Turbulent Boundary between Contrasting Air Masses Driving Severe Weather over Southern Plains
Recently, while driving home from a March trip to visit our friends at Malmay and Associates, we were struck by the contrast in temperatures in the short distance between Monroe, La and Baton Rouge, La. Leaving Monroe, the air was chilly and damp, and I joked that we should have packed sweaters. After a short drive south, the Baton Rouge air was hot and humid, and I broke my first sweat of the spring. This experience left me thinking about the short transition between cold and hot air masses, and whether the turbulent boundary would drive severe weather for the region. Sure enough, later that week a severe storm with tornadoes, hail, and wind damage launched what would be a month long and counting trend of severe weather that brought floods, strong wind, hail, and tornadoes to eastern Texas and southern Oklahoma along with parts of Missouri, Arkansas, and Louisiana.
When I started writing this blog post, about a month after the trip to Monroe, Denver was under a winter storm warning with a high of 49°F while the high in Dallas was 80°F and under a flood warning . Approximately 700 miles apart, the temperature dropped roughly 7 degrees every 100 miles. Not long after, extreme rainfall throughout east Texas caused what is being termed the Memorial Day Weekend Storm of 2015. Taken all together, starting in late March and currently ongoing the Southern Plains has experienced an extended, regional disaster with deadly and devastating outcomes.
Indeed, I’ve been posting about severe weather, floods, and tornadoes on a near daily basis for the last month. At this point, this system seems to have peaked with the Memorial Day Weekend storms, but it is unclear how long the current pattern will persist and if more severe storms are on the way.
El Niño and a Pinched Jet Stream
The spring months comprise the severe weather and tornado season across the plains and mid-west, so obviously this seasonal trend has been a major factor. However by all accounts this has been a wetter and more active tornado season than typical. From my understanding of climate processes, two interlinked factors have driven this activity: El Niño and a Pinched Jet Stream.
El Niño is the term used to describe an irregularly occurring period of above average surface temperatures along the central-equatorial Pacific Ocean. (La Niña is the term for the cooler phase.) Through a process called teleconnections, El Niño is known to have profound effects on weather patterns across the globe. For the U.S. Gulf Coast and Southern Plains, one effect is a steady supply of warm, moist air that drifts east from the Pacific Ocean.
El Niño also impacts the polar jet stream, which is a fast moving, high altitude air current that transports arctic air south over the central United State. As with any processes that moves air masses, the jet stream also impacts weather patterns over the Southern Plains. This year, the dominant trend has been a narrow and concentrated jet stream over the nation’s mid-section, causing a large cooler air mass to extend south like a giant lobe stretching across the nation’s mid-section. Interesting, the pinched polar jet stream also played into the extreme snowfall that Boston received over the winter, and the rash of storms that hit California in December.
The two air masses – one warm and laden with Pacific moisture and the other bringing cold arctic air south – mix across a broad, fluctuating boundary. Along this boundary, friction between the two different air masses concentrates energy into turbulent flows. These turbulent flows manifests themselves as severe storms with strong winds and tornadoes.
In the three short hours between Monroe and Baton Rouge, we drove from weather with the distinctive arctic chill of the cold air mass to the very distinctively hot and humid feel of the warm air mass. A week later, the cold air mass put Denver under a winter storm warning while the warm air mass brought 80°F high temperatures to Dallas. These conditions created two a turbulent boundary between two contrasting air masses – one dry and cool, the other warm and moist. When this happens, turbulence spins off energy concentrated in storms with strong winds and tornados, while the moisture laden air from the pacific resulted in an extended period of heavy rain that eventually lead to widespread flooding. All indications are the El Nino will continue strong for the near future. With a steady supply of warm moist air near certain, the persistence of the severe weather trend largely depends on how long the jet stream continues to transport cold air across the region.
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Here are a Few Maps from DisasterMap.net Depicting Precipitation, Flooding, and Wind Reports