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June hurricane season outlook

By: Dr. Jeff Masters, 7:37 PM GMT on June 15, 2007

An area of disturbed weather with heavy thunderstorm activity has developed along a broad trough of surface low pressure in the western Caribbean between Mexico's Yucatan Peninsula and Cuba. Satellite imagery shows a steadily increasing amount of heavy thunderstorm activity, but no signs of a surface circulation. NHC canceled the Hurricane Hunter flight scheduled for this afternoon, and has not scheduled a mission for Saturday.

NHC no longer thinks highly enough of this system to offer their suite of early model tracks, but the latest runs of the GFS, NOGAPS, and UKMET models all indicate that the disturbance will move north then northeastward over the weekend, bringing heavy rains of 2-4 inches over Central Cuba, extreme South Florida, and the northwest Bahama Islands. Some isolated amounts of up to 6 inches are possible over Cuba and the Bahamas. All of these models indicate the possibility that a tropical or subtropical depression could form by Sunday near South Florida or the Bahamas. There is enough wind shear that an extratropical storm could form, instead, though. Wind shear over the disturbance has decreased from 20-25 to 10-20 knots today, and is forecast to remain at similar levels for the next 48 hours. There is some dry air one can see on Water vapor satellite loops over the western Caribbean and southern Gulf of Mexico slowing development of the system, and this will continue to be a factor that will slow development over the next two days. Surface pressures are not falling over the region, and thus any development is going to be slow to occur. I give the system a 20% chance of becoming a depression. Wind shear is a very high 20-40 knots from central Florida northwards, which should act to keep the top winds of any storm that does form below 50 mph.

Last half of June climatology
The last half of June is usually one of the quietest portions of hurricane season. In the 12 years since the current active hurricane period began in 1995, only four tropical storms formed in the last half of June. Thus, recent history gives us a 33% chance of a last-half-of-June named storm. None of those four storms since 1995 became a hurricane, and hurricanes are quite rare in June. Only one major hurricane has 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 primary breeding grounds for last half of June tropical storms is the western Gulf of Mexico (Figure 1).

Figure 1. Tracks of all tropical storms and hurricanes since 1851 that formed June 16-30. The western Gulf of Mexico is the preferred location for storm formation in late June. Interestingly, the eastern Gulf of Mexico sees the most early June storms.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have remained about 0.5-1.0 �C above average over the tropical Atlantic over the past two weeks. An area of cooler than average SSTs that surrounded Florida in early June has shrunk, and the entire Gulf of Mexico is now warmer than average. However, while SSTs are above normal, they are still far cooler than the peak temperatures that occur in August-October. This will limit the regions where tropical storm formation can occur this month to the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida, where water temperatures are warmest (Figure 2). 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.

Figure 2. Sea Surface Temperature (SST) departure from average for June 14, 2007. Image credit: NOAA.

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 less heat energy available this year than in 2005, which recorded the highest SSTs and TCHP 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 June 14 2005 (top) and June 14 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 still quite high, but lower than in 2005. 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, top image) has been above 20 knots over most of the breeding grounds for June tropical storms--the Gulf of Mexico, Western Caribbean, and Bahama waters. While the shear has been below average (Figure 4, bottom image), any wind shear above 20 knots is high enough to discourage tropical storm formation. This is very typical for June, when the jet stream is still very active and quite far south. The jet stream will gradually weaken as summer progresses, bringing lower wind shear and greater chances for tropical storm formation. The extreme southwestern Caribbean has seen shear below 10 knots, but no tropical waves or remains of old cold fronts have moved into this region to trigger tropical storm formation. The latest two-week forecast from the GFS model predicts that wind shear will be near normal levels across the Caribbean, Gulf of Mexico, and tropical Atlantic for the remainder of June.

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 that wind shear has been below average over most of the tropical Atlantic and Gulf of Mexico over the past 11 days.

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, though.

Steering currents
The steering current pattern for the first half of June featured a pattern much like we saw in 2006, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. I expect this pattern to continue for the remainder of June, and the troughs should be 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. The atmosphere often stays locked in to a particular steering pattern for an entire summer.

Recent history suggests a 33% chance of a named storm occurring in the second half of June. Given that the current SST pattern and two-week wind shear forecast look fairly typical for June, and we have a system out there now that has a small chance of becoming a named storm, I'll go with a 40% chance of a named storm forming during the last half of June.

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.