Hurricane Idalia: Day before Florida Landfall Analysis

Idalia is now a hurricane, and projected to make landfall tomorrow (midmorning on Wednesday 30 August). Here is the latest impact swath based on the 5am ET National Hurricane Center (NHC) forecast, using my TAOS/TC model:

Here is a link to Key Messages regarding Hurricane Idalia (en Español: Mensajes Claves). Let’s take a look at the “where, when, how bad” in order. Where is somewhat straightforward, but there are some nuances to the NHC forecast, and we sort of have to combine looking at “where” and “how bad” to understand the forecast. You’ll understand why this matters, especially for the Savannah area, at the end (hopefully!). Here is what the basic model tracks that NHC is using looks like. Notice the dark red line of the official track (OFCL) is to the right – of the rest of the tracks:

On the NHC track the worst of the storm (eyewall) should make landfall along the more sparsely populated area of the Big Bend between Steinhatchee and Cedar Key. But only a slight wobble to the right takes out Crystal River, and the Tampa/Clearwater corridor is also at risk. This is probably why NHC is keeping to the right of the track envelope – that is the “forecast of least regret” in this case, making sure people in those vulnerable areas take action. A wobble to the left has fewer consequences as it puts the worse of the storm in the Big Bend wildlife management area and a more sparsely populated area – obviously bad for them, but in the emergency management business you have to play the numbers game. But there’s more.

For “how bad”, the problem is that as noted before the Gulf of Mexico has so much energy in the upper layers of the water that a storm, if the rest of the environment is favorable, can very quickly intensify. In this case the expectation is that Idalia will go from a minimal 65 knot (75mph) hurricane to a major Category Three storm with 105 knot (120mph) winds. Recall that kinetic energy (wind in this case) increases with the *square* of the speed, so a 120mph wind isn’t 1.60 times stronger than a 75mph wind (120/75), it is 2.56 times stronger ({120*120}/{75*75})!

Not all of the intensity models show Idalia gaining strength so rapidly, however, our ability to predict intensity has not improved as much as our track prediction. Another aspect, and why NHC is shading their forecast to the right, is that the models that do show the rapid intensification are the ones (especially ensemble members from ECM) to the right. So … if you believe that the storm is going to rapidly intensify (and conditions are ripe for that), you will put a bit more trust in the tracks that also show that. This dilemma gives you a glimpse of the complexity of hurricane forecasting, and why you really shouldn’t trust folks who don’t do this all the time. Tropical Cyclone Forecasting is a complex business, and just like you wouldn’t have a General Practitioner do neurosurgery, be a bit cautious of “every day” forecasters – even good ones – who jump in to hurricane forecasting, especially if they are disagreeing with NHC.

So what to do? If in Florida, follow the advice of your local emergency management. This is a bad storm, and that part of the coast can get high storm surges. The area around Cedar Key down to Crystal River will see 6 to 12 foot storm surges at the coast. So those evacuation orders make sense, even down to the Clearwater area given the potential for a wobble (which would mean a stronger storm – see above). There are a lot of mobile homes in that part of Florida, many of which may not be as secure as they should be. In any event, even inland there and in to south Georgia (Valdosta – Waycross corridor) mobile home residents should seek shelter in stronger structures.

For Coastal Georgia and South Carolina, don’t be duped by the fact that landfall will be in Florida mid-morning Wednesday – given the size of the storm, by then conditions will be deteriorating across South Georgia. Brunswick/Glynn and Camden Counties will see things deteriorating around sunrise, in Savannah a bit later by the 10am to noon time frame, and Beaufort area a couple hours later (noon to 2pm). The worse of it should again progress south to north – peaking in Brunswick around 6pm, Savannah 9 to 11pm, and Beaufort a couple hours after that.

The inland track and “wobbles” will matter a lot. The discussion below is based on the National Hurricane Center scenario, which is what you should use for planning purposes. We *might* not get so much if the storm does not develop that rapidly, or decays faster after landfall, but while we can hope for that, hope isn’t a good plan …

On the most likely track, the storm center will pass just inland of the coastal counties. As you can see in the swath map above, that’s not great, as the worst impacts extend from about 20 miles left of the track to 60 or 70 miles to the right of the track. So, using the NHC track as the worst case, and given how storms decay, Savannah will get tropical storm conditions – 40mph sustained winds in open areas, gusts probably to the 60mph range inland (higher right on the water). That’s enough to cause big limbs to come down, widespread power outages, that sort of thing. Heavy rain, 6-10″, are also possible on that track. Idalia should be fast moving, so while there will be flooding in the usual places in town, it should be in the “bad but not catastrophic” category. So in short, in coastal GA/SC Idalia should be hazardous, you don’t want to be out in it, but not dangerous if you are in a typical residential structure or commercial-residential (apartment complex). Mobile homes are a bit iffy – this is at the threshold where you should consider moving to a sturdier shelter, especially if tornados start to crop up since even a weak one is bad news for manufactured housing. And speaking of tornadoes …

Tornadoes: landfalling storms in the southeast do produce tornadoes as they decay. They tend to be on the weaker side – F0 to F1 – so these are not the Kansas, Wizard of Oz type tornadoes that sweep homes off their foundations. But they do cause damage, and be alert for warnings. This is why I strongly recommend having a battery powered weather radio handy – cell phone and networks often go down in storms, and it’s the *only* RELIABLE and timely way to get warnings.

Flooding: The latest forecast shows the peak water levels at the Fort Pulaski tide gauge at about 10.5 ft MLLW (remember, that’s only about 3 feet above our normal high tides), so that’s the threshold of “major coastal flooding”. The GFS based tide forecast shows 10.1ft tomorrow at 9pm (high tide). According to the NWS/NOAA guidance …

• 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.
• At 10.5 ft MLLW, major coastal flooding occurs. Damaging flooding is expected, expanding along the entire southeast Georgia coast. Flooding will likely cause the closure of HW-80, isolating residents on Tybee Island. Several other island communities will also likely become isolated due to flooded and impassable roadways. On Tybee Island, widespread significant flooding is expected with numerous properties impacted.

Irma and Matthew were both well over 12 feet, so we’re not expecting that level of flooding.

Check local media for closures and emergency manger guidance. Most of the schools will be closed Wednesday and likely Thursday for cleanup. Given the labor day weekend, some may well say forget it and not bother to restart until Tuesday depending on how extensive the power outages and cleanup is. My expectation is things in GA/SC will be back to normal by the weekend except in isolated cases where there was damage. Florida, of course, is looking at weeks of cleanup and recovery if the expected rapid intensification develops.