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Area Forecast Discussion
492 FXUS62 KCHS 280748 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 348 AM EDT TUE JUN 28 2016 .SYNOPSIS... A cold front will approach from the northwest today, becoming nearly stationary and lingering nearby Wednesday and Thursday. The front will lift back north on Friday as Atlantic high pressure builds across the region. Another cold front will approach the area early next week. && .NEAR TERM /THROUGH TONIGHT/... Today: The overall pattern today will favor a more active afternoon and evening with increased coverage of showers and thunderstorms. Aloft, the persistent mid/upper level ridge will break down and dissipate as shortwave energy approaches from the northwest. This shortwave energy is on the southern periphery of a sharp upper trough that will move through the Great Lakes region today. As the larger scale flow becomes more cyclonic and the shortwave energy approaches, it will help to push a cold front into the area from the northwest this afternoon and evening. Moisture will pool along and ahead of this front, and when combined with the aforementioned upper support and low level convergence along the boundary will result in at least numerous coverage of showers and thunderstorms this afternoon and evening. The models seem to be in pretty good agreement that initiation will take place to the north and northwest across the Midlands, and will then propagate southward into the area. The main time period for thunderstorms will be from roughly around 2 pm for inland locations to around 8-9 pm along the coast. Confidence is high that most, if not all, locations will see measurable rainfall today. But pops have been capped at the 70 percent range due to uncertainties with regard to exactly when the storms develop and how quickly the proceed southward. POP adjustments will likely need to be made through the day as radar trends become evident. Regarding impacts, the main threat for today appears to be locally heavy rainfall. Precipitable water values surge to in excess of 2 inches and the presence of embedded heavy thunderstorms could result in rainfall totals of 1-2 inches. It appears that these storms will have enough forward speed to preclude a significant flooding threat, but we could still see isolated areas with locally excessive rainfall that produces minor flooding issues. Given the expected movement, a mention in the Hazardous Weather Outlook is not needed at this time. Finally, regarding the severe potential, the forecast area is in a general thunder from the Storm Prediction Center. Model soundings do not reveal a particularly conducive atmosphere given the deep moisture and lack of steep mid level lapse rates. However, one thing to keep an eye on is if these storms can get any organization as the form across the Midlands and put out much leading outflow. The severe threat is certainly on the low side given the marginal thermodynamics and modest CAPE values of 1000- 1500 J/kg, but we could still see an isolated strong to marginally severe storm or two. Tonight: In the evening the bulk of the ongoing activity is expected to focus along the coast and be on a diminishing trend with the loss of diurnal heating. Still, with the front in the vicinity, shortwave energy aloft, and deep moisture, isolated to scattered showers and storms could continue for much of the overnight. POPs feature a diminishing trend but we do maintain mid to high range chances for many areas through the period. Lows will fall into the low to mid 70s. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... Wednesday: An amplified and broad large scale pattern will exist aloft across the country, featuring a strong anticyclone near the Four Corners in the west, and the eastern trough that stretches South into the Gulf of Mexico. At the surface we find an undulating cold front near or over our north and northwest zones. An abundance of moisture will prevail within a deep southwest feed of sub-tropical air evidenced by Pwat that is near or in excess of 2 inches. Throw in some weak upper divergence and subtle mid level perturbations, convergence and large scale forcing for ascent from the proximity to the cold front and the typical sea breeze boundary, and an elevated convective rain chance will be prevalent. Our gridded forecast depicts 40% chances of showers/t-storms north and north to 50% south and southeast. The overall thermodynamics are modest at best, but with some 20-25 kt of bulk shear in the 0-6km layer and MLCAPE upwards of 1500 J/kg, there could be a low end risk for isolated marginally severe storms. Probably a bigger concern will be the potential for locally excessive rains with weak storm motion and the high Pwat. Maximum temps will be held somewhat in check by the higher rain chances, generally in the upper 80s or near 90. Thursday: A broad and large scale trough will persist in east and southeast, while a Piedmont trough and a rather weak cold front wavers nearby. The proximity to the front and trough, the sea breeze boundary, plus forcing for ascent due to subtle impulses aloft and some divergence in the upper atmosphere will support yet another day with higher than typical convective rain chances. We again have 40- 50 Pops across the entire CWFA, and locally heavy rains are again a problem since storm motion is less than 10 kt and Pwat is up near the 90th percentile for this time of year. There may also be a slightly greater risk for isolated severe storms with better instability and DCAPE on the order of 1200-1600 J/kg. With the high moisture content this may allow for a few marginally severe storms with wet downbursts. Temps will be near or a tad above normal. Friday: The flow aloft starts to flatten and becomes quasi-zonal with small height rises to occur locally by late in the day. The cold front will have either dissipated or lifted far to the north, allowing for sub-tropical high pressure to build from the Atlantic. While there is the lack of any significant forcing, given the sea breeze and the Piedmont trough still in place and still plenty of deep moisture, scattered Pops are in order. With the higher heights and an expansion of low level thickness, temps will reach at least into the lower 90s before storms develop. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... The weekend will feature more typical summer weather, with the Bermuda-Azores High and Piedmont trough to encompass the synoptic pattern. Zonal flow aloft and mid level heights up near 5920-5940 meters will limit the amount of convection both Saturday and Sunday to generally 30 or 40 percent. Low level thickness and 850 mb temps are a little above normal, so max temps will peak around 93- 95F. With dew points in the lower and middle 70s will generate heat indices as great as 103-105F each afternoon, but well below our July 1st Heat Advisory criteria of 110F or greater heat index. An upstream cold front will attempt to approach early next week, but the flow aloft stays fairly zonal, which indicates that the front will struggle to get this far south. For that reason we won`t go any higher than 30-40 Pops for Independence Day and next Tuesday. && .AVIATION /07Z TUESDAY THROUGH SATURDAY/... The main forecast problem for the next 24 hours revolves around the potential for direct impacts from thunderstorms this afternoon and evening. A cold front will approach the area and showers and thunderstorms are expected to develop to the north. These showers and storms will then move southward and near the terminals by the afternoon. Confidence is relatively high that both sites will see a thunderstorm at some point. The time period for KCHS appears to be 20z through 00z and the time period at KSAV appears to be 22z through 02z. Have introduced a TEMPO group for TSRA with just MVFR conditions for now. At least brief IFR conditions will be possible as will some low end wind gusts. Once the storms push south and dissipate overnight, there will be the potential for fog and/or stratus development. The best chances for fog and/or stratus appear to be beyond the current TAF window. Extended Aviation Outlook: Flight restrictions are possible at least periodically in SHRA/TSRA Wednesday through Thursday as a cold front lingers nearby. While there are less chances for direct impacts Friday and Saturday, there still does remain the potential for SHRA/TSRA. && .MARINE... Today and tonight: A cold front will approach from the north and northwest today and then stall in the vicinity of the coast overnight. Winds will primarily be south to southwest through the period, generally 15 knots or less. Seas will range 1-3 feet, highest from 20 nm and beyond. We could see thunderstorms approach the coast this evening and even push out into the waters tonight. As such, mariners should be prepared for possible thunderstorms and lightning and gusty winds. Wednesday through Thursday: An oscillating cold front will be situated near or just north of the marine community during the middle of the week, with the Atlantic ridge suppressed across Florida. A fairly light pressure pattern will prevail, although with occasional small surges due to the sea breeze circulation and nocturnal jetting, winds will be up near 15 kt at times. Seas for the most part will average 2 or 3 ft through the period. Given a rather slackened wind field both Wednesday and Thursday morning, and the close by cold front and the expectation of convergent cloud lines, there might be an enhanced risk for waterspouts. There is also the potential for greater coverage of showers and t-storms than what is typical. Friday through Sunday: the Atlantic ridge will become the dominant system, although inland a Piedmont trough will prevail. A general southwest synoptic flow will prevail at or below 15 kt, with seas 3 ft or less. && .TIDES/COASTAL FLOODING... The upcoming Perigee occurs early on Friday, and the New Moon occurs on Independence Day. This could allow for another bout of "King Tides" and we`ll see if enough departure will occur to generate shallow coastal flooding late this week into early next week. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...BSH SHORT TERM... LONG TERM... AVIATION...BSH MARINE...BSH TIDES/COASTAL FLOODING...