KCHS issues Area Forecast Discussion (AFD) valid 2019-12-09T17:45:00Z 418 FXUS62 KCHS 091745 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1245 PM EST Mon Dec 9 2019 .SYNOPSIS... A coastal warm front will lift northward through the forecast area today. A cold front will approach from the west on Tuesday then push through the area Tuesday night into early Wednesday. Strong high pressure will build in thereafter, followed by a storm system late week. High pressure will return late weekend. && .NEAR TERM /THROUGH TONIGHT/... Early this afternoon: Overall, not significant changes through the rest of the afternoon. Some patchy dense fog is still floating around portions of the South Carolina coast, and we have a Special Weather Statement handling that through 2 pm. Late this morning: Area web cams, observations, and satellite imagery all show the fog is steadily improving across the area. Therefore, the Dense Fog Advisory was allowed to expire at 10 am. There could still be some patchy dense fog along the Charleston and Beaufort county coasts, but further improvement is expected over the next hour. Clearing is taking place from the south as noted by the fact that the Georgia coast is now clear and the clearing line is steadily progressing northward. Observations show that the coastal warm front is lifting northward and it should speed up its forward progress with the erosion of fog and stratus over the next few hours. Once the front gets north of the area a rapid warm up is expected and highs in the mid to upper 70s look on track. Overall, the forecast area remains in an area of weak large scale forcing, so the forecast remains dry. Tonight: A 5890-5900 meter mid level ridge will amplify over Andros Island and the Straits of Florida, as a broad and positively tilted trough covers much of the central and eastern states. Although the flow aloft backs to the southwest across our area, deep ridging over the Atlantic will keep the next upstream cold front off near and west of the Appalachians. Any fog that develops will come from two sources. The first will be the risk of sea fog that advects onshore of locations near and east of US-17. The second will be as a result of fog advancing in from south of the Altamaha River late. For now we have mention of patchy fog. Lows will be some 10-12F warmer than early this morning, which is around 15-20F above climo. && .SHORT TERM /TUESDAY THROUGH THURSDAY/... The forecast area will be solidly within the warm sector on Tuesday ahead of an approaching cold front that will be moving towards the East Coast. The best forcing stays west of the local area with some subsidence noted in model soundings. This will keep dry conditions across much of the area through the day. The exception would be across far inland zones where a couple showers could sneak in late in the day. Deep south/southwest flow will promote temperatures well above normal. Highs will be in the upper 70s to around 80, maybe even a couple spots in the low 80s closer to the Altamaha. While highs appear to be shy of daily records, it will be within a few degrees (see climate section below) Tuesday night, lows will mainly be in the mid to upper 50s. Latest visibility guidance shows sea fog drifting onshore and expanding, especially across our northern zones. Thus, a mention of fog was included in the forecast. The cold front will push through late Tuesday night into early Wednesday. Precipitation associated with the front looks to be in a weakening state as it moves into the area, but there will still be some showers around. Rain chances in the 30-40% range were maintained. Strong high pressure will build in from the northwest in wake of the front, and eventually become wedged down the east side of the Appalachians on Thursday. A tight pressure gradient will make for a fairly gusty day, especially along the coast. While the atmosphere will dry out upstairs, plenty of moisture remaining in the lower levels will result in an abundance of clouds. Increasing isentropic ascent could produce a few showers, but best rain chances look to hold off until Thursday night and beyond. Much cooler temperatures in the wedge set-up, with highs topping out in the low to mid 50s. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Models have started to come into better agreement for the late week period. The pattern continues to favor cooler and unsettled conditions. A wedge of high pressure will linger inland Thursday night into Friday, while a coastal trough/warm front develops offshore. Meanwhile, low pressure will organize over the Gulf of Mexico. The low is expected to lift northeast over or near the area sometime late week. While there are still some timing issues, plenty of moisture and forcing will support higher than normal rain chances, with the potential for periods of moderate to heavy rainfall. Late weekend once the low passes through, drier conditions and temperatures closer to normal are expected. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... VFR conditions have returned at both KCHS and KSAV and should prevail through the evening. Overnight, the forecast challenge once again revolves around fog and/or stratus development. Model guidance is once again advertising IFR and even LIFR conditions which makes sense given the atmosphere and generally light winds in place. The forecast features MVFR fog and stratus starting around 07z, then lowering to IFR around 09-10z. There is certainly a chance for IFR to occur sooner, but overall confidence in timing isn't high. Similar to this morning, whatever conditions develop overnight will likely linger into the middle of Tuesday morning. It looks like any rainfall with the approaching cold front will be after the 18z Tuesday. Extended Aviation Outlook: Flight restrictions possible in fog/stratus Tuesday night into Wednesday. Additional flight restrictions possible in showers late week. && .MARINE... Today: A warm front will lift onshore of the waters this morning, allowing for E winds to clock around to the SE and then the S-SW this afternoon. Speeds will average no more than 10 or 15 kt throughout. Seas remain elevated from recent days, and are still as large as 3-5 ft. Fog conditions continue to improve, but web cams and the Charleston Pilots report that patchy dense fog is still out there. Will allow the Dense Fog Advisory to expire at noon but have added patchy dense fog to the forecast for the South Carolina waters and the Charleston Harbor through the early afternoon. Will also issue a MWS to handle the fog for the next couple of hours. Tonight: The waters will be under the fringes of Atlantic high pressure, causing S and SW winds at and below 15 kt. Seas will again average 3-5 ft. Conditions do not seem to be ideal for sea fog, since there might be too much wind and we'd like to see dew points a little higher. But if winds are less than anticipated and dew points are just a bit more, the risk of sea fog would increase significantly. We'll carry patchy fog for now over the 0-20 nm waters and in Charleston Harbor. Tuesday through Saturday: Southerly winds will prevail on Tuesday ahead of an approaching cold front. The front will pass through early Wednesday, turning the winds to the north then eventually northeast. Speeds will increase and seas will build, and Small Craft Advisories appear likely for all waters by Wednesday evening. A period of gale-force gusts is possible late Wednesday night into Thursday, and Gale Watches and/or Warnings could be needed for at least portions of the waters. Winds will improve some on Friday, however it will take seas longer to subside. Advisories could persist into Saturday. Sea Fog: An abnormally warm and humid air mass will move into the area today and Tuesday. Provided that winds aren't too strong, sea fog formation will be possible across at least the nearshore coastal waters and at times even into Charleston Harbor, primarily beginning Monday night and potentially continuing into Wednesday morning prior to cold frontal passage. && .TIDES/COASTAL FLOODING... Elevated tide cycles are expected late week due to increasing astronomical influences with the full moon on Thursday and strong northeasterly winds. Minor coastal flooding will be possible with the morning high tides Thursday and Friday. Coastal Flood Advisories could be required. && .CLIMATE... Record Max Temps December 10th... KCHS...82F set in 1972. KCXM...79F set in 1943. KSAV...82F set in 1972. && .EQUIPMENT... KCLX will remain down until further notice. A technician will return to the radar site this morning to further troubleshoot. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM...ETM LONG TERM...ETM AVIATION...BSH/ETM MARINE...33/ETM TIDES/COASTAL FLOODING... CLIMATE... EQUIPMENT...