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Zow! It's Zeta!

By: Dr. Jeff Masters, 4:39 PM GMT on December 30, 2005

The unbelievable Hurricane Season of 2005 has decided to ring in the New Year with a final tropical storm, and a final record. Satellite imagery indicates that an area of low pressure in the eastern Atlantic has developed into Tropical Storm Zeta about 1000 miles south-southwest of the Azores. Zeta is the latest tropical storm on record to form in the Atlantic. The previous record was held by Hurricane Alice of 1954, which also formed on December 30, but earlier in the day than Zeta.

Like its Greek cousins Delta and Epsilon of November, Zeta formed in conditions one would not normally expect a tropical storm to form--sea surface temperatures are only 24 C (2 1/2 degrees below the 26.5 C minimum usually needed), and under wind shear of 20-25 knots (usually, wind shear of 15 knots or less is required.) However, all these storms formed from pre-existing low pressure systems, and these type of systems are more resistant to shear thanks to their well-developed circulations.

Zeta is trapped under a strong ridge of high pressure that will force it to move on a slow westward track for the next three days. A trough of low pressure moving across the central Atlantic may weaken the ridge enough to turn Zeta northwards and recurve it on Monday, but the ridge may be strong enough to insulate Zeta from this trough and keep the storm moving westward through at least five days, when another, stronger trough of low pressure should be able to turn Zeta northwards and dissipate it.

Zeta has an impressive satellite signature today, with good spiral banding and upper-level outflow. There is 20-25 knots of wind shear over the storm, and this shear is expected to increase substatially to 40-50 knots by New Year's Day. This may make Zeta's existence a very short one. The SHIPS intensity model is calling for Zeta to dissipate by January 1. If Zeta can survive past January 1, shear levels are expected to relax, and Zeta could hang around for most of the week. I don't expect Zeta to threaten any land areas, or make it to hurricane strength.

Jeff Masters

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