Keeping A Close Eye on Hurricane Irma

As of 5 pm ET, Hurricane Irma is a category 3 storm (115 mph sustained winds) located at 17.3° N and 34.8° W and is moving west northwest at 12 mph. Irma was only a tropical storm this morning but underwent rapid intensification. Irma has remained in a relatively low-shear environment; however, this kind of quick intensification was actually a bit surprising. While sea surface temperatures were above the threshold for tropical development (above 26°C/79°F) when Irma quickly intensified, these temperatures were not overly warm. While the National Hurricane Center has Irma strengthening further over the next day, I actually wouldn’t be surprised if there is some slight weakening or at the least, no additional strengthening for about a day. There is a bit of dry air out ahead of the system, and as I mentioned, sea surface temperatures are just above the threshold for development. Regardless, Irma will eventually move over much warmer sea surface temperatures (with higher ocean heat content), and given that vertical wind shear should remain light, there’s no reason that this system won’t rapidly intensify into a stronger hurricane, possibly reaching category 4 status over the next few days. To sum it up, after a brief break in strengthening, Irma should strengthen further.

dry air hurricane irma

Figure 1: There is some dry air that Hurricane Irma could temporarily encounter.

We are going to be tracking Irma for a long time, which gives us a decent amount of time to figure out where Irma is going. Also, if Irma were to directly impact the contiguous U.S., it won’t be for at least 10 days, give or take a day. It could even be a bit longer. What I’m basically saying is NOT to cancel any plans just yet, but instead, follow the latest tropical updates very closely if you live near the coast or have vacation plans.

Hurricane Irma projected path

Figure 2: The National Hurricane Center’s 5-day projected path of Hurricane Irma

Hurricane Irma Discussion:

The big question over the next several days will be to determine if Irma could be a threat to the U.S. Along with a mid to upper-level low (northwest of Irma) to the south of the Bermuda ridge creating a weakness, numerous shortwave features will ride along the northern periphery of the Bermuda ridge, keeping this ridge on the weaker side over the coming days. In addition to this, the latest guidance indicates that a longwave trough could develop over the eastern U.S. next week. If at this point Irma were to be far enough west, then it would likely recurve and miss the East Coast. However, it appears that the eastern trough will move out in time, and Bermuda high pressure will build westward, possibly extending along the East Coast. This would increase the threat of Irma eventually impacting the East Coast, but also, some guidance builds the Bermuda ridge far enough west and southward that Irma slides between Florida and Cuba into the Gulf of Mexico. At this point, there are a plethora of scenarios, but two of several things to watch closely will be the strength of the Bermuda ridge and how long the longwave trough remains established over the eastern U.S.

If you’re going to follow the forecast models, don’t actually be too focused on where the models show Irma going at this point. The model guidance will continue to vary quite a bit on exact track, so as stated above, the focus should be on the following two features: the strength of the Bermuda ridge and the longwave trough that will be building over the eastern U.S. Also, keep a close eye on how far north Irma gets over the next several days, because a more northward track increases the odds of either impact to the East Coast OR a recurvature out to sea. If the trough fully moves out and Bermuda ridging builds westward quickly enough, then we potentially have a problem in the Gulf of Mexico.

From the looks of it, Typhoon Sanvu in the western Pacific may play a major role in influencing the downstream pattern over the U.S. in about a week. Sanvu is going to gain latitude with time and make an extra-tropical transition as it gets absorbed into the westerlies. Recurving typhoons in the western Pacific can sometimes cause the jet stream to buckle as excess amounts of latent heat gets transferred to the atmosphere, resulting in a wavy jet stream downstream over the U.S. I suspect this is the cause of the western ridge/eastern trough pattern that will evolve over time, and while this would be a great pattern for steering hurricanes away from the U.S. (assuming the hurricane remains high enough in latitude), the forecast model guidance just doesn’t have this pattern sticking around long enough. Some models lift the trough out quickly but leave a piece of energy behind over the southern U.S., and some fully lift out the trough. The problem is that as soon as this trough moves out, the Bermuda ridge will most likely build westward and become well-established. That’s what we don’t need to happen.

Jet stream hurricane irma

Figure 3: Model guidance shows a western ridge/eastern trough configuration over the U.S. next week.

Bullet Point Summary Of “Stuff” To Watch Over The Next 5 to 7 Days:

  • Watch the strength of the Bermuda ridge and see how the upper level low to the south and extra-tropical systems to the north influence its strength.
  • Watch to see if Irma gains some latitude, given that any amount of latitude gain could increase the chance of an East Coast impact OR a recurvature out to sea.
  • Watch the building trough over the eastern U.S. and see how quickly or slowly it will be moving out of the region.
  • Watch how Typhoon Sanvu influences the downstream pattern in about a week, given that this will probably be the culprit behind the western ridge/eastern trough configuration
  • Understand that models have a difficult time handling such meridional flows (wavy jet stream patterns). Throw a hurricane into the mix, and it gets very complicated.
  • Don’t really bother looking at where the models have Irma going beyond 4 to 5 days at this point in time. Focus on the above bullet points for now.

If you read this article and have come to the conclusion that we have no clue what’s going to occur, then, to an extent, you’d be correct. I could wait several days to write these articles until there is a bit more certainty, but I like to go ahead and introduce the Firsthand Weather audience to what I’m actually watching. I will be referring to features such as the Bermuda ridge, the eastern trough, etc. over the next week to two weeks, so I wanted to go ahead and give everyone a very early introduction to what’s going on. It will only make the forecast better over time and will help all of you with your planning for this potential hurricane.