textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

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 today late week.

Rest of Today: Strong ridging aloft will remain the main synoptic feature influencing the local forecast area today. At the surface very weak troughing is in place along the coastline. The ridging aloft has led to a northwesterly flow today, pinning the sea breeze to the coastline. The delayed inland progression of the sea breeze will allow temperatures this afternoon to soar into the upper 90s across most locations, with a few spots touching 100. This northwesterly flow will allow for some mixing of Tds across the region, however once the sea breeze does push inland Td values are expected to reach into the mid to upper 70s across coastal locations. Heat index values of up to 106-112 are forecast inland and 113-119 is forecast across the coastal corridor. Additionally, limited convection this afternoon will provide little relief from the heat. An Extreme Heat Warning remains in effect from 11 AM to 8 PM today for SC/GA coastal counties and also includes Dorchester County, SC and Effingham County, GA. A Heat Advisory remains in effect from 11 AM to 8 PM today for all remaining inland areas as well.

Wednesday into Friday: The ridging aloft will begin to weaken slowly starting on Wednesday. This will allow for convection to increase in coverage each afternoon. Heat index values are still forecast to reach Heat Advisory criteria (>108) along the coastal corridor where dew points will be highest. A Heat Advisory has been issued from 11 AM to 8 PM Wednesday for the coastal counties. Additional Heat Advisories will likely be required Thursday and possibly again on Friday. Convective coverage will increase into Friday as a shortwave trough aloft moves through the Mid-Atlantic states. Friday could see the greatest coverage of showers and thunderstorms, possibly hindering temperatures from reaching their peak values and negating the need for any heat headlines.

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

This afternoon and evening: The background synoptic-scale setup features a front that mostly stretches from west to east situated north of the forecast area. This is driving west to west-northwest surface flow across the area, which will force the sea breeze to be late developing and pinned very close to the coast. Given that we are devoid of any forcing mechanism for convection other than differential surface heating and the sea breeze, the environment doesn't support much in the way of coverage. This is well supported by the hi-res model runs as well as the 18/12z HREF, all of which show little to diurnal convection. But, if a storm or two can get going there will be a low risk of a strong to marginally severe storm.

Wednesday and Thursday: A similar environment is expected each day favoring isolated to scattered convective coverage at most. We will still be detached from any meaningful forcing so we will continue to be dependent on surface heating and convergence along the sea breeze. The typical summertime rules apply in that any storm that develops could become marginally severe, produce locally heavy rainfall, and also frequent lightning.

Friday through the weekend: The expectation remains that the pattern will become more active as we move into the weekend. A slow moving front is expected to settle into the area and help provide more of a triggering and focusing mechanism for thunderstorm development. With continued elevated precipitable water values, this increase in coverage should bring the potential for locally heavy rainfall and several rounds of thunderstorms across the forecast area each day.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

The 18z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. Overall, VFR conditions are expected to prevail through 18z Wednesday. The only small concern is the low chance that a shower or thunderstorm could develop this afternoon or evening, primarily near KSAV. However, the overall likelihood appears quite low and we will not include any explicit mention in the TAF.

Extended Aviation Outlook: Prevailing VFR through Thursday. Afternoon showers and thunderstorms could bring brief flight restrictions starting Friday.

MARINE

Through tonight: West winds will continue until the late developing sea breeze gets going. Southwest flow will prevail this evening, turning more westerly through the late night hours. Winds should mostly top out in the 10-15 knot range this evening, diminishing more to be 5-10 knots late tonight. Seas are expected to average 2-3 feet.

Wednesday through Sunday: Overall, a pretty typical summertime pattern is expected to prevail across the local waters through the weekend. Winds will locally surge with the sea breeze each day, mostly into the 10-15 knot range and perhaps a few gusts up to 20 knots. Over the weekend there could be some support for more 15-20 knot southwest flow thanks to a bit more gradient. But, overall conditions across the local waters should remain below Small Craft Advisory thresholds through the period.

CLIMATE

Record High Temperatures:

August 18: KCHS: 101/1995 KCXM: 97/1995 KSAV: 100/1995

August 19: KCHS: 98/1983 KCXM: 98/1895 KSAV: 99/1900

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

August 21: KCHS: 100/1983 KSAV: 101/1902

Record High Minimum Temperatures:

August 18: KCHS: 80/2017 KCXM: 82/2017 KSAV: 78/2010

August 19: KCHS: 79/2010 KCXM: 83/2021 KSAV: 79/1878

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

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

August 22: KCHS: 79/2023 KCXM: 82/2014 KSAV: 79/1902

August 23: KCXM: 81/2019 KSAV: 78/2016

CHS WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory until 8 PM EDT this evening for GAZ087-088-099- 100-114-115-137. Extreme Heat Warning until 8 PM EDT this evening for GAZ101- 216>219-238>241. Heat Advisory from 11 AM to 8 PM EDT Wednesday for GAZ217>219- 239>241. SC...Heat Advisory until 8 PM EDT this evening for SCZ040-042-043. Extreme Heat Warning until 8 PM EDT this evening for SCZ044- 045-147>152. Heat Advisory from 11 AM to 8 PM EDT Wednesday for SCZ148>152. MARINE...None.