Archive | May 2013

Seasonal Tropical Cyclone Forecasts Are Coming In – But Are They Worth It?

Citizen scientists working on Cyclone Center are working with a few thousand tropical cyclones which have developed since 1978.  Beginning just a few years later, Dr. Bill Gray at Colorado State University (CSU) first began issuing forecasts for the number of tropical cyclones that will develop in the Atlantic Ocean for the upcoming Atlantic season (June 1 – November 30 each year).  Since that time, several other groups, including the U.S. National Oceanic and Atmospheric Administration (NOAA), have also developed similar techniques to predict seasonal activity.  With the official start of the Atlantic season just a couple of weeks away, this year’s predictions are in.

Hurricane Andrew

Hurricane Andrew, a devastating Category-5 storm, occurred during an extremely quiet Atlantic hurricane season.

The CSU forecast, issued in April of this year,  predicts 18 named storms (those achieving at least Tropical Storm strength), 9 hurricanes, and 4 major hurricanes (Saffir-Simpson Category 3 or higher).  This is well above the long-term average for the Atlantic.  The NOAA forecast, which relies on similar parameters to predict activity (e.g. warm ocean temperatures, El Nino phase),  puts the chances of an active season at 70%.  Groups in other parts of the world also produce seasonal forecasts for their own region.  For example, the Bureau of Meteorology in Australia issues a national as well as regional seasonal outlooks.   Recently, other groups such as the United Kingdom Met Office have begun issuing “dynamical” forecasts, which explicitly count tropical cyclone-like features in weather models rather than relating environmental conditions to past activity.

Seasonal forecasts receive quite a bit of publicity, despite questions about their skill and usefulness.    Statistical schemes such as the CSU forecast, rely on past connections between environmental factors and TC activity.  They fail especially in predicting extreme seasons, such as the 1995 or 2005 Atlantic seasons, because the models just don’t know about hyperactive years like that.   Dynamical predictions, which theoretically can predict record breaking years since they do not rely on past seasons, have been shown to have better predictive skill than statistical techniques for seasonal TC prediction.

But even if a model were 100% accurate, would it really make a difference?  The majority of systems that do develop into tropical cyclones do not affect land.  Predictions of landfall are made by several groups but have not shown any skill so far.  For any given location of coastline, the chances of a TC impact in any given year are very small.  So if a homeowner hears that the upcoming season will be active, should any action be taken?  Does it really matter if we’re going to get 12 storms this year or 11?  Remember that some of the most devastating hurricane events in U.S. history, such as Andrew in 1992,, occurred during inactive seasons.  In the end, how do seasonal forecasts help society?

One could argue that any publicity that gets people to assess their readiness is good – but I think that most will not do anything.   Perhaps more effort should be invested in determining how the nature of tropical cyclones will change in our warming world.  Cyclone Center is going to provide researchers with new data that will help determine if and by how much the nature of global tropical cyclone activity has been recently changing.  With stronger tropical cyclones predicted in the Atlantic and other parts of the world – along with rising sea levels – time and energy is better spent developing plans for mitigation for the big ones rather than issuing forecasts with little or no value for coastal residents.

– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville

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Diary of 1949 Typhoon Allyn: Part 4 – The end

If you missed it, read the introduction to this article and part 1 and part 2 and part 3.

Admin Bldg. #4 ComMar after Typhoon Allyn. Guam 20 Nov. 1949

Admin Bldg. #4 ComMar after Typhoon Allyn. Guam 20 Nov. 1949

Typhoon Allyn was one of the most destructive to pass Guam. Although the eye passes some distance to the South, damage was severe over most of the island. the following is the conclusion of a diary of events during the 10 hours of the storm’s passage over the island.

“1705K: The eye of the typhoon passed approximately 45 miles to the south of the station between 1615 and 1630K. Our pressure has risen two millibars now and should continue to rise as the storm moves on.

“1720K: An item of interest might be that when the air freight terminal went to pieces, there were several cars wrecked. One in particular, a Ford, is folded up just like an accordion and is one complete wreck. Several other cars are almost as bad. The front end of the terminal building here is beginning to sag and the operations sections has a large hole in it. Doesn’t look good if these winds continue as they are.

“1800K: Forecast diminishing winds and eye passage given to radio at 1715K to be relayed to [other bases]. Courier from Harmon arrived 1730K and was given forecast. Wind at this timeis definitely swinging to south presently southeast at 70 knots with gusts to 85 knots.

“1915K: Forecast given … that winds would be 30 knots by [midnight], present winds SE at 50 knots with gusts to 60 knots.

“2200K: The winds are slowly dying down, but still holding steady around 40 knots with gusts in the 50s. Our pressure is on the rise still and by 6 hours everything should be well under control. Light rain is continuing to fall with intermittent heavy showers.”

So in the time span from noon until 10 p.m., the conditions went from showers to dangerous winds back to rain showers. The result was catastrophic. Nearly $20 million (1949 dollars, about $200 million in 2012 dollars) was done to military installations. The economy of Guam was also severely impacted.  Most – if not all – of several important crops were destroyed: breadfruits (100% lost), vegetables (90%), banana (75%) and so on.