Watching Irma: September 3, 2017

Strong hurricane approaching the Leeward Islands, expected to strengthen. U.S. impacts? Wait and see

Hurricane Irma on GOES-16 via CIRA. GOES-16 data is preliminary and non-operational.

With another tropical system on the horizon that appears to pose a threat to the East Coast, it’s time to dust off Medium and begin some regular updates. With any luck, this series will end with a whimper — but let’s get prepared regardless.

National Hurricane Center advisory for Hurricane Irma as of 5PM, September 3, 2017.

5 PM Advisory

As of the 5 PM advisory from the National Hurricane Center, Irma is a major hurricane with 115 MPH winds. It’s moving west at 14 MPH. This movement continues into Monday before turning back more toward the west and northwest. Irma is expected to continue to strengthen; NHC has it as a Category 4 hurricane by tomorrow evening. On its present path, the storm poses a threat to the Leeward Islands, Puerto Rico, the Dominican Republic, and then the Bahamas over the upcoming week.

NOAA Hurricane Hunters are flying the first of what will be many missions into Irma this evening. Data from these missions will be used in the models for improved initialization and (hopefully) better forecasts. If following these missions is your jam, Levi Cowan’s Tropical Tidbits is an amazing site for doing just that.

Steering Players

Ocean Prediction Center analysis valid 2PM Eastern Daylight Time, showing high pressure to the north of Irma.

Irma’s largely being steered by a large ridge of high pressure to the north of the storm. The ridge is strong enough to send it a little southwest of due west at times, and it is this southward motion that is worth monitoring closely for impacts a few days from now. The chief concern in trying to figure out this track is that the further south the storm is, the more questions arise whether a trough that is expected to move in from the west early next week will be deep enough to pick up Irma and scoot it out to sea. This is, indeed, the big forecast question (and the one most of you care about). Unfortunately, it is still very much wait and see as everything comes together for what will seem like an eternity.

Dispelling Model Myths

As always, there’s a good bit of model hype going around social media. Let’s defuse some of that, and then get into some of the trends driving the current thinking around Irma’s ultimate path.

Consider operational runs suspect, for now

First, the standard disclaimer: It’s far too early to put much stock into an individual operational run of any of the major global models (yes, even the ECMWF, a.k.a. “King Euro”) for the period we are all interested in — namely early next week (Sept. 10–12), when Irma makes its closest pass to the southeast United States. Run-to-run continuity at that range is almost nonexistent. I guarantee you will see posts with these cherry-picked model maps on social media. Don’t bite. If it looks outrageous, it very well just might be.

A better way: ensembles

This far out, it’s better to use ensembles. In addition to the operational run of a model, each global model is run numerous times with slightly different initialized conditions. The tighter the agreement of the ensemble members, the more confident the forecaster can be in the particular model solution (and vice versa). However, even when using ensembles, the guidance envelope can and often does shift with each runtime, so it’s then important to analyze the trends across several runs of the ensemble members.

Finally, after all this, you have at best a semblance of an idea of what the threat may look like past seven days; however, the limits of skillful hurricane forecasting generally top out around 5 days, and the average error there is very significant — 211 nautical miles. (Average track error is how The Cone of Uncertainty is computed by the National Hurricane Center, by the way.)

So, yes, even with ensembles, we don’t have a good idea of the details for the Southeast yet — and anyone selling you that is full of it. What ensembles can tell us is where the threat may be highest, what scenarios are in play, and what scenarios aren’t.

What We Know

The ensemble runs over the past several days generally spell out the concern quite nicely:

  • A major hurricane will continue moving westward, with an eventual west-northwest turn, toward the United States.
  • Most locations along the Southeast coast — yes, including Charleston — are at risk of feeling effects of this hurricane, based on ensemble runs for the past several days.
  • There are still numerous ensemble members which take the storm out into the Atlantic, with no U.S. landfall.
  • There are still a few runs which take the storm into the Gulf, but this does not look to be a likely scenario at this point in time. It’s not off the table, however.
  • The degree to which a particular location feels effects is still unknown and will be for a few more days. Once we get into more forecastable range on Wednesday, we can really start to get the details nailed down.

Bottom Line

  • Swell from Irma will begin reaching the coast Tuesday. This is the only real clear impact from Irma we in #chswx can see right now. Rip current risk will increase as a result starting then (though Labor Day is looking fine so far). Monitor NWS Charleston, SC’s beach forecasts if you plan on getting in the ocean this week.
  • It is still too early to know additional impacts from Irma. We will have a better idea of where the storm is going after a few Hurricane Hunter missions and the passage of time help fine-tune the models. Give it a few days.
  • It’s not time to panic, but it is time to prepare. Regardless of Irma’s impacts (or lack thereof) on the Southeast coast, it is the peak of hurricane season. Review your hurricane safety plans. Make sure you know what evacuation zone you live in and where you will head if ordered to leave. Check on your hurricane kit and make sure you are all set. Don’t have a kit? Build one. Hopefully, Irma will serve only as a reminder that preparedness for tropical cyclones is an important part of living in our town. And if it serves as more than that, you’re ready!
  • Stay informed. Monitor reliable outlets of weather information, including the National Hurricane Center for official forecasts, for additional information on Irma over the next week. You don’t have to check obsessively — watching your local TV meteorologist is likely good enough! You can also follow along with me on Twitter, Facebook, Instagram, as well as here on Medium as I continue to update those platforms with the most relevant and reliable information I can.

Enjoy Labor Day

Yes, the specter of a hurricane so soon after Matthew is unnerving. I totally get it. But we in #chswx are watching it closely, and you should still enjoy your Labor Day holiday. Temperatures will remain in the upper 80s with lower humidity and rainfree conditions with a good bit of sunshine.

Next update will come tomorrow evening, or sooner as conditions warrant.


Follow Jared Smith’s Charleston Weather updates on Twitter and Facebook. An updated forecast and current conditions are always available at chswx.com, too. Bookmark it on your home screen!