Hurricane season is here!

The hurricane season of 2006 is here! The date June 1 has taken on a notoriety second only to 9/11 in the consciousness of many of us, and the arrival of summer now has an ominous flavor--thanks to the unbelievable Hurricane Season of 2005. As I sat at my desk back on New Year's Day this year writing a blog on Zeta, the 28th named storm of that season, I wondered if the Hurricane Season of 2005 would ever end. Would an endless series of tropical storms develop through the winter, making the traditional June 1 start of hurricane season seem meaningless? Well, I am happy to report that the atmosphere sometimes does behave in a logical and predictable way. We've had a normal five straight months of no tropical storm activity in the Atlantic, leading up to today's official start to the season. And if you're not ready for hurricane season yet, then the Atlantic Hurricane Gods have benevolently granted you an extension to your preparation period--this year's season will have a slow start.


Figure 1.Graph of hurricane frequency for the Atlantic Ocean hurricane season. Image credit: NOAA.

What is typical for June?
June is normally the least active month of hurricane season (Figure 1). There have been 32 named storms in June since reliable records began in the Atlantic in 1944--an average of one every two years. There have been 10 June hurricanes (one every six years), and only two June major hurricanes. One of these major hurricanes was the notorious Hurricane Audrey, a Category 4 monster that killed 550 when it slammed into the Texas/Louisiana border on June 27, 1957. The only other June major hurricane was Hurricane Alma, which struck Cuba on June 8, 1966. Alma moved just offshore Florida's west coast as a Category 3 hurricane before weakening to a Category 2 hurricane and striking the Big Bend region of Florida's Panhandle. Alma killed 90.

Last year, June was four times more active than normal. Two June storms formed--Tropical Storm Arlene, which formed on June 8 and hit Alabama on June 11 as a 70 mph tropical storm, and Tropical Storm Bret, which formed June 28 and hit Mexico the next day as a minimal 40 mph tropical storm. The record for most named storms in June occurred in 1936 and 1968, when three storms formed.

What areas are at risk in June?
As we can see from examining the plots for 1936 and 1968, the Gulf of Mexico and the western Caribbean are the primary regions of formation for June tropical storms. The Gulf Coast, Cuba, and Mexico's Yucatan are the primary targets for these systems. June systems typically form from the remains of a cold front or trough of low pressure that moves out over the Gulf of Mexico or western Caribbean. These systems typically are slow to form, and require two to four days of "festering" before they acquire enough thunderstorm activity and spin to make it to depression stage. The tropical waves coming off of Africa this time of year are too far south to make it into the Caribbean sea, so we shouldn't expect any tropical storms to form in the central or eastern Caribbean.

What about SSTs?
Sea surface temperatures (SSTs) last year at this time were about 2 - 3.5 degrees C above normal, and were the highest ever measured in late May. This year, SSTs are about 1 - 1.5 degrees cooler, but are still above the 80 F threshold needed to get a tropical storm going. All we need is an initial disturbance to start with, and plenty of low wind shear to allow the convection to grow around it. SSTs will not be a limiting factor this June for hurricane development.


Figure 2. The GFS model forecast for June 11, 2006, shows a strong subtropical jet stream continuing to blow over the Gulf of Mexico. The strong winds of this jet will likely create too much shear for any tropical storms to form in the Gulf.

What about wind shear?
High wind shear is going to be a severe impediment to tropical storm formation for at least the first two weeks of June. The jet stream has split into two branches--the polar jet, located over the northern U.S., and the subtropical jet, which is blowing over the Gulf of Mexico. As long as the subtropical jet is blowing over the Gulf of Mexico with 30 - 50 knots of wind like it is now, no tropical storm formation is likely in the Gulf. If we do get Tropical Storm Alberto in the next two weeks, it will have to form in the western Caribbean south of Cuba. Steering currents would then likely take the storm north across Cuba and then northeastward across the Bahamas and out to sea. The Gulf Coast from Texas to the Florida Panhandle will be protected from any tropical storms by the strong subtropical jet steam. I'm predicting only a 10% chance of a tropical storm in the Atlantic by June 15 this year.

The GFS model predicts that the subtropical jet will continue to generate high levels of wind shear over the prime June breeding grounds for hurricanes for at least the next 12 days. After that, I suspect the subtropical jet will weaken, and we will get one tropical storm forming in late June over the western Caribbean or Gulf of Mexico.

Vacation
Given that the next two weeks are likely to be the quietest time in what promises otherwise to be another long and busy hurricane season, I'm outta here. This will be my final "live" blog until June 13, as I'm taking my main summer vacation early. I plan to spend some time at Cape Hatteras before any hurricanes threaten! I've prepared a series of "canned" blogs, mainly Q and A from a newspaper interview I did last Sunday for a Florida newspaper. If Alberto does surprise us while I'm gone, the other meteorologists at wunderground will post the latest analysis here for you.

So long!
Jeff Masters