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Gulf of Mexico disturbance and the June hurricane outlook

By: Dr. Jeff Masters, 4:52 PM GMT on June 01, 2007

A large area of disturbed weather continues over the Western Caribbean and Gulf of Mexico in association with a non-tropical area of low pressure. There is no circulation evident on QuikSCAT, satellite loops, or the Tampa Bay radar. Wind shear is about 20-40 knots, which is unfavorable for tropical storm formation. The shear is expected to remain high over the storm for the next few days, and I don't expect it to develop into a tropical depression. However, the storm has a lot of tropical moisture with it, and it should bring rains of 1-3 inches over western Cuba and much of Florida over the next two days, as well as the threat of 50 mph wind gusts and a few weak tornadoes. A Hurricane Hunter aircraft is scheduled to investigate the system at 2pm EDT today.

Figure 1. Total rainfall from the Key West radar.

June outlook
June is typically the quietest month of the Atlantic hurricane season. The basin averages 0.5 named storms in June. Only one major hurricane has made landfall in June--Category 4 Hurricane Audrey of 1957, which struck the Texas/Louisiana border area on June 27 of that year, killing 550. The highest number of named storms for the month is three, which occurred in 1936 and 1968. In the 12 years since the current active hurricane period began in 1995, there have been nine June named storms. Four tropical storms have formed in the first half of June in that 12-year period, giving a historical 33% chance of a first-half-of-June named storm.

Figure 1. Tracks of all June tropical storms and hurricanes since 1851.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) are still quite cool in June, which limits the regions where tropical storm formation can occur. Typically, June storms only form over the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida, where water temperatures are warmest (Figure 1). This year (Figure 2), SSTs are below average in the region surrounding Florida, so we should expect any storms that do form to occur in the Western Gulf of Mexico or Western Caribbean. June storms typically form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance. African tropical waves, which serve as the instigators of about 85% of all major hurricanes, are usually too far south in June to trigger tropical storm formation. Every so often, a tropical wave coming off the coast of Africa moves far enough north to act as a seed for a June tropical storm. Another possibility is that the disturbed weather area in the Eastern Pacific Intertropical Convergence Zone (ITCZ) will push north into the Western Caribbean and spawn a storm there. This was the case for last year's Tropical Storm Alberto (which may have also had help from an African wave).

Figure 2. Sea Surface Temperature (SST) departure from average for May 31, 2007.

Tropical Cyclone Heat Potential
It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential (TCHP, Figure 3) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. As we can see, there is much less heat energy available this year than in 2005, which recorded the highest SSTs ever measured in the tropical Atlantic. I expect that the TCHP will continue to remain below 2005 levels this year, so we should not see as many intense hurricane as we saw in 2005.

Figure 3. Tropical Cyclone Heat Potential (TCHP) for May 31 2005 (top) and May 31 2007 (bottom). TCHP is a measure of the total heat energy available in the ocean. Record high values of TCHP were observed in 2005. TCHP this year is much lower. Image credit: NOAA/AOML.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart.

Wind shear over the past 11 days (Figure 4) has been very high over North America and the surrounding waters of the Gulf of Mexico and Atlantic. This is very typical for June, when the jet stream is still very active and quite far south. The jet stream will gradually weaken and retreat northwards as summer progresses, bringing lower wind shear and greater chances for tropical storm formation. Right now, the Caribbean is the only region with wind shear low enough to support tropical storm formation. The latest two-week forecast from the GFS model predicts that wind shear will remain high over the Gulf of Mexico for the first half of June, and I don't expect and tropical storms to form in the Gulf the next two weeks. Wind shear over the Caribbean is expected to fluctuate between hostile and favorable levels over the next two weeks, so it is possible we could get a tropical storm forming in the Western Caribbean.

Figure 4. Top: Average wind shear over the past 11 days. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). In most circumstances, wind shear above 20 knots (10 m/s, the blue colors in the top image) will act to inhibit tropical storm formation. Wind shear below 12 knots (6 m/s, the orange colors) is very conducive for tropical storm formation.
Bottom: Departure of wind shear from average for the past 11 days in meters per second. Note the higher than average wind shear values over the Gulf of Mexico, a prime breeding ground for June tropical storms.

Dry air and African dust
It's too early to concern ourselves with dry air and dust coming off the coast of Africa, since these dust outbreaks don't make it all the way to the June tropical cyclone breeding grounds in the Western Caribbean and the Gulf of Mexico. Developing storms do have to contend with dry air from Canada moving off the U.S. coast; this was a key reason why this year's Subtropical Storm Andrea never became a tropical storm.

Steering currents
The steering current pattern over the past few weeks has been much like 2006, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs are frequent enough and strong enough to recurve any tropical storms of hurricanes that penetrate north of the Caribbean Sea. Steering current patterns are not predictable more than about two weeks in advance, and there is no telling if we are in for a repeat of the favorable 2006 steering current pattern that recurved every storm out to sea. It is encouraging to note that in 2006 the steering current pattern locked into place in late May and stayed that way for almost the entirety of the hurricane season. I am hopeful that this pattern will occur again this year, but there is no way of telling at this point.

Recent history suggests a 33% chance of a named storm occurring in the first half of June. Given the current SST pattern and two-week wind shear forecast, the Western Caribbean is the most likely area for a storm to occur. Any storm forming in this region would likely move north or northeastward, impacting Cuba, Florida, and the Bahamas. Due to the high levels of wind shear expected over the next two weeks, I'm forecasting only a 20% chance of a named storm forming during this period.

Radio play
National Public Radio's The Story program will be airing a long interview with me today about my flight into Hurricane Hugo in 1989. The show is carried on NPR stations in MI, WI, IL, IN, IA, MN, NC, NY, VA, and WI, and airs live today at 1pm or 8pm EDT. Check http://thestory.org/Stations for local stations and times. You can also listen live on the Internet at NPR station wunc.org in North Carolina. The host, Dick Gordon, is a very gifted interviewer, and it should be an interesting program.

Last night, I was guest on the Barometer Bob Show. You can listen to a podcast of my 50-minute spiel at http://www.barometerbobshow.com/podcast/.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.