textproduct: Charleston

WHAT HAS CHANGED

All sections updated.

KEY MESSAGES

- 1) Dangerous heat will remain a concern today and tomorrow.

- 2) Isolated showers and thunderstorms expected this afternoon, before increasing in coverage on Friday through the weekend.

DISCUSSION

KEY MESSAGE 1: Dangerous heat will remain a concern today and tomorrow.

Rest of Today: Strong ridging aloft will remain the main synoptic feature influencing the local forecast area today. A Heat Advisory remains in place for the coastal counties and Berkeley County through 8 PM this evening. Heat index values within the Heat Advisory area could reach as high as 110. Convection this afternoon across the Charleston Tri-County will provide some relief from the heat to a few locations.

Friday: Mid-level troughing will dominate the east coast while a weak surface front approaches the local forecast area. Temperatures are still forecast to be above normal (mid 90s), but heat index values look to fall short of Heat Advisory criteria across most of the region. The only locations that could see heat index values of 108 or greater would be along the coastal counties, especially in SC. However, given the increased coverage of precipitation there is little confidence in hitting the heat advisory criteria. Therefore, one has not been issued at this juncture.

This weekend into early next week: Temperatures will cool slightly, with highs in the low 90s (still above normal for this time of year). The extended forecast looks to be heat-headline free with heat index values topping out in the 100-103 range each afternoon.

KEY MESSAGE 2: Isolated showers and thunderstorms expected this afternoon, before increasing in coverage on Friday through the weekend.

This Afternoon: As mid-lvl ridging remains stationed across the region, a weak coastal trough + the afternoon sea breeze will yield isolated showers and thunderstorms this afternoon. The latest HRRR/CAM guidance continues to indicate that the majority of convection will remain across the Charleston Tri-County. Initial development will likely develop along the afternoon sea breeze, before pushing inland where the sea breeze will interact with pre- existing boundaries. SBCAPE values of roughly 2000-3000 J/kg, with locally higher values, is expected by peak heating, while SBCAPE values should exceed 1000 J/kg. Therefore, any storm that does become established could quickly become strong, with localized damaging wind gusts as the primary concern. Deep-layer shear remains rather weak, so organized and/or long-lived severe convection does not appear to be in the cards this afternoon. Instead, the threat should consist of pulse-type storms and loosely organized multicell clusters. Coverage across southeast Georgia appears more isolated and will be dependent upon outflow boundaries propagating south/southwest from earlier convection. Also, heavy rainfall will be a concern as well with any slow-moving storm with PWATs ranging from 1.8 to 2.0 inches and weak steering flow present. While widespread flooding is not expected, rainfall amounts of a couple inches could occur beneath the stronger storms, yielding localized flooding of poor-drainage locations. Overall, convection this afternoon should be more localized than the next upcoming days.

Friday: A more appreciable change in the pattern is expected on Friday as this mid-lvl ridge finally breaks down and broad troughing begins to build across the eastern half of the CONUS. Several embedded shortwaves are forecast to round the base of this trough while a slow-moving surface front approaches the region from the west and northwest. This will provide greater convection coverage than the previous day (Thursday).

Scattered showers and thunderstorms are expected to develop mid- day/early afternoon, before becoming numerous and widespread in the afternoon/evening hours. Expect moisture to increase substantially ahead of this approaching front, with PWAT values forecast to rise above 2 inches (near the 90th percentile for late August at KCHS). Despite somewhat greater cloud cover and temperatures several degrees lower than the previous couple days, sufficient heating should still yield moderate to locally strong instability. The combination of instability, abundant moisture, and appreciable DCAPE values will support another risk for strong to severe storms. Thus, the SPC has highlighted the majority of the region in a Marginal Risk (Level 1/5) for severe thunderstorms on Friday. Again, damaging wind gusts will be the greatest threat (within the stronger storms), along with localized flooding possible.

Saturday: Another scattered to numerous shower and thunderstorm day as the front stalls near the region. The exact corridor of greatest coverage is less certain and will depend heavily on the frontal position. Strong wind gusts remain possible again, but training storms and localized flash flooding could become an increasing concern given PWATs near 2 inches and storms potentially tracking parallel to the boundary.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

KCHS/KJZI: A TEMPO group has been introduced from 18 to 21Z this afternoon to account for the threat of thunderstorms. Thereafter, prevailing VFR through the remainder of the 18Z TAF period. There is a chance that showers/thunderstorms could impact the terminals close to 18Z on Friday afternoon, however mention of any impacts has not been included with these 18Z TAFs.

KSAV: Prevailing VFR through the 18Z TAF period.

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

MARINE

Rest of Today/Tonight: No significant marine concerns are expected across the local waters through tonight, with winds and seas remaining below Small Craft Advisory criteria. A typical summertime pattern will dominate, with generally south-southwesterly flow around 5-10 kt becoming more south-southeasterly this afternoon as the sea breeze pushes inland. Winds could briefly increase to 10-15 kt along the immediate coastline and across the Charleston Harbor late this afternoon and evening. Seas should generally range from 2- 3 ft.

Extended Marine: Outside of showers and thunderstorms, overall marine conditions will remain fairly typical for late summer, with south-southwesterly winds generally 5-15 kt with seas ranging from 2- 4 ft through early next week. The main concern for mariners will be the increasing thunderstorm threat through the weekend as a front approaches and then stalls nearby.

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 until 8 PM EDT this evening for GAZ217>219- 239>241. SC...Heat Advisory until 8 PM EDT this evening for SCZ045-148>152. MARINE...None.