Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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117 FXUS62 KCHS 082328 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 728 PM EDT Wed May 8 2024 .SYNOPSIS... A cold front will approach the area Thursday night, then move offshore Friday. High pressure will prevail for the weekend into early next week. A storm system is possible by the middle of next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Convection off to the north and northwest of the forecast area will likely stay outside the forecast area with a steering flow almost due east, increasing CINH and poor low level lapse rates. Maybe something can sneak into northwest Dorchester and northern Berkeley County, but all guidance shows it dissipating before getting here. So no mention in the actual forecast. Otherwise, the only other concern would be late tonight, when some low stratus might be able to advect across the Altamaha River within the southwest synoptic flow. Even if it does, there is likely too much mixing for any fog concerns It`ll be another warm night for May, with most areas not falling below 70F. As of right now the low temperature at KCHS for May 8th has been 73F, which should it hold, would tie it for the record high for the date set in 2003. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/... Thursday and Friday: The mid-levels will consist of zonal flow over the Southeast U.S. Thursday morning. A shortwave will be embedded in the flow, moving over our area Thursday afternoon or evening. Troughing will strengthen over the Great Lakes Region on Friday, with amplification expected down towards our region by late Friday. At the surface, Low pressure will be near the OH Valley Thursday morning. This Low will gradually shift eastward with time, moving off the Mid-Atlantic coast by late Friday. A cold front attached to the Low will approach from the northwest Thursday. It`ll be located just to our northwest early Friday morning. The front should slowly move through our area from northwest to southeast Friday afternoon, shifting offshore Friday evening. The main focus will be the severe potential Thursday, followed by possibly a second round on Friday. Thursday morning should be dry, with any convection going on well to our west. This line is expected to shift eastward during the day, and into our area. There will be a deep plume of moisture ahead of this line with PWATs rising to ~2". This is well above normal for this time of year, above the 90% mark for CHS per the SPC Sounding Climatology. In addition to this moisture will be above normal temperatures. Highs will peak in the lower to middle 90s, which is near record values for this date. So there will be plenty of moisture and heat in place across our area, which will be supportive of severe thunderstorms. MLCAPE values should peak in the 1,500-2,000 J/kg range. Lapse rates will be steep, especially in the lower levels, Additionally, nearly unidirectional 0-6 km shear should be in the 40-50 kt range. So the main threat will be damaging winds, possibly widespread, large hail, and maybe even an isolated tornado. The SPC has upgraded portions of our area to an Enhanced risk, and this seems reasonable given the environmental setup. The main question will be the timing and evolution of the storms. The CAMs have a MCV moving through our area, ranging from the very late morning through the afternoon. The synoptic models also hint at this, more towards the later afternoon. Since any lines of convection tend to move faster than the models, we`re anticipating it to reach our far inland counties early in the afternoon, making it to our coastline by later in the afternoon, then offshore by the evening. The line will be moving at a decent speed, so the severe threat at any one location should only be around for maybe an hour or two. In addition to the already mentioned hazards, locally heavy downpours are expected, with flooding possible in the typical low-lying and poorly drained areas. But the risk of widespread areal flooding is low. Once the convection moves through, their should be some remnant showers persisting into the evening, with a potential lull for a brief time overnight. Then, another round of convection is possible Friday morning into the afternoon. The models are starting to hint that the instability and moisture setup could be similar on Friday compared to Thursday. But this doesn`t mean that we`ll have identical severe impacts on Friday. What happens on Friday will depend partly on what happens Thursday. The SPC has our area under a Marginal Risk for severe weather on Friday, and this seems reasonable. We wouldn`t be surprised if it got upgraded further. Otherwise, we have likely POPs for most of the day Friday with high temperatures near normal. Convection will finally end from inland to coastal locations Friday evening, as the front moves through. Friday overnight is expected to be dry. Clearing skies will yield temperatures from the mid 50s far inland to the lower/middle 60s along the immediate coast. Saturday: Mid-level troughing will persist over the East Coast. High pressure will build in from the west, bringing much drier conditions. Expect mostly sunny skies with highs in the upper 70s to lower 80s. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... High pressure will bring dry conditions and seasonal temperatures this weekend through early next week. Models then suggest that a storm system could move across our area starting late Monday and possibly lasting through Wednesday. There remains uncertainty in the models, so we didn`t stray away from the blend, which has a period of likely POPs. But this aspect of the forecast will need to be adjusted with future updates. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... VFR conditions are expected to prevail at KCHS, KJZI, and KSAV into early Thursday afternoon. Maybe a period of MVFR ceilings at KSAV Thursday morning as low stratus advances in from the southwest. But probabilities are not high enough to mention at this time. Convection associated with a cold front could impact the terminals from 18-24Z Thursday. For now we have VCTS at all sites, but flight restrictions and strong winds are possible. We can re-evaluate in later TAF issuances. Extended Aviation Outlook: A front will bring periods of flight restrictions Thursday night through Friday. VFR will return for the weekend. && .MARINE... Tonight: Winds will remain 10-15 knots with gusts to 20 knots (and a few gusts approaching 25 knots) will continue along the land/sea interface and in Charleston Harbor during the early evening hours. We will maintain enough gradient overnight to yield elevated south to southwest flow across the local waters. A solid 15-20 knots is expected for periods of time. Seas will increase, becoming 2-4 feet on average, and up to 5 feet in the outer Georgia waters. Thursday through Friday: A strong cold front will approach Thursday causing SW winds to increase. Marginal Small Craft Advisory conditions are expected across our Charleston waters Thursday afternoon and evening due to gusts around 25 kt, and again Friday night behind the front. Additionally, there will be the potential for strong thunderstorms Thursday afternoon and evening, following by a second round Friday late morning or afternoon. The timing of thunderstorms remains unclear, but any storms that do develop and move across the waters will be capable of producing strong wind gusts. Saturday through Monday: Marine conditions will rapidly improve this weekend as weak high pressure builds in from the west. By Saturday, winds will drop to 10-15 kt and seas to 3 ft or less through Monday. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...