textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Hot and humid conditions will continue today, with isolated to scattered thunderstorms this afternoon/evening, a few which could become strong to severe with damaging wind gusts being the primary threat.

- 2) Rain chances and thunderstorm coverage expected to increase Sunday through Tuesday, bringing cooler temperatures for a few days before temperatures rise again mid-week.

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

DISCUSSION

KEY MESSAGE 1: Hot and humid conditions will continue today, with isolated to scattered thunderstorms this afternoon/evening, a few which could become strong to severe with damaging wind gusts being the primary threat.

Another hot day is expected with the local area situated under ridging aloft and between sfc high pressure extending across the western Atlantic and lee troughing across the Midlands and Upstate region. A westerly flow will help mix out sfc dewpts late morning and afternoon, potentially into the mid-upper 60s away from coastal areas, and lowest west of I-95 across Southeast Georgia. Once a sea breeze begins to shift inland, dewpts will surge within the marine layer during peak heating hours, likely leading to heat index values up to 108-110 degrees along the coastal corridor. Given the setup, along with a potential delay in shower/thunderstorm activity until late day/early evening, a Heat Advisory will remain in effect for coastal areas from 11 AM to 7 PM today.

Regarding thunderstorms, coverage and environment remains similar to conditions experienced the previous day with moderate instability, ample DCAPE and strong low-lvl lapse rates supportive of a few strong to severe thunderstorms capable of damaging wind gusts. Convection could start early afternoon, mainly across the Tri-County Area near a pinned sea breeze closer to the coast. Should this activity develop, convective outflows could continue to produce additional shower/thunderstorm activity with heavy downpours as PWATs remain around 2.0 inches. Quite a bit of guidance suggests additional showers/thunderstorms developing upstream of the local area while h5 shortwave energy traverses across the inland trough, some of which could make a run toward inland locations of Southeast South Carolina late day/early evening. The overall strength of this convection appears dependent on the time of arrival, with a weakening trend possible if delayed until sunset and/or shortly thereafter. Regardless, the potential for a few strong and/or severe thunderstorms could persist into early evening hours before convection wanes in a weakly sheared environment.

KEY MESSAGE 2: Rain chances and thunderstorm coverage expected to increase Sunday through Tuesday, bringing cooler temperatures for a few days before temperatures rise again mid-week.

Sunday: Weak H500 troughing to our north will bring a weak surface low pressure off the Virgina/North Carolina coast and into the Atlantic. A weak cold front associated with the surface low will slowly sag down into the region, bringing increased chances for showers and thunderstorms to the region as the expected sea-breeze will provide additional lift for thunderstorm development. While agreement amongst the models could be higher, a few models continue to depict a weak surface low pressure developing along the cold front, moving across the area Sunday night before pushing offshore on Monday. With precipitable water values near 2 inches and slow storm motions expected, locally heavy rainfall will be possible. If a surface low pressure does develop, that would further increase moisture pooling capabilities, with a Marginal risk (level 1 out of 4) for excessive rainfall from WPC covering that threat.

Before storms start forming in the afternoon, the heat wave continues with afternoon highs in the mid to upper 90s. Dewpoints will be highest along the coast in the upper 70s, resulting in heat index values peaking near 110 degrees resulting in the possibility for another Heat Advisory being needed for Sunday. Inland areas will see a touch less moisture with dewpoints in the lower 70s, keeping heat index in the lower to mid 100s. As for the potential for strong to severe thunderstorms, 1500-2000 J/kg of CAPE will continue isolated chances, though shear remains low keeping chances for organized convection low. With astronomical high tide of 6.44 ft MLLW occurring shortly after 7 PM, any thunderstorm activity near the Charleston peninsula will increase chances for impactful coastal flooding.

Monday - Tuesday: Cooler air continues to filter down into the region as a surface high pressure moves towards/across the Appalachians, with the front remaining across/south of the region which will continue periodic chances for rainfall on Monday and Tuesday. As before, models continue to suggest another weak surface low pressure may develop along/north of the front. Precipitable water values continue to remain high in the 2-2.5 inch range, keeping the threat for locally heavy rainfall on the elevated side, which WPC agrees with as they have a Marginal risk (level 1 out of 4) for excessive rainfall on both days. Similar to Sunday, if heavy rainfall were to occur during evening high tide, this would increase impacts from any coastal flooding as astro tides peak at 6.57 ft MLLW on both days. The increased cloud coverage from rainfall chances will bring temperatures down into the upper 80s to lower 90s both Monday and Tuesday..

H500 heights begin to rise again on Wednesday as the ridge strengthens, with temperatures rising back up into the lower to mid 90s. Temperatures look to further climb into the mid 90s Thursday and Friday.

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 /07Z SATURDAY THROUGH WEDNESDAY/

VFR conditions should prevail at CHS/JZI/SAV terminals through 06Z Sunday. However, showers and thunderstorms could impact CHS/JZI terminals late afternoon into early evening hours. PROB30 groups remain at these terminals for MVFR conditions associated with thunderstorms between 21-24Z today. Outside convection, gusty west/southwest winds upwards to 15-20 kt are possible during afternoon hours at all terminals.

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

MARINE

Today through Tonight: Another round of enhanced southwest flow is anticipated during afternoon/evening hours, with gusts typically in the 15-20 kt range and perhaps a few gusts up to 25 kt, highest near the land/sea interface where a sea breeze develops then shifts inland. Conditions could become marginally supportive of a Small Craft Advisory in the Charleston County waters and possibly the Charleston Harbor, but are currently forecast to fall just short of criteria. Seas should range between 2-4 ft. Thunderstorms are expected to develop inland this afternoon and evening, and could eventually shift across local waters with frequent lightning and gusty winds. Special Marine Warnings could eventually be needed.

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

Record High Minimum Temperatures:

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

July 12: KCHS: 79/2020

CHS WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from 11 AM this morning to 7 PM EDT this evening for GAZ217>219-239-241. SC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening for SCZ045-148>152. MARINE...None.