Tuesday Idalia Update

KEY POINTS

  • With the 11 a.m. advisory today, NHC issued a tropical storm warning for the South Carolina coast south of South Santee River and a tropical storm watch for our coast northward into North Carolina. A Storm Surge Watch remains in effect for our coast south of South Santee River.
  • The track has been shifted slightly to the left, and additional slight shifts to the left may be needed. The expected forward speed is now slower, resulting in a longer duration of impacts.
  • There is a chance that Idalia strengthens more than expected over the Gulf of Mexico, making it stronger in South Carolina.
  • The change in track results in slight changes to the impacts, slightly lessening the wind risk but expanding the area expected to see extreme tropical rainfall and isolated tornadoes toward and even north of the I-20 Corridor.
  • Storm surge remains a significant risk for coastal areas due to high astronomical tides, and there is some uncertainty still on how well the peak storm surge will align with Wednesday evening's high tide.
  • We still expect at least a few dry and tranquil days starting Friday.

Idalia is now a hurricane over the eastern Gulf of Mexico about 625 miles southwest of Charleston, packing 90 mph winds and intensifying. Franklin remains a major hurricane about 575 miles east-southeast of Charleston.

Swells originating from Franklin will continue to cause dangerous surf with a high rip current risk along our coast and dangerous boating conditions on our coastal waters. However, there will be no direct effects as Franklin is making its closest approach to us today.

The changes to Idalia's forecast track and intensity since Monday are slight. The National Hurricane Center (NHC) has shifted the track slightly to the left. You'll also notice the timing to be a little slower than before.

So, there are some changes to the risks that we face. Here's how it's looking as of early afternoon.

Wind:

The only notable change to the wind risk is timing. Tropical storm force winds are likely to reach Jasper and Beaufort Counties late Wednesday afternoon and spread over our Coastal Plain Wednesday night. Winds will lessen from south to north late Wednesday night and Thursday, with tropical storm force winds likely ending around midday Thursday along the Grand Strand. The NHC forecast has winds of 65 mph at the 8 p.m. Wednesday point near Savannah, weakening to 50 mph winds at the 8 a.m. Thursday point along the Grand Strand. The strongest winds will occur along our coast, with peak gusts likely in the 60-70 mph range in the coastal Lowcountry and 50-60 mph along the Grand Strand. 

Sustained winds will be lower inland along the Coastal Plain, but frequent 40-50 mph gusts look likely. The slightly more inland forecast track means a better chance for gusts of 40 mph or so along I-20 Wednesday night into early Thursday.

Uncertainty is lower than before, but there's still a chance that Idalia becomes stronger than forecast before reaching Florida or weakens less than expected over land before moving into South Carolina. 

Rain:

Extreme rainfall in the 4-8 inch range, locally more, looks likely between I-20 (and I-95 past I-20's terminus) and the coast as Idalia moves through.

It's coming on top of some downpours parts of the state have seen in recent days. Also, a stationary front over the Upstate will generate heavy rainfall over much of the state this afternoon and evening.

Recent dry conditions in the Midlands, Catawba River Area, and parts of the Pee Dee help us handle the rainfall from Idalia better, but we still have a risk for flash flooding. The flooding risks are best related using the Weather Prediction Center's Excessive Rainfall Outlooks. Below is a daily breakdown.

Downpours this afternoon and night from the stationary front (interacting with thick humidity surging northward ahead of Idalia) will cause flooding downpours in some areas through tonight, especially in the Upstate. Flash flooding along our Coastal Plain from Idalia's rain could be widespread later Wednesday and Wednesday night. The shift in the forecast track extends the level 3 of 4 risk area into the I-20 Corridor. Rain will taper off from southwest to northeast late Wednesday night through midday Thursday.

River flooding in Idalia's wake should not be a big concern, but some rivers along our Coastal Plain will see minor flooding.

Storm Surge:

The current forecast storm surge is 2-4 feet south of South Santee River and 1-3 feet to the north. Combining this with high astronomical tides caused by our current full moon will result in major coastal flooding along the Lowcountry coast from Idalia. The forecast peak water level in Charleston has increased since yesterday.

Farther north along our coast, including the Grand Strand, minor storm surge flooding is likely at high tide this evening, then slightly worse flooding comes at Wednesday evening's high tide.

There remains some uncertainty on the timing of the peak storm surge. It looks like the worst surge will come after the high tide Wednesday evening, but if Idalia moves faster than the current forecast, the flooding will be worse. Tropical rainfall from Idalia tomorrow night will exacerbate the coastal flooding.

Tornado Risk:

The slight shift to the west of the forecast track expands the area of concern for isolated tornadoes, and the Storm Prediction Center (SPC) has enlarged its outlook areas for Idalia's passage.

The level 2 of 5 severe risk now covers the entire Coastal Plain. The risk will be highest close to the coast and begin tomorrow afternoon in the Lowcountry. The risk will spread northeastward through the evening, then end as Idalia's center passes by later Wednesday night into early Thursday.

If you're in these areas, the standard tornado safety rules apply. Decide on your best shelter area (a basement, interior room, or closet, under sturdy furniture) in advance. Have at least two ways to get these warnings (a weather radio or your phone's emergency alerts) that don't depend on plug-in power, which could fail during the storm. Ensure these will wake you up if you fall asleep during the storm. More detailed tornado safety information is available from the National Weather Service and Ready.gov.

Your storm preparations should be in high gear. If you need prep advice, you can always get that from the prep experts at SCEMD at hurricane.sc.