should see direction gradually turn more north/northeasterly by early Monday morning. This trend is expected to continue for much of the upcoming week, with seas generally ranging from 2 to 3 ft.

Rip Currents: A moderate risk for rip currents will remain along the SC/GA coast Saturday. Conditions will feature 2-3 ft swell with wave periods up to 9 seconds. In addition, SSW winds will likely generate a moderate longshore current, possibly supporting rip currents near piers and jetties.

CLIMATE

Record High Temperatures:

July 10: KCHS: 100/1986 KCXM: 99/1986 KSAV: 102/1879

July 11: KCHS: 102/1986 KCXM: 100/2001 KSAV: 103/1980

Record High Minimum Temperatures:

July 10: KCHS: 81/1986 KCXM: 82/1986

July 11: KCHS: 80/2016 KCXM: 82/2016

July 12: KCHS: 79/2020

CHS WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory until 7 PM EDT this evening for GAZ217>219-239- 241. Heat Advisory from 11 AM to 7 PM EDT Saturday for GAZ217>219- 239-241. SC...Heat Advisory until 7 PM EDT this evening for SCZ044-045-148- 149-151. Heat Advisory from 11 AM to 7 PM EDT Saturday for SCZ045- 148>152. Extreme Heat Warning until 7 PM EDT this evening for SCZ150- 152. MARINE...Small Craft Advisory until 10 PM EDT this evening for AMZ340. Small Craft Advisory until 2 AM EDT Saturday for AMZ360.