Overall SummaryIt looks like it’ll be an active Atlantic Basin tropical season in 2013 and, once again, the east coast is vulnerable to a direct hurricane hit and this includes the region from North Carolina to Maine. In a typical Atlantic Basin tropical season, there are about 12 named storms with six or seven reaching hurricane status and only two or three of these storms actually impact the coast of the United States. I believe, however, that we are headed for an active tropical Atlantic season this year with above the normal number of named tropical systems (perhaps 15-18) and as many as 9 or 10 hurricanes. As far as the Mid-Atlantic’s summer weather is concerned, it is likely to feature wetter-than-normal conditions along with normal-to-slightly above normal temperatures – not as warm as last summer. Meanwhile, much of the western US from Colorado to California appears to be headed for a hot summer with near normal precipitation amounts and this will likely lead to numerous wildfire situations during the second half of the summer in that part of the country.
East Coast Vulnerability Weather has a way of repeating itself and given the current oceanic temperature patterns in the Pacific and Atlantic Oceans (cold northern Pacific, warm northern Atlantic) which closely resembles the circumstances experienced in the 1950’s, I believe the east coast from North Carolina northward to Maine is again quite vulnerable this year to a direct hurricane hit which happened quite often over a several year period in the 1950’s. In fact, there were 8 major hurricanes (i.e. category 3, 4 or 5) that hit the east coast in a 7-year period from 1954 to 1960. Hurricane Irene (August, 2011) and Hurricane Sandy (October, 2012) affected the Mid-Atlantic coastline in important ways during the past two years and I believe this is indicative of the current oceanic temperature patterns that resemble closely what happened during the 1950s and this coastal storm threat will continue this summer - and perhaps for the next few summers.
Tropical Outlook Main Factors The following factors lead me to believe we are in for an active tropical season in the Atlantic Basin region:
1) predominately above normal sea surface temperatures across the tropical Atlantic from the west coast of Africa to the Caribbean Sea and the Gulf of Mexico 2) the continuation of a predominately negative North Atlantic Oscillation (NAO) pattern 3) the lack of an El Nino in the tropical Pacific 4) the below normal levels of vertical wind shear in the tropical Atlantic
Breakdown of Factors 1 and 2 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. Above normal sea surface temperatures in this region help to intensify tropical waves that come off of the west coast of Africa and move westward in the trade winds. The NAO has been primarily on the negative side in recent months and it is likely to continue that way for the foreseeable future. Research has shown that negative NAO patterns tend to favor active Atlantic tropical activity as it usually correlates to a weakened subtropical high pressure system which, in turn, leads to weaker trade winds across the tropical Atlantic. Weaker trade winds tend to result in less mixing of sea water which generally leads to warmer-than-normal sea surface temperatures in this important “breeding ground” region.
Last summer, there were pockets of cold water in the Atlantic Ocean just off the western coast of Africa and we did not have the usual number of tropical storms that took the long trip across the tropical Atlantic. Instead, we had a setup in which there were many “home-grown” tropical systems that developed "closer to home” over the Gulf of Mexico, Caribbean Sea or off the Southeast coast. This summer, however, I believe that given the warmer-than-normal sea surface temperature pattern in the tropical Atlantic, we will get back to the more conventional “long-trekking” type of storm that takes several days to cross the tropical Atlantic.
Breakdown of Factors 3 and 4 What goes on in the tropical Pacific does indeed have an effect on the tropical Atlantic. El Nino (warmer-than-normal sea surface temperatures) conditions actually tend to be correlated with less active Atlantic tropical seasons as research has shown a correlation between El Nino in the tropical Pacific and high levels of vertical wind shear in the tropical Atlantic. Wind shear, which can be defined as the change of wind speed and wind direction with height, is an inhibiting factor for tropical storm formation. Currently, there are neutral-to-weak La Nina (colder than normal sea surface temperatures) conditions in the tropical Pacific and there is no sign for the development of an El Nino. Indeed, several numerical model forecasts suggest that the current neutral-to-weak La Nina conditions will persist for the next several months.
Mid-Atlantic Summer Outlook Analog years to this year that also featured neutral-to-weak La Nina conditions in the tropical Pacific generally included wetter-than-normal precipitation amounts in the Mid-Atlantic region as well as normal-to-slightly above normal temperatures in the June through August time period. Of course, an active Atlantic tropical season would very likely enhance precipitation amounts in the Mid-Atlantic region; especially, during the second half of the summer when the tropical season really gets going. Also, Canada is expected to be quite chilly this summer and this could set up a temperature gradient across the northern US that could lead to many thunderstorm scenarios in the Mid-Atlantic region which favors overall wetter-than-normal conditions.