Hurricane Genesis Outlook Interpretation

Tropical Weather Outlook

Astride the hurricane watch/warning system is the Tropical Weather Outlook, a kind of early-alert system for developing hurricanes. It calls attention to tropical weather systems currently being monitored for development, capturing the process of tropical cyclogenesis (hurricane formation) as it happens.

Beginning with the 2008 season, an explicit genesis forecast, which predicts the formation of tropical cyclones in terms of probabilities, was added to this product, made possible by gradual advancements in the scientific understanding of how hurricanes form.

The Tropical Weather Outlook alerts the public as to whether tropical cyclogenesis is expected anywhere in the Atlantic basin during the coming 48 hour (2-day) period. The encircled cloud clusters labeled 1 and 2 are potential proto-hurricanes, or tropical disturbances that could become hurricanes. Some of these disturbances will develop into tropical depressions, tropical storms, and/or hurricanes, some will be assigned names and/or numbers, a few of them will enter the history books, and others will simply dissipate without ever having developed beyond the tropical disturbance stage.

The minimum structural and intensity threshhold a tropical disturbance must achieve to be recognized as a tropical cyclone is that of tropical depression, describing a closed surface low with maximum sustained winds of 34 mph or higher. Any tropical disturbance flagged for investigation is circled in the graphical product in yellow, orange, or red, and assigned a probability (low, medium, or high) of development into a tropical cyclone within the 2-day period.

Yellow represents the forecasters' judgement that a given disturbance has a LOW probability (less than 30 percent likelihood) of development, orange stands for MEDIUM probability (30-50 percent), and red describes HIGH probablitity (greater than 50 percent). The skill of the genesis outlook was determined to be not yet high enough to be able to narrow the probabilities down further than that at the present time.

Pictured below is satellite imagery of what later became Hurricane Dean of 2007. The first two images are of the system just as it was coming off the west coast of Africa. During the intervening two-day period, tropical cyclogenesis took place.

Enhanced infrared image of Tropical Disturbance INVEST 90L of 2007, later to become Category-5 Hurricane Dean

At left is the proto-Dean as a tropical wave (disturbance), as it was on August 12, 2007. At that time, it was known only as INVEST 90L, a resusable name of the kind assigned to tropical disturbances, used to identify and distinguish tropical features that have been deemed worthy of further investigation.

Enhanced infrared image of Tropical Depression 4 of 2007, later to become Category-5 Hurricane Dean

This is the same system two days later on August 14, now a tropical depression. Having passed that threshold, it was now recognized as a tropical cyclone and assigned its permanent number, tropical cyclone No. 4 in the Atlantic, or TROPICAL DEPRESSION FOUR. After crossing the 39 mph threshold, it became known as Tropical Storm Dean.

This last panel is the same storm just seven days later on August 21. It is a water vapor image of Category-5 Hurricane Dean at maximum intensity - 165 mph sustained winds and a central pressure of 907 millibars - immediately prior to making its first landfall along the Mexican coast.

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