Area Forecast Discussion
Issued by NWS Charleston, SC

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS62 KCHS 271434 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1034 AM EDT Wed Sep 27 2023 .SYNOPSIS... A wedge of high pressure will build into the region today and holds into early next week, while a broad trough of low pressure persists well offshore. && .NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Strong mid-latitude blocking remains across eastern Canada and the Great Lakes with a closed cut-off low centered near Chicago. West-southwesterly flow aloft along the periphery of the low is in place across the south/southeast states with an upper jet axis that extends off the mid-Atlantic coast. Strongest larger scale forcing for ascent and associated precip has moved well out into the Atlantic at this juncture. But the wedge is in with surface high pressure and cooler air wedging down through the Carolinas into Georgia behind a boundary that stretches from northern Florida northeastward into the Atlantic. Instability and baroclinically driving bands of showers are fairly widespread in the Atlantic waters...some of which have been streaming into inland portions of southeast South Carolina and southeast Georgia. Pattern changes very little over the next few days with cold air damming/wedging holding least through Thursday. Ongoing post-frontal f-gen forcing in the Atlantic is forecast to work further offshore through this afternoon...suggesting our overall shower threat will gradually diminish as we go through the afternoon and especially tonight. Bigger forecast challenge revolves around the low cloud cover and impact on temperatures. HRRR forecast soundings do suggest that ceilings will nudge upward through the afternoon with perhaps some breaks in the low cloud cover. But overall it will remain cloudy and I`ve lowered forecast max temps into the lower to middle 70 range...around 10 degrees below normal for late September. That said, any amount of sunshine this afternoon will bust our temp forecast. Cloudy skies persist through tonight with lows in the lower to middle 60s.
-- End Changed Discussion --
&& .SHORT TERM /THURSDAY THROUGH SATURDAY/... A closed mid level low near the Lower Great Lakes Thursday will shift to the the vicinity of Lake Erie and Lake Ontario Friday, then off Long Island and New England as it deepens Saturday. On Thursday, the local area will lie within a west-southwest flow between the low and a large mid level ridge from the Bahamas into the Atlantic. The flow aloft on Friday and Saturday is near the southern apex of the trough, while a weak closed low tries to form in the northeast Gulf of Mexico. At the surface a strong inland wedge holds firm Thursday, weakens some on Friday, then intensifies again later Saturday. A broad trough will also exist well offshore during this time, with maybe waves of low pressure to form on that feature. There is not much in the way of forcing for ascent, as we`ll have to rely on low level convergence to generate any showers. The better probabilities (20-30%) will occur from Hilton Head south to McIntosh County, where the better trajectories from off the ocean occur. There is too much dry air further north and west where we don`t see much chance anywhere else in the forecast counties. Not very excited about the chance for t-storms, but there might be just enough instability to produce a few stray, diurnal t-storms. daytime highs will run near or slightly below climo, while night time lows will be several degrees below typical late September norms. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... Another Rex Block looks to develop over the CONUS, with a large trough in the Atlantic, strong ridging covering much of the area east of the Rockies, and a deep cyclone out west. At the surface there is still wedging inland, with a broad trough far offshore. rain chances are no higher than 20-30% over the coastal corridor, and most especially southeast Georgia. Not much chance for anything further inland. Temps will average close to normal during each day, but below climo at night with light winds and some radiational cooling effects. && .AVIATION /13Z WEDNESDAY THROUGH SUNDAY/... Prior to the 12Z TAFs, the backdoor cold front will begin the day over the Savannah River, pushing to the SW. North of the front, building CAD should support IFR ceilings across KCHS and KJZI during the initial daylight hours, then MVFR ceilings will likely linger into the afternoon hours. At KSAV, IFR ceilings are forecast to develop between 12-14Z, highlighted with a TEMPO. MOS and forecast soundings indicates that MVFR ceilings will into this afternoon. Deepening mixing and an increased pressure gradient should result in NE winds strengthening to 10-15 kts with gusts around 20 kts. The gusty NE winds and at least MVFR ceilings are expected to continue until late this afternoon. This evening, conditions should favor steady NE winds around 10 kts and VFR conditions. The CAD should reinforce across the region tonight. Conditions should feature steady NE winds around 10 kts and MVFR ceilings developing between 5-8Z. Extended Aviation Outlook: Perhaps occasional flight restrictions in low ceilings late each night and through mid morning late in the week. Otherwise VFR will prevail. && .MARINE... Today, a backdoor cold front will push across the coastal waters of SC and GA today. In the wake of the front today, northeast winds will strengthen to around 25 kts with gust around 30 kts outside the CHS Harbor. Seas will begin the day between 3-5 ft, building to 4-7 ft by late this afternoon. The wind and sea conditions will remain nearly unchanged through tonight. Small Craft Advisories have been issued for all marine zones outside the CHS Harbor. Thursday through Monday: The marine area will remain situated between an inland area of high pressure, while a broad trough persist far offshore. There will be fluctuations in the strength of the inland high and the position of the trough, which will have impacts on the strength of the NE`erly winds and the resulting seas. For now we`re expecting Small Craft Advisories for all waters outside Charleston Harbor into Thursday, with the outer Georgia waters to continue into next week. Even if the advisories come to an end on the waters, it might only be temporary, as there is a decent pinching from time to time through the forecast period. Rip Currents: Gusty NE winds will create a strong longshore current with the surf zone of all beaches today. Breakers are expected to peak between 2-4 ft today, with periods around 7 seconds. In addition, experimental Rip Current MOS peaks in the high category for all beaches. Based on local risk guidelines, a moderate risk for rip currents is forecast for all beaches today. Moderate northeasterly winds, a small amount of swell energy, and the Perigean Spring Tides will lead to an enhanced risk of rip currents late this week. The risk could very well remain enhanced even into early next week with similar conditions. && .TIDES/COASTAL FLOODING... Gusty northeast winds should persist throughout the high tide cycle this afternoon into early this evening. The gusty winds should increase tidal departures, resulting in at least moderate coastal flooding along the coast of Charleston and coastal Colleton Counties. Tide guidance would indicate that tide levels for the 7:11 PM could reach or exceed 8 ft MLLW. However, this would require a tidal departure of 1.22 ft, which would be a significant increase from expected 0.85 ft departure this morning. The potential for major coastal flooding will be addressed in the HWO. Several days of at least moderate northeasterly winds, plus tides that are already elevated due to the Perigean Spring Tides, will result in a potential several day period of coastal flooding during each of the two daily high tides. Moderate to perhaps major coastal flooding can occur in Charleston and coastal Colleton, with minor to moderate from Beaufort to coastal McIntosh, including the Tybee/Savannah area. There is even the risk for minor flooding in Tidal Berkeley (a somewhat rare occurrence). With such a prolonged period of elevated tides, areas of beach erosion will also occur. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory until noon EDT Thursday for AMZ350-352. Small Craft Advisory until 5 PM EDT Thursday for AMZ354. Small Craft Advisory until 6 AM EDT Tuesday for AMZ374. && $$ NEAR TERM...Adam/NED AVIATION...NED MARINE...NED is the U.S. government's official web portal to all federal, state and local government web resources and services.