Hurricane Irma Should Be Monitored For Possible U.S. Impacts This Weekend/Next Week

As of 8 pm ET, Hurricane Irma is still a strong category 5 hurricane with maximum sustained winds of 185 and gusts up to 225 mph. It has unbelievably maintained that same strength since yesterday, despite some fluctuations in minimum central pressure. It is moving west-northwest at 16 mph and will be skirting the northern coast of Puerto Rico soon.

Forecast Discussion:

The biggest challenge to forecasting Hurricane Irma’s track through early next week remains the various features that either have or will impact its steering. Bermuda ridging over the Atlantic has kept Irma on a westward course, and this feature will continue to play a role on Irma’s movement. This ridge has strengthened and has even built southwestward with time; however, there are two main features to watch that could act to break down the westward extend of the ridge closer to the end of the week.

First, a longwave trough has established itself over the eastern United States with a ridge back to the west. For what it’s worth, this is actually the pattern that is responsible for bringing less humid and cooler air across the eastern half of the United States. Unfortunately, it appears that the trough is going to move out too quickly for it to actually steer Irma safely away from the U.S. east coast; however, as the trough lifts and then propagates eastward with time, it will keep a weakness established between the Bermuda ridge out east and a ridge that will center itself over the Four Corners region and extend into west Texas by this weekend. Once Irma gets far enough west, this should result in Irma taking a hard turn northward. This is actually well-advertised in the model guidance, but there has been a bit of spread over the last few days between the models on when that northward turn will occur. Some models take Irma into the eastern Gulf of Mexico, some northward through the peninsula of Florida, and some skirting Florida along its east coast before taking Irma into Georgia and South Carolina.

Figure 1: This is the latest model representations of where Irma could go.

Although Bermuda ridging may build southwestward over the next day or two, this feature should become less of a dominant steering mechanism at least long enough for Irma to start making the turn northwestward/northward towards the end of the week or early weekend. The second feature to watch will be Hurricane Jose. It’s difficult to say if that will have any influence on the southern extent of the Bermuda ridge as Jose treks in a northwestward direction, but it’s most certainly something to watch closely.

To complicate matters even further, the current ridge that is established over the western U.S. which extends northward into western Canada is going to actually break (imagine a wave in the ocean crashing), which will result in the formation of a cut-off low (the same trough I was referring to earlier). Another, weaker shortwave will be moving southeastward towards Mississippi and Alabama, and ridging may begin building over the northeast by this weekend. All in all, this continues to be a super complex forecast.

Aside from all of these features to monitor, the angle that Irma will be coming in relative to the coast makes for additional challenges. A jog fifty miles west or east can be the difference between major hurricane-force winds along the coast or barely tropical depression/storm-force winds.

Where I Think Irma Could Go And Who Should Be Preparing:

Now that you’ve listened to me spend quite a lot of time talking about the complexity of this forecast, I’m actually going to attempt to make a forecast. We still have the three scenarios on the table that I presented close to a week ago in my earlier articles. Irma could barely make it into the eastern Gulf of Mexico and ride along Florida’s west coast, Irma could ride up or along the eastern Florida peninsula into Georgia or the Carolinas, OR Irma could move just east of Florida and out to sea.

The least likely scenario is for this system to go into the eastern Gulf of Mexico. While Bermuda ridging will certainly continue to influence Irma’s steering, it likely won’t extend far enough westward. With a weakness established between the two ridges and with the possible influence of the eastern U.S. trough (even with it lifting out), this should open the door for Irma to make that turn northwestward and then northward before getting into the Gulf. There was quite a bit of talk in the meteorological community about how if Irma stayed below a certain latitude that it would increase the odds of an entrance into the Gulf, and while that may be true, Irma should gain enough latitude over the next couple of days for this scenario not to unfold. I still advise those along the Florida panhandle and west coast of Florida to closely monitor the latest forecasts regardless, due to the complexity of the forecast.

Figure 2: Hurricane Irma remains over warm waters.

Based on what I am seeing, the most likely scenario is for Irma to make the turn northwestward and eventually northward before reaching the Florida Keys. This would result in Irma riding along the Florida east coast, possibly causing tropical storm to hurricane-force winds for regions closer to the coast. It’s too soon to say if these winds will be tropical storm/low-end hurricane strength or closer to major hurricane-force winds. I’m not saying that because I’m predicting Irma to weaken considerably before reaching coastal regions of Florida, but because a slight difference in track west or east will make a significant difference in impact. That’s why all residents from the Keys to the east coast of Florida need to prepare for this event. Also, residents located in the Bahamas need to prepare to possibly be impacted by a major hurricane.

As Irma rides close to the coasts of Florida and Georgia, the upper-level low over the Southeast, the ridging over the Northeast, and the Bermuda ridge could cause Irma to eventually make a landfall somewhere between the north Georgia or South Carolina coasts. The strength of these features will ultimately play a role in how this evolves. Residents along the Georgia and Carolina coastlines (including North Carolina) need to monitor this situation very closely, and if model guidance begins to consistently support this forecast and the reasoning behind it, vacation plans for late this weekend and early next week will need to be canceled.

Figure 3: These are states that could be impacted by Hurricane Irma. Note: the black line represents the NHC’s latest projected path; however, the buffer around the black line is not the NHC’s cone of uncertainty, just a 2 degree buffer.

Bullet-Point Summary:

• The Bahamas need to prepare to potentially be impacted by a major hurricane. The Florida Keys to the east coast of Florida could be impacted by tropical storm to hurricane-force winds. A fifty-mile difference in track could result in huge differences in impact. Given that we’re about five days before Irma reaches Florida, it’s too soon to make a forecast with that kind of precision, given Irma will trek parallel to the state. In fact, it actually could prove to be a challenging forecast even 24 to 48 hours beforehand.
• While it can’t be ruled out that Hurricane Irma will go into the eastern Gulf of Mexico, it’s highly unlikely. Residents from the panhandle to the west coast of Florida still need to be aware of the latest forecasts in case anything changes.
• Residents along and inland from the Georgia and Carolina coasts need to prepare to possibly be impacted by a hurricane very late weekend into early next week. While it should be understood that this forecast remains to be complex, planning for this event should be ongoing.
• While some weakening could occur, Irma is expected to remain in an environment that will support little weakening through at least the end of the week. It seems unlikely that Hispaniola will majorly impact Irma’s strength.
• We will address other states that could be impacted by Irma over the next day or two.
• Again, as has been stated numerous times, this forecast is complicated. Modifications will have to be made, but there is nothing wrong with preparing for this event, despite the uncertainty that remains.

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