There are some key differences and drivers this year, compared to years past. Several of these have only recently become apparent, while a few were known prior to the start of the Pacific & Atlantic Hurricane Seasons.
Firstly, this entry is not looking solely at the Atlantic. And when considering potential U.S. impacts, some seasons just have the ability to challenge the imagination, and almost defy logic. It starts to become necessary to either reach way back into long-forgotten historical records, or contemplate the possibility that a new record may get set. 2015 could be shaping up to be just such a year.
With deference to the title, it is informative to look at the 2014 season in both the Atlantic & Pacific. Prior to the start of each, it was widely expected to be average to below average in the Atlantic, and slightly to moderately above average in the Pacific, as an El Niño was expected to slowly develop and be fully in effect by the winter of that year. That never happened. Waters in the Niño 3.4 region trended back to neutral (3MMAs ASO 0.2, SON 0.4, OND 0.6, NDJ 0.6, DJF 0.5, JFM 0.4). "Warm" Niño 3.4 region (i.e., 5oN-5oS, 120o-170oW) events are defined as 5 consecutive overlapping 3-month periods at or above the +0.5 anomaly.
However, throughout the year evidence began building that the AMO (Atlantic Multidecadal Oscillation) may have started trending negative (cool) for the first time in 20 years, and now many specialists believe this is indeed underway. If correct, the multidecade era of generally more active Atlantic hurricane seasons/less active Central-Eastern Pacific hurricane seasons may already be closing.
Also during the 2014 hurricane seasons, the wave train off of Africa was really subdued - the Saharan Air Layer (SAL) just drying out and polluting the entire stretch from east Africa well across the Atlantic. Hazy days with tan to reddish sunsets became frequent all the way to gulf states like Florida and Texas. Long bouts of high windshear and sinking, dry air effectively put the lid on most of the (relatively few) features that had a snowball's chance to form.
The 2014 Atlantic Hurricane Season ended with the fewest named storms since 1997, despite the fact that the much advertised El Niño never arrived.
So what's so different in 2015?
Well for starters, we look to be in the early to middle phase of a strong to very strong El Niño. For certain this time. In fact, indications are that this could rival our biggest El Niños of the past, and maybe even surpass them.
Another, it still looks as if that at the very least there could be a temporary pause - if not an absolute switch - from the positive phase of the AMO (warmer Atlantic), to the negative (cooler Atlantic).
Another, the trend of globally higher average SSTs (Sea Surface Temperatures) continues - something that has increasingly been expanding the tropics poleward - at a rate of about 0.7 degrees latitude per decade - and so often helping set the plate for conditions more favorable for subtropical and tropical cyclone development and sustainability at increasingly higher latitudes.
Interestingly, the SAL is still really abundant in 2015, but disturbances across Africa have just blossomed of late (Image below)
These thunderstorm complexes sometimes sustain as waves when they roll into the Atlantic, where they either die (over water or land), develop, or end up in the Pacific. The sheer numbers of them being created now is raising eyebrows - and not just for the Atlantic - because we are in a year when conditions are so favorable in the central and eastern Pacific, and waters so anomalously warm, that U.S. states and territories and vacation spots not accustomed to having to deal with tropical cyclones are definitely at greater risk.
So what all possibilities might we be wise to consider in such an unusual year?
* There is actually a chance that the Atlantic could end up with an average to even above average number of named storms and/or hurricanes, after all - and this is very reliably backed up in the historic record. 1972, which saw the third strongest El Niño in the most data rich period (since 1950), actually ended up with 19 Tropical Cyclones in the Atlantic basin.
* Like 2014, the Hawaiian archipelago is clearly at great risk - and possibly even more so than in 2014, when we saw the strongest tropical cyclone on record to make landfall on the Big Island, Iselle, wreak havoc there. Iselle, which made landfall as a powerful 60MPH tropical storm, was at its peak a 140MPH Category 4 hurricane prior to weakening on approach from the southeast. Winds gusted to to 72 mph at the Oahu Forest, and 91 mph on Mauna Kea. Parts of the Big Island picked up over a foot of rain, and flooding was widespread.
* Unlike 2014, the Hawaiian islands will see more approaches from the south, which lends to tropical cyclones traveling over much warmer waters and through more unstable air prior to crossing the latitudes of the islands. The most extreme example of this in recent recorded history is Hurricane Iniki (1992). Thousands of homes were destroyed and there were at least six confirmed deaths.
* Remembering 1992, it also makes one think about teleconnections. And while history doesn't necessarily repeat itself exactly, it does rhyme. Also in 1992? An August Atlantic tropical cyclone named Andrew.
* Years with unusually warm eastern Pacific waters can also favor cyclones traveling north or recurving north-northeastward during the late summer to fall period, which can and has put the Southwest at risk. The most extreme example of this in reliably reconstructed history is The 1858 San Diego Hurricane (85MPH). More recent examples, in the satellite era, include 1997's Nora, which after making landfall in Baja California, tracked into California and Arizona as a bona fide mid-range tropical storm, with Yuma, Az. reporting maximum sustained winds of 57MPH and higher gusts. Nora's remnant circulation even sheared off the tops of hundreds of large trees in southwestern Utah.
Title edited to better reflect this thread.