650 FXUS62 KCHS 101149 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 749 AM EDT Wed Oct 10 2018 .SYNOPSIS... Hurricane Michael will move north through the Gulf of Mexico before making landfall along the Florida Panhandle this afternoon. Michael will then gradually weaken to a tropical storm as it moves northeast across Georgia and South Carolina tonight and Thursday. Cooler and drier high pressure will then build in through the weekend before a coastal trough and cold front affect the area bringing a return of some rain. && .NEAR TERM /THROUGH TONIGHT/... Major Hurricane Michael will continue to approach the western Florida panhandle this morning, moving onshore that area during the middle of this afternoon. The major hurricane will gradually weaken tonight as it transitions to a Category 1 hurricane in southern Georgia and then to a Tropical Storm as it moves northeast toward the Central Savannah River Area (CSRA) by daybreak Thursday. This will occur in advance of a strong cold front stretching from the upper Great Lakes to the lower Mississippi valley and northwest Gulf. For today, a deep and strong South to Southeast flow between Michael and high pressure centered near Bermuda will continue to produce deep and strong moisture convergence and transport across the area today, aided further by an inverted trough near the coast this morning into early afternoon. Given Pwat that reaches as high as near 2.5 inches or around 200% of normal, subtle mid level differential vorticity advection, weak upper divergence will result in at least scattered to numerous showers, some of which will produce heavy rains. It looks like the highest chances will occur this morning into early afternoon, especially near and north of the Savannah River. Further south and elsewhere during the afternoon there are indications of somewhat of a lull as the inverted trough dissipates, and subsidence occurs in advance of the rain shield from Michael that doesn't get in here until tonight. While some localized flooding can occur, the potential for any significant flooding would be confined to downtown Charleston if heavy rains are ongoing near and at the time of the mid to late morning high tide. The overall instability and lapse rates are minimal early this morning, so we don't expect anything more than a few stray lightning strikes. There is better instability that develops later this morning and this afternoon, so we have included a slight chance of t-storms during that time. While there could be enough shear and helicity for the risk of isolated tornadoes toward evening over our southern Georgia counties, the higher potential exists during tonight. On average winds will be 15-25 mph and gusty over the coastal counties, and 10-20 mph with higher gusts further inland. However, with winds as high as 30-35 kt in the mixed layer, some of the showers and t-storms moving through will result in similar winds (30 to 35 kt) throughout the day. Overall skies will be cloudy today, but if any breaks are able to occur, given that we're already beginning the day with temps not much lower than the normal high, it won't take much to allow for max temps to reach into the middle 80s. Tonight: As poor as conditions will be today, they will steadily worsen tonight as Michael draws into closer proximity of the area. We anticipate tropical storm conditions will overspread the forecast zones from southwest to northeast, and depending on how quickly Michael diminishes in strength from a hurricane to a tropical storm, there is a low end probability that some hurricane force wind gusts might approach Candler, Jenkins, Sylvania and Allendale counties late. However, these odds are not high enough for a Hurricane Watch, so we continue with our Tropical Storm Warning. PWat will rise near 2.6 to 2.75 inches within a deep southerly flow as the tropical cyclone gets closer. This will allow for PoP to increase to as high as 70 to 100% probabilities by daybreak Thursday. We considered a Flash Flood Watch, but since the system is moving at a fairly steady northeast clip, we opted not to raise one at this time. There certainly could be Flash Flood Advisories issued, but the potential for any warnings seems low. The fast pace of the cyclone will lead to increased low level helicity and low level shear, with model guidance depicting a SIG TOR as high as 2 to 4 units and SHERBS3 near a unit of 1 that spreads in from southwest to northeast during the night. This will lead to a favorable environment for at least isolated tornadoes across the entire region. A Tornado Watch could become necessary. Elevated dew points within the warm southerly flow will prevent temps from getting any less than the mid or upper 70s, or about 15 to 20F above the normal low and close to normal daytime highs. In fact, record high minimums are in jeopardy at all 3 climate sites (See CLIMATE section below). && .SHORT TERM /THURSDAY THROUGH SATURDAY/... Based on the latest NHC forecast, weakening Tropical Storm Michael will be moving through the SC Midlands Thursday with the worst conditions across southeast SC/GA during this time. Several inches of rain for most places away from the GA coast and tropical storm force winds/isolated tornadoes everywhere are expected, beginning first in GA and then spreading into SC. Improving conditions are then expected Thursday night with diminishing winds and generally dry weather as Michael moves farther away. Long overdue cooler and drier conditions are then on tap Friday and Saturday as high pressure builds from the north. Temperatures should still remain at or above normal though through the period. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... Late this week cooler and drier high pressure will build back into the region behind Michael. A weak coastal trough will then develop early next week ahead of a cold front which could move into the area as early as Monday night. Rain chances are not expected until early next week. Temperatures look to remain near to above normal levels. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... KCHS: There will be periods of moderate to perhaps heavy SHRA at KCHS this morning as deep moisture continues to arrive from off the Atlantic, with activity becoming more sporadic in nature during the afternoon. Conditions will lower into the MVFR category at times, maybe even briefly down to IFR if heavy enough rainfall occurs. The airport will teeter near low-end VFR and MVFR the rest of the day into early tonight, before late night rains in association with TC Michael approaching the Central Savannah River Area will move in after 08Z Thursday. There is a low end potential for TSRA today and early tonight, with increasing chances late in the TAF cycle as shear and instability increase. Gusty SE and S winds will prevail through 12Z Thursday, with the chance for wind gusts of 35 to 45 mph that can occur in any of the heavier convection. KSAV: While there will be scattered SHRA and maybe a few TSRA around the terminal today into this evening, the timing on any direct impacts with possible flight restrictions are too uncertain at this time to include with the 12Z TAF. Thus we are maintaining mainly a VFR forecast through 03Z. The coverage of convection will then increase in association with TC Michael far inland across Georgia tonight, with even the potential for TSRA and isolated tornadoes. We include nothing worse than MVFR with SHRA for now until trends are more a certainty. Gusty SE and S winds will prevail through the entire period, with the highest winds to occur late tonight, and maybe in a few of the earlier convective rains that occur. Extended Aviation Outlook: Flight restrictions likely Thursday due to periods of moderate to heavy showers/tstms. Gusty winds expected Thursday as tropical cyclone Michael moves northeast to the west of the terminals. && .MARINE... Today and tonight: Dangerous marine conditions will impact the coastal waters, with tropical storm conditions to develop in response to Major Hurricane Michael that moves onshore of the western Florida panhandle this afternoon, and then into southern Georgia tonight, gradually diminishing in strength to a Tropical Storm late. The strongest winds and largest seas this period will occur tonight across the entire Atlantic waters and in Charleston Harbor. Mariners can expect isolated to scattered t-storms today and tonight, some of which will produce wind gusts of 40 or 45 kt. Plus there is an increased chance of at least isolated waterspouts given the favorable shear and helicity across the region, mainly late in the day and more so tonight. Thursday through Monday: Conditions will remain dangerous Thursday as Tropical Storm Michael moves northeast through the SC Midlands. Tropical storm force winds are expected during this time with wind gusts to near 50 kt and seas building to near 13 feet mainly off the Charleston County coast. Conditions will then improve late Thursday as the storm moves away and winds shift offshore. Cooler/drier high pressure will then build in from the north with some increase in winds/seas although likely staying below Advisory levels. A weak coastal trough should then develop early next week while a cold front approaches from the west, possibly arriving into the local area by the end of the period. Rip Currents: A high risk for rip currents continues today at all beaches due to persistent, strong onshore wind, some swell energy and astronomical influences. The rip current risk will remain elevated through at least Thursday or Friday. && .TIDES/COASTAL FLOODING... Coastal Flooding: Since the high tide last evening, levels have been running as much as 1.0 to 1.5 ft MLLW above predicted, and a similar trend will continue today and tonight, with even larger departures possible Thursday. As a result, we anticipate at least minor to moderate flooding will occur with the next three high tide cycles in Charleston, and at least minor flooding at Fort Pulaski during at least 2 or 3 of those high tides (including this morning). The actual high tide will likely occur around 30 minutes after the astronomical high tide, which will lead to the largest departures at that time. A Coastal Flood Advisory remains in effect for all but Tidal Berkeley at this time for the high tides today and this evening. There is however an increasing chance that we might require a Coastal Flood Watch and/or Warning for the South Carolina coast with the late morning high tide Thursday (for 8.0 ft MLLW or greater) and also the need for a Coastal Flood Advisory for Tidal Berkeley. These conditions will be considered at a later time. In addition, areas of erosion will occur due to strong onshore winds and elevated tide levels. High Surf: There will continue to be large breaking waves impacting the entire stretch of beaches today through Thursday, reaching as large as 5 to 8 ft, highest during Thursday with TC Michael at its closest approach. A High Surf Advisory will therefore remain in effect. && .CLIMATE... Record high minimums for 10 October: KCHS: 75/2017. KCXM: 78/2017 and previous. KSAV: 75/2017. Rainfall record for 10 October: KCHS: 1.82/1990. KCXM: 2.13/1990. KSAV: 5.68/1990. Rainfall record for 11 October: KCHS: 4.48/1990. KCXM: 5.46/1990. KSAV: 4.43/1885. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Tropical Storm Warning for GAZ087-088-099>101-114>119-137>141. High Rip Current Risk through this evening for GAZ117-119-139- 141. High Surf Advisory until 4 AM EDT Friday for GAZ117-119-139- 141. Coastal Flood Advisory until 2 PM EDT Thursday for GAZ117-119- 139-141. SC...Tropical Storm Warning for SCZ040-042>045-047>052. High Rip Current Risk through this evening for SCZ048>051. High Surf Advisory until 4 AM EDT Friday for SCZ048>051. Coastal Flood Advisory until 2 PM EDT Thursday for SCZ048>051. MARINE...Tropical Storm Warning for AMZ330-350-352-354-374. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE... TIDES/COASTAL FLOODING... CLIMATE...