Thursday, May 14, 2015

Professor Quibb's Picks – 2015

My personal prediction for the 2015 North Atlantic Hurricane Season (written May 13, 2015) is as follows:

9 cyclones attaining tropical depression status*,
7 cyclones attaining tropical storm status*,
3 cyclones attaining hurricane status, and
1 cyclone attaining major hurricane status.

*Note: Tropical Storm Ana formed on May 8, almost a month before official start of the hurricane season and before I published my predictions.

The above prediction anticipates a significantly below-average hurricane season, particularly relative to the past few decades. It calls for just over half the typical number of tropical storms, and less than half of the usual hurricanes and major hurricanes. If this prediction were to come true, 2015 would be the least active season since 1994.

Several factors are stacked against tropical cyclone formation this season. First, a weak El Nino event that developed in 2014 has persisted so far this year. In addition, predictions indicate that it will strengthen over the coming months. Already, the anomalously high sea surface temperatures in the Pacific associated with an El Nino have caused a stronger jet stream and stronger wind shear across the United States and the neighboring Atlantic.

The relative scarcity of Atlantic hurricanes over the 2013 and 2014 seasons also suggests that we finally may be entering the "cool phase" of the Atlantic Multi-Decadal Oscillation (AMO), an apparent cycle of above-normal and below-normal hurricane activity whose period is measured in decades. We do not completely understand this cycle and cannot predict it, but the elevated activity for the 15 years ending in 2012 and the subsequent lull could indicate a reversal. Currently, cool water temperatures abound throughout the tropical Atlantic, supporting the hypothesis that we are entering a "cool" period. Regardless, the low Atlantic sea surface temperatures will help to suppress cyclone formation.

Finally, sea-level pressures over areas of the north central Atlantic have been higher than normal over the past few months. Though I wrote elsewhere that a stronger high-pressure system over the central Atlantic tends to steer Atlantic hurricanes towards land and that the strong high appears with the La Nina, and not the El Nino, the truth is more complicated. Both its position and strength influence hurricane activity in different ways and the latter may not correlate with the ENSO cycle. This year, there is both an El Nino and a higher intensity Bermuda high, and both of this factors tend to reduce hurricane activity.

My estimated risks for different parts of the Atlantic basin are as follows (with 1 indicating very low risk, 5 very high, and 3 average):

U.S. East Coat: 1
A combination of a strong jet stream and cool ocean waters will result in a very low risk of landfall for the U.S. East Coast. Ironically, Tropical Storm Ana made landfall in South Carolina before the season even officially began. However, this was due to a blocking pattern that should occur only infrequently, particularly if the El Nino strengthens.

U.S. Gulf Coast/Northern Mexico: 3
The anomalies in water temperatures are less pronounced in the Gulf of Mexico, so this region is at greater risk for landfalling cyclones. The Bay in Campeche in particular may be a birthplace of one or more short-lived tropical storms this season.

Yucatan Peninsula and Central America: 3
The high sea-level pressures this season will enhance trade winds, causing the potential for westward-tracking, fast-moving systems through the central Caribbean and into the Central American states. Despite the expected quiet season, this region will have about average risk.

Caribbean Islands: 2
The Caribbean can expect some activity, but El Nino-related dry air may lead to the demise of some tropical systems before they can undergo much strengthening. This region probably has little to fear from Cape Verde-type strong hurricanes this year.

Overall, the 2015 season is expected to be quiet, and possibly historically quiet. The precise strength of the developing El Nino is the main uncertainty in predicting the level of activity. Regardless, even quiet years can have devastating storms, so be sure to always practice hurricane preparedness!


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