Forecasting hurricanes: Part 1
How good (or bad) are today's forecasts?
This is the first of a two-part series on hurricane forecasting. Part 1 details how reliable forecasts are expected to be when (and if?) the 2010 tropical season finally picks up steam. Part 2, to appear next week, will look at ongoing research into the development of tropical systems and the prospects for improved forecasts.
As reported in an earlier post by CWG's Greg Postel, the National Oceanic and Atmospheric Administration's latest update of its 2010 seasonal hurricane forecast calls for a significant chance that the remainder of the season will be very active, perhaps one of the most active on record. Whatever the number of tropical storms and hurricanes -- collectively referred to here as tropical cyclones (TC) -- most seasonal hurricane forecasts justifiably caution that it's impossible to reliably forecast when and where an individual storm might develop, nor its intensity and whether it will make landfall, before a storm even exists.
The specifics of genesis, strength, size and track of TCs fall in the domain of daily weather. As such, predictability is limited in theory and practice from a few hours to about a week or two at most (no one really knows for sure). These specifics, of course, are precisely those which are required by emergency managers who, for example, must decide if and when to order evacuations, and those which ultimately determine the impact of a TC on lives and property.
So, what are the current capabilities and limitations in the accuracy and utility of TC forecasts?
The foundation of the TC prediction and warning process is the track forecast. If the track forecast is off, so too will be predictions of other parameters, such as wind speed and direction, rainfall, and storm surge relative to landfall. This would be true even if it were possible to accurately forecast the distribution of winds, precipitation, etc., relative to the storm center. In reality, however, this is not possible (discussed below), which compounds the significance of track errors and further undermines the value of all aspects of the TC forecast.
The good news is that track forecasts -- on average -- have steadily improved. As seen in the chart below, the 72-hour average track error for the Atlantic Basin decreased between the 1970s and 2000s from about 385 nautical miles to 155 nautical miles. Following the lower dotted horizontal line from right to left shows that three-day forecasts are as skillful now as 48-hour forecasts were during the 1990s, and more accurate than 36-hour forecasts of the 1980s. Forecasts to five days out did not begin until 2001. Following the upper dotted horizontal line from right to left shows that the current decade's five-day track error (about 265 nautical miles) is somewhat less than that of the 2.5-day forecasts of the 1980s.
This overall improvement in prediction of TC tracks is an impressive accomplishment attributable to advances in observational systems and weather prediction models and, just as important, forecasters' improving ability to balance the capabilities and limitations of observations and models in the process of formulating official forecasts.
While these improvements in the average skill of track forecasts indisputably reflect major progress, they hardly can be considered good enough for having a high level of confidence on what, if any, action should be taken as a TC moves toward possible landfall. Moreover, the average errors obscure the fact that there is considerable variability in track forecast errors within a season. To account for this, National Hurricane Center (NHC) forecasts are displayed in the form of a "cone of uncertainty."
The width of the cone is statistically based and set so that it encompasses two-thirds of the track errors over the most recent five-year period. To put it another way, there is a 33.3% chance the actual track will fall outside the cone. For the 2010 tropical season, the width of the cone at days one, three and five is 124, 322 and 570 nautical miles, respectively. Needless to say, especially beyond a couple days ahead, the inability to be more precise in the forecast track generally requires the warning area for coastal areas to be considerably larger than the area ultimately affected. This often results in costly "over warning" (e.g., unnecessary evacuations) given the priority of safeguarding lives and property. (Informally and unofficially, this "better-safe-than-sorry" approach in the official track forecasts is frequently referred to as the "path of least regret.")
Note: While some storms are intrinsically more or less predictable than others, the cone of uncertainty's fixed width from storm to storm precludes it from communicating variability in the degree of uncertainty from one storm to the next. However, advances in ensemble prediction systems -- which produce an array of possible storm tracks by running the same model multiple times, each time with slightly different initial conditions -- will likely be factored into operational forecasts in the relatively near future (more on this in part 2 of this series).
While there have been notable improvements in predicting TC tracks, little progress has been made over the years in mean errors of intensity (wind speed) forecasts. For example, the average error in the 72-hour intensity predictions during the 2000s was insignificantly different from that in the 1990s (21 mph). Most importantly, the average errors do not reflect the very much larger errors when storms undergo rapid changes in intensity -- winds increasing or decreasing by more than 35 mph in 24 hours (approximately a two-category change in the Saffir-Simpson scale).
With virtually no ability to anticipate rapid changes in strength much more than possibly a few hours in advance, a run-of-the-mill TC approaching the coast could become a monster with virtually no warning and, thereby, put coastal communities at grave risk. Or, the imminent threat of a monster storm resulting in massive evacuations, for example, could become nothing more than a relatively minor annoyance (except for the outcry from citizens unnecessarily stuck in massive traffic jams heading away from the coast -- as they contemplate throwing brickbats and worse at forecasters).
By way of example, in October 1995 Hurricane Opal featured both an unexpected rapid intensification and weakening over the Gulf of Mexico as it approached the Florida Panhandle. Between Oct. 3 and 4, Opal intensified from a marginal hurricane (75 mph winds) to a 150 mph Category 4 major threat. But, within just a few hours of landfall, maximum winds fortunately decreased to a more moderate, though still dangerous, 115 mph. (aside: subsequent to landfall, Opal continued northeastward and spawned three tornadoes locally in Charles, Prince George's and Anne Arundel Counties. I was an eyewitness and almost victim of the Prince George's tornado as it lifted my car about a foot high before jolting it back to the ground).
It's important to recognize that the category of a storm describes only the maximum wind speed (about 30 feet above the ocean surface) anywhere within the TC. It conveys no information on the spatial detail in the size and structure of a storm, such as the radial extent and distribution of hurricane-force winds, nor on precipitation and storm surge relative to center of the TC. Suffice to say, accurate and reliable predictions of these aspects of a TC are currently beyond the capabilities of operational models and forecasters. However, in order to provide some possibly useful information in forecasts of intensity and other critical parameters (such as just mentioned), NHC now provides information on uncertainties in the form of probabilities derived from the historical record of forecast errors.
Obviously, much improvement is necessary in TC predictions to ensure timely issuance of TC watches and warnings and, thereby, reduce to the extent possible the loss of life and injury, as well as property losses and economic disruption. Intensive research efforts are currently in progress, including three major programs this summer, to address this need. The essential aspects of the research and where it might lead -- to the "Holy Grail" of hurricane research? -- will be the subject of part 2, coming next week.
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