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Area Forecast Discussion
971 FXUS62 KCHS 271729 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 129 PM EDT MON JUN 27 2016 .SYNOPSIS... A cold front will stall and dissipate south of the area today. Another cold front will approach from the northwest Tuesday, becoming nearly stationary and lingering nearby Wednesday and Thursday, before dissipating by Friday. Atlantic high pressure will then extend across the area next weekend, before another cold front approaches early next week. && .NEAR TERM /THROUGH TONIGHT/... No changes with the afternoon update. Radar is showing showers developing over Charleston County and that matches up well with the ongoing forecast. For the remainder of the day, the upper level pattern will feature upper level ridging that continues to extend eastward across the area from the large anticyclone that resides near the four corners region. For the most part, NVA will prevail, though there are some model progs that show a very weak embedded shortwave that pushes mainly off the North Carolina coast. At the surface, the weak surface low and associated trough will gradually dissipate leaving behind a rather nebulous pattern dominated by mainly onshore easterly flow. Model soundings and time heights reveal a very dry mid/upper level atmosphere. Also, temperature profiles in the mid/upper levels are quite warm and serve to effectively cap the atmosphere. Even without the cap the soundings indicate puny lapse rates and CAPE values struggling to reach 1,000 J/kg. As such, convective activity is expected to be quite limited and POPs have been limited to just slight chance for a small area mainly along the South Carolina coast. Tonight: The forecast area will be solidly positioned within a large region of NVA aloft ahead of the approaching trough from the northwest. An associated cold front will begin to approach late, but the forecast is dry. A few showers and/or thunderstorms may make a run for the far western zones by daybreak Tuesday. Otherwise, a quiet night with lows mainly in the mid 70s. && .SHORT TERM /TUESDAY THROUGH THURSDAY/... Tuesday: Mid and upper level ridging will break down atop the local area as the east coast trough begins to re-establish itself as strong short wave energy moves through the Great Lakes. This forces a cold front near the spine of the Appalachians early in the period to approach from the northwest. Compressional heating in advance of the cold front will allow for temps of 90F north to 95F south, before cloud cover and rain chances climb in response to the approaching front. Heat indices will peak at 100-103F south of I-16 in Georgia, but not high enough for our pre-July 1 criteria of 105F or greater for a Heat Advisory. Lift and convergence as the front draw closer will have ample moisture to work with as PWAT reaches as high as 2 inches, or near the 90th percentile for this time of year. Convective rain chances will build during the heating of the afternoon, climbing into the 50-60% range northwest tier, with 30- 40% probabilities elsewhere. A sluggish storm motion and the elevated moisture content supports a risk for localized flooding concerns in persistent showers/t-storms. While the overall thermodynamics are not overly impressive, dew points in the lower and middle 70s will support steep low level lapse rates and that along with 20-25 kt of 0-6km shear might supply just enough of a boost to some of the updrafts to support gusty winds with as much DCAPE as 800-1000 J/kg. Wednesday through Thursday: The large scale pattern aloft will feature a broad trough in the east and ridging in the west. The surface cold front will struggle to make too much progress to the south-southeast as it starts to become aligned parallel to to flow upstairs. Thus we look for that front to waver over or near the local zones during the middle of the week. While it is certainly difficult to pinpoint if and when any short waves will slide through the trough, better mid level lapse rates with the lower heights aloft and at least seasonable PWAT will support a better than average risk for showers and t-storms. Dependent upon where the cold front is able to establish itself Wednesday and Thursday, there is also the nearby proximity to the RRQ of the upper level jet and indications of modest QG forcing per various models. That could support even higher probabilities than the 40-50 Pop now shown in the forecast. With the higher rain chances does come slightly lower temps, with highs both days before the onset of convection to peak a degree or two either side of 90F. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... The pattern aloft flattens out with a quasi-zonal flow to develop over the local vicinity by late Friday into Sunday, with the cold front at the surface becomes diffuse and dissipates. This allows for an expansion westward of the sub-tropical Atlantic ridge, while Piedmont trough develops inland. This equates to a return to a more typical pattern for summer, featuring mainly diurnal scattered showers/t-storms. This allows for hotter temperatures and more uncomfortable heat indices to occur. For early next week another cold front may approach as the east coast trough begins to develop yet again. && .AVIATION /17Z MONDAY THROUGH FRIDAY/... Mainly VFR through 18Z Tuesday. Convection is developing along the sea breeze in SC, so added TSRA to the KCHS TAF. Brief MVFR is possible as the convection passes over the airport. No convection is expected for KSAV. There is also a small chance for late night fog and/or stratus at KCHS and KSAV. The probabilities are too low to include in the current TAFs. Extended Aviation Outlook: Flight restrictions are possible at least periodically in SHRA/TSRA Tuesday afternoon through Thursday. Less chance of direct impacts on Friday, but still a risk for SHRA/TSRA. && .MARINE... Today through tonight: A weak area of low pressure and associated surface trough just to our east will gradually dissipate through the day. Winds along the land/sea interface are increasing this afternoon with diurnal heating and could reach 15-20 knots for the Charleston Harbor. Then overnight winds will diminish to 10 knots or less and become southerly. Seas will be 1-3 feet, highest at 20 nm and further out. Tuesday through Thursday: A slow moving cold front will move into or near the marine area during the middle of the week, generating a fairly nebulous pressure pattern with winds and seas far below any Advisory thresholds. The proximity to the front will cause an increased chance of showers and t-storms, and the lighter wind fields plus sufficient instability, moisture and shear could support an increased risk for waterspouts. Friday through Saturday: The cold front will have dissipated and the Bermuda High will expand west across the local waters, allowing for a more typical pattern for early July, with slightly higher south- southwest winds, seas of 2-3 ft and a lower potential for showers and t-storms. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ SYNOPSIS... NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...