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Severe Storms Today?

9:52 am in Extreme Weather, Severe Storms, The Ozarks by Ted Keller

The Storm Prediction Center has put a portion of the Ozarks in a risk for severe thunderstorms today. There is a measureable tornado risk.

The threat area is mainly north of the yellow slight risk line shown on the enclosed map. This map also shows the higher (5%) tornado risk in purple. Ongoing storms and flash flood warnings as of 10:35 am are also indicated.

The current storms will continue to track east with locally heavy rain totals the main calling card. They are expected to weaken somewhat.

Later this afternoon, more storms will build in the same area as the heat and instability build.  The current area of storms will leave behind cooler and stable air but also a boundary for which future storms can interact.  This area would also be subject to the return of more unstable air later this afternoon and evening.

Table Rock Tornado/Saturday Storms, April 24, 2010

7:46 pm in Uncategorized by Ted Keller

Storm Front 4/24/10 - Hwy 65 - Amber Nicholson

The atmosphere did produce storms and severe weather, including a tornado, on Saturday afternoon April 24, 2010.  As reviewed in my blog post early on Saturday, the conditions did end up being favorable for a tornado but further west than I think anyone would have thought.

Several storm lines where pivoting around an upper level low on this day.  Sunshine in-between these lines was allowing air to become unstable, feeding the next line.  There were three of these yesterday.

The first was swinging through the extreme eastern and northeastern portion of the KOLR/KSFX viewing area.  One cell along this line was exhibiting rotation aloft in southern Phelps County (the height of the radar beam at that distance makes the direct detection of low level rotation impossible) and was enough to issue a tornado warning.

The second line formed in northern Arkansas and moved NNE.  This was the one that blew through Greene County (photo left).  Strangely, a tornado was generated by this by a cell in this line at Table Rock State Park at about 1:40 pm (official NWS report here).  This tornado not obvious on radar and I would classify this as a bottom-up tornado where some sort of rotation near the surface was “spun-up” into a strong thunderstorm updraft. This stretches the rotation, decreases the radius of the rotation and causes an increase in wind speed.

The storm at about the time or shortly before the tornado had a strong updraft as noted in the 3D image.  North is left, south is right in this picture.  A strong updraft will often suspend larger rain droplets (or hail) aloft, leading to “weak echo region” (WER),  This mini “WER” is circled in the image. In large supercells, this feature is often very large and tall.  In this case, all of this is under 10,000 feet (legend on left of image). 

3D Reflectivity at 1:30 pm

Weak rotations were detected at the surface but had virtually no depth to them.  The nature of this type of tornado makes it difficult to tag these circulations as tornadoes.  The vast majority of these weak circulations (and there are often very many in thunderstorm lines/areas) do not produce a tornado so it cannot be assumed that a tornado is present.

Just to let you know, these types of tornadoes may not be warned upon for a long time.  It would take smaller radars scanning continuously, combined with more knowledge of how these types of storms form AND a dissemination system quick enough to warn people!

Tuesday Night Tornado in February!

1:00 pm in Storm Summaries by Ted Keller

On this date in 2009,  a tornado intermittently caused damage to homes and businesses along with trees and power lines starting southwest of the interstection of Republic and Scenic extending northeast to the intersection of Fremont and Catalpa. Winds up to 100 mph…or EF1 on the enhanced fujita scale…was indicated by the damage.

  Ordinary radar echoes at 10:45 pm and 10:49 pm on 2/10/2009
    Storm-Relative Velocity images at 10:45 pm and 10:49 pm on 2/10/2009
    Normalized Rotation (NROT) at 10:45 pm and 10:49 pm on 2/10/2009 (standard MDA settings)

A tornado watch was in effect but Greene County was not under a warning of any type when this tornado occurred!  The reason has to do with the type of tornado which occurred.

Small tornadoes can occur if rotation which exists at ground level finds itself under the updraft of a storm. The updraft stretches the column of air and decreases the radius of rotation.  Since angular momentum must be conserved, the speed therefore must increase (cue obligatory ice skater example).

In theory, this can happen under any updraft.  As a practical matter however, meteorologist look for certain areas in thunderstorms that are favored for tornado development and would likely examine such areas more closely to see if there was rotation.

In this case, the rotation was overlooked because the radar signature gave no cause for concern (I call it a pimple on the radar screen!) and the rotation was so broad and short-lived that computer algorithms simply didn’t register it as a threat.  Even if perchance I had seen this rotation, I might have raised an eyebrow but I don’t think I would have pulled the warning trigger if I were the one with the warning responsibility!  Moreover if I had some magical gift of radar interpretation or inside knowledge and decided to cry tornado, would it have done any good?  The twister was on the ground for a total of five minutes!

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