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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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911 FXUS62 KCHS 171014 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 614 AM EDT Wed Jul 17 2024 .SYNOPSIS... High pressure will persist offshore, while a weak trough resides inland today. A cold front will approach the region late week and likely stall nearby, before high pressure returns next week. && .NEAR TERM /THROUGH TONIGHT/... Latest high-res data has pushed the axis of greatest coverage of showers/tstms a tad east closer to the coast, thus increasing the potential for outflow/sea breeze interactions. Coastal pops were increased about 10% as a result. Minor adjustments were made to hourly temperatures through mid-morning based on the latest T/Td trends. The rest of the forecast looks on track. Today: Influences from the upper ridge will steadily lessen today as the primary ridge axis shifts farther offshore. A broad southwest flow aloft will dominate the Southeast U.S. today with a variety of impulses progged to cross the area. A warm, moist environment characterized by PWATs in excess of 2 inches will change little as the day progresses with conditions becoming increasingly unstable as surface temperatures warm. Model soundings suggest convective temperatures will run a few degrees lower than the past several days. This will likely support an earlier convective initiation with isolated showers/tstms popping as early as late morning/early afternoon the across the coastal corridor in the vicinity of the sharpening sea breeze. The focus for scattered to numerous showers/tstms will then shift over the interior as the sea breeze propagates inland and the genesis of convective outflows begins to modulate the low-level convergence pattern. Modified soundings support solid mid-summer instability with MLCAPE values reaching 1500-2000 J/kg across far interior Southeast Georgia where surface dewpoints will mix out a bit and over 3000 J/kg along/east of I-95 owing to higher net moisture values. Pops today peak 50-60% away from the immediate coast with 20-30% at the beaches. Shear remains weak with atmospheric conditions supportive of a few classic, deep-summer pulse severe tstms. The primary hazards will be locally damaging winds, frequent/intense cloud-to-ground lightning and locally heavy rainfall capable of producing some minor flooding of low-lying and poor drainage areas. Highs will warm into the lower-mid 90s away from the beaches with a few upper 90s far inland, mainly across interior Southeast Georgia. Heat indices will peak in the 105-108 range with 100-105 at the beaches. A few instances of heat indices 108-109 can not be ruled out along east of I-95, but limited coverage and durations do not support the issuance of a Heat Advisory at this time. Breezy conditions appear likely at the beaches this afternoon as the sea breeze moves inland and the pressure gradient between high pressure offshore and a sharpening lee-side trough across the western Carolinas tightens. Tonight: Scattered to numerous showers/tstms across the interior will gradually weaken as the activity pushes east to the coast. How quickly convection wanes will be dependent on how much overturning occurs during the late afternoon hours. Pops were capped at 40-60% (highest inland) for now, but these will likely have to be adjusted depending how convection and its influence on instability evolves during the afternoon. Lows tonight will range from the lower-mid 70s across the interior to around 80 at the beaches and Downtown Charleston. && .SHORT TERM /THURSDAY THROUGH SATURDAY/... Broad troughing will prevail over the eastern half of the United States on Thursday and Friday, with high pressure beginning to build into the region from the east on Saturday. At the surface a cold front will approach the region Thursday into Friday, likely stalling in the vicinity of the Carolinas through Saturday. With PWATs over 2 inches as well as plentiful instability, scattered to numerous showers and thunderstorms are possible each afternoon, with widespread coverage forecast on Friday. With plenty of moisture and instability, a strong to marginally severe thunderstorm is certainly possible, as well as the threat for heavy rainfall. With increasing precipitation and cloud cover temperatures will likely be a degree or two less than earlier this week, although still hot and humid. Heat Index values are forecast to reach 104-107F each afternoon. However, afternoon convection could inhibit temperatures from reaching their full potential. Overnight lows are forecast to dip into the low 70s inland and mid to upper 70s along the coastal counties. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... High pressure aloft and at the surface will build into the forecast area from the east into early next week. PWATs are forecast to remain elevated along with instability. Scattered to numerous showers and thunderstorms will be possible each afternoon, generally due to boundary interactions and the afternoon sea breeze. Guidance continues to be rather bullish with PoPs through the period, so the current forecast maintains chance to likely PoPs, still above climatology. With above normal precipitation and increased cloud cover through the period, temperatures are forecast to return to near normal or even slightly below normal through the period. && .AVIATION /10Z WEDNESDAY THROUGH SUNDAY/... 17/12z TAF Discussion: KCHS/KJZI/KSAV: The main forecast challenge for today is the timing/coverage of tstms. Best chances for impacts this afternoon/evening will be at KCHS and KSAV as the sea breeze moves inland. TEMPO groups for MVFR conditions in TSRA were maintained at both terminals, but times were adjusted slightly, 20-23z at KCHS and 21-00z at KSAV. Data now suggest tstms may get closer to KJZI, but possibly hold just to the west, so VCTS was introduced 20-02z for now. Convection should gradually concentrate west of the terminals by early evening, but there are signals decaying showers/tstms could make a run for the coast by late evening. VCSH with some mid-level cigs were maintained to try and account for this. Further adjustments will be needed throughout the day as trends become more apparent. Extended Aviation Outlook: Scattered afternoon showers/tstorms could produce brief flight restrictions at all terminals this week. && .MARINE... Today: A modest nocturnal surge over the waters will weaken a bit as sunrise approaches. Southerly winds will diminish to 10-15 kt this morning. Winds will begin to pick up again this afternoon as a modest sea breeze develops along the beaches and the pressure gradient begins to tighten between high pressure offshore and a sharpening lee-side trough over the western Carolinas. Speeds will increase to 15-20 kt this afternoon as a result. Frequent gusts to 25 kt appear likely over the Charleston Habor as well as the nearshore waters from South Santee River to Edisto Beach by mid-afternoon so Small Craft Advisories have been posted for these waters. Seas will average 3-4 ft with a few sets as high as 5 ft. Tonight: The gradient will remain fairly static supporting 15-20 kt all waters again tonight. Gusts to 25 kt are the most likely over the South Santee-Edisto Beach nearshore legs, but could extend south to include all remaining waters. It is not exactly clear how frequent gusts to 25 kt will be south of Edisto Beach, so a second period Small Craft Advisory was not posted. The need for an advisory will be reevaluated later today. Seas will build 3-5 ft. Thursday through Sunday: High pressure will prevail across the marine zones while a cold front approaches the inland zones. Conditions are expected to remain below Small Craft Advisory criteria. Generally southerly winds 10-15 knots are forecast, with slightly stronger wind speeds are possible along the direct coastline in association with the afternoon sea breeze. Seas should average 3 to 4 ft. Rip Currents: Gusty onshore winds along with 2 ft swell every 8-9 seconds will support a moderate risk for rip currents today and Thursday. && .CLIMATE... Record High Minimum Temperatures: July 17: KCHS: 78/2023 KCXM: 82/2021 KSAV: 80/1883 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory from 2 PM this afternoon to 6 PM EDT this evening for AMZ330. Small Craft Advisory from 2 PM this afternoon to 6 AM EDT Thursday for AMZ350. && $$