Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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020 FXUS62 KCHS 021926 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 326 PM EDT Tue Jul 2 2024 .SYNOPSIS... A weak area of low pressure will linger across Southeast Georgia through Wednesday. Through the rest of the week, a hot and humid air mass will build over the region. A weak cold front could approach the region by early next week. && .NEAR TERM /THROUGH TONIGHT/... 02/18z mesoanalysis placed the stationary front just south of the Altamaha Sound. An area of concentrated showers with isolated tstms persists near the front, but overall, the risk for heavy rainfall for the remainder of the afternoon has mostly ended. The front will begin to buckle back to north across the coastal waters overnight while a wedge of high pressure holds over land. Increasing convergence near the front should promote an uptick in showers and tstms over the Atlantic later tonight. There has been a fairly consistent signal in directing the bulk of this activity into the far southern South Carolina coast and into the Savannah River Entrance early Wednesday morning. Pops up to 50% were highlighted in this region to account for this. There will also be a risk for locally heavy rainfall and minor flooding given some pretty hefty rain amounts have been observed here over the past 24-hours. Otherwise, 20-40% pops were held for the coastal counties overnight with far interior areas mostly remaining rain-free. Lows tonight will range from the lower 70s well inland to around 80 at the beaches. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... Wednesday: A weakening stationary front or surface trough will remain across southeast GA through the afternoon. The HREF indicates a pool of 1500 to 2000 J/kg of CAPE will remain along and south of the trough. At the surface, the center of a 1020 mb high will remain off the mid-Atlantic coast, resulting in broad east flow across the forecast area. As temperatures warm into the upper 80s to lower 90s, a seabreeze should develop and advance inland during the afternoon hours. Scattered to numerous showers and thunderstorms are expected to form over the coastal waters of GA early Wednesday morning, the convention is expected to spread onshore generally south of the Savannah River from midday through the afternoon. Independence Day: A large H5 ridge will expand across the Deep South and Southeast US region. Forecast soundings indicate a subsidence inversion centered at H75. This inversion should maintain at 30 to 50 J/kg of CIN through the heat of the afternoon. In addition, PW values should remain limited to 1.5 inches across the region. As a result, the forecast will feature dry conditions through much of the day. However, a SCHC PoPs are possible across portions of southeast GA, and the far inland counties of SC. Little to no QPF is expected. Given the position of the mid-level ridge, and expected partly sunny conditions, temperatures are forecast to peak in the mid to upper 90s with heat index values between 100 and 105. Friday: GFS indicates that the center of the +590 DM ridge will remain over the Deep South. Forecast soundings indicate that the subsidence inversion will remain centered at H75, resulting in at least weak capping. However, given the passage of a weak late afternoon sea breeze, isolated showers and thunderstorms may develop during the afternoon. High temperatures should range in the mid to upper 90s for most areas. In addition, dewpoints along and behind the sea breeze may increase into the mid to upper 70s, with a few spots reaching the low 80s. Heat index values may exceed 108 in spots, especially across portions of the SC Lowcountry. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... The mid-level pattern through the long term will be very interesting. The ridge will generally remain across the Southeast U.S. as a closed low associated with TC Beryl enters the western Gulf of Mexico. To the east, GFS and ECMWF indicate that a closed low south of Bermuda will drift toward the Southeast coast. As a result, moisture will gradually build across the region under the ridge, especially as the closed low over the Atlantic approaches from the east. At the sfc, a weak cold front will approach from the west, expected to become nearly stationary east of the Appalachians early next week. This pattern should support generally diurnal showers and thunderstorms, greatest coverage expected on Monday and Tuesday. Conditions will remain hot across the CWA, high in the mid to upper 90s with heat index values around Heat Advisory values. && .AVIATION /19Z TUESDAY THROUGH SUNDAY/... 02/18Z TAF Discussion: KCHS/KJZI: VFR through the period. There is a chance near MVFR cigs could develop near KJZI after midnight as shower activity off the lower South Carolina coast tries to develop. VCSH was highlighted at KJZI 08-14z for now. KSAV: MVFR cigs will improve to VFR 20-21z. VFR should then dominate through late evening. After that, the risk for MVFR cigs will increase again as shower activity off the coast draws closer to the coast. Extended Aviation Outlook: Brief flight restrictions are possible in showers and thunderstorms, mainly in the afternoon and evening through the period. && .MARINE... Tonight: Winds continue to frequently gust 25-30 kt across South Carolina nearshore waters and the Savannah River Entrance this afternoon as pinched gradient conditions persist. Models have not handled the evolution of the pinched gradient well at all today. Given the current trends, the advisory has been extended until 6 PM. Winds should diminish after this the front begins to buckle north and resulting pinched gradient relaxes a bit. Seas will diminishing to 2-4 ft through the night. Wednesday through Sunday: Winds are forecast to remain from the northeast on Wednesday, generally between 10 to 15 kts. Seas will range between 2 to 3 ft. Thursday through this weekend, south winds are forecast to remain less than 15 kts. Seas will favor values between 2-3 ft. && .TIDES/COASTAL FLOODING... Charleston Harbor: Minor coastal flooding is likely at Charleston Harbor this evening (606 PM) as departures remain around +1.35 ft above astronomical (as of 215 PM). Taking a look at the latest P- ETSS and ETSS guidance shows models running behind at initialization with a a 0.5/ 0.7 ft anomaly needed for correction. This is likely due to guidance not handling the tight pressure gradient/ stalled cold front along the coast well. Given the above, the latest forecast for Charleston Harbor is for a 7.4 ft (minor) high tide this evening (606 PM). A coastal flood advisory is currently in effect for Charleston and Coastal Colleton Counties. Another round of coastal flooding is then possible Wednesday evening as winds veer around from the southeast. Fort Pulaski: Tidal departures continue to hold steady this afternoon around ~1.65 ft (as of 215 PM). Forecast guidance also is running behind this afternoon, and this is likely due to global guidance (which the ETSS and P-ETTS use as forcing) not capturing the tightening gradient along the coast with multiple offshore reporting sites observing wind gusts just shy of 30 mph. The current forecast high tide at Fort Pulaski this evening (608 PM) is 9.5 ft (minor). As such, a coastal flood advisory is now in effect for Beaufort, Coastal Jasper, Coastal Chatham, Coastal Bryan, Coastal Liberty and Coastal McIntosh counties. There is some upside potential here as the tight pressure gradient from the stalled frontal zone remains in the region. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Coastal Flood Advisory until 8 PM EDT this evening for GAZ117- 119-139-141. SC...Coastal Flood Advisory until 8 PM EDT this evening for SCZ048>051. MARINE...Small Craft Advisory until 6 PM EDT this evening for AMZ350-352. && $$