The potential impacts of Michael on Florida have become somewhat clearer now that the storm has entered the warm putrid waters of the Gulf of Mexico (sorry folks who live on the Gulf, as someone who lives on the Atlantic coast, it just smells funny to me). The short version is that while Michael is still “only” a Category 1 with 90mph winds, this is a potentially serious storm that could reach 120 mph or Category 3 strength before landfall. If you live in the Florida panhandle in the warning areas take immediate action to protect life and property!
Not much to fuss over about the forecast track – the major model families agree on the big picture, with landfall near Panama City. Here’s the major models, along with the official track (in Red) and the watch and warning areas noted:
Intensity forecast is tricky. While there are conditions that are favorable for further strengthening, there are also some factors working against it. I’ve heard speculation saying Michael could reach Category 4 intensity (130mph), but I think that’s increasingly unlikely (in fact I doubt it will be much over 100mph at landfall). But for planning purposes people in the warning areas should plan on a Category 3 – if it’s not that strong, super, if more, it won’t likely be much more because NHC is pretty careful about underestimating landfall intensity.
As for impacts, there are three areas to talk about: the 80 mile wide (30mi left, 50mi right) swath right around where the eyewall hits land, the Gulf Coast (especially the Big Bend area), and the inland and Atlantic coast. The landfall area will get hit pretty hard by “traditional” hurricane impacts – extreme winds, rain, waves, and storm surge. If Panama City takes a direct hit it will be ugly, current estimate is $7 to $10 Billion in damage. Of course, wobbles and things breaking that shouldn’t (or stuff not breaking that in theory should) matter for those kinds of estimates. As the storm decays inland trees down, flash flooding, power outages, that sort of thing begins to dominate. Another factor for storms like this are tornadoes – these tend to spawn more than a few. Michael won’t be a Florence/Harvey kind of storm where most of the damage is due to rain flooding after landfall, this is more of a “traditional” 24 hour hurricane event. But that still means flash flooding along the track. The second area to watch are areas south of Perry FL, in the “Big Bend” areal The geometry of the shoreline and strong onshore winds could create some pretty impressive storm surges even away from the usual place you worry about such things to the 80 miles or so to the right of the location of landfall. If you live in coastal areas prone to flooding, pay attention and take action.
The third area to watch are places right on the coast of Georgia, SC, and NC. While they will see gusty winds and rain, places right on the shoreline are at risk of some flooding around high tide Wednesday Evening, and the two tides on Thursday. On the Georgia coast, high tides are already running a foot or so above normal because we are near the Fall Equinox, when the sun and moon are lined up and we have higher than average tides. Add a foot or two of water on top of that and it can cause problems in low lying areas. For example, in the Savannah area, using the Fort Pulaski tide gauge for reference, the average high tide is about 7.5 feet. The “no wind” forecast for the Thursday morning high tide (10:23am) is for about 8.4 feet. US 80 starts to flood around 9.4 feet, and parts of Tybee (14th street) flood at about 10 feet. So if Michael adds one to two feet to that tide, which seems reasonable at this point, US 80 will flood, and parts of Tybee may see some *shallow* flooding at the time of high tide. It won’t be dangerous unless you do something dumb, but will be hazardous and inconvenient.
So you see local knowledge is key here – beware of getting tide gauge references and topographic map datums mixed up! Tide gauges are referenced to something called “Mean Lower Low Water” or MLLW because boats generally care more about not enough water (eg how low) than how high over fears of running aground. Topographic maps used to be referenced to mean sea level (MSL) – which on the GA coast is 3.5 feet *higher* than MLLW. But today topography is usually referenced to the WGS 84 datum, which is close to but not exactly MSL. I often see local TV weathercasters screw this up and scare the crap out of people because they mixed up MSL and MLLW. Your local NWS office (www.weather.gov, then click on your location) usually has information to sort all that out in their “Hurricane Local Statements” and other products.