textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Coastal flooding will gradually increase along the Southeast Georgia and South Carolina coast through Saturday. Moderate, to potentially major, coastal flooding is possible along the coast of Charleston and coastal Colleton Counties on Saturday.

- 2) Hurricane Isaias is expected to track across the central Gulf today, before approaching the northern Gulf Coast on Friday. Although the center of the storm will remain well west of the region, heavy rainfall is likely across Southeast Georgia and the South Carolina Lowcountry this weekend. Flash and urban flooding is possible.

- 3) Increasing potential for a couple isolated tornadoes possible Saturday across Southeastern Georgia into South Carolina Lowcountry.

DISCUSSION

KEY MESSAGE 1: Coastal flooding will gradually increase along the Southeast Georgia and South Carolina coast through Saturday. Moderate, to potentially major, coastal flooding is possible along the coast of Charleston and coastal Colleton Counties on Saturday.

Today: Coastal Flood Advisory remains in effect until 8 PM this evening. Tidal departure at low tide this afternoon was slightly above 0.7 ft. Our local climatology tool indicates that departures may decrease toward 0.6 ft at high tide. Using the climatology information along with recent tide trends is appears that high tide 7:08 PM may crest around 7.1 ft MLLW.

Friday: Northeast winds will strengthen across the coastal waters as the pressure gradient begins to tighten. The gusty northeast winds may drive tidal departures high enough for minor coastal flooding during the high tides on Friday, with the highest astronomical tide in the morning.

Saturday: Winds are forecast to veer from the east as the center of Isaias tracks north across the Deep South. The pressure gradient between Isaias and high pressure wedged over the Carolinas will likely support gales across the nearshore waters. In addition, widespread moderate to heavy rainfall will begin during the overnight hours Friday night and should continue through both high tides on Saturday, around 8:20 AM and 8:30 PM. In addition, astronomical tides will increase due to the New Moon on Saturday. At this time, moderate coastal flooding is expected for both high tides along the coast of Charleston and Colleton Counties and minor coast flooding to the south. Tide guidance indicates that the Saturday morning high tide in the Charleston Harbor has a 10 percent chance of reaching major flood stage. In addition, any heavy rainfall during high tide will exacerbate any flooding, see rainfall key message for additional information. Minor coastal flooding is possible within the Charleston Harbor Sunday morning. Eventually, winds should turn offshore following a cold front Sunday into Sunday night, ending the coastal flood threat.

Rip Currents/High Surf/Beach Erosion: Increasingly gusty northeasterly winds combined with increasing wave heights will create a High Risk of rip currents along the southeast Georgia beaches on Friday, with a Moderate Risk of rip currents along beaches from Edisto, SC to Savannah, GA. Expect High Risk of rip currents across all area beaches on Saturday. High surf is possible on Saturday, especially during the high tide cycles, were breakers in excess of 6ft are possible along the GA and SC coasts. The large breakers and flooding high tides may lead to areas of significant beach/dune erosion.

KEY MESSAGE 2: Hurricane Isaias is expected to track across the central Gulf today, before approaching the northern Gulf Coast on Friday. Although the center of the storm will remain well west of the region, heavy rainfall is likely across Southeast Georgia and the South Carolina Lowcountry this weekend. Flash and urban flooding is possible.

A rather soggy and complex weekend is on tap as Hurricane Isaias makes landfall along the northern Gulf Coast and races inland towards Tennessee. Several main players will be in place to setup the potential for heavy rainfall starting late Friday night and persisting through early Sunday. The largest elephant in the room will be Hurricane Isaias, as it is expected to make landfall late Friday night into Saturday morning near the Alabama/Florida border into the northwestern Florida Panhandle. As Isaias quickly marches inland a slug of deep rich tropical moisture will come along with it as PWATS will climb above 2 inches and potentially approach 2.1 to 2.2 inches by Saturday. The next moving part is the cold air damming/Wedge that will build into the Carolinas on Friday with the surface backdoor front stalling somewhere near the SC/GA border and then arching along a surface trough just off the coast. This boundary will provide a low level convergence focus as we move through the night Friday and especially on Saturday.

Friday Night: Rainfall will likely begin across southeastern GA late Friday evening into Friday night as the backdoor front slides south and the leading moisture surges north. The backdoor front and associated wedge will act as sort of a ramp allowing for moisture to be lifted upward over the wedge producing widespread moderate rain to start. Some heavier bands will likely develop mainly right along the surface front where low level convergence is the highest. PWATs will not quite be at their peak; however, they should still be in the 2 inch range and with some elevated instability moving in heavier rain rates of 1 to possibly 2 inches per hour could develop late Monday night across Southeastern GA. Where exactly this band sets up is tough to pinpoint right now; however, some guidance is supporting some areas of highly localized 3 to 6 inches falling across Georgia Friday night. If these bands were to develop, an overnight flash flood risk could materialize. Most of Georgia would see 1 to maybe 2 inches through Friday night which will set the stage possibly for some more flooding concerns on Saturday.

Saturday: By Saturday, Isaias will have made landfall and should be making its rapid trek north across Alabama and moving into parts of Tennessee. As it moves north that surge of moisture will begin to spread across the Carolinas. At the surface, the aforementioned wedge front will begin to erode and lift northward throughout the day as a warm front. At the same time the coastal trough will move inland. Confidence is increasing on the potential for heavy rainfall across a good chunk of the area on Saturday, especially along the coastal areas of South Carolina including the Charleston metro/Tri-county area. With the lifting surface boundary and slowly advancing coastal trough, there will be a period where low level convergence will be maximized mainly Saturday afternoon. Widespread rain with heavier embedded banding features are expected to develop and slowly push inland from the Atlantic. Widespread rainfall totals of 2 to 4 inches look likely across most of the area with the potential for a narrow corridor of 4 to 6 inches mainly along the South Carolina coast. The biggest question will be if and where heavier banding does setup. Given the PWATs across the area and increasing instability throughout the afternoon, heavier bands will certainly be able to produce rain rates approaching 2 inches per hour in the heaviest of bands. If these bands sit over an area too long then localized flash flooding concerns will quickly materialize as rainfall totals could quickly pile up to near 8 inches. Flash flooding concerns will also occur in and around the two high tide peaks as the aforementioned coastal flooding will restrict rain runoff and compounding coastal and rain flooding would be likely to occur. This is especially true for Charleston county where coastal flooding may reach Moderate flood stage with just tide influences.

The biggest uncertainty for this event evolves around 1. where the coastal trough/heaviest banding sets up on Saturday and 2. whether or not a dry slot (pocket of dry air wrapping around Isaias) cuts into totals across Georgia and interior South Carolina on Saturday. The biggest question with the coastal trough is whether or not it stays offshore. If the heaviest band stays offshore then totals will likely run more on the 1 to 3 inch side and be a more widespread soaking rain. However; if the band moves inland then much more serious flash flooding concerns will develop. The dry slot on the other hand will likely limit how much parts of Georgia sees on Saturday. As Isaias lifts north it will wrap a big punch of dry air over Georgia and into central South Carolina. This dry slot will help limit storms Saturday afternoon and evening across interior parts and depending on the timing of this feature and how much rain is able to fall prior to its arrival will determine the flood risk for areas not along the immediate coastline. Given the uncertainties in both features we have elected to hold off on a Flood Watch at this time; however, one will likely be needed with an upcoming forecast package as details become clearer. Nonetheless, Concern for some localized flash flooding will persist through Saturday night for a good portion of the area, especially along coastal South Carolina.

Key Message 3: Increasing potential for a couple isolated tornadoes possible Saturday across Southeastern Georgia into South Carolina Lowcountry.

As if the flooding wasn't enough like most tropical systems there will be a risk for a few rotating storms capable of producing tornadoes mainly on Saturday. Two areas of concern seem to be shaping up on Saturday.

The first of which will be the slowly advancing coastal trough along the South Carolina coastline. Bands of storms will develop along the warm side of the boundary over the Gulf stream in a deep moist tropical environment. Given rather moist profiles and storm relative inflows around 25 ish knots, small transient supercells (AKA mini- spinnies) will likely be the storm mode. These mini-spinnies will approach the coastline throughout the day and then weaken as they cross the boundary. Typically in these setups, surface instability will be the biggest question as the surface front tends to hang up just offshore. However, if we manage to push enough instability onshore during the afternoon then the environment should be ripe with curved low level hodographs with plenty of streamwise vorticity for a couple of tornadoes along the immediate coastline from Savannah earlier in the morning to Charleston later in the afternoon. Given the deep tropical moisture any storms that would be tornadic would likely need some level of storm merging to produce and thus the storms to watch on Saturday will likely come in clusters of supercells within close proximity of one another.

The second area of interest will be across interior parts of GA into interior parts of South Carolina late Saturday afternoon and possibly another round Sunday. Several low level convergence bands will likely setup across central Georgia as Hurricane Isaias lifts north throughout the afternoon. As mentioned earlier, a pocket of drier air will begin to overspread Georgia which will help lead to some clearing in the mid and upper level clouds during the afternoon. Scattered storms could develop along these convergence bands along the nose of the lifting instability and moisture gradient throughout the afternoon. Given the environment will still be suitable for mini-spinnies capable of producing a tornado or two. The biggest caveat to this risk is 1. will we see enough instability to get strong enough updrafts and 2. the dry air not be too dry to prevent convection to develop. The big thing to watch would be any clearing or sunshine during the morning across Georgia. If we can get the instability to develop without being too dry then a decent risk could materialize across Georgia as usually the limiting factor in tropical cyclone tornadoes is instability. The same scenario could develop again on Sunday across parts of South Carolina as the same convergence bands lift northward with the dry slot. The only difference on Sunday is that the overall wind profile is not quite as conducive as the better low level shear lifts north of the area.

We will have to keep an eye on the mesoscale details over the next 24 to 48 hours to see how the tornado threat shapes up, but there is certainly a threat to keep a close eye on.

AVIATION /19Z THURSDAY THROUGH TUESDAY/

VFR conditions are expected at CHS/JZI/SAV terminals through 18Z Friday.

Extended Aviation Outlook: Flight restrictions are likely at all terminals Friday night through Sunday with widespread showers and/or thunderstorms, some capable of heavy downpours and gusty winds.

MARINE

Tonight: The sfc pattern will support northeast winds strengthening to 15 to 20 kts and seas between 3 to 4 ft.

Friday: See NHC forecast for the latest on Hurricane Isaias. For our region, the pressure gradient will increase between a high pressure to our north and the hurricane to our southwest as it travels northwards. This is expected to strengthen the northeast winds Friday morning into the afternoon hours, with wave heights building as well. Gale gusts may develop over the outer GA waters in the morning, spreading across the nearshore GA and lower SC waters during the afternoon.

Friday night into Saturday: As Isaias moves northwards, still expected to remain well west of the area in Alabama per NHC's latest forecast, the pressure gradient strengthens leading to a period where gale-force winds are possible across all waters outside the CHS Harbor. Gale Watches have been posted for the nearshore and outer GA/SC waters and Small Craft Advisory for the Charleston Harbor. Rough seas are anticipated on Saturday, in the 5 to 8 ft range from 0-20 nm out and 8 to 10 ft in the waters from 20-60nm out.

Lake Moultrie: Wind gusts across Lake Moultrie may increase to 25 kts on Saturday into Saturday evening. A Lake Wind Advisory is possible for Saturday.

Sunday into Monday: The surface high pressure gets pushed off into the Atlantic as the remnant surface low pressure of Isaias is expected to push off into the northeast states before moving offshore. This will likely keep winds breezy on Sunday as southerly winds move in, with breezy westerly winds on Monday. Currently, expecting at least Small Craft Advisory conditions to persist into Sunday, with a non-zero chance for them to continue to portions of Monday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...High Risk for Rip Currents from Friday morning through Friday evening for GAZ217-219-239-241. SC...Coastal Flood Advisory until 8 PM EDT this evening for SCZ149- 150. MARINE...Small Craft Advisory from 3 AM Saturday to 1 AM EDT Sunday for AMZ340. Gale Watch from Friday evening through Saturday evening for AMZ360. Gale Watch from Friday afternoon through Saturday evening for AMZ362-364. Gale Watch from Friday evening through late Saturday night for AMZ380. Gale Watch from Friday afternoon through late Saturday night for AMZ382. Gale Watch from Friday morning through late Saturday night for AMZ384.