By: Andrew J Siffert - May 31st, 2016
After two years of below normal named storm activity in the Atlantic basin most prognostication suggests that the 2016 Atlantic hurricane season will be above average. Many of these forecasts are citing a weakening El Niño and warmer than average seas surface temperatures over much of the Atlantic basin as a reason to expect conditions would slightly favor more named storm activity.
So far the prediction of an active season seems to be holding, with named storm Alex forming in January and most recently tropical storm Bonnie making landfall in South Carolina this past Memorial Day weekend. In fact, 2016 now joins the year 2012 as the only years, since reliable satellite coverage began; to have two named storms form prior to June 1.
Historically, the average number of named storms to develop in a season is 12, with six reaching hurricane status and three becoming major hurricanes. This year the overall number of named storms will likely be higher due to the lack of El Niño that tends to dampen hurricane formation in the Atlantic by increasing wind shear. In an environment with high wind shear, weak tropical disturbances have a more difficult time reaching into the atmosphere and forming into named storms. A La Niña looks to be rapidly developing and historically this climate forcer has lead to an above normal Atlantic hurricane season, which increase the chances of U.S. landfall named storm activity.
Like most hurricane seasons there are a few wild cards that could factor into more or less storm activity on top of general El Niño/ La Niña influences. One of these factors would be the Atlantic Multidecadal Oscillation (AMO) which has two phases: —a high hurricane activity phase and a low hurricane activity phase. Given the decrease of hurricane activity over the past few years, forecasters are uncertain as to whether the high-activity phase, which began in 1995, has ended. If it has, we could continue to see fewer hurricanes despite the otherwise favorable conditions of La Niña and warm ocean water.
Another wildcard could be the Saharan Air Layer (SAL) which is a mass of very dry, dusty air which forms over the Sahara Desert during the late spring, summer, and early fall and usually moves out over the tropical North Atlantic Ocean every 3-5 days. The SAL can have a significant negative impact on tropical cyclone intensity and formation. Its dry air can act to weaken a tropical cyclone by promoting downdrafts around the storm, while its associated strong winds can substantially increase the vertical wind shear in and around the storm environment also making it difficult for storm formation. The SAL can cover an area the size of the continental U.S. and has been tracked as far west as the Caribbean Sea, Central America, and the Gulf of Mexico.
Lack of Named storm Landfall forecasts
Many of the seasonal outlooks don’t predict how many storms could make landfall, but there has been a dearth of major hurricane landfalls over nearly the past decade, and even more so the lack of any hurricane landfall of any intensity in Florida since Wilma in 2005. In that time, more than 2.5 million people have moved to Florida who might have no experience preparing for or responding to a landfalling hurricane.
To get an idea of possible landfall areas for this up-coming Atlantic hurricane season some seasonal forecasts use current atmospheric and global sea surface temperature patterns to put together analogs years of past storm tracks. These analog years (1988, 1995, 1998, 2007, 2010, 2012) provide ideas of where storms have tracked when past condition were similar to this years conditions. This year it would appear there will be more named storm activity forming closer the U.S. coastline vs storms forming in the middle of the Atlantic Ocean in the main development region. This would yield a higher chance of named storm / hurricane landfall. The analog years point toward more storm activity in the Western Caribbean Sea which could increase the chances of a storm tracking into the Gulf of Mexico as well.
In summary NOAA’s outlook is in line with those from other organizations, both academic and private. Last month, Colorado State University research scientist Phil Klotzblach issued an outlook for a nearly-average season. Britain’s Met Office predicts a slightly above average season, as does private weather companies like WeatherBELL Analytics and WSI. The London-based weather consortium Tropical Storm Risk is forecasting a season 40 percent more active than the past 10 years, with 17 named storms, nine hurricanes and four major hurricanes. On the lower side would be the algorithm derived by University of Colorado Boulder which suggests only 6-12 named storms this season.