textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Scattered showers and thunderstorms today with an isolated strong thunderstorm possible until sunset.

- 2) Breezy and cooler weather arrives behind a cold front Wednesday and Thursday.

- 3) The threat for coastal flooding will increase mid to late week.

DISCUSSION

KEY MESSAGE 1: Scattered showers and thunderstorms today with an isolated strong thunderstorm possible until sunset.

Today: A mid-upper lvl low currently positioned off the Carolina Coast will slowly depart to the north-northeast, then shift further offshore while beginning to phase with a longwave trough shifting across the Mid-Atlantic and Northeast late day. Although this should result in some NVA locally, previous convection appears to have left multiple outflow boundaries in the vicinity, aiding convection to re- develop across Southeast South Carolina this afternoon. Further south, sfc heating has been stronger, leading to afternoon highs in the upper 80s/around 90. Scattered showers along with a few thunderstorms should continue to develop and/or persist across Southeast Georgia and into parts of Southeast South Carolina with some slight enhancement to coverage and strength of activity in the vicinity of an afternoon sea breeze. A stronger thunderstorm with gusty winds and localized heavy rainfall/minor flooding issues can not be ruled out with modest instability and deep moisture (PWATs 2+ inches) in place. However, severe weather chances are low given a lack of substantial forcing and weak 0-6km bulk shear.

Tonight: Shower/thunderstorm coverage will likely be on a downward trend early evening due to the loss of daytime heating with the bulk of activity diminishing within a few hours after sunset. A cold front will then approach the area from the north-northwest overnight, initially flipping winds northerly late evening, prior to a reinforcing wedge pushing the front across the area late night. Latest guidance indicates dry conditions for most areas through the night, but a few showers can not be ruled out with fropa late.

KEY MESSAGE 2: Breezy and cooler weather arrives behind a cold front Wednesday and Thursday.

A wedge of high pressure will strengthen across the Carolinas and Georgia Wednesday, likely forcing a cold front across far southern zones early-mid morning, then south of the Altamaha River before noon. The position of the front and strengthening wedge will play the largest role in precip coverage locally, limiting the risk to a few rain showers only across inland and northern areas. Best chances for precip should be along the I-95 corridor in Southeast Georgia, where warmest temps prevail early. Scattered showers along with a few thunderstorms are possible in this area, but the threat for a stronger thunderstorm has lowered based on an earlier time of fropa.

Cold air advection and a tightening pressure gradient associated with the nearby wedge will likely support breezy conditions Wednesday afternoon and Thursday. Northerly winds could gust upwards to 20-25 mph Wednesday afternoon/night, then up to 25-35 mph Thursday, strongest near coastal areas. Conditions appear to fall just short of Wind Advisory criteria over land, but could warrant a Lake Wind Advisory across Lake Moultrie, especially Thursday. Temperatures will be noticeably cooler than previous days post fropa, with highs only reaching the mid-upper 70s Thursday and Friday, prior to gradually warming into the 80s this weekend. Overnight lows should also dip into the low-mid 50s across the interior late week.

KEY MESSAGE 3: The threat for coastal flooding will increase mid to late week.

As a full moon approaches on 9/26, breezy northeast winds develop Wednesday and linger into Friday, which may result in coastal flooding issues as tidal departures rise. Will likely see minor coastal flooding Wednesday evening along the Charleston and Colleton county coast, and again on Thursday evening. Latest guidance would indicate even the Friday morning high tide may see minor coastal flooding, with another round expected for the evening high tide. Thursday evening and Friday evening both have potential for reaching moderate flood stage in the Charleston Harbor.

Impacts are less clear moving down the rest of the southeast South Carolina coast and into southeast Georgia, with the latest guidance keeping water levels below minor flood stage at Fort Pulaski. The two high tides where minor flood stage cannot entirely be ruled out yet are Thursday and Friday evening.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

TEMPO MVFR/IFR conditions are likely at SAV due to showers and thunderstorms between 18-22Z today. At CHS/JZI, confidence is lower in flight restriction activity, but have maintained TEMPO MVFR conditions at JZI between 18-19Z today due to vicinity of ongoing activity. VFR conditions should prevail at all terminals this evening and into early night until a cold front pushes through late with low cigs. Prevailing MVFR conditions begin at 08Z Wednesday at CHS/JZI, then 09Z at SAV. Expect MVFR cigs to persist through 18Z Wednesday thereafter. Gusty winds should also develop near or shortly after daybreak Wednesday, generally in the 15-20kt range at CHS/SAV and up to 20-25 kt at JZI by late morning.

Extended Aviation Outlook: Periods of flight restrictions possible due to low cigs associated with a wedge of high pressure Wednesday afternoon through Thursday. VFR conditions should prevail at all terminals this weekend.

MARINE

Through Tonight: Marginally breezy northeast winds this afternoon will remain at similar strength into the overnight hours, with periodic showers and thunderstorms capable of gusty winds and frequent lightning.

Wednesday-Friday: A cold front will be pushing through during the morning hours on Wednesday, with a strong surge in northeasterly flow as the pinched pressure gradient (aka the wedge) settles in. Breezy winds ramp up during the morning hours, with wind gusts of 25 to 34 knots remaining across the local waters into Friday afternoon. There is a 30-55% chance for Gales for areas 20+nm off the coast, with the trend being increasing probabilities for a larger spatial area. Given the breezy winds, a Small Craft Advisory is in effect for all near-shore zones, as well as the zone from Savannah to the Altamaha River, GA from 20-60nm out. A Gale Watch from Thursday evening into Friday was considered, but would like to see more data before committing to that. In addition, wave heights of 5-8 ft will likely accompany the breezy winds for areas 10+ nm away from the coast Thursday morning through Friday evening before winds begin to weaken.

Saturday-Sunday: The wedge continues to weaken, further decreasing the wind strength throughout the day. Seas will remain unsettled, with offshore zones (20+nm offshore) seeing 6ft+ waves continuing into the evening hours before decreasing below 6 ft.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from 5 AM Wednesday to 5 PM EDT Friday for AMZ360-362. Small Craft Advisory from 8 AM Wednesday to 11 PM EDT Friday for AMZ364. Small Craft Advisory from 8 AM Wednesday to 8 AM EDT Sunday for AMZ384.