Hurricane Season Is Almost Over – So Why Are Meteorologists Watching The Caribbean?

Image result for hurricanes caribbean

A report by Marshall Shepherd for Forbes.

It is the time of year when those of us in the United States are honoring our veterans or thinking about Thanksgiving turkey. However, an Atlantic hurricane season that has already given us Florence and Michael is not ready to exit stage left. A new disturbance in the tropics has formed, and it actually has the chance to develop into a pre-Thanksgiving week tropical or subtropical system.

A potential threat in the tropical Atlantic for pre-Thanksgiving week.NOAA

Meteorologists at the National Oceanic and Atmospheric Administration (NOAA) National Hurricane Center are keeping an eye on a tropical wave (below) that has formed just to the east of the Lesser Antilles. The Tropical Weather Outlook issued by the National Hurricane Center at 1:00 pm ESTon November 11th provides the latest information:

Showers activity has increased and become a little better organized today in association with a tropical wave located about 350 miles east-southeast of the Leeward Islands. Environmental conditions are forecast to gradually become more conducive for development by Tuesday, and a tropical or subtropical cyclone is expected to form by the middle of the week. The system will move westward to west-northwestward for the next few days, passing near or north of the Leeward Islands, Puerto Rico, Hispaniola, and the southeastern
Bahamas.

The Hurricane Center puts the chances of development at 50 percent within the next two days. Chances for development increase to 90 percent through 5 days (updated). The system is currently referred to as Invest 96L. An Invest is designated according to NOAA as “a weather system for which a tropical cyclone forecast center (NHC, CPHC, or JTWC) is interested in collecting specialized data sets (e.g., microwave imagery) and/or running model guidance.” If the storm further develops to be named, it would be Patty and the 16th named storm of the Atlantic season.

Tropical weather outlook from the National Hurricane Center.NOAA

Is it normal to have a named tropical storm within a week or so of the Thanksgiving holiday? It is not common, but it is also certainly not unprecedented either. The figure below illustrates the points of origin for named tropical cyclones in the Atlantic basin from 1851-2015. Data from the NOAA Hurricane Research Division also provides further context on November storms. Over the period 1851-2015, there have been 1619 tropical storms and 991 hurricanes. Only 89 (5.4 %) of the tropical storms and 59 (5.9%) of the hurricanes developed in November.

Where November named tropical systems have formed from 1851-2015.NOAA

Though November storms are possible, there are reasons that “turkey month” is typically a ramp down period for hurricanes. Wind shear starts to increase and water temperatures slowly start to cool. Hurricanes typically do not form in water temperatures cooler than 26.5 degrees C (80 degrees F). Current sea surface temperatures (below) are certainly warm enough to support tropical development. Such factors and the occurrence of hurricanes outside the “typical” season have raised questions about whether a “hurricane season” is obsolete. As I wrote last year in Forbes,

Experts note that the current length was established in 1965 based on the formation dates of 97% of tropical cyclone activity in the basin. However, research published in Geophysical Research Letters by Jim Kossin at the University of Wisconsin found that warming sea surface temperatures were leading to more “pre” and “post” season storms. A more recent study the Journal of Meteorological Research investigated relationships between sea surface temperatures and early onset. A 2017 study in the Journal of Climate found relationships between El Nino and the tropical cyclone season onset in the Pacific basin. 

As a huge college football fan, I will be watching the last few weeks closely but also the tropics.

Current sea surface temperatures in the Atlantic Ocean.NOAA

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