Andy Siffert, BMS’ resident Meteorologist, discusses Severe Weather in a Warming World

As we have all seen in the media it would appear that on May 15 Mother Nature has turned on the severe convective storm season, which to date had been historically quiet in terms of insurance losses and severe convective storm reports.

Since May 15 the preliminary tornado count stands at 305 tornadoes, but considering the nation is currently at the climatological peak of the severe convective storm season and the tornadoes are occurring precisely where historically they should occur, the impacts of the severe weather should be expected and can be easily explained by understanding the current weather pattern.

The reason we have seen the recent uptick in severe weather activity to more normal levels is the spring of 2013 has been climatologically cooler than normal over the eastern two-thirds of the country, which has kept instability levels low. This is most likely due to a weather pattern associated with the a negative phase of the Arctic Oscillation (AO), which has kept a low amplitude jet stream pattern in place over the eastern portion of the U.S., allowing cool air from Canada to spill southwards into the U.S. blocking warm moist air northward progression from the Gulf of Mexico.

Starting in mid May a shift in the jet stream winds resulted in a weather pattern that allowed for frequent weather systems to draw upon the warm moist air from the Gulf of Mexico to clash with the cooler, drier air moving east off the Rocky Mountains. This different weather pattern has provided the main ingredients necessary to produce what have been widespread multi-day severe weather events.

Understanding weather patterns can fluctuate explains the last three years of tornado activity, which have experienced both a record minimum and record maximum tornado count. These two extremes of recent tornado surplus (2011) and the current tornado drought are rare and considering they are back to back it makes the occurrence even more unusual. However, these patterns have resulted in several contradictory views on the impact a warming world might have on severe convective storms in the U.S. In a warming world should we experience more seasons like 2011 or fewer tornadoes like 2012? Are extreme tornadoes like Moore, OK, a result of this warming world?

In the latest BMS Introspect – Severe Weather in a Warming World and Its Impact to the Insurance Industry we attempt to answer these questions.