textproduct: Charleston

WHAT HAS CHANGED

The Aviation section was updated for the 12z TAFs.

KEY MESSAGES

- 1) Heat will remain a concern into late week.

- 2) Afternoon and evening thunderstorm coverage will remain isolated through Thursday. An increase in thunderstorm coverage is expected Friday through the weekend.

DISCUSSION

KEY MESSAGE 1: Heat will remain a concern into late week.

Finally, after baking on the surface of the sun for the past two weeks it appears our heat wave is finally coming to a close. The ridge that has plagued us has weakened as upper troughing begins to take control over the eastern US. We will still have one to possibly two more days of hot temperatures as we remain in a rather nebulous regime before the upper trough fully ejects eastward with the assistance of several embedded shortwave troughs. Overall, heat indices will be slightly lower today as the combination of some drier air mixing dewpoints into the 60s inland and the potential for an early start to afternoon showers and storms should help keep things more manageable in the 105 to 108 range. The only real concerns look to be predominately along the immediate coastline and into the Tri- county area behind the afternoon seabreeze. A few locations along the coast could reach heat indices around 110-112 degrees for a hour or two this afternoon and as a result a heat advisory remains in effect.

The gradual cooling trend will persist into Friday as the influences of the upper trough to our west slowly begin to inch their way eastward into the area. While air temperatures will likely climb back into the low to mid 90s, increased cloud cover and early storm initiation should help limit afternoon heating enough to keep most of the area below advisory criteria. The main areas would be along the immediate coastline where once again the overlap of near 80s dewpoints and temperatures in the 90s could result in heat indices in the 108-110 range behind the seabreeze. However, confidence in how long these heat indices will last and storm coverage are currently limiting the need for one final heat advisory tomorrow. We will likely revisit the need for an advisory with a later issuance as the potential or lack there of becomes clearer.

By the weekend, the upper trough will be in full control as a surface front moves into our area. An increase in shower and thunderstorm activity this weekend looks to cool us off enough to possibly go without any heat advisories/warnings, though high temperatures look to still peak in the lower to mid 90s.

KEY MESSAGE 2: Isolated to scattered thunderstorms this afternoon and evening, mainly across the Charleston Tri-County. Stronger and more widespread thunderstorms with locally heavy rainfall are expected Friday and Saturday.

Today: Mid-level ridging will prevail, leading to another day of above normal temperatures and stifling humidity. This will yield decent instability, generally 2,000-3,000 J/kg of SBCAPE, with locally higher amounts possible. Additionally, DCAPEs will be above 1,000 J/kg. All of this combined with lift from a coastal trough and the sea breeze should generate isolated to scattered thunderstorms this afternoon and evening. The CAMs and deterministic models generally have the convection across the Charleston Tri-County, so the forecast reflects this with up to chance POPs. The convection is most likely to form along the sea breeze and then shift inland, driven by outflow and boundary interactions. Given the conditions, strong to severe thunderstorms are possible with the main threat being damaging winds. Heavy rainfall is also a concern due to the very weak steering winds and elevated PWATs, so localized flooding is possible. Any convection will decrease this evening.

Friday and Saturday: Mid-level riding over our area will quickly dissipate Friday morning, being replaced by troughing over the eastern half of the U.S. The trough will strengthen/amplify during the weekend. At the same time, there will be a series of shortwaves rounding the base of the trough and moving over our region. At the surface, a slow moving front will approach from the west and northwest on Friday, possibly transitioning into a stationary front Friday night, and then meandering just to our west and northwest through the weekend. There will be a deep plume of moisture just ahead of the front, with PWATs rising above 2" Friday and the first half of Saturday, then remaining somewhat elevated into Sunday. These values are near the 90% mark for CHS per SPC Sounding Climatology. Even though temperatures will be a few degrees "cooler" than the previous few days, they'll still be well above normal, generating a decent amount of instability and DCAPE. All of the models are in generally good agreement indicating widespread showers and thunderstorms starting on Friday and occurring again on Saturday. Given the conditions, strong to severe thunderstorms are possible with the main threat being damaging winds. SPC has most of our area in a Marginal Risk of severe thunderstorms on Friday, and we wouldn't be surprised if this is repeated on Saturday. Heavy rainfall will also be a concern on Friday due to the very weak steering winds and elevated PWATs, then again on Saturday due to storm motions moving along the front. The exact QPF amounts will be highly dependent on where the thunderstorms form, how slow they move, and if there is any backbuilding, or training. But based on some of the ensembles, there is the potential for several inches of rainfall to fall in very localized areas Friday and Saturday. WPC has portions of our area under a Marginal Risk for Excessive rainfall both days. Flash flooding is certainly possible both days. While there should be some let up in the convection during Friday/Saturday evening and overnight, it may not completely dry out, especially closer to the coast. It's not until Sunday that the convection trends lower as the front slowly moves offshore.

AVIATION /12Z THURSDAY THROUGH MONDAY/

20/12z TAFs: VFR conditions will prevail throughout the forecast with light southwesterly winds this morning becoming southerly during the afternoon as the seabreeze moves inland. Scattered showers and thunderstorms could develop between 19Z and 22Z in and around CHS possibly leading to temporary restrictions in vsbys and ceilings. Storms should subside by the early evening with VFR conditions prevailing overnight. Winds will be light out of the south-southwest.

Extended Aviation Outlook: A front in our vicinity will bring showers and thunderstorms Friday into the weekend, leading to occasional flight restrictions.

MARINE

There are no large-scale marine concerns. Mornings will generally start out with south-southwesterly winds, increasing in strength as they become southeasterly in the afternoon, and back to south- southwesterly overnight. Seas mostly in the 2-3 ft range. There may be some isolated showers/storms that move out into the waters today and Thursday afternoon, with coverage increasing Friday.

Extended Marine: A typical summertime wind pattern is expected with winds/seas that are below Small Craft Advisory levels. Each day there will be some backing of the winds. They'll be the strongest along the land/sea interface due to the sea breeze. Each night winds will veer a bit. Seas will mainly be in the 2-3 ft range.

CLIMATE

Record High Temperatures:

August 20: KCHS: 97/2005 KSAV: 102/1900

Record High Minimum Temperatures:

August 20: KCHS: 78/2017 KCXM: 82/2017

August 21: KCHS: 79/2010 KCXM: 82/2023 KSAV: 80/1902

August 22: KCHS: 79/2023

August 24: KCHS: 76/2010 KCXM: 80/1975

August 25: KCHS: 77/1982

August 26: KCHS: 78/1998

CHS WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for GAZ217>219-239>241. SC...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for SCZ045-148>152. MARINE...None.