Idalia update, Monday morning 28 August

TLDR: The forecast track has shifted south and east, and the forecast intensity has gone up as well. Idalia is increasingly a threat to Central Florida. Potential impacts to Coastal Georgia have not changed significantly from yesterday’s estimates, details below, including some Science!

As always, your best “quick overview” of for any storm in the Atlantic are NHC’s Key Messages regarding Tropical Storm Idalia (en Español: Mensajes Claves). Overnight Idalia continued a slow consolidation and strengthening, and is now a strong tropical storm on its way to becoming a hurricane later today or tonight. Hurricane watches are in place for most of the west coast of Florida. If you are those areas please follow the advice of your local emergency management officials. Idalia has the potential to be a nasty storm.

Here is my TAOS/TC model run based on this morning’s NHC forecast. Looking in order of “where, when, how bad,” the “where” part has continued to shift south a bit. But recall that hurricanes are not points – they are more like lopsided plates, with the worst conditions to the right of the direction of motion in the northern hemisphere. So in this case, with a direction of travel to the northeast, the worst conditions will be to the east and south of the track …

click to enlarge.

“When” hasn’t changed too much: tropical storm conditions should reach the coast Tuesday morning, spreading across much of the northern third of the peninsula, from Orlando to the Georgia border.

It’s the “how bad” that has changed the most overnight. Recall from yesterday (and the discussion of the tropical depression that hit Texas two weeks ago) that the Gulf of Mexico is extremely warm; in fact so warm that all things being equal, there is enough extra energy that a given storm could in theory be a full Saffir-Simpson category stronger this year than an average year! So there is enough energy out there to support whatever intensity the rest of the environment will allow. The thinking was that there would be enough wind shear to prevent development much more than a Category 1, however, upper air conditions now seem to be more conducive for development.

One aspect that is mentioned by NHC in their discussion is a phenomena called upper air difluence. To understand what that means you have to know a little about how a hurricane works. Air spirals in to the center (the eye) of the hurricane at the surface, then spirals up in the eyewall, releasing energy (in the form of wind). But that air has to go somewhere – at the top of the storm, above 30,000 feet, the air spirals outward. One way hurricane development is inhibited is when that outflow of air is blocked. Difluence means there is a divergence in the upper air patterns that allows the rising air from the storm to more easily get away from the storm, meaning the storm can intensify. In this case, a major hurricane (Category Three) seems very possible.

Comparing with yesterday afternoon’s runs, the damage estimate has tripled and is approaching $10 billion dollars in some cases. On this track, storm surges over 3 meters (10 feet) are likely in the Cedar Key, Crystal River (including a direct hit by the eyewall on the the Crystal River Nuclear Plant), down through Spring Hill. A bit further shift south and the Tampa area starts to get increasingly severe effects, so that bears watching. There are evacuations being ordered across the region, so if you are in Florida in an area vulnerable to flooding, or in a weaker structure like a mobile home, please take precautions.

For coastal Georgia, this track presents some challenges. Look again at the map above and notice how the impacts are not centered along the track, and how quickly impacts drop off moving north as opposed to south and east. This means that even a few miles shift means drastically different impacts for the Georgia coast. Assuming this forecast is correct (and beware, I expect more shifts today as the storm begins the more direct northward motion), the picture isn’t really that different from that outlined for the Savannah/Beaufort area. The most likely scenario is still inconvenient, messy, hazardous in a few places where it usually floods, the inevitable power outages (but not severe network damage). With common sense not terribly dangerous unless you are very unlucky (which can, as we saw a few weeks ago, happen even in a run-of-the-mill thunderstorm). Only those in areas that have a history of flooding, right on the coast/marshes, or in unsecured mobile homes are at risk. Everybody else should be able to safely ride this one out. I wouldn’t expect evacuations other than perhaps around Brunswick, but that is a political as well as a public safety decision, so who knows. Expect a lot of businesses and schools to be closed Wednesday for sure.

The time to start making decisions will likely be this afternoon into evening for the GA/SC coast. The “exiting” coast (from Cape Canaveral north) will then be close to the 36 hours before tropical storm winds, and I would expect watches to start to be raised by NHC, and local emergency managers to start doing their thing. That’s plenty of time to take care of things tomorrow (Tuesday), in anticipation of conditions deteriorating Tuesday night, with the worst of it in the Savannah area mid-day Wednesday, and clearing out by Thursday. Brunswick on this track be of more concern, being closer to the center of the storm, and will see earlier and more severe impacts (overnight Tuesday). By far the biggest concern will be coastal flooding, since tides are already running above normal (full moon is Wednesday, an it is close to perigee, and worse, the equinox is in a couple weeks so it’s higher due to sun-moon alignment). Onshore winds starting tomorrow will start to build up water in the marshes and along the coast that can’t drain out, so that’s not a good thing.


  1. Always such great and sane information. As a coastal GA resident, I so appreciate you!

  2. Thank you for providing such clear and complete information! I just listened to the Weather Channel for 20 minutes and never heard an estimated day when the storm might be here, but not surprisingly, I found it here.

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