textproduct: Charleston

WHAT HAS CHANGED

Key Message 1 and the Aviation Section have been updated.

KEY MESSAGES

- 1) Moderate to Major coastal flooding expected with the Saturday morning high tide, with additional coastal flooding possible through the weekend.

- 2) Hurricane Isaias is expected to approach the northern Gulf Coast late today, before shifting across the Deep South on Saturday. 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 flash flood watch is in effect from 8 PM tonight through 2 AM Sunday.

-3) A few tornadoes are possible across Southeast Georgia and Southeast South Carolina Saturday.

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.

Saturday: A Coastal Flood Watch in effect for Saturday morning for coastal Charleston and Colleton Counties. The combination of gusty onshore winds, widespread rainfall, and broad coastal low pressure should drive the tidal waters well above the astronomical tide Saturday morning, with tidal departures increasing between 1.3-1.5 ft within the CHS Harbor. Fort Pulaski on the Savannah River is forecast to crest within minor flood stage Saturday morning. Ongoing heavy rainfall and gusty onshore flow should keep departures elevated through Saturday evening. Further south, high tide is most likely to remain below minor flood stage for the Savannah/Beaufort area.

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 building wave heights will create a High Risk of rip currents along the southeast Georgia beaches today, with a Moderate Risk of rip currents along beaches from Edisto, SC to Savannah, GA.

On Saturday: High Risk of rip currents for all area beaches. High surf is possible on Saturday, especially during the high tide cycles, were breakers in excess of 6 ft are possible along the GA and SC coasts. The large breakers and flooding high tides may lead to areas of significant beach/dune erosion.

Sunday: Lingering large swell waves and gusty southwest winds will yield a high risk of rip currents along Charleston County with a moderate risk to the south.

KEY MESSAGE 2: Hurricane Isaias is expected to approach the northern Gulf Coast late today, before shifting across the Deep South on Saturday. 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 flash flood watch is in effect from 8 PM tonight through 2 AM Sunday.

Hurricane Isaias is on its final approach to the northern Gulf coast and the current NHC forecast is for the hurricane to make landfall later tonight across the western Florida Panhandle. Several main players will be in place to setup the potential for heavy rainfall starting late tonight and persisting through late Saturday. As Isaias 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 has built into the Carolinas this afternoon 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 and especially on Saturday.

Tonight: Rainfall has begun to steadily encroach on southeastern Georgia this afternoon and into the evening. 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. By after midnight, some higher instability and elevated instability should begin to work its way into Georgia as we should see a uptick in heavier storms as we head into the early morning hours. As these storms start to work inland, localized bands of heavier rainfall could result in localized amounts of 3 to 5 inches across Georgia with more widespread 1 to 2 inches of rainfall falling overnight. Where exactly this band sets up is tough to pinpoint right now, however if these bands were to develop, an overnight flash flood risk could materialize. Further north, banding could approach the South Carolina coastline after midnight and especially as we approach daybreak on Saturday. However, the main heavy rain threat likely materializes during the day on Saturday. Any rainfall that occurs overnight across the Lowcountry will only just help "prime" the soils for potential flooding later.

Saturday: As Isaias moves north into Alabama on Saturday, 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 morning into early 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 to 10 inches in very localized spots. 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 significant 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. Nonetheless, Concern for some localized flash flooding will persist through Saturday night for a good portion of the area, especially along coastal South Carolina. Nonetheless the potential is there for flash flooding and a Flood Watch for flash flooding has been issued for our entire area beginning 8 PM tonight.

Key Message 3: A few tornadoes are possible across Southeast Georgia and Southeast South Carolina Saturday.

The overall tornado risk continues to remain the same as yesterdays forecast. Two areas of concern seem to be shaping up as early as late tonight into the day 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 beginning as early as tonight and persisting throughout the day. 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 /00Z SATURDAY THROUGH WEDNESDAY/

00Z TAFs: VFR to start the TAF period at KCHS/KSAV with KJZI bouncing between VFR/MVFR. Conditions will gradually worsen overnight as tropical moisture builds across the area associated with outer bands of Hurricane Isaias. Chances of thunderstorms will increase into daybreak Saturday, reflected in a PROB30 group at all terminals. Easterly wind gusts will increase through the TAF period, peaking around 20 to 25 kt Saturday. Additional restrictions are likely as it becomes more clear where rain bands will set up and additional amendments are likely over the next 24 to 36 hours.

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

MARINE

Tonight: 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 this evening, with wave heights building as well. Gale gusts may remain over the nearshore and outer SC waters outside the CHS Harbor tonight.

Saturday: Combination of inland ridging and developing coastal troughing as a warm front moves up from the south will maintain Gale conditions to the SC coastal waters through early afternoon. Strongest winds will be on the west side of a coastal boundary expected to linger offshore, with the GA waters remaining south of this coastal boundary and unlikely gust above 30 kt. Moderate southernly flow develops later Saturday as the warm front lift north of the area.

Lake Moultrie: Wind gusts across Lake Moultrie expected to reach 30 kts Saturday morning. A Lake Wind Advisory has been issued accordingly.

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.

CLIMATE

Record Precipitation:

October 10: KCHS: 1.82/1990 KCXM: 2.13/1990

CHS WATCHES/WARNINGS/ADVISORIES

GA...Flood Watch through late Saturday night for GAZ087-088-099>101- 114-115-137-216>219-238>241. High Risk for Rip Currents through Saturday evening for GAZ217-219- 239-241. SC...Flood Watch through late Saturday night for SCZ040-042>045- 147>152. High Risk for Rip Currents through Saturday evening for SCZ148>151. Lake Wind Advisory from 5 AM to 2 PM EDT Saturday for SCZ045. Coastal Flood Watch from 6 AM EDT Saturday through Saturday morning for SCZ149-150. MARINE...Small Craft Advisory until 1 AM EDT Sunday for AMZ340. Gale Warning until 5 PM EDT Saturday for AMZ360-380. Gale Warning until 5 PM EDT Saturday for AMZ362-382. Small Craft Advisory until 1 AM EDT Sunday for AMZ364-384.