textproduct: Charleston

WHAT HAS CHANGED

All sections and Key Messages have been updated.M

KEY MESSAGES

- 1) Risk for heavy rain and flooding this afternoon across parts of Southeast Georgia.

- 2) A fairly typical summertime pattern is expected through the middle of next week with daily shower and thunderstorm chances.

- 3) Hot and humid conditions likely to peak Monday and Tuesday. Heat Advisories could be needed.

DISCUSSION

KEY MESSAGE 1: Risk for heavy rain and flooding this afternoon across parts of Southeast Georgia.

Weak cyclonic flow will remain in place across the Southeast U.S. today as a series of shortwaves propagate eastward across the Ohio Valley into the Northeast States and Mid-Atlantic region. A weak backdoor cold front will drop south into the area later this morning and become stationary near the I-16 corridor later this afternoon as it becomes increasingly intermingled with the sea breeze. A pronounced change in airmass will occur behind the front with an increasingly drier and capped environment advecting south into the South Carolina Lowcountry. South of the front, warm and humid conditions will persist with PWATs holding above 2 inches and dewpoints in the lower-mid 70s.

20/00z guidance is similar in showing weak upper forcing occurring later this afternoon a region of weak channeled vorticity propagating east along the northern Gulf coast moves into Southeast Georgia. This coupled with building instability (MLCAPE 2000-2500 J/kg) and low-level convergence in the vicinity of both the front itself and the sea breeze should support isolated to scattered showers/tstms across far southern South Carolina with scattered to widespread showers/tstms across parts of Southeast Georgia. The highest coverage looks to occur near Altamaha River where moisture, lift and instability will be maximized. 850-300 hPa flow looks to remain fairly modest (20-25 kt) so storms should remain fairly progressive; however, given the degree of PWATs there is an elevated potential for torrential rainfall with a risk for isolated flash flooding across parts of Tattnall, Long and McIntosh Counties where 3-hr neighborhood probabilities for >3" are running 30-35%. At this time, the risk for more significant flash flooding looks to remain displaced south of the Altamaha River where as much as 4-10" of rain have fallen over the past 24-36 hrs. WFO Jacksonville has issued a Flood Watch for that part of Southeast Georgia as a result. A Flood Watch will not be issued for Tattnall, Long and McIntosh Counties at this time given this area has observed considerably less rain, but this will be carefully monitored throughout the day. In addition to heavy rain, an isolated strong/severe tstm or two can not be ruled out with localized strong/damaging wind gusts and frequent cloud-to-ground lightning being the primary non-rain hazards. Convection will quickly wind down this evening with a risk for isolated convection lingering across parts of Southeast Georgia through late evening.

Highs today will warm into the upper 80s/lower 90s. Heat indices could peak in the 100-104 range across south of a Reidsville- Beaufort line before the onset of convection. Overnight lows will range from the upper 60s/near 70 well inland to the mid- upper 70s at the beaches and Downtown Charleston.

KEY MESSAGE 2: A fairly typical summertime pattern is expected through the middle of next week with daily shower and thunderstorm chances.

The forecast area will become positioned between high pressure offshore and an approaching cold front early next week, before forecast to stall near or over the area midweek. Aloft, a series of weak disturbances will cross the Southeast, before larger scale troughing possibly settles into the eastern CONUS. Isolated showers and thunderstorms are possible both Sunday and Monday, with coverage increasing Tuesday through at least Thursday. Showers and thunderstorms are expected to peak in the afternoon and evenings when instability is highest. At this time, the threat for organized severe weather is low, but cannot rule out a couple of strong to severe storms, especially Tuesday onward.

KEY MESSAGE 3: Hot and humid conditions likely to peak Monday and Tuesday. Heat Advisories could be needed.

Already hot and humid conditions will intensify Monday and Tuesday as a ridge aloft strengthens across the southern CONUS and western Atlantic. High temperatures are forecast to reach the low to mid 90s on Monday, with temperatures climbing a few degrees higher on Tuesday. Some locations could approach or exceed the upper 90s Tuesday afternoon. Combined with dew points in the 70s, heat index values could reach 105-110F on Monday, especially across the coastal counties. Confidence is higher on Tuesday, when more widespread heat indices above 105F are expected. A few locations could reach max heat indices between 110-113F. Little overnight relief is expected Monday and Tuesday, particularly near the coast, where minimum temperatures could only fall into the upper 70s to lower 80s. In addition, NWS HeatRisk guidance indicates a Major Risk (Level 3 of 4) across portions of the area. Given the combination of extreme heat, high humidity, and limited nighttime cooling, Heat Advisories could be needed where heat indices meet or exceed our local criteria of 108F degrees. Forecast trends will continue to be monitored over the coming days.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

20/06z TAF Discussion: KCHS/KJZI: A period of MVFR cigs are expected to filter into both terminals over the next few hours as a weak backdoor cold front drops south through the area. Cigs are expected to mix out/lift to VFR thresholds by mid-morning with VFR prevailing through the remainder of the 06z TAF cycle.

KSAV: Confidence through daybreak is moderate. Guidance is mixed on the potential for MVFR and possibly IFR cigs developing across Southeast Georgia ahead of a backdoor cold front. Confidence is highest for MVFR cigs, but opted to include a TEMPO for high-end IFR cigs 10-12z with a little ground fog to trend. Cigs should lift to VFR by mid-morning. Convection may begin to pop near the terminal mid-late afternoon, but the greatest coverage should remain well to the south. PROB30 for MVFR conditions in TSRA was included 22-01z for now.

Extended Aviation Outlook: Brief periods of flight restrictions possible with any showers and thunderstorms.

MARINE

Through Tonight: A weak backdoor front will stall across Southeast Georgia and the nearby Atlantic waters today. Wind fields will become a bit chaotic later in the day as the afternoon sea breeze develops and intermingles with the front. Speeds should remain 10 kt or less. Southwest winds will turn more westerly overnight with speeds generally less than 10 kt. Seas of 2-4 ft this morning will subside to 1-3 ft by this afternoon with 1-2 ft overnight. Tstms could produce wind gusts in excess of 34 kt across mainly the Georgia and far southern South Carolina waters this afternoon/evening. A few Special Marine Warnings may be needed.

Sunday through Thursday: No marine concerns are expected Sunday. The coastal waters will become positioned between high pressure centered well offshore and an approaching cold front Monday and Tuesday, leading to increasing winds and building seas. Gusts could approach 25 knots at times especially in the afternoons, but looks fairly marginal for Small Craft Advisories at this time. Conditions should stay below advisory levels through Thursday.

CLIMATE

Record High Minimum Temperatures: June 23: KCHS: 78/2024 KCXM: 82/1998 KSAV: 77/2017

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...None.