Sea surface temperature anomalies; map courtesy tropicaltidbits.com, NOAA

Sea surface temperature anomalies; map courtesy tropicaltidbits.com, NOAA

Executive Summary

  • Near normal tropical season in the Atlantic Basin
  • Warmer-than-normal water in the western Atlantic Ocean and Caribbean Sea makes the southern and eastern US vulnerable to “home-grown” hits
  • Near normal to slightly below normal temperatures in the Mid-Atlantic
  • Near normal to slightly above normal rainfall amounts in the Mid-Atlantic
  • Main factors include weak El Nino in the equatorial Pacific and pockets of warmer-than-normal and colder-than-normal water in the Atlantic Basin
Numerous computer forecast models predict El Nino conditions in upcoming months; courtesy IRI/CPC

Numerous computer forecast models predict El Nino conditions in upcoming months; courtesy IRI/CPC

Atlantic Basin Tropical Outlook

The overall numbers are likely to be near normal this year in terms of the number of Atlantic Basin (includes the Caribbean Sea and Gulf of Mexico) tropical storms and the sea surface temperature pattern in the western Atlantic and Caribbean Sea makes the southern and eastern US somewhat vulnerable to “home-grown” tropical hits.  The major factors involved with this year’s tropical outlook include an unfolding weak El Nino in the equatorial Pacific and a mixed picture of warmer-than-normal and colder-than-normal patches of water across the Atlantic Basin.  

In a normal Atlantic Basin tropical season, there are about 12 named storms with 6 reaching hurricane status and only 2 or 3 actually reaching major status (i.e., category 3, 4 or 5) and that is the general outlook for this upcoming season.  There has already been one tropical system (Arlene) this year in the Atlantic Ocean which formed during the month of April and gave the tropical season an early start.

One note of interest, amazingly, the US mainland has not been struck by a major hurricane (i.e. category 3 or higher) since Hurricane Wilma in October 2005.  Although the landfall record gets muddy before the early 20th century, this is the first time since hurricane record-keeping began in 1851 that the United States has gone so long without at least a category 3 landfall. The previous streak was eight years, from 1861 to 1868.  Wilma made landfall in southwestern Florida during late October of 2005 as a category 3 hurricane.

Some models such as JAMSTEC predict strong El Nino conditions for this summer

Some models such as JAMSTEC predict strong El Nino conditions for this summer

Weak El Nino in the tropical Pacific Ocean

What goes on in the tropical Pacific Ocean does indeed have an effect on the tropical Atlantic Ocean. El Nino, which refers to warmer-than-normal waters in the equatorial Pacific Ocean, affects global weather patterns and it tends to produce faster-than-usual high-altitude winds over the tropical Atlantic Ocean.  This increase in the upper atmospheric winds over the tropical Atlantic Ocean is usually an inhibiting factor for tropical storm formation in the Atlantic Basin as it tends to “rip apart” developing storms. Currently, there are signs for a weak El Nino in the tropical Pacific Ocean over the summer months and this should somewhat inhibit storm formation in the tropical Atlantic Ocean. Numerous computer forecast models support the idea that El Nino conditions should prevail this summer in the tropical Pacific Ocean and some actually forecast moderate-to-strong strength (e.g., JAMSTEC).  I do not believe, however, that there will be a moderate-to-strong El Nino this summer and certainly nothing like the strong El Nino of a couple of years ago.

200-850 mb vertical wind shear anomaly 45-day average with higher-than-normal values (in blue) stretching from west coast of Africa to Caribbean Sea; courtesy NOAA

200-850 mb vertical wind shear anomaly 45-day average with higher-than-normal values (in blue) stretching from west coast of Africa to Caribbean Sea; courtesy NOAA

Mixed signals in sea surface temperatures ACROSS Atlantic Basin

The main breeding grounds for Atlantic Ocean tropical systems are in the region between the west coast of Africa to the Caribbean Sea and the Gulf of Mexico and there are mixed signals in terms of sea surface temperatures all along this route.  Above normal sea surface temperatures in this region generally help to intensify tropical waves that come off of the west coast of Africa and move westward in the trade winds. In recent weeks, there has been a persistent pocket of colder-than-normal sea surface temperatures in the central Atlantic and this would typically inhibit the formation of tropical storms in that part of the tropical Atlantic Ocean.  On the other hand, there are warmer-than-normal pockets of water just off the U.S. east coast and just off the west coast of Africa and these anomalous regions should aid in the development of tropical systems.   

Soil moisture content is quite high in the eastern US thanks to recent rains; courtesy NOAA

Soil moisture content is quite high in the eastern US thanks to recent rains; courtesy NOAA

Mid-Atlantic Outlook

As far as the Mid-Atlantic region is concerned for this summer, I believe there is little chance for an excessively hot summer with any kind of serious drought.  One important factor that should greatly inhibit the chances for an excessively hot summer with serious drought conditions is the fact that soil moisture is quite high around here thanks to recent soaking rainfall events and there is no reason for that to change dramatically in upcoming days.

Temperature anomalies for June, July and August based on analog years; data courtesy NOAA

Temperature anomalies for June, July and August based on analog years; data courtesy NOAA

Soil Moisture

Normal-to-high soil moisture content this time of year tends to reduce the chances for extensive summertime drought and excessive heat as some of the sun’s energy is used up in evaporation rather than in the heating of the ground (which it turn heats the lower atmosphere).

Precipitation anomalies for June, July and August based on analog years; data courtesy NOAA

Precipitation anomalies for June, July and August based on analog years; data courtesy NOAA

Temperature Outlook

In terms of temperatures, analog comparisons clearly suggest it could actually turn out to be cooler-than-normal; however, given the very warm waters just off the east coast, I believe the “analog suggestion” of cooler-than-normal will be offset somewhat resulting in an overall summer featuring normal-to-slightly below normal temperatures in the Mid-Atlantic. The analog years used here feature weak La Nina conditions early in the year that transition to weak El Nino conditions during the late spring and summer seasons.  In terms of rainfall for the Mid-Atlantic, analog comparisons suggest near normal-to-slightly above normal rainfall amounts and sea surface temperature pattern in the western Atlantic (i.e., warmer-than-normal) supports that idea.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com