TD10: the picture three days before Florida landfall

Tropical Depression Ten continues to organize while hovering right on the coast of Yucatan. It should start moving north over the Gulf later today, and as time goes by the situation is certainly evolving, especially the intensity estimates. Here’s what we know as of Sunday Morning …

First, as always, the best official quick summary for any Atlantic storm maybe found in NHC’s reallly nice Key Messages regarding Tropical Depression Ten (en Español: Mensajes Claves). They are one page graphics that summarize the forecast and threats expected from a storm. The “Forecast Discussion” is a bit more technical, but has lots of details in it such as how confident the forecast is and what factors went in to the forecast. And if you like a more “impact driven” assessment, you can always check in here. Will try to do a post on what happens in the Enki offices, but for now, here is what our key summary output looks like, the “plain language” impact swath and economic impact summary, which takes the NHC forecast and tries to figure how what impact the storm will have on people and things in the way:

On this latest forecast track, TD10 should become Tropical Storm Idalia later today (possibly even this morning), and a hurricane by Monday. What is different about this forecast as opposed the the first forecasts yesterday is that Idalia is now expected to continue to strengthen until landfall.

With any storm, there are three major questions: where, when, and how bad. The “where” (which sort of controls both “when” and “how bad”) for TD10 is a bit clearer than yesterday, more concentrated on the “Big Bend” area of the Florida Panhandle. The major track models are more concentrated there, although until the storm moves away from land a bit and has a more distinct center this could shift around some. Here is what they look like at the moment …

The storm should make landfall there sometime overnight Tuesday night in to Wednesday. On that timeline, expect tropical storm warnings to go up in Florida either overnight tonight or early Monday morning. So that’s the “when and where” as best we know it this morning, but how “bad”?

That’s a harder question. Certainly the Gulf of Mexico is a vast pool of warm fetid water this year. There is enough energy in it to support pretty much whatever intensity of storm the rest of the environment will allow. Recently that hasn’t been much: there has been a lot of wind shear – in other words, the winds at different levels in the atmosphere are moving in different directions. That makes it difficult for a storm to form or intensify, since the thunderstorms that make up the storm are torn apart. What has changed in the last 48 hours since we started watching the potential development of TD10 is that a frontal boundary/trough that is passing through the Gulf early next week does not look like it will be as disruptive as previous forecasts indicated. This should allow the storm to develop and, most importantly, rather than begin to decay *before* landfall, continue to develop. Recent forecast trends by the dedicated hurricane models have been for a more intense storm, and NHC (the black line in this graphic) has followed the upper bounds of that trend for the most part:

The “spikes” by the HWRF and HMON models are because they slow the storm a bit more before landfall so landfall is “later” in the time line, at 80+ hours rather than 72 hours for the NHC and other forecasts. The official forecast has landfall now at an 80 knot (90mph) storm, almost a Saffir Simpson Category Two.

So what does all this mean as far as impacts? Actually not terribly different than yesterday’s estimate. The storm is a bit stronger and more organized, but the wind field estimate is smaller, and the landfall location isn’t heavily developed or populated.

Right on the coast near landfall, depending on the intensity, storm surges of a up to 5 or 6 feet above normal high tides can be expected. I wouldn’t expect widespread evacuations, but there will be disruptions across North Florida. There will be power outages, trees down, mobile homes in particular are vulnerable. This kind of system can spawn a tornado or two, so depending on the exact track parts of North Florida and South Georgia might get a few of those, certainly will see watches and warnings. If you end up in an area that has a tropical storm or hurricane watch and warning, please follow the advice of your local emergency managers and take proper precautions, especially if right on the coast or in a weaker inland structure like a mobile home.

For Coastal Georgia and the Low Country of South Carolina, by far the most likely impacts no matter the exact track and intensity are rain and maybe some gusty winds, nothing worse than we’ve had already this summer during thunderstorms, only of a bit longer duration. In Savannah, starting mid-morning Wednesday through Thursday afternoon look rainy and gusty with the worst about midnight; expect that about 6 hours earlier towards Brunswick, and six or so hours later up towards Charleston. The only real concern for these areas are the high tides – spring tides associated with the Full Moon are coming up this week, so several days of sustained wind out of the east can add a couple of feet to already high water levels, so the usual areas that flood can anticipate that. No need to panic over that, again we’ll know more by Monday so if that looks likely time to prepare.

Again, it’s worth revisiting your hurricane plan (link to FEMA), but otherwise this is something to watch but nothing to get super worried over. On my scale, it is “hazardous but not dangerous with a little common sense.”

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