Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 242014 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 414 PM EDT Mon Sep 24 2018 .SYNOPSIS... A stationary front will linger over the area through Tuesday. Low pressure will also approach from the Atlantic tonight and Tuesday, then weaken as it moves northward toward the North Carolina coast Tuesday night. A cold front will approach from the west Wednesday and Thursday, then stall nearby into Friday. A second cold front is expected over the weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... This Evening and Tonight: Despite sufficient instability late this afternoon, forcing associated with a stationary front remains limited under a mid-lvl ridge of high pressure extending from the western Atlantic over the Southeast United States. Latest trends indicate a few to scattered showers persisting for the next couple hours while sfc temps remain in the mid/upper 80s over most areas and around 90 degrees over southern areas in Southeast Georgia. Thereafter, activity should wane into evening/early night hours with the loss of diurnal heating around sunset. Overnight, the main issues will be the possibility of patchy fog inland and/or showers arriving on the outer skirts of low pressure approaching the Southeast coast late/near daybreak. At this time, overnight lows should dip into the upper 60s well inland to low/mid 70s closer to the coast. These cooler temps and light winds under high pressure in combination with sfc dewpts in the upper 60s to lower 70s suggest the potential for at least some patchy fog inland late tonight. Warmer sfc temps and clouds arriving from the Atlantic late should limit fog potential closer to the coast. A few showers could drift onshore by daybreak, mainly brushing northern locations of Charleston and Berkeley Counties late tonight and into daybreak. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/... Tuesday: Broad low pressure in the Atlantic and drawing closer to the SC coast, is still expected to stay offshore as it reaches its closest approach about 175-200 nm east of Charleston in the afternoon. The latest indications are that there is a medium chance of this system acquiring tropical characteristics, but fortunately our forecast region will lie on the western side, with the bulk of any stronger winds and heavier convection to stay over the Atlantic. The low is being driven around the western side of a H5 5950 meter anticyclone centered near Bermuda, and will eventually get picked up by a stronger west-southwest flow aloft in association with a broad trough in the central U.S. during Tuesday night. There is quite a bit of dry air and subsidence over much of the forecast zones during the day to the west of the system, and this will put a cap on convective chances. However, there is enough forcing and better moisture with PWat near 2 inches over Charleston, Berkeley and Dorchester counties due to closer proximity to the low, and also the interaction of the sea breeze. It is here where we have our highest PoP in the 30-50% range. MUCAPE reaches in excess of 1500 J/kg most areas, even greater than 2000 J/kg in the Charleston tri-county area, so we have mention of a slight chance of t-storms. Storm motion is somewhat restricted, so some pockets of heavy rains are possible near and north of I-26. H8 temps are yet again above normal, and as warm as 17-18C. This will support max temps ~90F most places inland from the coast and outside of the Charleston tri- county district, where max temps will be in the mid and upper 80s. Wednesday: An abnormally strong and deep layered oceanic ridge will be the main weather feature, as it expands west across the region. Upstream is a continuation of a broad trough covering a large portion of the central states, with an associated cold front to stretch from New England to the TN valley and to coastal TX by the end of the day. The better forcing stays outside of the CWFA, but due to adequate moisture and some instability, isolated to scattered showers and a few t-storms will form along the sea breeze and due to boundaries arriving from outside the area in the afternoon. A deep southwest flow and H8 temps of 17-18C will support max temps in the lower 90s inland from the immediate coast. This is around 6-9F above normal for late September, but a few degrees shy of any records. Thursday: The cold front will edge closer to the area from the northwest, but will struggle to get in as it becomes aligned parallel to the southwest flow aloft, and blocked by a solid 5920- 5930 meter anticyclone off the Southeast coast. Moisture profiles as evidenced by cross-sections are deeper than recent days, and stronger nearby forcing with the upper jet close to the Appalachians. This will support scattered showers and t-storms most places, with the highest PoP along the NW tier. The overall thermodynamics are better than of late, with Lifted Indices as low as -6C, MUCAPE in excess of 2500-3000 J/kg and 0-6 km Bulk Shear of 20-25 kt. If this comes to be, then some organized storms could develop, perhaps some with strong or severe winds with forecast DCAPE in excess of 1000 J/kg. We`ll knock off a couple of degrees from Wednesday`s highs due to more cloud cover and higher rain probabilities. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Thursday night into Friday a front is expected to be stalled over or near our area, bringing above normal POPs. A cold front is then forecast to approach from the northwest this weekend, continuing the rainfall potential. Models differ on the exact location of this second front and that will dictate the forecast. For now it appears to equate to above normal POPs. However, more adjustments to the forecast are expected. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... VFR conditions should prevail at both CHS and SAV terminals through 18Z Tuesday. However, a few showers and/or isolated thunderstorm could impact either terminal this afternoon, but chances appear best at the CHS terminal. Tempo MVFR conditions remain at CHS between 19-21z this afternoon to account for showers at this time. Models hint at some low clouds/patchy fog at both terminals late tonight/around daybreak Tuesday. However, confidence remains too low to include at either terminal at this time. Extended Aviation Outlook: Temporary flight restrictions are possible in mainly diurnal convection at KCHS Tuesday afternoon, then at both KCHS and KSAV late Wednesday or Thursday through Saturday. && .MARINE... This Evening and Tonight: A broad easterly flow regime will hold in place through early tonight while a stationary front remains over/near the coastal waters and Atlantic high pressure prevails aloft. Expect winds/seas to remain below Small Craft Advisory levels through tonight. In general, east winds will average between 10-15 kt with gusts to around 20 kt in northern South Carolina waters and the CHS Harbor into late evening/early overnight hours, before tipping northeast and remaining at or below 10-15 kt overnight. Seas should be no higher than 2-4 ft. Tuesday and Tuesday night: The latest forecast shows broad low pressure coming close to the local waters, but staying about 150-175 nm east of the AMZ350 waters at its closest approach around 21Z Tuesday, before it lifts off to the north and northeast thereafter. Provided that the system remains weak enough and doesn`t acquire tropical characteristics, winds will be NE or E at or below 15 kt Tuesday before veering to S-SW at even lighter speeds Tuesday night. Seas will hold under 4 or 5 ft, a mix of 2-3 ft swells and 2- 3 ft wind waves. Wednesday through Sunday: Sub-tropical ridging will be the dominant feature into late in the week, before a cold front makes it through during the weekend and is replaced by high pressure to the north. Unless there is some swell energy that arrives late in the period from Sub-tropical Storm Leslie spiraling in the central Atlantic, t here is no significant concerns regarding winds and seas. Rip Currents: We opted to go with a Low risk of rip currents at area beaches Tuesday. But any slight increase in winds and/or swell energy will bump conditions into the Moderate range. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...DPB SHORT TERM... LONG TERM... AVIATION...DPB MARINE...DPB CLIMATE...

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