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Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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980 FXUS62 KCHS 180738 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 338 AM EDT Sun Aug 18 2024 .SYNOPSIS... A cold front will move through the area tonight. High pressure will then gradually build in from the north through the remainder of the week. && .NEAR TERM /THROUGH TONIGHT/... Through This Evening: The broad longwave trough over the eastern CONUS will steadily sharpen through tonight as a series of shortwave propagates through its base. Shortwave energy moving across the Tennessee Valley this morning will push across the Carolinas late this afternoon/evening along the nose of a northwest-southeast oriented H5 jet streak. This will help drive a cold front into the area overnight. Ahead of the front, a warm and increasingly moist airmass will support the development of scattered showers/tstms with coverage peaking late afternoon into this evening as forcing with the passing shortwave and associated jet streak pivot through. As is typical this time of year, the mesoscale pattern will help drive how convection will evolve. The 18/00z CAMs are painting a number of possible convective scenarios as they all appear to be handing the placement of outflow boundaries that are meandering over the area early this morning differently. While the hourly specifics are still unknown, there is broad consensus that the best chances for showers/tstms will extend across much of Southeast Georgia into south coastal South Carolina, roughly in the 21-03z time frame, possibly a tad earlier across interior Southeast Georgia. Pops will were capped at 30-50% this afternoon/evening with the greatest coverage clustered across the coastal counties. Modified soundings yield modest instability with MLCAPE peaking 2200-2700 J/kg during the late afternoon. Convection should exhibit some degree of upscale growth as it becomes embedded within a belt of 30 kt of 0-6km bulk shear. This coupled with some mesoscale boundary interactions should yield several pronounced tstm clusters with some degree of organization. Given the strength of instability and shear, expect isolated to perhaps scattered severe tstms to occur with the greatest risk occurring from roughly 4PM-11PM with the greatest chances occurring across the coastal counties and areas roughly along/south of the I-16 corridor aligned with the axes of highest H8 theta-e and mixed-layer instability. The primary hazard looks to be damaging winds with potential maximum wind gusts approaching 75 mph per latest WINDEX calculations. Frequent cloud-to-ground/water lightning will also occur and there will be a risk for locally heavy rainfall. Thankfully, convection looks to remain fairly progressive, but given how wet soils remain in the wake of Tropical Storm Debby, it will not take much to get some minor flooding of low-lying and poor drainage areas. Highs will warm into the lower-mid 90s away from the coast this afternoon. Low-level moisture advection ahead of the front will help drive dewpoints into the lower-mid 70s this afternoon with a corridor of upper 70s possible across parts of Charleston into eastern Berkeley Counties. Heat indices look to peak in the 105-108 range--just a tad below Heat Advisory criteria. Overnight: The ill-defined cold front will meander through the area early Monday, pushing offshore just after daybreak. The bulk of the convection should shift offshore by midnight with rain chances ending quickly from west-east. As high clouds thin, some patches of low stratus could develop across the interior late. Lows will range from the lower 70s well inland to the mid-upper 70s at the beaches. && .SHORT TERM /MONDAY THROUGH WEDNESDAY/... Aloft, a large trough extending from extreme eastern Canada down to the Florida Keys will sit nearly stationary, with bouts of vorticity traversing the base through the period. A weak cold front is expected to reach the Atlantic coast Monday with a slight stall before it pushes offshore Tuesday as high pressure begins to build in from the north. The forecast looks mostly dry behind the front, with the exception of an isolated showers/thunderstorms near/along the coast with the help of upper lift and moisture offshore. Modest subsidence will limit the overall coverage of convection, especially inland. Another limitation for convection is the relatively weak sea breeze, with little land/sea temperature differential especially Wednesday. Low to mid 90s are forecast for most locations Monday. Overnight temperatures will drop into the upper 60s/lower 70s inland and mid 70s along the immediate coast. High temperatures will be somewhat cooler the following days, reaching the upper 80s/lower 90s Tuesday and mid/upper 80s Wednesday. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... The upper trough will lift northeast Wednesday night and the main feature will be the strengthening surface high pressure building in from the north, forming a wedge pattern. Meanwhile, a broad upper low looks to detach from the departing trough and form over the northern Gulf of Mexico. High pressure over the Atlantic will expand southwestward tilting the axis of deep moisture across the Southeastern U.S. Scattered showers and thunderstorms could develop mainly along the coast during the afternoon each day with the return of moisture and upper forcing back in play. High temperatures will gradually lower Thursday and Friday, but begin to warm into the upper 80s by next weekend. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... 18/06z TAF Discussion: KCHS/KJZI/KSAV: VFR for much of the period. Scattered showers/tstms are expected to develop late this afternoon into this evening for the coastal corridor. Confidence is somewhat diminished as a number of the near term convective models are showing a number of possible scenarios for both coverage and timing for convection. There seems to be a broad consensus that the periods from roughly 22-02z at KCHS/KJZI and 23-03z at KSAV will have the best chances for impacts at the various terminals. For now, the 06z TAFs were trended to use VCTS in the above mentioned periods. The introduction of TEMPO or prevailing groups for TSRA may need to be considered in later TAF cycles as confidence in timing/coverage increases. Extended Aviation Outlook: VFR will prevail through the week. && .MARINE... Today: South to southwest winds will increase this afternoon ahead of an approaching cold front. Winds look reach as high as 15-20 kt over the South Carolina waters and closer to 15 kt over the Georgia waters. Seas will build through the day, reaching 3-5 ft for the waters within 20 NM and 5-6 ft over the Georgia waters 20-60 NM. A Small Craft Advisory continues for the Georgia waters 20-60 NM offshore through tonight. Convection will begin to move into the coastal waters by late afternoon/early evening. These storms could produce winds in excess of 34 kt and frequent cloud-to-water lightning. Special Marine Warnings may be needed for some areas. Tonight: Assuming convection pushing offshore does not completely disrupt the pre-frontal wind field, southwest winds will increase to 20 kt with gusts to 25 kt over the South Santee-Edisto Beach nearshore leg overnight with seas building 4-6 ft. A Small Craft Advisory has been issued beginning this evening and continuing into daybreak Monday given these conditions. Elsewhere southwest winds 15-20 kt will occur with seas 3-5 ft in the Edisto to Altamaha Sound out 20 NM area with 4-6 ft over the Georgia 20-60 NM leg where a Small Craft Advisory remains in effect until daybreak Monday. Mariners should remain alert for possible strong to severe tstms into the evening hours. Special Marine Warnings may be needed for some areas. Monday through Friday: Conditions will improve throughout the day Monday as a weak front along the coast gradually departs further into the Atlantic. Southwest winds will drop to 10-15 kt and seas will drop to 2-4 ft. By Tuesday, winds will shift out of the northeast through the remainder of the week as inland high pressure builds from the north. A surge in both winds and seas is possible beginning late Wednesday night into Friday with a modest pressure gradient in place over the waters. Small Craft Advisories could be needed for most of the marine zones for seas 6 ft or greater and gusts around 25 kt. Rip Currents: Sunday: A high risk for rip currents continues for all beaches today given the continued long period swells from Tropical Storm Ernesto impacting the Southeast U.S. coast. Monday: Long period swell around 11 seconds will gradually subside, but continue to bring impacts to the coast. A moderate risk of rip currents is in place for all beaches. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...High Risk for Rip Currents through this evening for GAZ117-119-139- 141. SC...High Risk for Rip Currents through this evening for SCZ048>051. MARINE...Small Craft Advisory from 6 PM this evening to 8 AM EDT Monday for AMZ350. Small Craft Advisory from 5 AM early this morning to 8 AM EDT Monday for AMZ374. && $$