KCHS issues Area Forecast Discussion (AFD) valid 2019-12-09T09:05:00Z 505 FXUS62 KCHS 090905 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 405 AM 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/... This morning: A weakening high pressure wedge will continue to erode as a warm front that is still off the coast lifts steadily onshore and finally to the north and northwest of the local counties by 16-18Z. Isentropic lift and considerable low level moisture within a southerly boundary layer flow that is trapped beneath the strong inversion will result in widespread stratus and areas of fog through at least 10 am. There appears to be just a little too much mixing for us to raise a Dense Fog Advisory. But locally dense fog with visibility down near 1/4 mile or less will occur, mainly near and east of I-95. A Special Weather Statement is in effect as a result. Temps will begin the day at their warmest for most of us since the 1st of the month. For the balance of today: In wake of the warm front, Atlantic high pressure will build west across Florida and southern Georgia. Simultaneously aloft we find zonal flow with H5 heights climbing to as great as 5810-5840 meters, or close to the 90th percentile for this time of year. That along with strong low level warm advection that generates H85 temps reaching 12-13C, will boost afternoon temps into the mid and upper 70s most communities. The exceptions will be near and along the coast where a southerly flow off the cooler ocean will hold those temps down in the mid 60s north, upper 60s central and lower 70s south. There are indications that a few late day showers could make a run for our far southwest counties in association with a swath of higher theta-e air. But this is not enough to include any explicit mention of rainfall in the gridded forecasts. 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 SUNDAY/... 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 /08Z MONDAY THROUGH FRIDAY/... Low stratus and fog will impact the KCHS and KSAV terminals into the middle of this morning, resulting in LIFR conditions and occasionally near and below airfield minimums through 14Z. Conditions will improve back to MVFR during the late morning, before VFR prevails this afternoon and evening. Sea fog could pose a problem tonight, especially at KCHS. But this will be addressed in futuristic forecasts. 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 NE and 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. A Dense Fog Advisory remains in effect through 10 am. Although technically not from sea fog, but rather from the build down of low stratus, it will still cause visibilities under 1 nm over the entire 0-20 nm Atlantic waters, Charleston Harbor and the Port of Savannah. Vessel traffic will be significantly impacted with local visibilities close to zero at times. 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...Dense Fog Advisory until 10 AM EST this morning for AMZ330-350- 352-354. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE... TIDES/COASTAL FLOODING... CLIMATE... EQUIPMENT...