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Tornadoes of 2011

Mar 05, 2012

Tags:Dr. Grey,Dr. Klotzbach,Earthquake,East Pacific,El Nino,Hurricane Irene,Hurricane Outlook,Hurricane Prediction,Hurricane Season 2011,Hurricane Season 2012,HUrricane Season Update,La Nina,Meteorology,Polar Vortex,Tropical,Tropics,Weather,Winter

Tornadoes of 2011

Five of the major weather disasters in the U.S. last year involved outbreaks of strong and violent tornadoes. These predominantly occurred in the middle to southeast part of the country during April through June.

A review of weather data analyses at various levels of the atmosphere during that 3-month period showed a convergence of several features that are key to the development of supercell (rotating) thunderstorms that can produce tornadoes.

These key ingredients are described as follows (see figure below):

1.Warm, moist air in low levels from warmer than normal Gulf of Mexico waters

A stronger than normal flow of air within 5000 feet of the ground off the warmer than normal western Gulf of Mexico into the southern and central Mississippi River Valley provided warm, moist air that fueled and maintained large thunderstorms.

2.Hot, dry air in mid levels from drought stricken Texas

A stronger than normal flow of hot, dry air from the southwest launched off the drought-stricken Edwards Plateau of northwest Texas and southern plains lofted eastward at 5 to 10 thousand feet above the Mississippi River Valley added to the depth of instability and wind shear in the environment. Dry air at this level when cooled by rain helped to strengthen the rear flank downdrafts of supercell thunderstorms that are often key to the formation of tornadoes.

3.Cold air aloft

A persistent broad upper level low pressure trough over the northwestern US provided a source of colder air aloft and further increased the depth of instability in the atmosphere. Deep instability enhances vertical movement of air and strengthens thunderstorm updrafts and downdrafts.

4.Stronger than normal jet stream

A stronger and more persistent polar jet stream aloft from over the North Pacific Ocean across the southern and central Rockies into the middle US remained focused and locked in position by the broad upper level trough over the northwestern US and above normal high pressure ridging over northern Mexico.
A stronger than normal jet stream aloft causes rapidly developing low pressure systems or cyclones that induce thunderstorm development. This scenario is often associated with La Niña, cooler than normal equatorial waters in the Pacific.

5.Enhanced upward vertical motion in central US

A broad area of enhanced upward vertical motion over the central U.S. from the Great Lakes to the northern Gulf States helped maintain organized lines and clusters of thunderstorms. Enhanced upward vertical motion is dynamically induced in the exit region of a strong jet stream aloft.

The fact that these key ingredients are clearly depicted in analyses of weather data averaged over a 3-month period (not shown), April-June 2012, suggests that the dynamical features were predominant and repetitive. It also suggests persistent blocking in the hemispheric flow pattern kept upper level troughs and ridges locked in position for long periods of time. With a persistent blocking pattern, similar weather features (highs, lows, and frontal boundaries) can predominate or repeat over the same area for several weeks or even months leading to extremes in temperature, precipitation, and/or dryness.


What about 2012?

It is difficult to predict from one year to the next the amount and intensity of severe weather we are apt to experience across the U.S. Climatology suggests that the greatest threat of significant springtime tornadoes will continue to be centered in Oklahoma stretching eastward across the Deep South and northward into Iowa. And even if the configuration of circulation patterns change, there are many ways to bring key ingredients together to trigger supercell thunderstorms and the associated elevated risk of significant tornadoes.

That said, there are differences in the pattern this year from last year. The La Niña has weakened and is expected to transition to neutral conditions this spring. However, as the recent outbreaks of tornadoes in the middle US has shown us, the residual La Niña pattern may occasionally reappear with branches of the jet stream across the Pacific merging to produce accelerated flow and energy to disturbances crossing the continental U.S. (see figures below).


Typical La Niña jet stream pattern.


Jet Stream configuration 2/29/12.

Rod Gonski, Author

Kermit Keeter, Editor

Jeremy Gilchrist, Reviewer


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