Blue bar at far right represents the record-breaking number of days in the US without any major hurricane landfalls (Note-the streak is projected out to the beginning of the next hurricane season which officially will begin on June 1, 2017); source Dr. Roger Pielke, Jr.

One of the best ways to measure tropical activity is with “accumulated cyclone energy” (ACE) which takes into account both the strength and longevity of tropical systems. ACE is a commonly-used metric for assessing tropical activity because it is not dependent on exact numbers of named storms or hurricanes, but rather is based on both the intensity and longevity of all tropical storms and hurricanes (so a long-lived tropical storm could contribute as much ACE as a short-lived storm that reached hurricane intensity). As of early December, the ACE measured this tropical season in the Atlantic Basin is at 135% of normal as calculated by Weather Bell Analytics with a large portion of the contribution coming from the long-lasting and powerful Hurricane Matthew (data source: Weather Bell Analytics (Dr. Ryan Maue).

One final note, one of the reasons the Southeast US suffered through very dry conditions this summer and fall (and wildfires) was largely a result of the lack of tropical moisture as many systems stayed out over the Atlantic Ocean. An important portion of the yearly precipitation totals in the Southeast US comes from the remains of tropical systems.

Tornadoes

Despite a spike in tornado activity during the latter part of November, the year as a whole is well below normal and is on pace to be the second quietest in the last decade. While November may not seem like a normally active month for tornadoes, there are on average 58 tornadoes during November (1991-2010) so a spike at the end of the month was not unprecedented. In fact, only one November back to the 1950’s completed without any tornadoes and that was in the year 1976. There are really two peak time periods in the US for tornadoes with the first being in the spring and the second in the fall which tends to peak in the month of November. One of the reasons for this double-peak of tornadoes is that there is a sharp temperature contrast across the US in both the spring and fall seasons. The difference is instead of retreating cold and advancing warmth in the spring, the reverse holds true during autumn, but both seasons can feature tornadic activity. This down year in tornadoes across the US continues the recent trend of down years when compared-to-normal.