Hurricane Season Off to Slow Start
No signs yet of season heating up
By Dr. Gregory Postel, Tropical Weather Expert
The long-range forecasts for the 2009 Atlantic Hurricane season have thus far come to a general consensus. They nearly unanimously agree on a near-average or slightly quieter-than-average season, with the number of named storms expected to be around 12 and the number of hurricanes about half that.
Now that these statistically based predictions have served their purpose by providing an estimation of what to expect, we can turn our attention to the shorter-term factors that strongly influence what we will actually get. While the developing El Nino may continue to play a significant role in creating a less-than-optimal setting over the Atlantic and Caribbean basins (see Andrew Freedman's post from July 8), there are other atmospheric processes we must watch on a daily/weekly basis that could ignite the otherwise sluggish start to this season.
Let's look at some of the factors that can affect tropical activity on time scales less than a month...
Certain phases of a pattern known as the Madden-Julian Oscillation (MJO), for example, are known to foster tropical cyclone development over the Atlantic. Capable of altering rainfall (and the vertical circulations that go along with it) and wind shear (a change in wind direction with height in the atmosphere that can rip apart developing tropical systems), the MJO's influence over the tropical Atlantic can be linked with upswings and downswings in storm frequency during hurricane season.
The North Atlantic Oscillation (NAO, a north-south oscillation in atmospheric pressure over the Atlantic Ocean) is another large-scale driver of tropical cyclone patterns over the Western Hemisphere. Often cited for its ability to steer hurricanes, the NAO can also at times create a dusty, highly sheared environment hostile to tropical storm formation.
One way to look at the structure and evolution of the MJO and NAO is through the lens of the weather prediction models; those very same models that forecast whether or not it will rain on your town tomorrow. While there is obviously much more to the complicated puzzle of hurricane formation than just knowing the phase of the MJO/NAO (or the status of El Nino, for that matter), the weather models are not yet showing a favorable configuration in any of these factors for Atlantic development.
Despite the fact that the sea-surface temperatures are warm enough for storms to form, and in fact warmer than "normal" over large portions of the basin, the environmental conditions are just too windy, dry and dusty. And they will probably remain that way for at least the next week -- perpetuating the already quiet July. Stay tuned ...
Dr. Gregory Postel is the lead meteorologist for a weather-risk management firm in Overland Park, KS. He earned his Ph.D. in atmospheric sciences from the University of Wisconsin-Madison, and conducted research on factors leading to the development of tropical cyclones. He's an avid hurricane chaser and has been known to drive more than a thousand miles to intercept land-falling hurricanes.
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