366 FXUS62 KCHS 172151 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 551 PM EDT Sun Sep 17 2023 .SYNOPSIS... A cold front will move through tonight, then high pressure will rebuild for much of the upcoming week. Coastal low pressure could affect the region late week into the weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... The last of the deep convection will pull offshore early this evening, and much of the rest of the night will be rainfree. We didn't make any changes to where fog occurs, but feel that almost any place can get some, as skies clear overnight. We will wait until later guidance to determine if we need to expand the coverage. The diurnal temp curve has been altered due to the recent rains, and we adjust the hourly curve as a result. Little to no changes to min temps though. Previous discussion... A longwave trough can be analyzed extending southward from the Great Lakes into the southeastern states. SPC Upper Air Analysis shows a 250 hPa jet streak extending from northern Mississippi into the Mid-Atlantic. Surface low pressure has formed in the right entrance region of this jet streak, in the upstate of SC. A cold front associated with this surface low extends southward and will push through the forecast area late this afternoon/this evening. The shower/tstorm activity seen on the KCLX radar is likely the cold front as it progresses eastward. CAMs depict the showers/tstorms congealing and pushing off the coastline this evening. Convective parameters remain favorable this afternoon for an isolated severe thunderstorm this afternoon, although some areas have been worked over. Instability remains between 1000-1500 J/kg according to SPC mesoanalysis. In addition to the instability, shear values are expected to also be favorable, although not super impressive. The 12Z KCHS RAOB Sounding showed 33 knots of Effective Shear, which is forecast to remain steady through the afternoon. The main hazard will be damaging wind gusts. This hazard has been highlighted by a Marginal Risk in the Convective Outlook by SPC. The threat for heavy rainfall continues this afternoon, with a couple Flood Advisories issued earlier today. The PWAT value with the 12Z KCHS RAOB Sounding was 1.80 inches and is forecast to only increase as moisture pools ahead of the approaching warm front, surface low pressure, and associated upper level disturbance. Forecast soundings depict PWAT values upwards of 2.0 inches this afternoon. The HREF continues to be impressive with it's potential rainfall totals, depicting a 30% chance of over 3" in 3 hours over southeastern SC. This risk has been highlighted by a Marginal Risk of Excessive Rainfall by WPC. Conditions will need to be monitored for additional Flood Advisories this afternoon. Temperatures this afternoon will be highly dependent on convection. Generally temperatures will reach into the low to mid 80s, with cooler temperatures during and right after convection moves through a location. Tonight: The surface low will lift to the northeast, with its associated cold front pushing offshore. Rain/tstorms will quickly diminish with clear skies moving in. Some fog is possible with the clearing skies and recent rainfall, especially further inland. Temperatures will drop into the low 60s further inland with upper 60s to near 70 closer to the coastline. && .SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/... A sharp and deep trough aloft on Monday will be followed by zonal aloft aloft Tuesday, then short wave ridging Wednesday. At the surface, low pressure off the Delmarva Monday gives way to the return of high thereafter. After morning fog Monday, considerable sunshine will prevail the rest of the day and again on Tuesday. There might then be some coastal showers developing Wednesday afternoon within a deeper easterly flow off the ocean. Max temps will be near or slightly above climo Monday and Tuesday, then near climo Wednesday. Good to excellent radiational cooling both Monday night and Tuesday night will allow for some far inland places to get down into the 50s, with temps closer to the coast about 6 to 10 degrees warmer. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... Although we are not yet certain as to when this will exactly happen, at some point later in the week a cut off low will form, most likely near or just offshore the Southeast coast. This will likely allow for some weak surface low to form along an old baroclinic zone, near the position of the upper low. While it is still much too early to make any conclusions about the exact strength and positioning and movement of the of low, it will likely result in greater moisture and rain chances across the region, along with the potential for stronger onshore winds. Temps look to be near or below climo through the period. && .AVIATION /22Z SUNDAY THROUGH FRIDAY/... Showers/tstorms will push offshore this evening, with skies clearing early tonight and prevailing VFR conditions through the overnight. There is a chance of fog across the region, however confidence in impacts to the terminals is low, as it appears the fog may stay inland of the terminals. Extended Aviation Outlook: VFR expected Monday through early Thursday. Maybe some flight restrictions in showers and t-storms thereafter. Gusty winds are expected Friday due to an enhanced gradient between inland high pressure and a weak low and trough offshore. && .MARINE... A cold front will push through the marine waters this evening, with showers and thunderstorms across most of the marine zones. Winds will be strong and gusty in and near t-storms early tonight, shifting to the SW and eventually the NW by daybreak after the frontal passage. Winds just before daybreak will likely surge to around 10 to 15 knots as high pressure builds in behind the departing cold front. Seas should average 2 to 3 ft, with 3 to 4 ft across the 20-60 nm offshore GA waters. Monday through Wednesday night: High pressure will be the main feature through the period. Larger seas from distant TC Nigel will start to arrive late wednesday or Wednesday night. However, nothing larger than 5 feet is expected. Thursday through Friday night: The gradient will tighten between strong high pressure to the north and a sharpening trough and possible weak surface low to the south and southeast. This will lead to stronger winds and much larger seas, where we will experience at least Small Craft Advisories across the entire marine area. Rip Currents: Despite lingering swell energy at the beaches Monday, given the offshore flow, we have a Low Risk of rip currents. There might be an enhanced risk Wednesday and Thursday if enough swell energy from distant TC Nigel is able to reach the coast. At some point late in the week there could also be the potential for high surf and beach erosion due to a prolonged onshore flow associated with inland high pressure and a weak surface low offshore. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...