textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Isolated to scattered thunderstorms are expected to develop along the sea breeze this afternoon. These thunderstorms will be capable of producing gusty winds, locally heavy rainfall, and cloud to ground lightning.

- 2) Warm and humid coniditons will remain across southeast Georgia and the South Carolina Lowcountry this weekend. This environment will support scattered to numerous thunderstorm each afternoon and evening. Some of these slow- moving storms may produce localized excessive rainfall and gusty winds.

DISCUSSION

KEY MESSAGE 1: Isolated to scattered thunderstorms are expected to develop along the sea breeze this afternoon. These thunderstorms will be capable of producing gusty winds, locally heavy rainfall, and cloud to ground lightning.

The front that has been in/near the region the past couple days remains draped across our southeast GA counties, with a weak low pressure expected to remain along the front. Aloft, weak northwesterly flow will continue throughout the day. This pattern will continue the near-normal temperatures today, with highs in the lower to mid 90s across the area. Dewpoints for inland areas have mixed out into the mid to upper 60s, keeping heat index values in the mid 90s. Areas along/near the coast are expected to keep dewpoints in the lower to mid 70s, resulting in heat index values in the upper 90s to lower 100s.

The afternoon sea-breeze in combination with the weak aforementioned surface low is expected to continue and expand the coverage of the ongoing showers and thunderstorms into the rest of the afternoon hours. The environment today is progged to have 1500-2000 J/kg of MLCAPE, though weak 700-500mb lapse rates and weak flow aloft will keep the threat for strong-severe thunderstorms low. However, with dry mid-upper levels the DCAPE values are in the 1000-1500 J/kg range, bringing a low chance for strong wind gusts to near 50 mph. While PWATs are rather low in the 1.2-1.6" range, the weak flow aloft will keep storm motions rather slow, resulting in locally heavy rainfall being possible, which also aids the chances for strong wind gusts. Convection will likely struggle to spread inland thanks to the aforementioned lower dewpoints. Storms should mostly run their course by the late afternoon and steadily dissipate through the early evening hours.

KEY MESSAGE 2: Warm and humid coniditons will remain across southeast Georgia and the South Carolina Lowcountry this weekend. This environment will support scattered to numerous thunderstorm each afternoon and evening. Some of these slow-moving storms may produce localized excessive rainfall and gusty winds.

Saturday: High resolution guidance indicates that a weak warm front will approach from the south tonight. Recent runs of the HRRR suggest that the warm front could reach the Altamaha River during the pre-dawn hours Saturday morning. 12Z run of the HREF indicates that PW values will continue to increase through the daylight hours on Saturday. By Saturday afternoon, PW values should exceed 2 inches across southeast GA and between 1.8 to 2 inches across the SC Lowcountry. Forecast soundings indicate only a weak subsidence inversion, as a closed low settles over the Ohio River Valley. As a result, surface-based CAPE may exceed 2,500 J/kg along the coast and between 1,500 and 2,000 J/kg inland. This environment should allow isolated to scattered storms to develop along the sea breeze early Saturday afternoon. Storm coverage should peak across southeast GA late Saturday afternoon and evening as a thunderstorm cluster and associated cold pool sourced from central GA interacts with the sea breeze over the I-16 corridor between Metter, GA and northern Chatham County. Storms across southeast GA may only have storm motions of 10-15 miles an hour. The slow storm motions combined with deep atmospheric moisture could lead to localized areas of 2-4 inches of rainfall. HREF 3-hour QPF probabilities highlights the I- 16 corridor with up to a 30% chance of 3 inches in a 3-hour period. These rainfall rates may require Flood Advisories and possibly Flash Flood Warnings. Temperatures will remain seasonable with highs in the low to mid-90s.

Sunday: Forecast area will remain between an H5 closed low over KY and TN and a very strong ridge centered over Bermuda. At the surface, a broad area of low pressure will remain across SC. Forecast soundings indicate that deep moisture will persist across the region. However, thick cloud cover and falling mid-level heights should limit instability to 1,000 J/kg or less. The combination of surface moisture convergence within the broad trough and afternoon sea breeze should trigger scattered to numerous showers and thunderstorms. Some high-resolution guidance indicates clusters of thunderstorms over south-central GA could drift into southeast GA in the late afternoon hours. These storms should develop the tallest updrafts and may possess some severe wind gust threat. SPC has highlighted this area with a Marginal Risk for severe thunderstorms.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

18Z TAFs - Scattered showers and thunderstorms will be the main aviation concern this afternoon, with the observed coverage expanding along the sea-breeze into the afternoon hours. Latest guidance suggests the rainfall coverage should be inland of the SC coast, so have removed the PROB30 group for JZI, but have changed the PROB30s to TEMPO groups for both SAV and CHS given latest model and radar trends. Thunderstorms this afternoon will be capable of producing gusty winds with possible MVFR/IFR vsbys/cigs. Overall quiet conditions expected overnight, though some guidance is hinting at stratus and/or fog possible across southeast GA, though confidence is too low to include at the SAV TAF at this time. Chances for scattered showers and thunderstorms return Saturday morning along the Charleston county coastline, with coverage expanding to the south into the afternoon hours.

Extended Aviation Outlook: The probabilities for periodic flight restrictions due to showers and thunderstorms will increase this weekend into early next week.

MARINE

Through tonight: A stationary front will remain draped across the Southeast and extend into the local waters through tonight. This will support relatively light winds with seas generally around 1 to 2 ft.

This weekend through Wednesday: Pressure gradient will gradually increase across marine zones Saturday into Sunday. Sea and wind conditions should remain below Small Craft Advisory criteria on Saturday. Wind gusts in excess of 25 knots may develop across the near and outer waters by late Sunday afternoon into Sunday night. In addition, wave heights are forecast to increase to 4-6 feet Sunday night, Small Craft Advisories are possible. Conditions should gradually improve on Monday, with benign wind and sea conditions expected for Tuesday into Wednesday.

Rip Currents: A moderate risk of rip currents have been posted for the SC coast for Sunday. Rip currents may result from gusty southwest winds and building swell between 6 to 9 seconds.

CLIMATE

Record High Minimum Temperatures:

August 1: KCHS: 81/1999 KCXM: 86/1999 KSAV: 79/2020

August 6: KCHS: 79/1991 KCXM: 82/2006 KSAV: 80/1899

CHS WATCHES/WARNINGS/ADVISORIES

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