Hurricane Irma – History, Current Status, and Future Impacts

With Southeastern Texas still reeling from Hurricane Harvey, it feels a little surreal to be talking about another major hurricane impacting the United States. But with Hurricane Irma, that may well be the case. In this blog, I’ll give an overview of Irma’s history and some background on Cape Verde cyclones, the current status of Irma, and the forecast for the next several days.

History:

Hurricane Irma first originated as a tropical wave on August 26th over Western Africa, and by August 30th, it became a named tropical cyclone approximately 300 miles west of Cape Verde. It strengthened dramatically from a high end tropical storm into a category 3 hurricane by early August 31st, and has since oscillated between category 2 and 3 strength as eyewall replacement cycles temporarily disrupt the storm’s evolution. The latest National Hurricane Center reconnaissance mission determined Irma to be a low-end category 4 storm with 1-minute sustained 130 mph winds.

Cape Verde is a notorious tropical cyclone formation region, and storms that form near Cape Verde are known as “Cape Verde cyclones.” Cape Verde cyclones tend to be steered by a large, semi-permanent area of high pressure that hangs out by the Azores during the summer.

Since the flow around an area of high pressure is anticyclonic (clockwise in the northern hemisphere, counterclockwise in the southern hemisphere), the Azores High helps initially direct Cape Verde cyclones to the west, with these cyclones often curving northward as they approach the Caribbean.

Depending on the strength and location of the Azores high and other neighboring upper-level high and low pressure systems, these storms can head due west into Central America (like Hurricane Joan in 1988), curve to the NW into the Gulf of Mexico (like Hurricane Ivan in 2004 or Ike in 2008), head into the Southeastern U.S. (like Hurricane Hugo in 1989), or completely miss land altogether (like Hurricane Gaston in 2016). The below graphic from NOAA shows how Cape Verde cyclones tend to travel westward at first but diverge as they approach North America.

Due to their long track over a favorable environment with warm water and little wind shear, Cape Verde cyclones are commonly some of the longest-lived and most intense cyclones in the Atlantic Basin. Unfortunately, they are also some of the most difficult to forecast due to uncertainties in the strength of upper-level high and low pressure systems such as the Azores high. This has been particularly apparent with Hurricane Irma – models have been gung-ho in developing Irma into an intense hurricane, but tracks have run the gamut from into the Gulf of Mexico to harmlessly out into the North Atlantic. Unfortunately, models have recently converged on bringing Irma as a major hurricane into Florida, but there is still a fair bit of uncertainty on that front as well.

Current Impacts:

Hurricane Irma is now a category 4 hurricane with winds of 130 mph. It is currently in a favorable environment for intensification with low wind shear, moist air at mid-levels, and increasing ocean heat content.

Despite this, intensity should fluctuate over the next few days due to frequent eyewall replacement cycles, where the inner eyewall of a hurricane dissapates and is replaced by an outer eyewall, weakening the storm in the process. A NOAA Hurricane Hunter reconnaissance mission earlier today revealed two local wind maximums, giving clear evidence of two eyewalls and thus an ongoing eyewall replacement cycle. Current models to not have the resolution to adequately model eyewall replacement cycles (in fact, our short term hurricane intensity forecasts have not improved over the past 30 years!), but the fact that this storm is entering an area of increasingly favorable conditions for enhancement and is predicted to largely stay over the ocean gives us confidence that it should continue to increase in strength over the next several days.

Forecast:

Over the next 24-36 hours, Hurricane Irma is expected to make landfall at or just north of the Northern Leeward islands as a major hurricane, with life-threatening storm surge, wind, and rain. A trough over the eastern U.S. will move to the northeast, allowing the large subtropical ridge that has been steering Irma to shift westward and keep Irma heading on a WNW trajectory towards the US/British Virgin Islands and Puerto Rico, and eventually Hispaniola, the Turks and Caicos, Cuba, and the Bahamas by the end of the week.

U.S. Impacts:

Models have come into much better agreement regarding the track of Irma over the next couple days, and the majority of them keep Irma just east of the mountainous terrain of Puerto Rico, Hispaniola, and SE Cuba and take Irma into Southern Florida as a major hurricane. Mountains are a hurricane’s Kryptonite, and a track as shown above would allow Irma to maintain strength as a category 4 hurricane or even intensify into a category 5 as it heads for southern Florida. However, it’s worth noting that there has been a substantial trend westward over the past few runs, so I wouldn’t be surprised if Irma ends up tracking over these islands and weakening substantially as a result.

Most models keep Irma as a category 4 hurricane for at least the next four days. After this, models diverge, with some keeping Irma at category 4 strength as it makes landfall by southern Florida, while others weaken it significantly before it encounters the U.S. mainland.

One more thing: some weather geeks out there (myself included) have been closely watching the GFS operational run, which has been repeatedly showing Miami experiencing a direct hit from what would be the strongest landfalling cyclone ever recorded in the Northern Hemisphere.

The good news is that the GFS is an outlier and the situation shown above is extremely unlikely… only a handful of cyclones have ever made it below 900 mb. Unfortunately, even a category 3 storm would do billions of dollars of damage to southern Florida, and a category 5 storm – which is not out of the question with Irma – would be catastrophic.

Bottom line:

Given the favorable conditions for intensification, I’m inclined to think that Irma will remain a major hurricane up to landfall, which is likely to be near Southern Florida. However, the proximity of Irma to the mountainous terrain of Puerto Rico, Hispaniola, and SE Cuba introduces some of uncertainty into the equation, and a slight westward deviation from its current predicted trajectory would result in a much weaker storm.

I’ll keep you posted with the latest developments on this tropical cyclone when time allows. In the meantime, check out the National Hurricane Center and Levi Cowan’s Tropical Tidbits blog/website for updates.

Happy Labor Day!
Charlie