Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 030908 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 408 AM EST Wed Mar 3 2021 .SYNOPSIS... Low pressure will move across the area this morning and push farther offshore later today. High pressure will then prevail with another storm system likely brushing the area this weekend. High pressure will return again early next week. && .NEAR TERM /THROUGH TONIGHT/... Low pressure is starting to deepen off the southeast Georgia coast as it moves across the warmer Gulf Stream waters. The low will intensify a bit this morning, before it pulls further away to the east this afternoon, as higher pressure starts to build in from the west late in the day. The low is being driven by a strong mid level short wave that will pass through this morning, and into the ocean for the afternoon. Upper level divergence/difluence within the right entrance region of the jet aloft, plus strong frontogenesis through about 15Z will support widespread rainfall this morning. PWat as great as 1.25 to 1.75 inches, plus a deep, warm cloud layer will support efficient rainfall rates, with as much as another 1.00 to 1.75 inches of rainfall to occur. However, there will be locally higher amounts, especially given the probabilities from the HREF which depict as much as a 30-50% chance of amounts exceeding 1 inch in 3 hours times early this morning. There is even a 10% chance of more than 3 inches in a 3 hour block through 12Z, centered in the Charleston region. These expected rain amounts will generate minor flooding concerns, mainly for the already swollen rivers, as well as the typical flooding in urban and poor drainage locations. Elevated instability will produce isolated thunderstorms over mainly the coastal counties early this morning. Rain chances are a solid 100% prior to daybreak, with coverage to steadily decrease from the west-southwest through the morning as the short moves through. Probabilities will be down to just 20/30% by 17-18Z, then rainfree thereafter. Temperatures are certainly a challenge, dependent upon how much clearing sets in for the afternoon. It`s somewhat surprising that all guidances seems to support max temperatures reaching within a couple of degrees either side of 60F. Since that meshes well with our surrounding WFO`s, we went with that in the forecast. But given that much of the energy from the insolation that develops will be utilized to evaporate the standing rainwater, we wouldn`t be surprised if these values are too warm. Conditions will be breezy to windy along the coastal counties this morning, when the best pressure falls followed by pressure rises will occur in response to the proximity of the surface low. North and northeast wind gusts will reach as high as 25 or 30 mph at times, especially along the coast. Lake Winds: Conditions will be close to advisory criteria this morning. But given only weak cold advection and a more stable mixing profile, we capped winds at 15-20 kt. We`ll continue to monitor for a Lake Wind Advisory. Tonight: An expansive and consolidating region of high pressure from southern Canada to the Gulf of Mexico will build into the area, leading to clear skies and a mostly decoupled atmosphere. This looks to produce good radiational cooling, maybe offset a little by the wet grounds. Still we are showing minimum temperatures in the upper 30s and lower 40s inland, with mid- upper 40s along the coast and in downtown Charleston. One concern that is not shown in the forecast will be the possibility of some fog. Given that the grounds will still be wet, we will have to watch for its formation during the late evening and overnight. && .SHORT TERM /THURSDAY THROUGH SATURDAY/... Moderate to high confidence through Friday night due to decent model agreement, then low confidence Saturday as models diverge a bit with respect to the impact of an upper trough moving through. High pressure is expected to begin the period but a relatively weak backdoor cold front looks to push through Thursday night with stronger and chillier high pressure returning Friday into Saturday. At this point it looks to dry so the front should come through rain- free. Later in the day Friday we should start to see higher clouds increase from the west ahead of an upper trough. Deeper moisture and better upper forcing are expected Friday night into the first part of Saturday so there could be some showers, mainly in GA, although confidence in coverage/amounts remains low. Temperatures should generally be near to below normal, mainly 50s/60s for highs and 30s/40s for lows. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... High confidence this period as models are in decent agreement keeping high pressure in control. The main forecast challenge will revolve around temperatures, which look to be near or below normal until Wednesday. Still possible to see some frost and/or freezing temperatures inland each morning through Tuesday. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... Flight restrictions will persist overnight into Wednesday afternoon, as low pressure passes off the nearby coast. Ceilings will be down in the IFR/LIFR range, while visibilities will vary from VFR to MVFR, maybe even occasionally IFR in light to moderate rains. While TSRA can`t be ruled out, it will be limited enough in areal coverage to not include with the 06Z TAFs at KCHS. However, recent radar and lightning trends within an area of elevated instability, supports the inclusion of VCTS at KSAV early this morning. As the rains taper off during the late morning at KSAV and by early afternoon at KCHS, conditions will quickly approve as the atmosphere dries out from the top, down. VFR is currently forecast again by 21Z at KCHS and KSAV. NE winds into early Wednesday will gradually back around to the NW during the afternoon. Depending upon the exact track and strength of the surface low, winds will be gusty at times. Extended Aviation Outlook: High confidence in VFR through Friday and again starting Sunday. However, flight restrictions are possible Friday night into Saturday night, mainly at KSAV, as a storm system possibly impacts the area. && .MARINE... Today: Forecast soundings, guidance from the HRRR, RAP and NBM, plus 70-90% probabilities of greater than 30 kt per the HREF supports the issuance of a short duration gale Warning for the Charleston County Atlantic waters and the outermost Georgia waters beyond 20 nm out. These conditions occur during the morning, when low pressure off the southeast Georgia first deepens, followed by increasing pressure rises as it pulls further out by afternoon. The highest speeds look to be around 7-11 AM on the AMZ350 waters, and from 9 AM to 12 PM over the AMZ374 waters. The other marine zones will be close to Gales at times, but have opted for solid Small Craft Advisories on the other waters. Seas will be as high as 5 to 8 ft within 20 nm of the shore, but up to 7 to 9 ft further out. Conditions will gradually improve this afternoon as pressure climbs slow down, the low pulls further into the ocean, and more of an anticyclonic pressure pattern develops. Once the Gales come to an end early in the afternoon, they`ll need to be replaced with Small Craft Advisories. Mariners should also be alert for isolated thunderstorms this morning, the combination of the nearby low and also energy in the mid and upper levels of the atmosphere passing through. Tonight: Continental high pressure will continue to slowly build in behind the surface low that will pass out near Bermuda. Winds will diminish to less than 15 or 15-20 kt, but the elevated seas will take much longer to drop to 5 ft or less; probably not until around 10 PM to 12 AM within 20 nm, and not until after 2 or 3 AM on the outer Georgia waters. Thursday through Monday: Moderate to high confidence that conditions will be below Small Craft Advisory levels most of the period except Friday night into Sunday when a storm system may track close enough to the area to push winds to at least 25 kt and seas to at least 6 ft, especially in the offshore GA waters. && .TIDES/COASTAL FLOODING... Tidal departures continue to increase along the coast, and dependent upon how quickly winds back more northerly will be key in determining levels with the late morning high tide. We currently anticipate that it won`t reach Coastal Flood Advisory criteria, but given tidal anomalies of 1.8 to 2.1 ft MLLW early this morning, conditions will be close. We will address this concern in the Hazardous Weather Outlook. && .CLIMATE... Record rainfall for March 3: KCHS: 1.81 inches/2012 KCXM: 1.73 inches/2012 KSAV: 2.70 inches/1948 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory until 1 PM EST this afternoon for AMZ330. Small Craft Advisory until 7 PM EST this evening for AMZ352- 354. Gale Warning until 1 PM EST this afternoon for AMZ350-374. && $$ NEAR TERM... SHORT TERM...RJB LONG TERM...RJB AVIATION... MARINE... TIDES/COASTAL FLOODING... CLIMATE...

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