The following is an analysis of the situation related to the potential landfall of Hurricane Irma this weekend in the United States. Let’s take a look at things currently (as of 5 pm CDT Thursday)-
Hurricane Irma is a violent Category 5 hurricane with maximum sustained winds at 175 mph (150 knots). Numerical models have been struggling on the exact path of the hurricane, but the general scenario is clear; there appears to be high confidence that the hurricane will make landfall in South Florida early-Sunday morning. The National Hurricane Center has the hurricane intensity forecast as a high-end Category 4 hurricane near landfall (max winds 150-155 mph with gusts up to 180-190 mph). This is after the hurricane impacts the southern Bahamas and the north Coast of Cuba with hurricane-force winds Friday-Saturday. After landfall either near or just north or south of Miami (this has been fluctuating in forecasts), the system is expected to advance northward spreading destructive winds, heavy rain, and (along the coast) damaging surf/surge northward up the Peninsula of Florida.
I will not sugar coat this. This is most one of the most serious hurricanes Florida has faced in a very long time. Certainly since Charley in 2004 (Category 4) and perhaps since Andrew in 1992 (Category 5). Enhanced locally by issues such as sea-level rise (climate change) and especially land use changes and urban population growth and land cover changes (geographic changes). In addition sea-surface temperatures (SSTs) were the 3rd highest on record globally for the month of August behind 2016 and 2015 (extraordinary considering no El-Nino in the tropical Pacific…much of the Atlantic basin is running above normal; in the long term trend, SSTs are warming because of climate change, however, shorter term climate variability, including the continued shorter multi-decadal cycle for enhanced SSTs and hurricane activity in the Atlantic contributing). The enhanced SSTs and deep warm waters (oceanic heat content) are leading to Irma having ample fuel to remain extremely powerful for a very long period of time. In fact, it became the longest tropical cyclone ever observed in the satellite era (since 1966) to have a strength of 160 knots (185 mph for 37 hrs straight). It has been a category 5 storm operationally since 8 am EDT Tuesday Sept 5th (now third longest streak on record in the Atlantic Basin as of this post, being out 2004 Hurricane Ivan). All this is from warm SSTs, low wind shear not sufficient to disrupt the circulation and low land interaction with the terrain of Hispaniola, which it managed to largely avoid last night (minus minor inflow of drier air off the mountainous terrain to south of the eye, lowering its intensity slightly).
A HURRICANE WATCH has been issued for much of the coastal areas, including the Keys and Lake Okeechobee (see NHC advisories for updates). Hurricane warnings will likely be issued by tomorrow night as the hurricane continues to approach. Evacuations have been called for portions of the Keys and for Miami-Dade County and other areas. If you are in South Florida and in an evacuation area for storm surge and haven’t evacuated, I would highly suggest you do so or call 311 for aid if you are disabled and have difficulty leaving your home. Surge in portions of South Florida from this hurricane could reach 10 ft or more. This is simply not survivable if you’re along the open ocean coast. It is twice as high as my height (I am 5′ 6″ tall). Surge is dependent upon the track and strength of the storm as well as the tidal heights…and if you stay, you basically put your life in the hands of a lifeless beast…
In addition, I do have serious concern Irma could make landfall in South Florida stronger than forecast. The system currently has much going for it. Thus far, the hurricane has exhibited INCREDIBLE resilience. It’s been unable to be significantly impacted by outside forces (weak vertical wind shear which would hamper weaker systems) or internal fluctuations in the inner core region surrounding the eye where the strongest winds are located. This is why Irma has remain so powerful for so long. In addition, beginning basically tonight into tomorrow, it will be entering a region of increasing sea surface temperatures and oceanic heat content (increasing heat with greater depth) and at the same time, the relatively modest vertical wind shear which has attempted to disrupt (largely unsuccessfully) the “inertially stable” inner core will remain modest or even weaken. This means, unless the system moves farther west than forecast and hits Cuba (which would be a catastrophe for them) before turning north, it would will likely (in my opinion) be a Category 5 hurricane as it approaches South Florida last Saturday night-early Sunday. So at least 160 mph sustained with gusts perhaps over 175 mph within the inner eyewall. The surge would be higher than currently forecast and as a result more lethal along the coast, extending farther inland. The winds would also be more destructive at landfall and with greater extent farther inland.
With that said, regardless, of Category 4 or 5, the south-to-north track up the peninsula as a violent hurricane moving at a relative speedy clip (over 15 mph) is a worst-case scenario at it exposes many significant populations to hurricane-force high wind event. A category 4 hurricane at landfall is a catastrophic storm for all hazards as well. Obviously if you are inland and far from the potential surge areas, be ready for days of outages with the necessary supplies and gas (which is quickly running low in many areas!).
If you are in parts of Georgia and South Carolina…keep track of Irma’s track…while there was some talk of it actually making its first landfall in those states, it appears the Florida chances of first landfall are increasing. However, damaging wind gusts (as well as very heavy rain and inland flooding) associated with the landfalling hurricane may spread northward into these states Sunday night-Monday. Track the latest forecasts and be ready for deteriorating weather conditions by the end of the weekend.
I will continue to have frequent updates on things on Twitter and Facebook. I’ll see about more extensive updates on the blog in the coming few days. Hopefully all this writing is wrong and this system misses Florida completely. There has already been incredible damage in Barbuda, Anguilla, the British Virgin Islands, as well as extensive impacts in the US Virgin Islands and Puerto Rico. The Turks and Caicos Island are suffering a direct hit this afternoon/evening. A truly awful storm we can all hope comes to a conclusion soon. But unfortunately the US may be the last on the list of targets folks in the path must be prepared to action to protect life and property.
–Meteorologist Nick Humphrey
EDIT at 7:50 pm CDT: Excuse the typo in the shear analysis graphic. That should say “Hurricane Irma”.