Grading NHC's Tropical Weather Outlook
The National Hurricane Center's Tropical Weather Outlook (TWO) is a text-only product that rates the potential of disturbed areas of weather to turn into tropical depressions or tropical storms. The outlooks are issued four times daily, at 5am, 11am, 5pm, and 11pm EDT. I've found them to be an excellent guide to what to watch out for. But how accurate are these outlooks? To find out, Jamie Rhome and Dan Brown, who are two of the hurricane specialists that write the Tropical Weather Outlook, verified the accuracy of all the outlooks issued in 2005 and 2006. They used a three-tiered classification of threat based on the following language appearing in the TWO:
High: "A tropical depression could form tonight or the next day."
Medium: "Some slow development is possible."
Low: "Tropical storm formation is not expected."
These forecasts were then graded by looking at the "best track" database of Atlantic hurricanes and seeing if a tropical depression formed within 48 hours of each TWO issued. The results, shown below, reveal that for the Atlantic in the years 2005 and 2006:
-When the TWO said, "A tropical depression could form tonight of the next day," a depression formed within 48 hours 53% of the time.
-When the TWO said, "Some slow development is possible," a tropical depression formed within 48 hours about 20% of the time.
-When the TWO said, "Tropical storm formation is not expected," a tropical depression formed within 48 hours only 3% of the time.
Figure 1. Verification of the Atlantic and Eastern Pacific Tropical Weather Outlooks issued in 2005 and 2006. Image credit: Jamie Rhome and Dan Brown, National Hurricane Center.