textproduct: Charleston

WHAT HAS CHANGED

Key Message 1 and the Aviation section have been updated.

KEY MESSAGES

- 1) Less thunderstorm activity expected today as a cold front moves through, ushering in cooler and drier air into the middle of the week.

- 2) Minor coastal flooding is possible along the Charleston and Colleton County coastline with the upcoming evening high tide.

DISCUSSION

KEY MESSAGE 1: Less thunderstorm activity expected today as a cold front moves through, ushering in cooler and drier air into the middle of the week.

Aloft, a well defined trough will drop southward from the Upper Midwest and central Appalachians into VA and the Carolinas late in the day. This will push a surface cold front southward as well. The front is currently analyzed from west to east across southeast SC. The front will continue to make steady southward progress through the day, likely near the Savannah River by midday and clearing the Altamaha River this evening. Regarding convective chances, for southeast SC the best chance of lingering showers will be in the morning before the front clears the area and notably drier air filters in. The bigger question mark is the southeast GA area ahead of the front this afternoon. Outflow from the early morning convection has moved through and with widespread cloud cover has helped to stabilize the environment. If destabilization can occur and we get some thunderstorm activity, there could be a conditional severe threat as the combination of peak heating and moisture pooling ahead of the front could align across southeast GA in the afternoon hours. However, recent hi-res model runs have definitely trended toward little to no convective development and coverage this afternoon. If a few storms do develop, the environment could yield an isolated damaging wind gust threat for the region along and south of I-16. For now, this seems like a low probability of occurrence.

Behind the front, considerably cooler air will be in place overnight. Lows are forecast to fall into the low to mid 50s away from the coast (about 10 degrees below normal). Then for Wednesday, highs are forecast to only rise to around 80 (6-8 degrees below normal). With dewpoints forecast to fall into the 40s for much of the area, conditions should be exceedingly comfortable for early June.

KEY MESSAGE 2: Minor coastal flooding is possible along the Charleston and Colleton County coastline with the upcoming evening high tide.

Gusty northeast winds will develop along the southeast SC coast today behind a passing cold front. This northeast flow will produce elevated tide levels, especially around the time of this evening's high tide (occurring around 1015 PM). The astronomical tide level is 5.5 ft MLLW, so it would require a rather anomalous 1.5 ft departure to reach the minor coastal flood threshold of 7 ft MLLW. For context, historical analysis shows that a departure of 1.5 ft or higher has only occurred with ~7% of all minor coastal flood events on record. Still, minor coastal flooding is possible, and a Coastal Flood Advisory could be needed for the Charleston and Colleton county coasts.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

The 12z TAF period begins with mostly VFR conditions at KCHS, KJZI, and KSAV. The cold front has moved through KCHS and KJZI and the first forecast challenge is determining if MVFR or IFR ceilings will spread in. Some upstream observation sites are reporting MVFR/IFR stratus but it is unclear to what extent it will impact KCHS and KJZI. Current thinking is that it will move in and stick around through midday. Breezy northeast flow will spread in as well, with frequent gusts into the 18-22 knot range expected. At KSAV, there remains a low shower/thunderstorm risk this afternoon until the front moves through. Northeast winds will become breezy as well. Any issues with restricted ceilings will end this evening and the latter half of the forecast is VFR.

Extended Aviation Outlook: Primarily VFR.

MARINE

Tuesday through Saturday: A cold front will push southward across the coastal waters this morning through early afternoon, ushering in breezy northeast winds. Accordingly, SCAs go into effect across the coastal waters from north to south today with 6-8 ft seas prevailing across the coastal waters by sunset. Charleston Harbor will likely see breezy conditions as well, with 20-25 kt wind gusts through much of the day, though the frequency of 25+ kt gusts looks to low to justify SCAs at this point.

Then, later this evening through the overnight hours, high pressure ridging in the Mid-Atlantic and Southeast will usher a reinforcing backdoor cold front/wind surge, with gusts increasing to 30-35 kt across the coastal waters - especially beyond 10 nm offshore - and seas peaking around 6-9 ft. Gale force gusts are currently expected to be infrequent enough to preclude the need for a warning, but further monitoring is necessary. Additionally, later tonight into early Wednesday morning will bring an increased chance for gusts to 25+ kt in the Charleston Harbor, and an SCA may be issued later today.

Marine conditions gradually improve through the week as high pressure continues to build in from the north. The ridging moves nearly overhead by the end of the week, with exceptional marine conditions possible Thursday into early next weekend.

Rip Currents: Strong northeast winds will spread along the southeast SC/GA coast today as a cold front moves through. The combination of strong northeast winds and increasing swell energy will result in a Moderate Risk of rip currents at all beaches today.

Looking ahead to Wednesday, northeast winds will remain strong and swell energy and wave action will be higher than Tuesday. This will result in a Moderate Risk for the Charleston County beaches and a High Risk from Edisto Beach southward.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from 11 AM this morning to 5 AM EDT Thursday for AMZ360. Small Craft Advisory from 1 PM this afternoon to 5 AM EDT Thursday for AMZ362. Small Craft Advisory from 4 PM this afternoon to 2 PM EDT Thursday for AMZ364. Small Craft Advisory from 4 PM this afternoon to 11 PM EDT Thursday for AMZ384.