Idalia evening update Monday 28 August

NHC has issued their 5pm advisory; as usual the links to the Key Messages regarding Tropical Storm Idalia (en Español: Mensajes Claves). Still a borderline tropical storm/hurricane, but could cross that threshold (64 knots/74mph) at any moment and probably will as soon as I hit “send” on this post 😛 . The track has been nudged west a little again; here a comparison of the 11am and 5pm tracks, so you can see it’s not very much difference …

Tropical storm watches have been extended so they now include all of the Georgia coastline, and South Carolina up to just south of Myrtle Beach.

As for impacts, NHC also “nudged” up the landfall intensity by 5 knots, to 105. Here is the impact estimate using the official forecast as generated by my TAOS/TC model:

A Five knot increase (6 mph) may not seem like much, but in terms of dynamic wind pressure it’s a bit more than it seems. 5 knots is only a 5% wind increase, but it is a 10 percent increase in dynamic wind pressure (the force the wind exerts on trees, buildings, etc). The damage estimate did drop though, because the shifts during the day reduced the winds over the “target rich” Tampa/Clearwater corridor.

As for the Savannah area, it still looks like tropical storm conditions but NHC has bumped up the inland winds as well, and again the small increase has an out sized impact. Unfortunately the track shifts have taken the storm inland, placing the Georgia Coast on the “bad” side of the storm. It’s still not in the “dangerous” territory, but it means more power outages, more limbs down, a greater chance for a tree or two failing and causing some damage.

While NHC rightly notes in their discussion that the storm is paralleling the Florida coast and a slight wobble changes the landfall location a lot, those of us further north are in a similar predicament. However, as previously discussed, small wobbles can make a huge difference because by the time the storm reaches the Georgia/South Carolina coast, the left hand side should be getting pretty ragged. So if the center is a little offshore, that will make Savannah experience a lot less wind and somewhat less rain. The current track is less favorable, so we’re edging in to extended power outages and more debris territory, and we’re getting close to the point where folks in mobile homes here on the coast need to think about seeking shelter in a more sturdy structure.

Aside from the risk of big limbs and the occasional weaker tree coming down in the gusts and rain, the biggest serious hazard I’m seeing at the moment is coastal flooding. The current projections are for the high tide at 7pm tomorrow to brush 9.5 feet, and the 8pm Wednesday to be the highest, right at 10 feet Mean Lower Low Water (MLLW, the datum used by the tide gauges). The current NWS guidance says this:

At 9.5 ft MLLW, minor coastal flooding occurs. Flooding will begin to impact Shipyard Road to Burnside Island. Parts of Ft Pulaski National Monument will begin to flood, including several trails. Flooding will also begin to impact Tybee Island including Catalina Dr and Lewis Ave. In Bryan County, water could breach docks near Ft McAllister and flooding will impact portions of Mill Hill Rd. In Liberty County, flooding impacts the Halfmoon Landing area and Cattle Hammock Rd near Bermuda Bluff subdivision. At 10.0 ft MLLW, moderate coastal flooding occurs. Shipyard Rd will be impassable, isolating residents on Burnside Island. Water will start to encroach on HW-80 and as the tide gets closer to 10.5 ft MLLW, could begin to cover portions of the roadway. Flooding will expand on Tybee Island and Catalina Dr and Lewis Ave will be impassable. Flooding will also impact Wilmington Island, the Coffee Bluff community, Ossabaw Island, Sapelo Island, and portions of HW-17 south of Darien.

As before, Brunswick should see impacts earlier (before dawn Wednesday), and Beeufort a bit later than Savannah, which should see tropical storm force winds arrive mid-morning on Wednesday, and depart sometime early Thursday morning.

Next big update will be in Tuesday morning, when hopefully some of this will be a bit more nailed down, as by then Idalia should be a hurricane, and the tracking should settle down.