Area Forecast Discussion
Issued by NWS Charleston, SC

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564 FXUS62 KCHS 181439 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1039 AM EDT Wed Jul 18 2018 .SYNOPSIS... A cold front will approach the region from the northwest today, then stall over or nearby our area through Thursday night. The front will slowly lift north on Friday. Another cold front should approach from the northwest early next week. && .NEAR TERM /THROUGH TONIGHT/... Morning sounding, as expected, show very moist conditions with PW around 2 inches. A little more surprising, was that modifying the morning sounding for afternoon conditions came up with DCAPEs of just over 1000 J/Kg, with CAPEs of over 3000 J/Kg. In addition, a weak surface cold front, associated with a broad upper trough, will slowly progress southward toward the region. By this afternoon, this front is expected to be over/close to the northern area. The combination of the front, high PWs and typical instability, should result in scattered to numerous showers and thunderstorms by this afternoon and lingering into this evening. Feel the CAMs are handling the situation better, with holding off convection until this afternoon. Given the DCAPEs of over 1000 J/Kg, cannot rule out an isolated strong to low end severe storm this afternoon. Model soundings indicate that the steering flow will weaken as the front approaches. Therefore, the threat for locally heavy rainfall may increase this afternoon, especially closer to the coast where the late day sea breeze interacts with the approaching cold front. Highs will be held to around 90 to the lower 90s, due to increasing clouds/convective coverages this afternoon. Warmest temperatures are expected over the GA region, where clouds may take a bit longer to fill in. Tonight: The weak front, aided by convective cool pools, should slowly drift SE across the forecast area. Convection coverage is expected to decrease through the overnight hours. However, given the position of the front and lingering instability, will keep a mention of scattered showers/thunderstorms during the period. Temperature timing is difficult to pin down, largely dependent on the position and timing of thunderstorms. The temperature curve will reflect a diurnal trends with this package, but will refined with later updates. Low temperatures should range in the low to mid 70s. && .SHORT TERM /THURSDAY THROUGH SATURDAY/... The mid levels will consist of a trough just off the East Coast Thursday morning. This trough will get pushed further offshore as ridging builds to our north during the day, generally leaving our area with semi-zonal flow. The ridging will be short- lived because Friday it will get pushed offshore as an amplifying trough centers itself from the Great Lakes Region down into the Deep South. This trough remains in roughly the same location into Saturday while amplifying further. At the surface, a stationary front is forecasted to meander across our southernmost counties Thursday and Thursday night. Friday the front will transition into a warm front and slowly lift northward across our area. It`s forecasted to be just north of our area by Friday night. Saturday the warm front will continue it`s slow progress northward, away from our area. Meanwhile, troughing will start to develop inland. Plenty of moisture will be in place. Models show PWATs exceeding 2.25" across our entire area Thursday and Friday. Though, their are hints of pockets up to 2.5" with the front itself. SPC climatology indicates these values would be in record territory for CHS for this time of the year. Values finally drop below 2" Friday night and Saturday as the front moves to our north. Given all the moisture in place, lift from the front, and good model agreements spitting out plenty of rain, we adjusted POPs higher, but kept them capped at likely Thursday and Friday afternoons. Saturday a vast majority of the area is in the chance category. Though, it must be stressed that these POPs are highly dependent on the exact location of the front. An unexpected shift further north or south would necessitate changes to the POPs. The severe threat remains low due to the limited instability. Models have changed quite a but, now showing less instability. Thursday and Friday GFS BLCAPEs struggle to reach 1,000 J/kg. They do increase to the 1,500-2,000 J/kg range on Saturday with pockets of higher DCAPEs approaching 1,000 J/kg. But this is highly dependent on the exact location of the front. We continue to forecast a chance of thunderstorms with a low severe threat. The main concern continues to be the rainfall potential. The deep moisture and slow storm motions resulting from light steering winds higher up should allow for periods of heavy rain Thursday and Friday. Local flooding is possible both of these days. High temperatures will be a few degrees below normal Thursday and Friday due to the showers/thunderstorms and cloud cover. Highs will rebound to normal on Saturday. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... The models are in good agreement showing a strong trough over the East Coast Saturday night through Monday, with it weakening on Tuesday. However, they don`t agree as much with the surface pattern. The models hint at a trough inland Saturday night into Sunday. Another cold front will slowly approach from the northwest Sunday into Monday. Then, they disagree over whether the front makes it to our area or stays to the north. Regardless, the end result will be a rainy weather pattern. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... GOES-East detected stratus across KCHS and adjacent inland terminals. I will indicate IFR stratus at KCHS through 14Z, then lifting to VFR. KSAV is expected to remain VFR early this morning. A slow moving cold front will advance toward the Coastal Plain from the Foothills this afternoon. A weak frontal wave is forecast to develop across the mid-Savannah River Valley late this afternoon and evening. Once the wave develops, the front is expected to slow or become nearly stationary. Deep instability and moisture on the east side of the front will favor thunderstorms this afternoon and evening, highlighted with a TEMPO from 19Z-23Z. Remnant showers and thunderstorms could linger well into this evening. Storms motions are forecast to range from 5-10 kts. Light SW winds, scattered showers, and thick VFR debris clouds are expected through tonight. Extended Aviation Outlook: A series of fronts will bring unsettled weather to the region. Periodic flight restrictions are likely in convection, mainly during the afternoon and evening hours. && .MARINE... A cold front will approach the coast today, remaining inland through sunset. Scattered to numerous thunderstorms are forecast to develop along a sea breeze early this afternoon, possibly drifting over the Intracoastal Waterway. Scattered thunderstorms should drift or develop over the marine zones from mid afternoon through this evening. Otherwise, the pattern will support SW winds between 10-15 kts with gusts near 20 kts. Wave heights are forecast to range between 2-3 ft within 20 NM to 3-4 ft beyond 20 NM. The cold front is expected to slide off the SC coast tonight, tracking SSE. Winds will shift in the wake of the front from the north. Winds south of the front should veer from the WSW and decrease to around 10 kts. Scattered thunderstorms are expected to linger through most of the night. Wave heights will generally range between 2-3 ft. A stationary front will meander over or nearby our area through Thursday night. This front will transition into a warm front and slowly lift northward on Friday, becoming located to our north by Friday night. A trough will develop inland and prevail into Sunday. Meanwhile another cold front will slowly approach from the northwest early next week. Even with this active weather pattern, no Small Craft Advisories are expected. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...RFM SHORT TERM...MS LONG TERM...MS AVIATION...MS/NED MARINE...MS/NED is the U.S. government's official web portal to all federal, state and local government web resources and services.