Irma is a historic storm with the potential for historic damage to Florida and perhaps other Southeastern states, depending on its track.

Over the past week we’ve all seen a lot of maps with a lot of possible scenarios for Irma’s eventual landfall here in the U.S.  We’ve seen what this storm is capable of already in the Caribbean.  For the general public, it can be difficult to understand what you are looking at with all the lines on some of those maps.  Many of the lines and the maps are not even that useful and are just shown for illustrating an idea, rather than a specific forecast.

Irma moving through the Atlantic Thursday. NOAA

Forecasters aren’t going to know until sometime Saturday where Irma is going to go with regard to Florida. Even then, there will still be a level of uncertainty. Different models and different versions of the same model are showing meteorologists a variety of possible solutions for where this storm will go.

Hurricane-force winds are in a relatively small area around the eye compared to the overall clouds. Dave Epstein

The eventual exact path is going to determine whether major, perhaps even catastrophic damage occurs or not.  One of the reasons for this is because although the cloud shield of a hurricane can cover hundreds of miles, the inner core of strongest winds doesn’t go out very far from the center, at least comparatively speaking. Travel 50 miles in any direction from the center of the storm, and winds rapidly drop below hurricane force.   

The uncertainty means a much wider area needs to be warned than will actually be impacted by the most ferocious part of any hurricane, including this one.  Also, the wind field will actually expand over Florida, so a large area could potentially see gusts to hurricane strength, but not Category 5 winds.

Winds over 74 mph could be felt over some of the most populated areas of Florida by early Sunday. WeatherBell

Worst Case Scenario

For southern Florida and the big cities of Miami and Fort Lauderdale a track where Irma came onshore around Key Largo would be devastating.  This would bring the strongest winds and the biggest storm surge in that area.  Although building codes have improved over the past 25 years, newer construction remains untested in the force of such a storm.  We’ve never seen a storm this powerful move over such a populated area.

Best Case Scenario

The storm moves east of Florida keeping the strongest winds and storm surge over the ocean.  This would be similar to Matthew’s track a year ago.  The storm would likely then move into Georgia or the Carolinas, sparing the densely populated region of Florida, but obviously being horrible for states to the north.

Other Scenarios

The storm could also move farther west and head up the western side of Florida.  This would, depending on the exact path cause big issues for the western part of Florida, but the storm would weaken inland and pose less of a catastrophic hit farther north.  

A Lot Of Models

You often hear about the two major models: the one from Europe (Euro) and the one here in the United States (GFS).  These are “run” every 12 hours for the Euro and every 6 hours for the GFS.  That’s when we receive a new idea of how the models think the atmosphere will behave.

Not all models are very good at forecasting a hurricane.

Some models try to forecast the hurricane, but aren’t made for this type of forecasting and aren’t really worth looking at to make a forecast, yet they are often shown on TV or on the web.  It’s important to remember this when viewing these so-called “spaghetti plots.”

Meteorologists look for trends in these models in terms of a storm’s track and strength.  We also know certain models tend to have their own biases, such as moving a storm too fast, too far in one direction or even being too wet or dry.   The Euro has been the most consistent model and, while not perfect, I would offer that it does outperform the GFS overall.

The European mode has been the most consistent so far with Irma’s track. (Philippe Papin‏ University of Albany)

Different Versions Of The Same Model

Another tool we are using is called ensemble forecasting.  This takes the same model, like the GFS or Euro and runs it with slightly different conditions.  Think about using the same recipe by changing the oven temperature by a degree or two to see if there is any difference in the outcome.  When the ensembles agree (same outcome) we gain more confidence in the forecast.  The ensembles are still not converging on a singular solution for how the storm impacts Florida.  Notice the map below, all those circles are different spots the ensemble members of the GFS believe Irma will be this weekend.  It can’t be in all those spots at once, but we have to make a forecast to the best of our ability.

The GFS ensembles still have a wide range of possible tracks for Irma. WeatherBell

Consensus Track

The Hurricane Center is taking all of this information and more and coming up with a best guess as to the future track of Irma.  You can see this track shift east or west every 3 to 6 hours when a new forecast is made.  The shifting is based on the models and a lot of meteorologists conferencing on why a particular model may be better and thus have more influence on the consensus track.

The track of Irma is likely to go over south Florida early Sunday. Florida Water Management

The cone of uncertainty takes into account all these variables to let residents know that although there is a predicted, most likely track, it can shift 100 miles or more in a given direction.  I would love nothing more than for this storm to actually stay off the coast and then weaken before making any eventual landfall, but authorities need to let people know of the worst-case scenario because as we just saw in Houston, that can actually happen, regardless of the low probability or not.

Irma is a powerful Category 4 storm Friday morning. NHC

Follow Dave Epstein on Twitter @growingwisdom

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