Area Forecast Discussion
Issued by NWS Charleston, SC

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360 FXUS62 KCHS 230211 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1011 PM EDT Tue May 22 2018 .SYNOPSIS... High pressure will remain to the east through Friday while a trough of low pressure lingers to the west. Developing low pressure over the Gulf of Mexico will slowly move north this weekend through early next week, potentially maintaining wet weather. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Deep moisture convergence will persist a little longer than previously anticipated across some locations near and west of US-301, with convection still ongoing due to upper difluence and PVA near the local region. We now have slight chance/chance PoP through about 1-2 am from about Millen to Metter and Reidsville, but with little chance of activity elsewhere. The SREF probabilities of low stratus after 09Z is now around 30-40% inland from I-95, perhaps suggesting that it could be more than just ground fog closer to daybreak. For now though we have continued not mentioning fog in the forecast since condensation pressure deficits are no lower than about 10-20 mb. temps are about the same as 24 hours ago, so we have maintained persistence regarding lows; upper 60s far inland, and lower to mid 70s closer to the coast. Previous discussion... For most of the region the risk of convection has ended. The exceptions will be near and west of US-301 across interior Georgia, where boundary interactions will still allow for scattered to locally numerous showers and a few t-storms through mid to late evening. The overall thermodynamics are weak, but with PWat around 160% of normal (1.90 inches) and a slow steering motion toward the N-NE, locally heavy rains can still occur near and west of a line from Reidsville to Portal and Hilltonia. Latest guidance suggests that some of the oceanic convection will make a run toward the upper Charleston County coast, but we have held any mention of PoP off the land given Forward Propagating Vectors that are offshore. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/... Atlantic high pressure will remain to the east while a weak surface trough lingers inland. Aloft, the area will remain between a ridge to the north and a trough to the south. Tropical moisture will stick around for several more days, with PWATs in excess of 1.75". Extensive cloud cover will limit surface based instability during the period. We expect scattered to numerous showers, especially during the afternoon hours away from the coast which will be closer to the surface trough and thus better convergence. Thunderstorms will be fairly isolated given the limited instability, but high PWATs favor some locally heavy rainfall. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... Not much pattern change late this week into early next week with high pressure remaining to the east and and low pressure to the west. Will be watching for potential tropical cyclone development in the south-central Gulf of Mexico. Models have been fairly agreeable lately that the low will track toward the north-central Gulf Coast region this weekend. This pattern will continue to maintain a tropical air mass across southeast SC/GA and lead to higher than normal rain chances. Temperatures should stay near to above normal. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... Both KCHS and KSAV will be VFR through a large percentage of the 00Z cycle. However, there is some risk for low stratus and/or ground fog at both sites between about 08Z and 13Z Wednesday, and a chance for SHRA/TSRA in the afternoon. Any potential for prolonged flight restrictions is too limited at this time to show in the forecast. Extended Aviation Outlook: A persistent tropical air mass will lead to higher than normal rain chances through the weekend, mainly each afternoon. Thunder chances will be relatively low however. Some restrictions will also be possible from morning low clouds and/or fog. && .MARINE... Tonight: No concerns as the region remains along the western flank of Atlantic high pressure. Southerly winds 10-15 kt will prevail with seas averaging 2-4 ft. Scattered showers and a few t-storms will occur through the night, especially out near the west wall of the Gulf Stream, with nothing more than isolated convection in closer to shore. Wednesday through Sunday: Broad Atlantic high pressure will persist to the east through the weekend. Meanwhile, a weak surface trough will exist inland through Friday, then surface low pressure will move into the central Gulf coast region over the weekend. Winds will remain onshore 15 kt or less through Saturday afternoon, then potentially increasing to 15-20 kt Saturday night and Sunday due to a tighter gradient. A modest afternoon sea breeze will affect coastal areas each day. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM...JRL LONG TERM... AVIATION... MARINE... is the U.S. government's official web portal to all federal, state and local government web resources and services.