Analyzing Irene’s Forecast

Predicted Track Was On The Mark While Intensity Forecast Shows Need For More Improvement

With clean-up well underway throughout the East Coast of the United States,    some are still wondering what the fuss was all about with Hurricane Irene.  At one point in its journey through the Atlantic, Irene had intensified to a Category Three storm on the Saffir-Simpson Scale.   Fortunately though, the storm began to gradually weaken before moving over the Outer Banks of North Carolina, New Jersey, and New York City.

Overall, the forecast was very good.  The forecast track of the storm from Tuesday, August 23rd to Sunday, August 28th was right on target.  The models were locked in on the Carolinas, Mid-Atlantic, Northeast, and New England, and they were right.  However, there are still flaws in the intensity forecast.  

At the time Irene was moving through the Bahamas, the storm was projected to become a Category Four storm with 135 mile per hour winds.  The system was also forecast to reach South Jersey as a Category Two.  Thankfully, Irene never got to that point, but it will still go down as one of the worst storms to affect the Garden State as well as much of the Eastern U.S..

Initially, insurance damage estimates are at $10 billion dollars, but there are indications that those numbers will be much higher.  As bad as things were though, it could have been much worse.  We emerge from this storm very lucky, but why?  How come the intensity prognostications were wrong?  The reason is because the science of forecasting hurricane intensity is not as good as predicting the future track of such storms.

Forecast Track Improvement

Several days ago, ABC News pointed out that since 1990, the forecast track of tropical storms and hurricanes have improved by 60 percent.  The models have become so good that forecasters are able to hold off on watches and warnings that they would have had to issue in a similar scenario ten years ago.  

For example, lets take a look at another storm that took a similar path as Irene.  Hurricane Floyd came up through the Bahamas parallel to the Florida East Coast in September 1999.  The storm grew to a powerful Category Four storm with 150 mile per hour winds.  Back then, forecasters weren’t sure that Floyd wasn’t going to keep pushing west into Florida and the Southeast coast.  

As a result, the largest peacetime evacuation took place as some 3 million coastal residents evacuated.  It was a very costly exercise.  Fast forward to August 2011, and Hurricane Irene.  In a similar spot, Irene didn’t prompt any watches or warnings from Florida to much of South Carolina.  Only some advisories for heavy surf and rip currents.

At one point during its coverage, the Weather Channel was receiving messages on its Facebook and twitter feeds asking why weren’t there any watches or warnings being issued even though Irene was coming very close to the Florida coast.  The NHC still believed that Irene would begin to turn.  Forecasters have become that confident in their projected tracks.  This confidence in the future storm path ended up saving Emergency Management officials in Florida, Georgia, and South Carolina a lot of time, effort, and money.  

Storm Intensity Predictions Still Prove Difficult

While there have been major advances in predicting the future direction of a storm, more improvement is still needed in projecting its future intensity.  In terms of forecasting the future track, forecasters have a better understanding of the upper level patterns around the hurricane.  Tools such as the NOAA Gulfstream aircraft are able to analyze the upper atmosphere in the vicinity of the storm, and get loads of data that help them better understand where the storm will go and why.

Hurricanes have always been known to be unpredictable.  From the early days of the NHC to today, tropical storms and hurricanes have had a mind of their own.  However, more and more it is because of the changes in intensity these storms take rather than where the storm goes.  While there are models such as SHIPS and GFDL that project future storm intensity, they don’t have all the environmental variables that determine whether or not a hurricane will strengthen or weaken.

One classic case of problems forecasting future hurricane strength is when you have a scenario that involves rapid intensification.  In this situation, you have a modest tropical cyclone in the form of a mild tropical storm or hurricane that suddenly explodes into a powerful Category Four or Five hurricane within 24 to 48 hours time.  Forecasters still struggle with what causes this to happen.  Hurricane Katrina was a prime example of this prior to coming ashore along the Gulf Coast six years ago this week.

Another example is forecasting weakening.  Look what happened this past week with Irene.  While sea surface temperatures were more than adequate and upper level winds were favorable toward Irene staying as strong as a powerful Category Two or Three hurricane when it moved up the Eastern Seaboard, the storm lost its punch and ended up being a strong Category One storm upon landfall along the Outer Banks of North Carolina.  Why did this happen?  The weakening occurred because dry air got entrenched in the storm.  Tropical storms and hurricanes don’t like dry air.

The dry air entrenchment wasn’t anticipated by the models.   Intensity models are not sophisticated enough to anticipate all the different variables and possibilities that can impact the future strength of a tropical cyclone.  There are such things as dry air and warm water eddys such as the Loop Current that scientists are still struggling to understand.  As research improves and computer power continues to increase, more intelligent and robust intensity models will be made to better predict how strong these storms will get in the future.

We have come a long way in our abilities to track and understand how hurricanes and tropical storms behave.  Since 1900, we have developed tools such as radar and satellite that detect tropical storms and hurricanes well out at sea, and computer models that better forecast where these storms will go.  However, there is still more work to be done as far as predicting the future strength of these powerful storms.  The story of Hurricane Irene is another lesson in why predicting the future strength of these storms is still difficult.