Gulf Coast Hurricane Tracker


Thursday, March 01, 2007

Improved hurricane forecasting possible

There are two aspects of forecasting hurricanes, where and how strong.

The path of a hurricane or tropical storm is fairly well predicted within a 3 - 5 day window. Storm tracks across the water or as the storm approaches landfall prove to be rather accurate. The critical period, of course, is as the storm makes landfall. As bad as the destruction of New Orleans was, it would have been worse had the Hurricane Katrina not veered slightly to the east just before landfall. Rita was predicted to make landfall around Freeport Texas (just west of us here). Instead it took a sharp right hook and hit Sabine Pass (at the border of Texas and Louisiana). While this difference of around 150 miles seems like a lot, it was well within the "cone of uncertainty". Basically if any part of the shaded area of the forecast track is heading towards you there is a potential that the storm will make landfall in your area - So get out!

The other part of the storm's prediction is its intensity. Just how strong a storm is when it makes landfall determines just how much damage it may do and even if evacuation is required. Just because a storm hits Cat 4 or 5 while over water does not mean it will be that strong at landfall. Both Katrina and Wilma in 2005 hit Category 5 status before dropping to a Cat 3 before making landfall. Likewise a storm can build suddenly as it approaches land as Wilma initially did when it crossed the Yucatan earlier in its existence.

Predicting the intensity of a storm is much more difficult. The intensity of a strong can be estimated from the radar echoes but the only true measure of the storms winds is for a "Hurricane Hunter" aircraft to fly through the eye of the storm and take on the spot measurements. Typically one plane at any given time flies through the storm and measures the wind speeds and barometric pressures along the radius of the storm.

In 2005, two planes from NOAA and one from the Navy flew in and around hurricanes Katrina, Rita and Ophelia. "It marked the first time that three Doppler radar-equipped aircraft flew simultaneously in and around these parts of a hurricane."

Data from Hurricane Rita showed for the first time that rapid intensity changes can occur when clouds outside the wall of the eye of the hurricane coalesce to create a new eyewall.

They saw a band of dry air develop around the existing eyewall as the hurricane swirled into a tighter spin -- similar to a moat around a castle.

A new ring of thunderstorms then formed around the existing eyewall, and the new outer eyewall contracted and strangled the inner eyewall, leading to a weakening of the storm. But as the new eyewall contracts, the storm may re-intensify quickly.

In less than a day, Rita grew from Category One, the least intense ranking, to a Category Five, the most intense.

"We comprehensively documented the processes that were taking place in the storm during the eyewall replacement," Robert Houze, a University of Washington atmospheric sciences professor and the study's lead author, said in a telephone interview.

"And we did this by flying the aircraft in a unique pattern, a circular pattern, with the plane flying between the two eyewalls and looking out either side with its onboard radar."

The researchers also developed a conceptual model to assist computer simulations to predict hurricane intensity changes.

Houze said these storms undergo intensity changes in a span of about 12 hours with an eyewall replacement cycle.

If a hurricane is within half a day of making landfall, it would be valuable to know how intense it will be when it comes ashore to guide authorities in their emergency management decisions such as evacuations, the researchers said.

Hurricane expert Hugh Willoughby of Florida International University, who was not involved in the research, said the findings represent a key milestone on the road to more skillful intensity forecasting. He said current intensity forecasts are not much better than simple statistical extrapolations.

Reuters Photo (right)A computer model image shows outer rainbands starting to form a new eyewall around the existing eye (top) and then the new eye forming over a large area (bottom). Scientists using data from planes flying into a large hurricane have documented changes inside these swirling storms that can quickly alter their intensity, and say these insights can improve forecasting. (University of Miami/Handout/Reuters)
The insights obtained from this study will help to improve hurricane forecasting. Knowing the intensity of a storm as well as its track will improve the ability of local officials to make the correct decisions in the proper amount of time to save lives and protect property.

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