760 FXUS62 KCHS 221127 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 627 AM EST Fri Feb 22 2019 .SYNOPSIS... A backdoor cold front will slide south across the area through this evening, then shifting back north as a warm front late Saturday and and Saturday night. A cold front will then move through Sunday followed by high pressure into mid week. Low pressure will then likely affect the area late next week. && .NEAR TERM /THROUGH TONIGHT/... As of 620 AM: IR satellite and ongoing upper air observation indicated a large patch of mid clouds, bases between 8.5-9.0 kft, across the CHS Tri-county area. The passage of the mid clouds has improve sfc visibilities, resulting in patchy dense fog. Elsewhere, light south winds and patchy fog remained inland with widespread dense fog along the coast. I will keep the current placement and timing of the Dense Fog Advisory. As of 355 AM: Latest sfc analysis indicated a wavy stationary front across the Deep South, curving from near the Fall Line of SC and east across the Grand Strand. This front is expected to slide south over the SC Lowcountry this morning as a backdoor cold front. The front should accelerate southward through the daylight hours as cold air damming strengthens across the central and western Carolinas. The front is timed to push south of the forecast area by early this evening. In the wake of the front, sfc pressures should gradually rise and winds will remain steady from the northeast. Fog: Sfc observations, Web cams, and IR satellite indicated widespread sea fog across the shelf waters and over the coastal counties. Inland, IR satellite and observations indicated a large ragged patch of mid clouds. These clouds and a few weak showers have kept fog limited to a half mile visibility or greater. Near term guidance has indicated over the past several runs that dense fog will develop inland. However, the persistent mid clouds has not shown signs of dissipating or moving significantly. I will keep the Dense Fog Advisory for the coastal zones until 10 AM. The case for fog tonight is low given NE flow and dewpoints falling into the 50s. Temps and PoPs: Temperatures will remain challenging giving the dense fog along the coast and the approach and passage of the backdoor cold front. As a result, the high temperatures forecast this afternoon will feature mid 60s across Berkeley County to low 80s along the Altamaha River. Using a blend of MOS, low temperatures will range in the low to mid 50s, with a corridor of around 60 along the Altamaha River. && .SHORT TERM /SATURDAY THROUGH MONDAY/... Moderate to high confidence this period. Weak high pressure wedging is likely to give way to a warm front later Saturday and Saturday night before a cold front moves through Sunday. Cooler high pressure will then move in for Sunday night and Monday. Some showers are expected Saturday and Saturday night with the warm frontal passage but not much rain is likely. A better chance of rain is then expected Sunday with some higher amounts, although mainly less than a quarter of an inch. Can't completely rule out a few storms but chances appear too low to mention at this time given the generally weak instability. Lastly, fog could develop Saturday night and/or push inland from the Atlantic into Sunday. Temperatures will likely stay above normal through the period, with the warmest temperatures on Sunday when record highs/high mins are possible depending on the timing of the cold front. && .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/... Moderate to high confidence through Tuesday with low confidence thereafter. Dry high pressure will likely persist through at least Tuesday before a coastal trough and/or upper disturbance possibly bring a chance of rain back to the forecast through mid week and eventually a low pressure system later in the week. So chances are pretty good that at least a portion of the mid-late week period will be unsettled but timing and rain amounts are quite uncertain. Temperatures should be near to above normal through at least Wednesday before possibly dropping below normal. && .AVIATION /11Z FRIDAY THROUGH TUESDAY/... A marine layer will likely persist between the sfc and 1 kft. The 12Z CHS sounding indicated a weak inversion from the sfc to around 1000 ft agl. The capped marine layer should support fog and low clouds at KCHS into the early daylight hours. I will initialize the 12Z KCHS TAF with LIFR conditions, then transitioning to IFR conditions through 14Z with daytime mixing. KSAV will begin the 12Z TAF period with IFR conditions. LIFR vis and ceilings mat develop over KSAV between 12-14Z. Based on latest MOS and forecast soundings, conditions at KSAV should return to VFR by 14Z. A backdoor cold front is timed to reach KCHS by 19Z and KSAV around sunset. The approach and passage of the cold front should support at least MVFR ceilings, ENE winds, and scattered showers. Extended Aviation Outlook: Restrictions likely Saturday into Sunday afternoon at KCHS/KSAV. Strong winds also possible Sunday. More restrictions possible toward the middle of next week. && .MARINE... Today: Sfc observations, Web cams, and IR satellite indicated widespread sea fog across the shelf waters and over the coastal counties. The sea fog will continue across the marine zones until a back door slides south across the waters today. Based on the timing of the front, a Dense Fog Advisory is in effect for the Charleston Harbor and Charleston County near shore waters until 10 AM, AMZ352 until 1 PM, and AMZ354 until 4 PM. In the wake of the front, winds will shift and strengthen from the NE. Wave heights are forecast to range from 2-3 ft within 20 NM to 3-4 ft beyond 20 NM. Tonight: High pressure will strengthen following the passage of the cold front, resulting in a 2 mb pressure gradient. NE winds are forecast to strengthen to 15 to 20 kt across AMZ350 with seas building to 4 to 5 ft this evening. At this time, conditions are expected to remain below Small Craft Advisory levels. The case for fog tonight is low given NE flow and dewpoints falling into the 50s. Saturday through Wednesday: A warm front will move north through the area later Saturday and Saturday night with a cold front to move through later Sunday. Conditions could reach marginal Advisory levels near the Gulf Stream Sunday and then again late in the period. Otherwise, some dense sea fog could develop Saturday in the GA waters and then across the SC waters Saturday night into Sunday until the cold front passes. && .TIDES/COASTAL FLOODING... It's not out of the question that we could reach Coastal Flood Advisory levels at Charleston Harbor with the high tide this evening. The 5.9 ft MLLW astronomical tide would only require a 1.1 ft anomaly which is quite possible based on the latest forecast for 15-20 kt NE winds along the Charleston County coast. && .CLIMATE... Record Highs for February 24... KCHS: 81/2017 and previous KCXM: 81/1930 KSAV: 86/2012 Record High Mins for February 24... KCHS: 60/1992 and previous KCXM: 63/2018 KSAV: 64/1980 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Dense Fog Advisory until 10 AM EST this morning for GAZ117-119- 139-141. SC...Dense Fog Advisory until 10 AM EST this morning for SCZ048>052. MARINE...Dense Fog Advisory until 1 PM EST this afternoon for AMZ352. Dense Fog Advisory until 10 AM EST this morning for AMZ330-350. Dense Fog Advisory until 4 PM EST this afternoon for AMZ354. && $$ NEAR TERM...NED SHORT TERM...RJB LONG TERM...RJB AVIATION...NED/RJB MARINE...NED/RJB TIDES/COASTAL FLOODING... CLIMATE...RJB