Historic Day in Hurricane History
Today will go down as a fairly historic day in hurricane history on many fronts. It is still too early to determine what the insured losses from Irma in the U.S. will be, but the indications from modeling companies suggest they could be historic. Irma made landfall in the U.S. between Big Coppitt Key and Big Bine Key at 9:10 a.m. EDT, and is now just hours away from another landfall along the western coast of Florida as a Category 4 hurricane with winds of 130 mph and a pressure of 929 mb. In fact, only six U.S. landfalling hurricanes have had a pressure of less than 928 mb, including some well-known names such as Camille, Andrew & Katrina most recently. To my knowledge, this is also the first time in recorded history that the lower 48 states have had two Category 4 hurricanes make landfall in the same year. Also, it is interesting that Irma is making landfall on the same day and in the same general area that Hurricane Donna did in 1960, which I used as an analog earlier in the week. Lastly, today is September 10, which is generally considered to be the climatological peak of the Atlantic hurricane season. I guess that could be viewed as good or bad.
Irma’s Size and Winds
Irma is still a large hurricane with hurricane-force winds extending 80 miles from the center of the storm. With Florida being about 100 miles at its southern tip across Alligator Alley, most of the southern part of Florida, including Miami, will experience hurricane-force winds. As Irma tracks northward, the far eastern part of the state likely won’t experience hurricane-force winds. However, with tropical storm-force winds currently extending 220 miles from Irma’s center, the entire state of Florida will likely experience tropical storm-force winds as Irma tracks northward over the next 24 hours.
A few things to keep in mind, however, are that most of the weather models don’t pick up on the very intricacies of the friction/roughness that occur at the surface. These intricacies can have a profound effect on the wind speed observed at a particular location. Some types of surface friction, such as an urban environment, can speed up winds due to the Bernoulli effect down city streets.
Although Hurricane Matthew impacted the eastern part of the state just last year, the full state has not had a storm of this magnitude in a very long time. This is important because there are many trees and structures that, for the lack of a better term, have not been cleaned out. Storms are a way to clean out older weaker trees and structures, and this will likely compound the amount of damage that occurs across Florida. In fact, social media is already showing many images of tree fall from the weaker outer bands of Irma, and this will only get worse as the winds increase over the next 24 hours.
Given that Irma’s track is expected to go right up the west coast of Florida, the major cities along the Gulf coast are likely to experience the highest winds Irma has to offer since the right side of the storm moving northward is always the strongest. Matthew last year had a different effect, where the east coast of Florida saw winds from its weak side.
Irma has currently wobbled east of the NHC official forecast track, which means that it will likely make landfall around Naples, Florida at approximately 12:00 p.m. locally. However, with an eye that is 20 miles wide, many cities could experience calm conditions as Irma’s eye passes very close to the coastline on its northward track. This means that the high winds will come from many different directions over the course of the day, causing the possibility of additional damage. It should be noted that in the past with some hurricanes that are close to land, friction along its path could cause the path to deviate right with jog to the left, as Irma did along Cuba’s northern shoreline. This is not well understood, but it is a possibility as Irma tracks northward.
— poweroutage.us (@PowerOutage_us) September 10, 2017
Hurricane Hermine just last year showed us that a great deal of tree fall and long lasting power outages can occur with even relatively weak Category 1 winds. The Big Bend region in the northern part of Florida has a high density of tree cover that will compound the power outages and wind damage related to tree fall.
Storm Surge Impacts
Irma’s current track position to the west of Florida is not ideal for creating large amounts of storm surge along the Florida’s west coast. This is because much of the water is being pulled away from the coastline as Irma tracks north. The back side of Irma will likely bring the largest storm surge as the winds switch to onshore flow, allowing water to build up along the western coast of Florida.
— Kait Parker (@WeatherKait) September 10, 2017
The highest storm surge forecast is currently found along Caple Sable island to Captiva, which is more southern facing, allowing for the water to build up along the coastline as Irma pushes northward. Surge values here could be over 15 feet above ground level. Storm surge into Tampa Bay is not the worst-case scenario for a Category 4 storm tracking towards Florida. Surge values here are expected to be 3 to 6 feet above ground level, causing some flooding in urban areas that have already been evacuated.
As expected, street and coastal flooding is already occurring in Miami. This should not be a surprise, as the Miami area floods even in high king tide situations. However, Irma’s rainfall and east side surge are compounding the problem as up to 15 inches of rain is still forecasted for much of the state.
Longer Term Forecast
Irma will track up the western coast of Florida over the next 24 hours, after which it will then move up into Georgia and Alabama early next week. During the middle of next week, the remnants of Irma could stall out over the Ohio river valley and create flooding issues.
Jose will loop up into the Bermuda Triangle, but it is still too early to determine whether it will impact the southeastern coast of the U.S. The probabilities of this occurring are low based on the current ECMWF ensembles.