Wednesday, September 15, 2010

2010 Atlantic Hurricane Season... So far

I'd like to take a blog to discuss a little about the 2010 Atlantic hurricane season to date. Prior to the start of the season, the majority of the agencies, universities, and private groups that provide seasonal forecasts suggested that the 2010 season would see above normal activity. This was not a surprise given that ocean temperatures across the Atlantic basin were and still are significantly above normal and we were entering a La Nina phase in the equatorial Pacific which typically enhances tropical cyclone activity in the Atlantic basin. Although seasonal forecasts from NOAA and Colorado State University have shown some reasonable skill at predicting overall activity, currently there is no skill in determining if a season will feature more landfalls along the US coastline or not.
2010 got off to a quick start with Hurricane Alex, shown making landfall in the radar image to the left. This storm made landfall only a 130 miles south of the Texas/Mexico border as an intensifying category 2 hurricane at approximately 02 UTC on the 1st of July. Alex featured a very low central pressure of 946 mb, which is often supportive of a stronger hurricane, but the wind field had not responded yet and had the system had another 12-24 hours over water it would have likely been a major hurricane. In any event, Alex was the second strongest June hurricane on record. Fortunately for the residents along the lower Texas coast a strong ridge of high pressure over the Gulf of Mexico kept Alex from making landfall further north.

Despite the quick start, the season slowed considerably with short lived storms: Bonnie and Colin. Although the tropical wave activity continued to roll off the African continent, waves had to deal with a variety of inhibiting factors. The most dominant was a very dry eastern Atlantic due in part to a significant amount of African dust, known as the Saharan Air Layer (SAL). Several recent scientific papers have focused on the SAL as both an inhibiting factor and a feature which can aid in tropical development. In this case, the SAL appeared to inhibit development by providing a very dry and stable airmass across much of the main development region. Coupled with the SAL is an easterly jet feature which is often present in the middle layers of the troposphere across the eastern Atlantic. This can induce wind shear on a developing tropical cyclone, thus not allowing the system to organize. Also the presence of a parade of upper-level low pressure areas across much of the central Atlantic increased the wind shear levels as well. All of this contributed to a relative lull in the tropical activity from July through mid August.

Activity quickly picked up as Hurricane Danielle developed. I have stated this before but it seem as if the atmosphere switched the tropical cyclone genesis switch into the "on" position. What would ensue was a parade of tropical cyclones most of which developed from tropical waves emerging off the western coast of Africa. These systems are often referred to as Cape Verde's due to the development region's proximity to the Cape Verde Islands. Many of the major hurricanes in our historical record were of the Cape Verde variety. Danielle would go on to become the first "major" hurricane (cat 3 or higher) of the 2010 season but recurve harmlessly out into the north Atlantic. Danielle was followed by Earl (image below) which made a much closer approach to the US before finally making landfall as a tropical storm in Nova Scotia. Earl, like Danielle, intensified to a major hurricane just after passing near Anegada in the northern Leeward Islands. Earl would eventually pass approximately 100 miles east of Cape Hatteras and the outer banks of North Carolina. Up next were short-lived tropical storms Fiona and Gaston.

Up next was Hermine, a storm that rapidly intensified from a tropical depression to a strong tropical storm in 21 hours in the Bay of Campeche. Hermine came right up to the brink of being classified a hurricane at landfall in Mexico, well south of the Texas/Mexico border. Hermine however reminded us of the threats that even tropical storms can pose as the system proceeded northward after landfall into central Texas. Hermine dumped nearly 10 inches of rainfall across the Austin, TX metro area and rainband convection spawned several tornadoes in the DFW metroplex. Tornadoes associated with tropical systems are typically found in the rainband regions and can occur well after landfall. In fact, one of our graduate students and TTUHRT blogger Scott Gunter is currently analyzing data collected from the SMART-radars during the landfall of Hurricane Frances (2004). His dataset focuses on two individual cells within an outer rainband of Frances that contained rotational signatures.

Moving on in the 2010 Atlantic season brings us to Hurricane Igor, Hurricane Julia, and Tropical Storm Karl which were all active at the time this was written. Igor (image to the right) and Julia both developed from vigorous tropical waves almost as soon as they exited the African coast. Both also went through significant phases of rapid intensification. Whereas Karl developed in the western Caribbean, making landfall as a Tropical Storm near the Belize/Mexico border. Igor and Julia are both forecast to turn northward into the open Atlantic and will likely not threaten the east coast of the United States. Although Igor may be a potential threat to Bermuda. At the time this blog was being written, both Igor and Julia were category 4 hurricanes. The last time two category 4 hurricanes were active in the Atlantic basin was in 1926.

Looking back, most of our Cape Verde systems over the past month have fortunately followed a similar track and recurved out into the open Atlantic and have not been significant threats to the United States. Why is this? well hurricanes are steered by the flow of air over a large depth of the troposphere. They typically move around large and deep areas of high pressure (ridge). Well typically during the peak of the hurricane season a well established ridge is present over the central Atlantic, known as the Bermuda-Azores High. Often tropical systems follow the southern extent of the ridge westward as this is the "path of least resistance". If the ridge does not extent far enough westward the system will simply rotate around the ridge and move more northward. This has been the case this season, as a series of upper-level low pressure areas or troughs have passed through the flow and weaken or erode the western side of the Bermuda High, thus allowing tropical systems to follow the "easy" path northward around the ridge. In 2004 and 2005 this was a different story as the ridge extended far enough westward to simply push systems toward the continental United States. Although this is a very simplistic representation of the steering regimes of tropical cyclones it gives a general idea how Cape Verde storms often progress. Predicting this pattern prior to the season is quite difficult, thus estimating the risk to areas of coastline before the start of a hurricane season is a tremendous forecasting challenge. As far as activity for this season, despite the slow start it is well on its way to being a very active year. So far this season has featured 11 named storms, 5 hurricanes, and 4 major hurricanes (Danielle, Earl, Igor, and Julia), an average season has 10-6-2. So with about 45% of the season remaining it looks like the seasonal forecasts of an active year will verify.

In the next blog, I'd like to highlight a little about the ongoing research within our group here at Texas Tech.

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