Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 182323 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 623 PM EST Sat Nov 18 2017 .SYNOPSIS... A cold front will pass through the area late tonight into Sunday morning then will stall south and east of the region. High pressure will build over the area and will prevail through early next week. A series of low pressure systems could then track northeast along the stationary front, producing unsettled conditions later Tuesday through mid to late week. High pressure will expand over the region late week or next weekend. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... A pronounced cold front is moving across Tennessee into Deep South early this evening and is delineated by a sharp line of showers/tstms. The cold front and accompanying line of convection will move steadily east tonight, reaching far interior areas around 3 am and coastal areas by 6 am. Do expect at least some weakening of the line as it moves across Southeast South Carolina and Southeast Georgia during the instability minimum, but upper forcing associated with the right entrance region of the upper jet will remain robust. RAP Showalter values go slightly negative just ahead of the front suggesting the presence of some weak elevated instability which may be just enough to support a risk for a few tstms despite no surface based instability. Plan to introduce a slight chance of tstms for the early morning hours. High resolution models are similar in showing a distinct line moving west-east across the forecast area, so 30% pops may be a bit low. Will keep pops capped at 30% for this update, these may need to be increased with subsequent updates. Lake Winds: Winds will increase on Lake Moultrie tonight ahead of a cold front, then lull late as the front passes. At this time it appears as though winds will mainly stay below Advisory levels /25 kt/ in the warm advection regime given the lake water temperatures in the 50s leading to less than favorable vertical mixing profiles. The strongest gusts will likely occur around the lake shores. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/... Clouds and a few showers could linger early Sunday; otherwise, the shortwave trough and associated surface cold front will push quickly offshore Sunday morning, followed by sunshine and drier conditions. Downslope flow and insolation will combine to offset cold air advection, pushing high temps into the mid/upper 60s most locations. The cold front will eventually stall south/east of the region, and this stationary front could play a role in our weather beginning Tuesday. Dry high pressure will expand over the area Sunday night, yielding clear skies and light winds. Strong radiational cooling will result, allowing temps to drop into the mid 30s inland and upper 30s to lower 40s closer to the coast. Accordingly, we introduced a mention of frost into the late Sunday night/early Monday forecast, and we may need to consider a Frost Advisory for Sunday night for some portion of inland zones where the winds would be more likely to go calm. A few of the chilliest inland locales could even bottom out briefly around 32F early Monday, but the probabilities for freezing temps remain too low to justify a Freeze Watch. Cool high pressure will persist Monday, limiting high temps to the upper 50s north to the mid 60s far south. Monday night into Tuesday: East of an amplifying longwave upper trough, forcing for ascent, moisture advection and a coastal trough will combine to produce increasing/thickening clouds, and showers should eventually develop. Latest guidance suggests that sufficient dry air will persist to maintain rain-free conditions at least through Monday night. Then, the chance for showers should increase Tuesday. Coverage of showers remains somewhat uncertain due to a persistent south/southeast to north/northwest moisture gradient. The latest forecast advertises Tuesday afternoon POPs ranging from likely far south/coastal waters to slight chance well inland. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... The period looks potentially unsettled though there are substantial differences in the global models. A series of shortwaves/jet maxima will dive into the longwave trough anchored over the eastern U.S, pushing the trough deeper into the Gulf of Mexico. Meanwhile, a surface stationary front will persist south/east of the forecast area. This scenario could support serial surface cyclogenesis over the Gulf, with individual lows tracking northeast along the stationary front and just off the southeast coast through mid to late week. Given ongoing uncertainty regarding the evolution/positions of key features, maintained ongoing/ conservative POPs in the 20-30% range into late week. && .AVIATION /23Z SATURDAY THROUGH THURSDAY/... Primary concerns: * Wind shift associated with cold front * Low-end risk for LLWS A cold front is on tap to cross the KCHS and KSAV terminals early Sunday, roughly in the 09-11z timeframe. A broken line of showers will accompany the front, but some weakening is possible prior to the front reaching the terminals. Will carry VCSH 09-12z with low-end IFR cigs for now. There is a low-risk for tstms given the presence of some elevated instability, but the risk is too low to justify a mention. Pre-frontal low-level jetting will intensify this evening, but there looks to be enough surface winds to keep conditions below low-level wind shear thresholds. Winds will shift to the northwest behind the front with steady clearing expected. Extended Aviation Outlook: Periods of flight restrictions are possible later Tuesday or Tuesday night through Wednesday. && .MARINE... Tonight: High pressure will be giving way to an approaching cold front. Elevated south/southwest winds will occur ahead of the front, although will be limited somewhat given the warm advection pattern leading to less favorable vertical mixing profiles. Advisories will go into effect this evening across the Charleston County nearshore waters which will be first to see 25 kt wind gusts and 6 foot seas, then around midnight for the rest of the waters, except Charleston Harbor which should hold off until a bit later. Sunday through Thursday: In the wake of cold fropa, offshore winds will diminish, and Small Craft Advisory conditions should end by midday Sunday across nearshore waters and around mid- afternoon beyond 20 nm. Between high pressure building from the northwest and an offshore stationary front, expect an extended period of northeast winds next week. As a series of surface low pressure systems develop and track along the stationary front/off the Southeast coast, the pressure gradient is expected to increase mid to late week. Small Craft Advisory winds and/or seas could develop over portions of the waters Wednesday night through Friday. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory from 2 AM to 7 AM EST Sunday for AMZ330. Small Craft Advisory from 11 PM this evening to noon EST Sunday for AMZ352-354. Small Craft Advisory until noon EST Sunday for AMZ350. Small Craft Advisory from 11 PM this evening to 4 PM EST Sunday for AMZ374. && $$

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