Alberto analyzed; June tropical outlook

Alberto has been downgraded to a tropical depression, and continues heading northeast across the Carolinas towards the Atlantic Ocean. The storm is undergoing the transition from a tropical system to an extratropical storm, and is expected to intensify into a powerful non-tropical low pressure system with 50 mph winds on Thursday once it moves out over the open Atlantic. The main threat from Alberto today remains heavy rain and tornadoes, and several tornado warnings have already been issued today for coastal North Carolina. Six tornadoes touched down in South Carolina yesterday, causing minor damage and some injuries. Alberto pushed a storm surge of 4-5 feet in the Big Bend area of coastal Florida, but no significant damage from this flooding has been reported. Perhaps this large surge surge from a mere 50-mph tropical storm will make local planners leery of permitting a controversial 7,000-unit condominium complex to be built in Taylor County where Alberto came ashore. I'm all for sensible development, but building in coastal wetland subject to large storm surges is certainly not sensible--especially with hurricane activity in the Atlantic expected to be higher than usual for at least 10-20 more years.

Analyzing Alberto's life
Alberto formed from a tropical wave that moved off of the coast of Africa on May 30. The wave tracked farther north than usual for June, entering the eastern Caribbean on June 5, and the western Caribbean on June 8. The wave interacted with the unsettled weather of the Intertropical Convergence Zone (ITCZ), which has been able to push unusually far north for this time of year. The interaction between the ITCZ and the African wave produced Alberto on June 9. It is uncommon for a June tropical storm to form from an African wave; usually, the left-over remains of a cold front or trough of low pressure serve as the seed for June storms. However, last year's Tropical Storm Arlene also formed from an African tropical wave at about the same time of year. It's worth noting that both the GFS and Canadian models made very good forecasts of the genesis of Alberto. The best track forecasts were made by the GFS model, but the official NHC forecast outperformed all the models.


Figure 1. Track of Tropical Storm Alberto (with winds speeds in mph plotted every six hours) overlaid on a plot of Tropical Cyclone Heat Potential (TCHP) from June 12, 2006. The TCHP is a measure of the total heat content of the ocean, and high values of TCHP have been shown to aid hurricane intensification. In this image, the high heat-content waters of the Loop Current are visible as the lighter shades of green extending from the Yucatan Channel northward into the Gulf of Mexico. Note that Alberto spent much of its life over the Loop Current. Image credit: NOAA/AOML.

Alberto struggled over its entire life with wind shear of 20-30 knots overhead. Why, then, was it able to put on a surprising burst of intensification on Monday morning over the Gulf of Mexico? One possibility is that the a brief lull in the wind shear allowed Alberto to take advantage of the warm waters of the Loop Current. As seen in Figure 1, the Loop Current was pumping a long tongue of waters with high heat content into the central Gulf of Mexico. Alberto spent much of its life over this high heat content water. Just as Alberto moved away from the Loop Current, wind shear appeared to drop by about 10%, based on satellite estimates I viewed at the University of Wisconsin's CIMSS site. This small relaxation in shear may have been enough to allow Alberto to take advantage of the warm Loop Current waters and put on a burst of intensification. Shortly thereafter, the shear increased by 10%, Alberto left the Loop Current, and the intensification stopped.

Tropical outlook for the rest of June
Past history has shown that an active June in the Atlantic has no correlation with hurricane activity later in the season. However, the model forecasts over the past few days from the reliable GFS, NOGAPS, and Canadian models are showing a weather pattern more typical of mid-July developing over the tropical Atlantic. This may make for a exceptionally active June. The Intertropical Convergence Zone (ITCZ) is forecast to be far more active and further north than usual, and the GFS model has been predicting that one or two tropical cyclones may form in the mid-Atlantic from African waves interacting with the ITCZ. This is almost unheard of in June. Wind shear over the Caribbean and tropical Atlantic is expected to be much below normal, and with sea surface temperatures 0.5 - 1.5 degrees C above normal, it would not surprise me to see two more named storms this June. One saving grace is that the subtropical jet stream is expected to stay active and relatively far south, with should act to bring hostile wind shear to any storm that might move into the Gulf of Mexico. In addition, a series of strong troughs are forecast to move across the Atlantic Ocean the remainder of the month, which should act to recurve any storm that might form there away from land.

While there is nothing threatening looking out there today, we should keep an eye on the ITCZ just off the coast of Africa south of the Cape Verde Islands, and the region just north of Panama, in the coming days.

Jeff Masters