Altering Characteristics of Tornadogenesis: The Impact of Global Climate Change & Spring 2018 Outlook (Photo Credit: NOAA's National Severe Storms Laboratory)
A tornado northeast of Cheyenne, Wyoming on June 12, 2017.
As spring is spinning up, tornado season is already at an active start. Despite deadly winter outbreaks earlier this year, the U.S. is running behind on the number of tornadoes this year- 58 through February 28th (compared to the average of 107). A strong blocking pattern over the western Atlantic (North Atlantic Oscillation) has allowed cold fronts to drop into the South, limiting the flow of warm, moist air from the Gulf and keeping instability low.
PDO (Pacific Decadal Oscillation) remained somewhat positive throughout the winter, helping California, the Southwest, and even the Great Plains to be drier than normal (negative PDO regimes allow storms to dig southward to bring moisture across the central and eastern U.S.). Drought conditions can lead to greater dew point depressions at the surface (having been warmer and drier than normal) and higher cloud bases. The higher the cloud base, the harder it is for a circulation to reach the ground to become a tornado. In addition, a moderate La Niña can favor increased tornadic activity, as the jet stream shifts farther north and drags up moist air from the Gulf. This pattern of a positive PDO, borderline moderate La Niña, and positive NAO leans towards a below to near normal tornado season and favoring an active start of tornado season across the Southeast and Mississippi Valley. A quieter than normal May is not out of the question for prime chasing territory across the Great Plains due to the weighting of a below normal season so far.
But, what would tornadogenesis look like in the future? Climate change may affect the frequency and intensity of tornadic events, as a warmer atmosphere can hold more water vapor and changing the amount of energy available in these storms. Above normal winter temperatures can also be an indication of events to come. Since climate change influences the onset of seasons, tornado season may shift earlier in the year and continue longer than normal. A strong tornado outbreak in late January of this year producing 81 tornadoes across the southeastern U.S. was the second largest January tornado outbreak and the third largest winter tornado outbreak since 1950. Climate change could be the only plausible explanation as to why extreme events, such as widespread tornado outbreaks, are happening with increasing frequency, but a single event cannot necessarily be attributed as a direct result of these changes.
This discussion is only meant to be a best-guess outlook and should not be used as a basis for interpretation without checking your local short-term forecast first.
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©2018 Meteorologist Sharon Sullivan