High wind shear over the Gulf of Mexico

April 2006 was the warmest April on record in the U.S. since record keeping began in 1895. The U.S. has now had two "warmest ever" months this year, January and April. The nationally averaged April temperature was 56.5�F (13.6�C), which was 4.5�F (2.5�C) above the 1901-2000 (20th century) mean. Globally, April ranked as the 7th warmest April on record, and the period January through April ranks as the 6th warmest such period on record globally. From Figure 1, we can see that the entire tropical Atlantic region where hurricane formation occurs was warmer than average during April, and this region has remained about .5 - 1.5 degrees C above normal over the past few months.


Figure 1. Temperature Anomalies (difference of temperature from normal) for April 2006.

With all this warmer than normal water over the Atlantic, one might expect that hurricane season could have an earlier than normal start. However, that will not be the case this year, because high levels of wind shear will dominate the regions where June tropical cyclones typically form--the Gulf of Mexico. The Gulf is the primary genesis region in June because that is typically the only region where we get initial disturbances that can set a tropical storm spinning up. These initial disturbances in June are usually the remains of an old cold front or upper-level trough that stalls out over the Gulf and festers for a few days, gradually developing deep convection and spinning up into a tropical depression. June is too soon to get a tropical storm in the Caribbean or tropical Atlantic, since the tropical waves that typically serve as the initial seed for a storm are still too far south. The tropical waves coming off of Africa right now are at about six degrees latitude, and they need to be at nine degrees latitude or higher before they are far enough from the equator to serve as a seed for a tropical storm.

For the start of this year's hurricane season next week, the GFS model (Figure 2) is forecasting that there will be strong upper-level winds over the Gulf of Mexico. These winds are part of the so-called Subtropical jet stream. The jet stream--the band of high velocity winds that circles the globe--always has at least one branch, the polar jet. As its name implies, the polar jet lies close to the pole, and circles it entirely. Sometimes the jet splits, and a branch called the Subtropical jet blows across subtropical latitudes, where the Gulf of Mexico lies. As we can see from the GFS forecast for June 3 in Figure 2, both the polar and subtropical jets are apparent where the color coding indicates strong winds at the 300 millibar level (the jet stream occurs at an altitude in the atmosphere where the pressure ranges between 300 mb and 200 mb). The strong winds of the Subtropical jet will create too much wind shear for a tropical storm to form in the Gulf of Mexico next week, and the jet is expected to remain strong for at least the next two weeks. So, an early start to hurricane season looks unlikely this year.


Figure 2. GFS forecast for June 3 2006 at 300 millibars, the altitude where the jet stream is found. The polar and subtropical branches of the jet stream are clearly visible where upper-level wind speeds are highest. An area of light upper level winds and low wind shear is forecast to develop over the southern Caribbean Sea.

What about the southern Caribbean Sea, where the GFS model is predicting very light upper level winds, and where wind shear is likely to be low? Well, we will have to watch this area for tropical storm formation, but as I indicated before, the tropical waves one needs to act as the seed for a storm are usually too far south in June. Tropical waves usually do not start entering the Caribbean until July.

Have a worry-free weekend, everyone!

Jeff Masters