035 FXUS62 KCHS 191716 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 116 PM EDT Sun Aug 19 2018 .SYNOPSIS... The area will be situated between an inland trough and Atlantic high pressure through Tuesday. A cold front will approach the area from the west on Wednesday, then stall over or near the region through the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Today: No significant changes were made with the latest update. Satellite imagery indicates plenty of cloud cover, mainly of the mid/high variety and mostly across SC near a mid-level shortwave. Radar imagery indicates some showers south of I-16. Otherwise, mid- level ridging is expected to strengthen a bit during the day from the southeast and low- level convergence will be limited given the sluggish sea breeze. Similar to the past few days we think the best rain chances will be inland closer to the deeper moisture and some additional weak shortwave energy. Kept rain chances no higher than 40 percent through the afternoon, highest in GA, but higher POPs may eventually be needed as we see how mesoscale features play out, including any differential heating boundaries. Highs should reach the lower 90s inland with heat indices peaking near 100 degrees most locales. The 12Z modified CHS sounding indicates similar conditions to the last few days so we don't anticipate a significant risk for severe thunderstorms, although plentiful moisture could lead to heavy rainfall and localized minor flooding. && .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/... Tonight: Convection will gradually diminish through midnight with dry conditions expected to prevail during the early morning hours Monday. Debris cloudiness will thin with time, but it may take a good portion of the night before significant clearing occurs. Lows will range from the lower 70s well inland to the upper 70s/near 80 at the coast, but locally cooler conditions could occur where pockets of rain-cooled air develop. Monday: In the morning the mid-levels will consist of broad high pressure in the Atlantic, with its periphery stretching into our area. This high will connect to a ridge that will stretch to our north. Meanwhile, a trough will be over the Plains States. As the day progresses the ridge will move eastward and get absorbed into the overall synoptic flow. Meanwhile, the broad high in the Atlantic will try to strengthen, with it's periphery reaching more into our area. This will help to delay the arrival of the eastward moving trough, which will stretch from the Great Lakes Region into the Deep South by the nighttime period. At the surface, low pressure over or near MO in the morning will move to the Great Lakes Region by the overnight hours. This low will pull a warm front (already to our north) with it and slowly drag a cold front (far to our west) towards us. Meanwhile a trough is just inland of our area. Additionally, high pressure will be hovering over Bermuda. A moisture gradient sets up across our area. PWATs are forecasted to be above 2" across our northernmost counties. On the flip side, PWATs could drop as low as 1.5" across our southernmost counties. The resulting POPs generally reflect this, being the highest across the Charleston Tri-County area and tapering as one heads south. It's possible their may be enough dry air to the south (especially in the mid-levels) to create a rainfree day. Instability is not very impressive for August. The best instability is across the Charleston Tri-County area where Showalter values are minus 2 and BLCAPEs struggle to exceed 1,000 J/kg. Even DCAPEs there are barely a few hundred and shear is lacking. Hence, the wind threat is low. Even the heavy rainfall threat could be decreasing as the area of deep moisture is small and mid-level winds should be strong enough to move the storms along, hence limiting the flood potential. With less convection and possibly more sun, highs could be a few degrees above normal. Tuesday: In the morning the mid-levels will consist of a weak ridge just off the East Coast, getting absorbed into the synoptic flow. Broad high pressure will be off the Southeast coast while a trough stretches from the Great Lakes Region into the Deep South. As the day progresses the high will get pushed away by the eastward moving trough, which is forecasted to stretch from the Great Lakes Region down into the Southeast overnight. At the surface, low pressure over the Great Lakes Region in the morning will quickly move into Canada as the day progresses. The low will drag with it a cold front, which is forecasted to be just west of our area during the overnight hours. Also, in front of the front will be a weak surface trough. Moisture is forecasted to increase just ahead of the front. A small swath possibly up to 2.25" may move through our area overnight. This is above normal for CHS for this time of year. Though, instability is lacking. GFS BLCAPES struggle to reach 1,000 J/kg during the daylight hours, inching upwards in the evening. Though, DCAPES inch upwards as well, which is worth watching for wind potential. The heavy rainfall potential will remain limited due to higher winds aloft helping to move along any storms that do develop. For now, we're only going with slight chance to chance POPs during the day. POPS go up overnight as the front approaches. Temperatures are expected to be a few degrees above normal. Wednesday: In the morning the mid-levels will consist of a trough stretching from the Great Lakes Region down into the Southeast. This trough is expected to remain in place into the night and amplify. At the surface, a cold front will be just west of our area in the morning. It will slowly move eastward during the day, and become located over or just off our coast by the overnight. The highest POPs follow the front and it's exact location will determine how much or little convection we get. Expect more changes as this gets narrowed down. The same with temperatures. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... Models show a long wave trough over the eastern half of the country Wednesday night. This trough will gradually shift eastward and then lift to the north into Friday, followed by zonal flow on Saturday. At the surface, a front is forecasted to stall over or just offshore of our area during this time period. This will lead to above normal POPs and near or maybe even below normal temperatures. But if the front positions itself further offshore, then large changes in the forecast would be needed. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/... 18Z TAFs: Generally VFR conditions expected. There is a small risk of MVFR cigs at KCHS/KSAV this afternoon. Otherwise, the best chance of impacts from showers and thunderstorms looks to be late this afternoon into early evening. Extended Aviation Outlook: Small probabilities of brief flight restrictions Monday and Tuesday due to convection. Higher probabilities of brief flight restrictions due to convection associated with a front Wednesday and Thursday. && .MARINE... Through Tonight: Typical late-summer southerly wind regime will hold in place. Speeds will average 15 kt or less today then increase to 15-20 kt overnight as a noctural surge develops. Seas will average 2-3 ft, although a bit higher off the northern Charleston County coast. Monday through Thursday: The coastal waters will be situated between an inland trough and Atlantic high pressure through Tuesday. A cold front will approach the area from the west on Wednesday, then stall over or near the region through the end of the week. This will lead to mainly southwest winds through Wednesday, followed by changing wind directions on Thursday. The highest winds are expected during the first half of Tuesday night due to an elevated pressure gradient from the approaching front. Wind gusts at this time should peak around 20 kt. Though, a few gusts could briefly approach 25 kt across the easternmost portion of the Charleston waters and the offshore GA waters beyond 30 nm. No marine headlines are expected. Seas will generally be in the 2-4 ft range this week. The exception will be Tuesday night when some 5 footers are possible in the areas where the wind gusts will be the highest. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...RJB SHORT TERM... LONG TERM... AVIATION...RJB MARINE...RJB