Irma: Bad, Very Bad

We’re less than 48 hours away from Irma making landfall in Florida. Confidence is now high in a landfall somewhere in South Florida. Currently a category 4 hurricane with winds of 155 MPH (which borderline Cat 4/5), the system has weakened some from it’s Cat 5 intensity it maintained for some time. The environment ahead remains very favorable for at least maintaining its intensity, if not strengthening again to a Cat 5.

The official hurricane center’s track moves Irma into Southern Florida Sunday Morning as a Category 4 hurricane, moving up the Florida Peninsula, into Georgia (still as a strong Tropical storm, or weak Cat 1 hurricane) and then spins down in the Tennessee Valley.

If we look at the guidance today, we’ll see a tight clustering of the models over S. Florida, suggesting a high confidence forecast. This solution is slightly better for Miami, but very bad for areas like Naples and the Everglades. Much of S. Florida will see winds over 100 MPH for several hours Sunday Morning, and as the system moves north, much of Florida will experience wind gusts that are likely hurricane force (74 MPH) (wind gusts through 8PM Sunday picture below).

Inland areas won’t see as strong of winds thanks to Friction. Quick physics 101 lesson: when wind encounters land it slows down, as it runs into trees, buildings, etc…, things that don’t exist over the open water. Friction is the same reason your car slows down if you’re on the highway and take your foot off the gas.

Coastal areas, however, will likely still be experiencing hurricane force winds well after the system moves up the Florida Peninsula.  It’s also why these areas have a big concern with rough surf. Computer models are depicting 30+ ft waves on the east coast of Florida.

Further north, as the system continues to spin down and enter Georgia, some models are still suggesting hurricane force wind gusts (like the European model) and coastal Georgia & South Carolina are likely to experience rough surf & 20-30 ft waves.

Euro model showing 60-80 MPH wind gusts over SE US. Euro sometimes overdoes wind gusts, but the potential is still certainly there.

So while the worst will be contained to southern Florida, quite a large population is expected to see some major impacts from Irma (either wind, rain, storm surge, or some combination).

In Maryland

So you’re probably also thinking  what about us here in Maryland? As Irma continues to spin down, there will be some waves of showers and thunderstorms that will move across the Mid-Atlantic. It will also likely be a little breezy next Tues/Wed. And because of the tropical nature of Irma, I can’t rule out a few brief spin-up tornadoes either. Not a significant threat, but something to watch.

Euro showing some showers from Irma

In addition, rough surf and rip-currents will also be an issue going up the coast next week.

 

Why the challenging forecast?

So a reasonable question is, why has this forecast been so challenging. And the answer has to do with a little piece of energy we call a shortwave in the weather business. This piece of energy will drop south and ‘capture’ Irma to pull it north.

This turn to the north has been the key to the forecast and where it goes, and correspondingly, who sees the worst impacts. A few days ago, that shortwave wan’t very pronounced on the models, and they were having a hard time with it. Part of that is because it was still out over the Pacific where there is very little observational data to input into the models. Remember, the models are only as good as the data we put into them. Now that the energy is being sampled, the models are handling it better, and why we’ve seen an increase in the confidence of the current forecast.

 

Hopefully things turn out better than feared in south Florida & hopefully everyone is following the evacuation orders.

-Andrew