textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Few to scattered showers with an isolated strong thunderstorm possible across interior Southeast Georgia this afternoon.

- 2) Minor coastal flooding expected along portions of the Southeast South Carolina coast with the evening high tide cycles today and Monday.

- 3) Mostly dry conditions expected through midweek with increasing rain chances to occur late week.

DISCUSSION

KEY MESSAGE 1: Few to scattered showers with an isolated strong thunderstorm possible across interior Southeast Georgia this afternoon.

High pressure extending across the western Atlantic will maintain a strong influence on the region today as mid-upper ridging prevails aloft. The bulk of guidance continues to show a pronounced northwest- southeast oriented theta-e axis setting up from east-central Georgia into far Southeast Georgia this afternoon, with MLCAPE approaching 2500-3000 J/kg during peak diurnal heating. Instability could support few to scattered showers along with thunderstorms later this afternoon while h5 vort energy rounds the western periphery of the Atlantic high near the timing of an inland moving sea breeze. However, deep-layer shear is weak, suggesting little organization to convection, unless activity congeals and/or outflow boundary interactions give a temporary boost to thunderstorms along and/or just ahead of a sea breeze across far interior of Southeast Georgia.

Although most convection that materializes is expected to remain in the form of general shower and thunderstorm activity and just inland to the local area today, a stronger thunderstorm can not be ruled out if the timing of convection aligns with the sea breeze passage mid-late afternoon across far interior Southeast Georgia. Although a risk for hail is present with mid-lvl lapse rates near 7 C/km and large -10C to -30C CAPE in place, deep layer shear is weak under the synoptic high and the environment more supportive of small hail if realized. Gusty winds will be the primary concern with a stronger thunderstorm given 8-8.5 C/km low-lvl lapse rates and DCAPE around 1000 J/kg during peak heating of the day. The Storm Prediction Center does highlight far interior Southeast Georgia in a Marginal Risk for severe weather today, but the threat remains quite conditional and more likely just inland to the local area based on activity developing ahead of a sea breeze and along the western edge of the Atlantic high prior to sunset.

KEY MESSAGE 2: Minor coastal flooding expected along portions of the Southeast South Carolina coast with the evening high tide cycles today and Monday.

Astronomical influences associated with the new moon (5/16) and lunar perigee (5/17) along with a modest southeasterly onshore flow will continue to drive elevated tide levels early this week, leading to minor coastal flooding along the southeast South Carolina coast and potentially along the southeast Georgia coast. The greatest risk for minor coastal flooding will be at the Charleston Harbor tide gage, while a much lower potential exists at Fort Pulaski. The latest forecast advertises a peak high tide around 7.1 to 7.3 ft MLLW in the Charleston Harbor this evening, then 7.0 to 7.2 ft MLLW Monday evening. A Coastal Flood Advisory has therefore been issued for Coastal Colleton and Charleston Counties between 7 PM to 11 PM this evening. Additional Coastal Flood Advisories could be needed Monday. Astronomical tide levels then begin to fall and the potential for minor coastal flooding will diminish.

KEY MESSAGE 3: Mostly dry conditions expected through midweek with increasing rain chances to occur late week.

Surface high pressure will extend across the region through midweek, while ridging largely prevails aloft. Convection potential will be quite limited, with most areas staying dry Monday through Wednesday.

Beyond Wednesday, the pattern could become more active, although there is still a good bit of inconsistency between models. Shortwave energy will advance toward the area, but will compete with the deep Bermuda High offshore, which could deflect the better forcing north of the region. At the surface, a cold front will approach the area, with its eventual progression likely dependent on the strength of the ridge. Our current forecast indicates a slow progression of the front into southeast SC/GA over the course of several days (Thu- Sat), which would support increasing moisture and higher rain chances across the area. However, if the ridge remains stronger, the front and better forcing could remain farther to the west and north, resulting in lower rain chances than currently forecast. Ensemble mean indicates the probability for accumulated rainfall of >1" ending Saturday evening of only 10-40% (highest inland), so widespread significant rainfall seems unlikely at this time.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

VFR conditions will prevail at CHS/JZI/SAV terminals through 18Z Monday. However, gusty winds (14-17 kt) will accompany an afternoon sea breeze today.

Extended Aviation Outlook: There are no high confidence concerns.

MARINE

Today and Tonight: High pressure will remain the dominant weather feature across local waters throughout the day and night, resulting in a typical summer-like southerly flow regime. In general, wind speeds will be no higher than 10 kt across most waters, although localized enhancement along the land/sea interface and Charleston Harbor is anticipated with any sea breeze enhanced this afternoon (~15 kt). Seas should range between 2-4 ft, largest beyond 20 nm from the coast overnight.

Monday through Friday: No marine concerns are expected. High pressure to the east will drive onshore east-southeasterly flow each day, with wind speeds mostly topping out in the 10-15 knot range. Each afternoon and evening there could be some local enhancement along the land/sea interface with the diurnal sea breeze. Seas should average 2-4 feet through the period.

Rip Currents: The combination of influences from the new moon cycle and persistent onshore flow will produce an elevated risk of rip currents through midweek. A Moderate Risk of rip currents is in effect at all southeast South Carolina and southeast Georgia beaches Monday and Tuesday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...Coastal Flood Advisory from 7 PM to 11 PM EDT this evening for SCZ149-150. MARINE...None.