First tropical wave of the year; Postcards V: Hurricane warning probabilities

The first respectable tropical wave of the year rolled off the coast of Africa into the tropical Atlantic today. While sea surface temperatures are too cold this time of year to support development of this tropical wave, today's event does serve as a reminder that hurricane season begins in just one month. Hurricane season in the Eastern Pacific begins May 15, and tropical waves such as this one sometimes serve as the nucleus for May tropical storms, once they cross Central America and enter the Eastern Pacific.


Figure 1. Visible satellite image from 12pm EDT May 2, 2008, showing the year's first African tropical wave. Image credit: Navy research lab, Monterey.

Hurricane conference news
The 28th Conference on Hurricanes and Tropical Meteorology in Orlando is over. A final postcard from the conference is below, and next week, I'll write some long letters about some of the presentations, focusing the hurricanes/global warming debate.

Hurricane warning probabilities
Hurricane specialist Michelle Mainelli of the National Hurricane Center evaluated the usefulness of NHC's new hurricane wind probabilities product. The product, which was developed via funding from the immensely valuable Joint Hurricane Testbed research project, provides users with information regarding the chances of experiencing winds of tropical storm force and hurricane force at specific locations within the five-day forecast period. They also indicate, in probabilistic terms, the range of possibilities regarding when these wind conditions could begin at specific locations (an important factor in the timing of evacuation orders). While these products are primarily intended for use by emergency managers (for decision-making) and the media (for communication of risk and uncertainty), the availability of the products via the NHC website makes it possible for anyone in the general public to use them for their own decision-making.



One of the significant challenges with the probabilities thus far has been relating them to coastal watches and warnings issued by the NHC. A hurricane watch is issued when hurricane conditions are possible within the watch area, generally within 36 hours. A hurricane warning is issued when hurricane conditions are expected within the warning area, generally within 24 hours (or less in some cases). Between 2000-2006, an area of coast under a hurricane watch actually received hurricane-force winds about 20% of the time, while areas under a hurricane warning got hurricane force winds 25% of the time. Mainelli's research shows that at the first issuance of a watch or warning, the new wind probability product typically gives a 10% chance of hurricane force winds impacting the end points of the watch/warning area. These probabilities increase to 32% at the center of the warning area. Her research will be used to help develop an automatic technique that will recommend to NHC forecasters exactly where to place their watches and warnings.

What's your forecast?
For those of you who have a hunch about how many tropical storms and hurricanes we'll see this year, you can log your forecast as part of our first ever "Wundercast" competition. It's a friendly competition between weather enthusiasts to determine the best of the best in weather forecasting. Don't worry if you do not have any experience in forecasting, learn from Weather Underground meteorologist--as well as experienced website community members--to become an expert forecaster. It's free, and you can sign up at http://www.wunderground.com/wundercast/. The first forecast day is Tuesday, May 13, and competitors will be forecasting for San Francisco, California for the first two weeks. Thereafter, you'll be asked to forecast for Omaha, Seattle, Philadelphia, and Miami.

Jeff Masters