textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Showers and thunderstorms will bring a risk for heavy rainfall and localized flooding through this evening. A few strong to marginally severe thunderstorms could also occur.

- 2) Wet conditions will bring the potential for heavy rainfall and localized flooding Monday, with the threat possibly lingering into Tuesday.

- 3) There is a High Risk for rip currents for the Charleston County beaches through this evening.

DISCUSSION

KEY MESSAGE 1: Showers and thunderstorms will bring a risk for heavy rainfall and localized flooding through this evening. A few strong to marginally severe thunderstorms could also occur.

Recent SPC mesoanalysis indicated that PW values across SE GA and SC ranged between 2.1-2.3 inches. As temperatures peak in the upper 80s to low 90s, instability should increase across the region. MLCAPE values are forecast to range between 2000-2500 J/kg, with the greatest values over inland GA/SC. Given ample instability and little to no inhibition, showers and thunderstorms should develop through the rest of the afternoon into this evening. KCLX VWP and forecast soundings indicate very little vertical shear across the area. Based on radar trends and recent runs of the HRRR, multi-cell storms over southern GA early this afternoon are forecast push over SE GA this afternoon, reaching the SC Lowcountry late this afternoon and evening. However, given the lack of shear, the outflow boundary may outpace the deep convection as it reaches the Savannah River Valley. Given the deep moisture, these storms will likely bring rounds of heavy downpours. If the storms are able to train or produce frequent cell mergers over the area of Saturday's heavy rain, excessive rainfall may develop. In addition, a few storms may experience collapsing rain-loaded updrafts, producing some risk for damaging wind gusts.

KEY MESSAGE 2: Wet conditions will bring the potential for heavy rainfall and localized flooding Monday, with the threat possibly lingering into Tuesday.

A mid-level trough moving over the Northeast U.S. Monday morning will generate a cutoff Low over the Southeast. At the surface, a cold front will be over the Appalachians Monday morning, slowly shifting eastward. Ahead of the front will be deep moisture. PWATs will peak around 2.3" during the first half of Monday, or roughly two standard deviations above climatology, before gradually decreasing into the 2.0-2.2" range by the evening. Although instability is expected to diminish to generally less than 1,000 J/kg, forecast soundings continue to display long, narrow CAPE profiles, favoring efficient warm-rain processes and the potential for high rainfall rates despite the weaker instability. Models continue to focus the highest QPF along the coastal corridor. Given antecedent rainfall, localized flooding concerns will increase, especially where storms move over the same locations. Urban and poor- drainage areas, including the Charleston metro area, could be particularly susceptible, potentially warranting Flood Advisories or Flash Flood Warnings. WPC updated their Excessive Rainfall Outlook to reflect this. The risk for heavy rainfall may continue into Tuesday. Though, PWATs are expected to be lower, which could support lower rainfall.

KEY MESSAGE 3: There is a High Risk for rip currents for the Charleston County beaches through this evening.

The combination of building seas and increasing winds will support a High Risk for rip currents for the Charleston County beaches. A Moderate Risk is in place from Edisto Beach south into the Georgia coast.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

18Z TAFs: Warm and humid conditions may result in a shower or thunderstorms at any time this afternoon. However, a cluster of thunderstorms sourced over southern GA is timed to reach the terminals later this afternoon and evening. The primary time windows will be highlighted with TEMPOs, 18-22Z at KSAV and 20-24Z at KCHS/KJZI. These storms may restrict visibility due to heavy rain and bring the possibility of 25 kt gusts. Following the storms, southwest winds should decrease to 5 to 10 kts. Showers may redevelop along the coast late tonight, possibly drifting onshore between 6-12Z, highlighted with a PROB30. On Monday, some guidance indicates the potential for restrictive ceilings. At this time, the TAFs will indicate VFR conditions with SCT MVFR cloud bases.

Extended Aviation Outlook: Periodic flight restrictions are expected in showers and thunderstorms this week.

MARINE

Tonight, the marine zones will remain between a Bermuda high and a cold front across the southern Appalachians. The pressure gradient between these features should support frequent gusts between 25 to 30 kts and seas building generally between 5 to 7 ft. Small Craft Advisories are in effect for all marine zones tonight. In addition, thunderstorms may reach the nearshore waters early this evening, possibly requiring the issuance of Special Marine Warnings.

Extended Marine: Winds and seas will trend lower on Monday, so the Small Craft Advisories will gradually expire during the day and into the evening. A typical summertime wind pattern is expected Tuesday through the end of this week. No Small Craft Advisories are expected.

Rip Currents: Monday: For our SC beaches, our internal calculator barely has a Moderate Risk of rip currents, while RCMOS has a High Risk. For our GA beaches, our internal calculator has a Low Risk of rip currents, while RCMOS has a Moderate Risk. Lifeguards have not reported any rip currents outside of Tybee through this (Sunday) afternoon. With all of this in mind, we opted to go with a Moderate Risk at all of our beaches.

High Surf: NWPS breaking wave height guidance suggests a few Charleston County beaches could see a breakers as high as 5 ft at times, mainly from late afternoon into the tonight period. These instances look pretty sparse and with most breakers holding in the 3- 4 ft. Since the majority of the large breakers will occur after 8 PM, no High Surf Advisory will be issued with this package.

CLIMATE

Record High Minimum Temperatures:

August 2: KCHS: 79/2022 KCXM: 84/1999 KSAV: 80/1886

August 5: KCHS: 79/1980 KCXM: 82/1991 KSAV: 80/1883

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

August 7: KCHS: 79/2007 KCXM: 83/2007 KSAV: 80/1879

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...High Risk for Rip Currents until 8 PM EDT this evening for SCZ150. MARINE...Small Craft Advisory until 2 AM EDT Monday for AMZ340. Small Craft Advisory until 9 PM EDT Monday for AMZ360. Small Craft Advisory until 6 PM EDT Monday for AMZ362. Small Craft Advisory until 8 AM EDT Monday for AMZ364. Small Craft Advisory until noon EDT Monday for AMZ384.