Irma Heading Into The History Books
I mentioned in Friday’s BMS Tropical Update that Irma would become a major hurricane, possibly a Category 5, near the Leeward Islands. As of this morning, Irma can be found 180 miles east of Antigua and moving at 14 mph, officially a Category 5 hurricane. In terms of wind, Irma is the strongest hurricane in the Atlantic Basin since Felix 2007, and in terms of pressure, the strongest since Igor 2010. Regardless of history and what records might be broken by Irma over the next few days, the fact is that it’s a large and dangerous hurricane not seen in the Atlantic Basin in a long time, and it likely won’t weaken below a major hurricane this week as it tracks closer to the U.S. coastline.
In the short term, Irma will remain a major hurricane as it impacts the northern Leeward Islands later tonight and into tomorrow. These small islands will likely suffer a significant amount of damage as it has been a very long time since a hurricane of this magnitude has impacted these islands (e.g., David 1979 and Sept 1928).
Irma will track into the southern Bahamas toward the end of this week. There is a chance Irma could weaken slightly due to land interaction with the northern Caribbean islands, but the water in Irma’s path has plenty of energy, which will limit any weakening in the coming days.
Meteorological Rule of Thumb
In my last update, I mentioned that Irma’s ultimate track could be influenced by a trough of low pressure that is now moving into the Upper Midwest and will be hanging around the East Coast for the next several days. Below is a great illustration I found by Philippe Papin, a PhD student studying atmospheric sciences at the University of Albany.
— Philippe Papin (@pppapin) September 5, 2017
As Philippe mentions, Irma will be moving along the southern edge of the Bermuda high pressure over the next several days. At the same time, a deep trough of low pressure will be moving to the East Coast. This steering flow around the high and the deep trough will begin to turn Irma to the northwest later this week. However, the combination of the trough of low pressure, which gets elongated along the East Coast later this week, and the strength of the Bermuda high likely won’t allow for Irma to turn north before it gets to the Florida coastline. However, what will likely happen is that it will allow for an abrupt northward turn in Irma’s track later this weekend as the storm is near south Florida, when Irma is on the southwestern periphery of the Bermuda high. This abrupt change in track northward means Irma could significantly slow its forward motion as it turns northward late this weekend into early next week. This increases the chance of major inland flooding due to its slower forward motion.
All of this meteorological assumption is based on an understanding of large scale atmospheric motion 120 hours from now, which is five days. Although five day forecasts are improving, small details in the large scale motion can have a big impact on where Irma ultimately makes landfall. In fact, the NHC track errors on a four and five day forecast are 175 and 225 statute miles, respectively.
— Jim Cantore (@JimCantore) September 5, 2017
According to the University of Albany, which keeps track of model forecast errors, the NHC’s five day forecast error is currently running about 200 miles.
Keep your eye on the ECMWF model, which currently has a very low track error, but overall it’s bound to errors as well. Just look at how the 10-day ECMWF ensemble forecast (an ensemble is a forecast of 52 similar forecasts with different settings) has already evolved for Irma since Friday.
Irma has been consistently defying the modeling of a northward turn up the East Coast, which was the most common model ensemble solution last Friday. Today fewer ensemble forecasts track Irma in between Bermuda and the East Coast.
In fact, as I talked about the recurving typhoon rule in my Friday post, I was thinking of another rule that is taught in meteorology school – don’t forecast against the trend. In this case, the trend has been for Irma to track further westward, meaning the chances of Irma ending up in the eastern Gulf of Mexico early next week are much higher today than even just a few days ago. Regardless, Irma will likely have an impact on south Florida along its forecasted path. Based on the latest forecast, there is an 80% chance that Florida will see a major landfalling hurricane.
Initial Insurance Analog Events For Irma
The probability is high because Irma will be a very large hurricane this weekend, with a large eye and hurricane force winds that could extend out from the center for up to 50 miles. With the southern part of Florida only being separated by just over 100 miles from east to west, it seems very likely that some part of southern Florida will be exposed to hurricane force winds.
At this point, the insurance industry should be preparing for another U.S. landfalling hurricane. To help understand the potential impacts at this early stage, a few analogs come to mind. Donna 1960 could be a good analog to the forecasted track of Irma over the next few days. If Donna occurred today, the Florida wind and surge insured losses could amount to $13.6 billion according to one catastrophe modeling firm. Potentially a more extreme analog might be the 1935 Labor Day Hurricane, which today would cause an estimated insured loss from wind and surge for Florida at around $40.7 billion.
There is still a great deal of uncertainty in the forecast for later this weekend, and there are many different scenarios that could play out. One thing for certain is that there has been a large population growth in Florida with a 27% increase in the number of homes since 2000, according to the U.S Census. The exposure is significant, and if Irma arrives with major hurricane force winds, the current building codes over much of the state will likely be challenged.