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Zeta approaching hurricane strength

The National Hurricane Center is finally warming up to my previous forecasts. Specifically, forecaster Pasch seems to have dispelled his own doubts about Zeta's ability to become an incredible January hurricane. Personally I think that Zeta is a great name for a hurricane. The latest official advisory lists Zeta as a tropical storm with maximum sustained winds of 65 mph. Hurricanes have sustained winds of at least 74 mph.

0:10am EST NHC advisory

Zeta is maintaining a healthy area of deep convection near the center of circulation. This is evident by the broad area of cloud tops around -60 deg C (the green area in the satellite image below).

1:Infrared satellite image

Microwave imagery indicates the presence of a ring that looks much like a forming eye.

2:Latest microwave imagery

When you overlay the ring indicated in the microwave images on top of the visible satellite, two things become evident. First, there is very healthy cirrus outflow at upper levels along the northern half of the storm. Secondly, the indicated eye is displaced to the east of where I would have otherwise have placed the center of circulation. This could be due to two reasons: 1) While the circulation is very strong, the center is not very well defined. 2) It is very common for subtropical and hybrid storms (Zeta developed from the former) to have a circulation pattern that is tilted with height.

3:Microwave / visible overlay

NHC's latest forecast discussion has finally come around to what has been my forecast for a while now. Although to be fair, Avila was hinting at this a couple days ago even though Pasch abandoned it yesterday.

SO FAR THE STRONG WINDS IN THE UPPER-TROPOSPHERIC ENVIRONMENT HAVE NOT HAD MUCH OF A DISRUPTIVE INFLUENCE. AN UPPER-LEVEL TROUGH...THAT IS CURRENTLY NEARING ZETA...IS PREDICTED BY THE GLOBAL MODELS TO SPLIT AND THIS COULD BRIEFLY CREATE A MORE FAVORABLE ENVIRONMENT FOR STRENGTHENING TODAY. THE OFFICIAL FORECAST WILL SHOW NO WEAKENING UNTIL A LITTLE LATER IN THE PERIOD...WHEN STRONG UPPER-LEVEL NORTHWESTERLIES IN THE WAKE OF THE TROUGH ARE EXPECTED TO BEGIN TAKING THEIR TOLL ON ZETA.

Pasch also finally recognized the "EYE-LIKE RING" that I first brought up yesterday. Given the bifurcation of the trough that NHC noted, I expect to see Zeta make a run for hurricane intensity today. Zeta remains over waters of about 24 deg C. While traditional wisdom had held that waters of at least 26 deg C were needed for hurricane development, this hurricane season has forced us to reexamine that rule of thumb. Indeed it seems that if colder air is present aloft, a hurricane can develop with cooler surface waters as a hurricane is basically a natural Carnot heat engine.

3:GFDL forecast track and SSTs


Posted by Bryan Woods | Permalink