Interesting Weather Information

Wednesday, June 5, 2013

Occluding Mesocyclones - Evidence from the El Reno, OK Tornado, Part I - The Meteorological Set Up

This is the first of two posts on the El Reno - Yukon, OK tornado of May 31, 2013.
Part II: Radar Loops and 3D - Just What is Going On Here?
These posts are semi-technical and should be easy to understand for weather enthusiasts.


Left click on any image for a larger view.

It's official! The El Reno, OK tornado is the widest tornado on record to touch down the United States with a width of 2.6 miles and it is another EF5, the 60th in the United States since 1950 and number 8 for Oklahoma making it the state with the greatest number of F5 or EF5 tornadoes with a total of 8 since 1950.

Courtesy: NWS, Norman, Ok.

The damage path of the El Reno tornado as seen by the MODIS (Moderate Resolution Imaging
Spectroradiometer) Satellite. Modified from the original by Steve Horstmeyer.
Courtesy: Space Science and Engineering Center, University of Madison, Wisconsin.

The upgrade from EF3 to EF5 came about when the National Weather Service consulted with both Howard Bluestein and Josh Wurman.  They are on my list of Severe Storms Rock Stars (my post of March 9, 2013). Their Doppler on Wheels indicated extreme wind speeds and Dr. Bluestein's crew clocked winds at 296 mph while Dr. Wurman's crew, from a different vantage point,  measured winds from 246 to 258 mph.

Along with the damage path surveyed by the National Weather Service, Dr. Bluestein's crew measured the width of the tornado at 2.6 miles using their high resolution Doppler on Wheels.

A 2.6 mile wide tornado is BIG. Take a look at what would happen if one would move north through Downtown Cincinnati. Yellow shows what a 2.6 mile wide path would cover and everything from Lower Price Hill to Mount Adams would be affected.

A 2.6 mile wide path across Downtown Cincinnati.

The Satellite View

The El Reno tornado was a monster and satellite images and loops tell an interesting story. On these loops you can see the storms grow in a band from northwestern Illinois to southern Oklahoma.  As a former colleague liked to say, "... it looks like they are growing out of the ground". On the wide view notice the small cumulus clouds streaming northward from the Gulf of Mexico as warm, moist tropical air streams into the supercells.

Satellite Loops -If not looping <ctrl> re-load

Courtesy: NOAA

They look like flat round pancakes ( I have heard some meteorologists say they look like cow pies). As the storms bump into the tropopause at the base of the stratosphere the cloud cover spreads laterally blown by high altitude winds. Unless there is a big push the cloud cannot penetrate into the stratosphere. 

The big push comes in the rotating updraft. You can see the "over shooting tops" on the close up loop as rough, textured looking areas. As the air rushes upwards it forces its way into the stratosphere. The overshooting top has also been called the updraft dome. This term is rarely used today but if you look at older literature you may run into it.

Courtesy: Space Science and Engineering Center, University of Wisconsin - Madison.

The Role of the Jet Stream

If not looping <ctrl> re-load

The animation just below, courtesy of the College of Dupage ( shows why the storms were growing where they did and why they were oriented in a line from northeast to south west.

The severe storms were being powered by what you have heard TV meteorologists call an "upper level disturbance", UAD for short.  A UAD is a bundle of energy embedded in the jet stream. The bundle travels with the jet flow and air accelerates into the disturbance and slows coming out of the disturbance.

You can track the UAD moving into the central Great Plains from the west coast in the animation below. The map for 00z 01 June 2013 (8PM EDT, 7PM CDT the evening of May 31 in the U.S.) is enlarged below to show the UAD. 

A UAD  may go by other names too:  velocity max, vorticity max, speed max, impulse, jet streak and short wave. All of these are used by TV meteorologists to describe a bundle of energy moving in the jet stream.

Upper level disturbances in the jet stream before, during and after the El Reno, OK tornado.
Courtesy: College of DuPage,

The map below shows the UAD at 00z (7 PM CDT) Friday May 31, 2013. Yellow is the core of the disturbance with the highest wind speeds. The highest velocity is 80 knots (92 mph). From the center of the yellow oval the wind speed decreases outwards.

The weather map at the 500 mb (or 500 hPa) level at 00z 31May2013.  The height of this pressure level varies but a good target height is 18,000' above sea level and at thye base of what we call "jet stream level". The yellow core of the UAD or velocity max stretches from northwest Illinois to northeast Kansas. Courtesy: College of DuPage,

A UAD is like a wind tunnel in the jet stream - air accelerates into the wind tunnel through the entrance region and slows as it leaves the wind tunnel through the exit region.  Looking downwind, the wind is at your back, we can divide the UAD into quadrants. Air rises from the lower atmosphere to jet stream level beneath the Left Front Quadrant and beneath the Right Rear Quadrant. The reasons this happens in these quadrants are well known but beyond the scope of this post. Rising air means falling surface pressure and those towering, churning thermodynamic machines we call supercells are just low pressure cells that get stronger when a UAD provides lift.

The same map as above but annotated. Courtesy: College of DuPage,

On the map above the double headed arrow marks the middle of the jet streak half way from entrance to exit region.  The axis of the jet is indicated by the yellow arrows, longer arrows represent higher wind speeds. When air enters the UAD it accelerates to the middle of the UAD then slows as it heads from the middle towards the exit region.  For dynamic reasons air rises (the same as surface pressure dropping) in the Left Front and Right rear Quadrants.  The tornadic thunderstorms in Oklahoma were powered that night by lift provided by jet streak's right rear quadrant.

The Surface Map 

The surface map at 7PM CDT om May 31, 2013 was complex with a quasi-stationary front across Oklahoma from southwest to northeast.  South of that a dryline separating the dry desert air from the warm, moist tropical air off the Gulf of Mexico.
The map date/time information says 21Z which is universal time coordinates (UTC) and it is based on Greenwich, England. This map 
is for 4PM CDT 31 May 2013 based on the U.S. Central Time Zone. The cool outflow from the thunderstorms was plotted as 
outflow boundaries.

Regional Surface Maps
Surface observations at 23z May 31, 2013. Station data use the standard plotting model. The heavy line is the 60° isodrosotherm (dew point line) and yellow represents where dew point temperatures are 60° or greater, orange shows areas with dew point temperatures of 70° or higher. Created with Digital Atmosphere,

Same color scheme as above with streamlines added to show the flow of tropical moisture into the developing supercell complex.
A Surface Pressure overlay on the map with the same dew point color scheme as in the maps above. A stationary front to the north was increasing surface convergence. The high south of the front is a thunderstorm induced "bubble" high pressure system. The dryline separates desert air from very warm and moist tropical air that originated over the Gulf of Mexico. The trof (meteorological version of trough) from the low at the north end of the dryline is probably a result of the analysis scheme and the  the developing thunderstorms over southern Oklahoma you can see in the image below.

23z (6PM CDT) KTLX Radar (Oklahoma City). Note the hook echo east northeast of the radar site.

To sum the situation up:
1. A quasi-stationary front with warm moist air to the southeast over most of Oklahoma provided surface convergence and therefore lift.
2. The right rear quadrant of a moderately strong upper level disturbance was moving over the area also providing lift.
3. Instability was high, at Norman, OK the lifted index was  -7.69 and CAPE was 3260.19 at 00Z (7PM CDT). The atmosphere was primed to explode.

Next Part II - Interesting Radar Loops and Radar in 3D - Just What is Going On Here?

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