2016 Tropical and Mid-Atlantic Summertime Outlook

Highlights:
1)    Odds favor a hot and dry summer in the Mid-Atlantic region
2)    Based on analog years, it looks like Atlantic Basin tropical activity will be slightly above normal this year with approximately 14 named tropical storms, 7 hurricanes and 3 "majors"
3)    Primary target area of Atlantic Basin tropical activity should be the Southeast US, Gulf of Mexico and Caribbean Sea regions
4)    Tropical system rains not likely to “help out” much in the Mid-Atlantic region during the summer months

Major factors:
1)    Weakening El Nino in the tropical Pacific Ocean and the beginning of La Nina conditions
2)    Mixed sea surface temperature signals across the Atlantic Ocean and Gulf of Mexico
3)    Six analog years featuring similar "El Nino-to-La Nina" transition: 1954, 1964, 1988, 1998, 2007, 2010 

Sea surface temperature anomalies as of January 4th, 2016; courtesy NOAA

Atlantic Basin tropical activity and Mid-Atlantic summertime conditions
The major factors involved with the "2016 Tropical and Mid-Atlantic Summertime Outlook" include a quickly weakening El Nino in the equatorial Pacific Ocean, the likelihood for the onset of La Nina by later in the summer, and the current “mixed” sea surface temperature signals across the Atlantic Ocean.   In a typical Atlantic Basin tropical season, there are about 12 named storms with 6 or 7 reaching hurricane status and only 2 or 3 actually reaching "major" status (i.e., category 3, 4 or 5).   Based on six analog years chosen in large part for their similar "El Nino-to-La Nina" transitions, this year may feature slightly above normal tropical activity in the Atlantic Basin with around 14 named storms, 7 reaching hurricane level, and 3 attaining “major” status.   While this would be a slightly above normal season - and noticeably more active than recent relatively quiet years - it is not likely to turn out to be a mega-year given among other reasons the numerous cold pockets of water across the Atlantic Ocean.  Last year was generally a quiet hurricane season in the Atlantic Basin with 11 named storms including four hurricanes, two of which reached “major” status.  

Sea surface temperature anomalies as of April 18th, 2016; courtesy NOAA

Based on the chosen six analog years, odds favor a hot summer in the Mid-Atlantic region with temperatures averaging at least 1-2 degrees (F) above normal and also a drier-than-normal summer with rainfall totals some 2-4 inches below the June/July/August standards. One bit of good news is that despite an extended dry spell earlier this spring, soil moisture is in relatively decent shape in much of the Mid-Atlantic region thanks to recent rains.  Given the relatively decent soil moisture content as we head towards the summer season, chances for widespread extreme drought are not terribly high in my opinion despite the prospects for hot and dry weather conditions.  Also,  relatively decent soil moisture content at this stage of the spring in the Mid-Atlantic region reduces the chance for an excessively hot summer.  The reason for this is that with some moisture in the ground, the sun's rays are partially "used up" in the evaporation of ground-level moisture rather than in directly heating the ground which, in turn, heats the atmosphere.   

El Nino, La Nina and the Southern Oscillation (ENSO) predictions by numerous dynamical and statistical computer models; courtesy International Research Institute for Climate and Society and Columbia University, NOAA

Collapsing El Nino in the equatorial Pacific Ocean 
What goes on in the tropical Pacific Ocean can indeed have an effect on the tropical season in Atlantic Basin. The current El Nino, characterized by warmer-than-normal waters in the eastern and central part of the tropical Pacific, started early in 2015 and reached a peak at the end of the last year as one of the strongest such episodes in the past 50 years.  In fact, the characteristic warm anomalies are still evident when looking at the latest map of SST anomalies in the Pacific Basin, but they have faded significantly since the winter and some colder-than-normal water (blue) is showing up just off the west coast of South America.  El Nino is an important predictor for hurricane activity because it generates stronger upper-level westerly winds that can tear storms apart in the tropical Atlantic Ocean and Caribbean Sea. In other words, El Nino years are often connected to weak hurricane seasons in the Atlantic Basin.  It is now in a state of collapse and all indications suggest it will flip to La Nina (colder-than-normal) by later in the summer or early this fall.  Computer forecast models generally are in agreement that either neutral or cooler-than-normal conditions (i.e., La Nina) will be in evidence for the latter part of the tropical season.  Therefore, given the collapse of El Nino and the likely formation of La Nina by later this summer or early this fall, the “beneficial” effect of an El Nino is likely to diminish or indeed completely disappear and this increases the odds for more activity compared to last year.  

Atlantic Basin sea surface temperature pattern 
The main breeding ground region for Atlantic Basin tropical systems extends from the west coast of Africa to the Caribbean Sea and Gulf of Mexico. Tropical cyclones are like giant engines that use warm, moist air as fuel.  In general, sea surface temperatures of 80 degrees or higher are required in this region for the formation of tropical storms and colder water inhibits or prevents storm development.  In the Atlantic Basin, there are somewhat mixed signals as we approach the onset of tropical season with respect to sea surface temperatures which play an all-important role in the potential formation of tropical storms.  

To begin, there are currently colder-than-normal waters near the west coast of Africa, the western part of the Gulf of Mexico and across a large part of the North Atlantic.  Typically, colder-than-normal water in the far North Atlantic is indicative of a negative phase of the Atlantic Multi-decadal Oscillation (AMO) which is usually associated with quieter periods for the Atlantic Basin.  Specifically, the cold waters in the far North Atlantic tend to force higher pressure in the tropics and stronger low-level trade winds and this can, in turn, eventually make it cooler in the tropical Atlantic Ocean.  In addition, clockwise flow of air around high pressure could push dry air from the Saharan Desert region of Africa into the eastern Atlantic and this would be an inhibiting effect on tropical storm formation and/or intensification.  On the other hand, in the bulk of the “breeding grounds” region of the Atlantic Ocean, it is currently warmer-than-normal which is a favorable sign for the formation and/or intensification of tropical systems.  

Composite map of temperature anomalies averaged for June, July and August from the analog years of 1954, 1964, 1988, 1998, 2007, 2010; courtesy NOAA

Analog years
There are six analog years that featured a transition of El Nino-to-La Nina in the tropical Pacific Ocean which is expected this year and they are as follows: 1954, 1964, 1988, 1998, 2007, 2010. In those analog years, the temperature and precipitation composite maps for June, July and August indicate hot weather dominated much of the country and drier-than-normal conditions extended from the Mid-Atlantic to the Southeast US.  

Composite map of precipitation anomalies averaged for June, July and August from the analog years of 1954, 1964, 1988, 1998, 2007, 2010; courtesy NOAA

In addition, tropical activity in these particular six analog years was slightly above normal and the number of named storms, number of hurricanes and number of major hurricanes is summarized in the following table for each of these years (data courtesy Wikipedia):

Yearly tally of the Atlantic Basin tropical activity during the six analog years; data courtesy Wikipedia

The yearly “storm track” maps (below) for the six individual analog years indicate that, in general, most systems stayed "out-to-sea" or impacted the Southeast US, Gulf of Mexico and Caribbean Sea - and not in the Mid-Atlantic region.

Storm tracks of Atlantic Basin tropical activity during six analog years; data courtesy Wikipedia

Amazing on-going streaks
Fortunately, the US mainland has not been struck by a "major" hurricane since Hurricane Wilma made landfall in SW Florida during late October of 2005 - a record-breaking year in the Atlantic Basin.  This is certainly one of the longest stretches ever recorded without a "major" hurricane hit in the US since record-keeping began. In addition, and perhaps even more amazing, is the fact that the state of Florida has not been hit by a hurricane of any intensity in more than ten years since Wilma and the population has grown in the interim by more than 2.5 million people.  Let's hope these streaks continue during the 2016 tropical season.

Recent history has been kind to the US with regard to hurricane strikes; data courtesy NOAA/Hurricane Research Division/HURDAT Re-analysis at http://www.aoml.noaa.gov/hrd/hurdat/All_U.S._Hurricanes.html; realclimatescience.com

Final thoughts
We’ll continue to monitor all of these factors over the next couple of months to see if any significant changes develop or on-going trends continue which could alter the tropical and Mid-Atlantic summertime outlook.  For example, if the colder-than-normal sea surface temperature pattern in the North Atlantic Ocean continues to trend southward - as it has done in recent months - then this could result in a quieter Atlantic Basin tropical season.  

Meteorologist Paul Dorian

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