Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 170822 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 422 AM EDT Fri Aug 17 2018 .SYNOPSIS... The area will stay between an inland trough and high pressure in the Atlantic through early next week. A front will approach the area from the west towards the middle of next week. && .NEAR TERM /THROUGH TODAY/... The synoptic pattern will change little today with the region aligned along the western flanks of subtropical high pressure. Convection will once again be clustered near and inland of the afternoon resultant sea breeze circulation. Similar to yesterday, the strongest sea breeze convergence looks to setup along the Georgia coast where both the RAP and H3R show the greatest concentration of convection by early-mid afternoon. Activity will gradually develop inland mid-late afternoon as convection along the immediate sea breeze weakens/dissipates and additional convective outflows emerge. Closer to sunset, activity along the northward moving Gulf Coast sea breeze will make a run for the Altamaha River. There are signals in a few of the CAMs that some of this convection will push north of the river and impact much of interior Southeast Georgia, but confidence how this scenario will evolve is only moderate at best this far out. Will carry 20-50% pops this afternoon through sunset with highest average pops clustered along the Georgia coast and areas roughly along/south of the I-16 corridor, including the Savannah Metro Area. Highs will warm into the lower 90s under partly to mostly cloudy skies. Modified soundings yield little in the way of meaningful DCAPE with values averaging 500-700 J/kg so the risk for damaging, wet microbursts is low. PWATS will approach 2.25 inches later today which will support a locally heavy rainfall with a potential for minor flooding, mainly in low-lying and poor drainage areas. && .SHORT TERM /TONIGHT THROUGH MONDAY/... Tonight: Convection, mainly over interior Southeast Georgia, will gradually diminish through midnight. The risk for isolated showers/tstms will shift back into the Atlantic during the early morning hours Saturday. Extensive debris cloudiness will be slow to thin with mostly cloudy skies likely to hold for much of the night. Lows will range from the lower 70s inland to upper 70s/near 80 at the beaches. Saturday: In the morning the mid-levels will consist of broad high pressure to our southeast and a trough to our west. The trough`s axis should stretch from the Great Lakes Region down into the lower MS Valley. The trough will move eastward during the day, pushing the ridge further offshore. The trough should pass over and to our north during the evening hours, then deamplify overnight. Additionally, there appear to be several weak waves/disturbances embedded within the trough. At the surface, our area will be sandwiched between synoptic features consisting of a trough inland and high pressure in the Atlantic. The trough should nudge towards us throughout the day, helping push the high further offshore. PWATs are expected to start out below 2", then increase to above 2" by the evening hours. Given the synoptic setup, instability that will be in place, and the disturbances in the trough, showers and thunderstorms are a good bet. We have high end chance values in the afternoon, but this may be too low. More refinements will be made when this day transitions into the near term. Heavy rainfall is a concern along with flooding of low-lying and poorly drained areas. But higher winds aloft may move these storms along at a good enough clip to limit this potential. Temperatures will reach their normal values for this time of year. Sunday: In the morning the mid-levels will consist of a weak trough to our north, generally stretching from the Mid-Atlantic States into the Northeastern States. To our east will be a broad high offshore. To our west will be a ridge stretching across the Upper and Mid MS Valley. As the day progresses the weak trough will lift into New England. The broad high offshore is forecasted to build towards the coast. Meanwhile, the ridge will move eastward, passing to our north late at night. At the surface, a trough will be inland. An east to west stationary front is forecasted to stretch from the Mid-Atlantic States into the TN Valley, meandering through the day and night. Meanwhile, high pressure will be in the Atlantic. PWATs will range from 2-2.15" or towards the higher end of the climo scale for CHS for this time of year. The setup is similar to Saturday, so once again we are expecting shower and thunderstorms. POPs are in the upper chance range and it`s not out of the question they may need to be raised further. Heavy rainfall is expected. Though, the flood potential is too early to determine at this point. Monday: In the morning the mid-levels will consist of broad high pressure in the Atlantic. 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 quickly move into the Northeastern States and weaken as the broad high in the Atlantic tries to strengthen and inch its way towards us. Meanwhile, the trough will intensify and move eastward, stretching from the Great Lakes Region into the Lower MS Valley 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 drag a cold front to our west towards us. All this while a trough is just inland of us. And of course high pressure will be hovering over Bermuda. PWATs will hover ~2". Similar to the weekend, POPs are in the chance range. Though, they conditions don`t seem as prime for convection, so they are on the lower end of the range. && .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/... A long wave trough will be over the East Coast while a front slowly approaches from the west and then passes or stalls over our area. The current models seem to indicate a wet weather pattern with below normal temperatures. But it`s still a few days out and more adjustments are expected. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... VFR. Guidance suggests tstms could be close to KSAV this afternoon as the sea breeze approaches the terminals. However, there is not enough agreement on timing/placement in the various high resolution guidance to support a mention of TSRA just yet for the 06z TAF cycle. Will reevaluate with the 12z cycle as additional CAMs can be evaluated by then. No issues are expected at KCHS. Extended Aviation Outlook: Brief flight restrictions are possible due to showers/thunderstorms, mainly in the afternoon/evening. && .MARINE... Through Tonight: Typical summer southerly flow regime will remain in place. Winds will be 15 kt or less with seas 2-4 ft. Saturday through Tuesday: The coastal waters will be sandwiched between an inland trough and high pressure in the Atlantic through early next week. A front will approach the area from the west towards the middle of next week. The aforementioned synoptic pattern will lead to mainly southwest winds. The highest winds are expected Saturday afternoon into Saturday night due an elevated pressure gradient. Wind gusts at this time should mostly top out in the lower 20 kt range. Though, a few gusts could briefly touch 25 kt across the easternmost portion of the Charleston waters overnight. However, no marine headlines are expected. Seas during the extended period will generally peak at 3 ft. The exception will be Saturday due to the winds. Additionally, there will be the occasional 4 footers near the Gulf Stream. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$

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