Monday, June 16, 2014

Twin Tornadoes in Nebraska Highlight Another Day of Severe Weather in the Plains

It has been a turbulent few days in the Central Plains. For the second time in three days severe thunderstorms have hammered the area. On June 14 numerous storms producing damaging winds and a few tornadoes swept across southern Nebraska and northern Kansas. 

Storms returned again today, but this time the focus was on eastern Nebraska across northern Iowa.

The surface weather map this morning had a warm front extending southeast from a low over northern Wyoming along the Kansas-Nebraska border and east across northern Missouri. South of the front was very warm and humid air and strong south to southwest winds aloft. The area along and north of a warm front like this is often a prime location for severe storm development.
Surface weather map at 7:00 a.m. CDT, June 16, 2014
 In anticipation of the potential for significant severe weather, the Storm Prediction Center placed an area from eastern Nebraska across northern Iowa in a Moderate Risk for severe weather today.

Convective outlook issued at 8:00 a.m. June 16, 2014
At 7:00 a.m. this morning the upper air sounding, which profiles temperature, moisture, and wind vertically through the atmosphere indicated weak instability. Below is what is called a Skew-T plot of the sounding. The temperature is the red line, dew point the green line, and the brown dashed line (generally to the right of the temperature line in this plot) indicates the ascent of a saturated parcel of air. In this plot the line indicates that the saturated parcel is warm than the environment aoound, meaning it is more bouyant and will continue to rise. The area between the brown dashed line and the temperature line represents CAPE, or the Convective Available Potential Energy, or CAPE. In basic terms, CAPE is a measure of instability in the atmosphere, expressed in Joules per kilogram of air. The higher the CAPE, the more unstable the atmosphere.  The CAPE in this sounding (based on a parcel of air rising from the surface) was 468 J/kg, indicating weak instability at that time. However, the atmospheric profile was expected to modify during the day and conditions were expected to become more unstable.

Skew-T plot for 7:00 CDT sounding from Omaha, NE
At 1:00 p.m. CDT the warm front extended from north-central Nebraska to just south of Omaha.

Surface map at 1:00 p.m. CDT June 16, 2014

The National Weather Service in Omaha launched a special extra upper air sounding at 2:00 p.m. to determine the latest atmospheric profile. As expected the atmosphere had become very unstable, with surface CAPE at 5765 J/kg, well into the extreme instability category. Note also how the winds turn from the SSE at the surface to west at 10 kilometers. This is directional shear.

Skew-T plot of 2:00 p.m. upper air sounding at Omaha, NE

At 3:10 p.m. the Storm prediction Center issued a Tornado Watch for northeastern Nebraska. This was a PDS watch (Particularly Dangerous Situation), indicating the possibility for supercells with large hail and damaging tornadoes.

A supercell developed southwest of Norfolk, NE between 2:00 and 3:00 p.m. and moved northeast. The first tornado was reported south of Norfolk about 3:40 p.m. The storm moved east of Norfolk and approached Stanton and Pilger, NE. Two distinct and separate tornadoes were on the ground about a mile apart (according to the National Weather Service). Here is the radar image at about the time the tornado struck Pilger, NE.

Radar image at 4:04 P.m. CDT from Omaha, NE. The left image is the base reflectivity, and the image on the right is the Storm Relative Velocity image. The location of the tornado circulation can be seen where the blue color is adjacent to the red.
Below are some images of the tornadoes, including a brief video (last).

June 16, 2014 twin tornadoes in northeast Nebraska.
Photo credit: Tony Laubach/Facebook
Twin tornadoes in Nebraska.
Screen capture of video from
View of twin tornadoes from Wisner, NE.
Photo credit: Taryn Vanderford on Twitter

One of the tornadoes destroyed 50 to 75 percent of Pilger according to the county sheriff.  As of the time of this post there has been one fatality reported and more than 16 people injured, many of them critically. Tornadoes also caused damage in Cuming and Wayne counties.

Here is the map of storm reports for June 16 as of  8:50 p.m. CDT.

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