Tuesday, May 27, 2014

Sunday Afternoon Landspout

Sunday afternoon around 3:00PM a tornado spun up in Dunklin county just west of the intersection of HWY 102 and HWY 25 north of Malden. 

Tornadoes usually form with rotation severe thunderstorms and usually have a few minutes warning time before they develop.  But there is one type of tornado that usually develops quickly and without warning.  That type of tornado is called the landspout.  A landspout tornado forms much in the same way its cousin the waterspout develops over the water.  Instead of a rotating severe thunderstorm in which rotation form the mid levels is brought down to the surface, these tornadoes start as a swirl on the ground that is stretched under a storm’s updraft. 
Two dust swirls on the ground.  Photo Leslie Asher.

Sunday afternoon there was a thundershower located over Dunklin County that collapsed around 2:40PM sending a boundary of rain cooled air across the Bootheel.  Along and immediately behind this boundary little eddies or swirls form.  Picture your arm moving along the top of the water in a swimming pool.  Right behind your arm you will see little swirls form.  This same action occurs in the atmosphere as boundaries move through.  

Video above from Brandon Shelton.

As the boundary approached Malden a thundershower was rapidly developing.  This thundershower’s updraft encountered the swirls on the ground stretching them making the rotation stronger.  The stretched rotation is pulled into the thundershower’s updraft and a tornado forms.  

Dust Swirl stretched into the storm. Photo Leslie Asher.

Now landspouts are usually very weak, EF0, but in some cases have reached EF3 strength depending on the storm’s updraft strength.  Looking at the video seen these were very weak, maybe technically not reaching the 65MPH wind speed needed to be classified as an EF0 tornado.  But with that said anytime there is a rotating column of air in contact with a storms updraft by definition, it’s a tornado.  Not sure if this will go down as an official tornado but to all who sent video and photos to help me investigate, thanks.  You can tell all your family and friends you saw a tornado. 

Thursday, May 15, 2014

Hail Possible Today

Although temperatures are below average for this time of the year we are still expecting a few hail showers this afternoon.  Right now it does not look as though we will see much in the way of severe hail, quarter size or larger, but a few thundershowers will contain a lot of pea size hail, and in some places could cover the ground.  So why are we seeing a threat of hail when in May cooler temperatures usually mean no storms?

The satellite above shows what the Heartland looked like from space around 11:30 this morning.  We see lots of sunshine but we are also seeing cumulus clouds developing across our northwestern counties.  These are the clouds that will grow into thunderstorms.  The reason is because there is an area of very cold temperatures in the mid levels of the atmosphere, around -15 degrees, moving across much warmer air at the surface.  This causes the air to rise rapidly which leads to thunderstorm development.

Above is a profile of the atmosphere, a skew-T diagram.  The blue line represents the temperature of the air as it is lifted.  The red line is the actual air temperature.  The area between the blue and red lines where the blue line is on the right represents the amount of CAPE (convective available potential energy) for thunderstorms development.  At first glance it is not very impressive. We call this a short skinny CAPE which usually leads to a few scattered showers.  But a closer look shows that most of the CAPE lies in an area where temperatures are below -10°C or the hail growth region.  So hail will form very easily in these showers.  But since it is a skinny CAPE, the updraft of the storm will be weak meaning the hail will not be able to grow to a very large size.  As of this writing we have already seen pea size hail across parts of central Missouri and I expect that to increase as the afternoon progresses.   

The above image shows a forecast for where storms should be around 4PM.  Don't pay attention to the exact location, just note there will be scattered thunderstorms with small hail throughout the Heartland later this afternoon. So a few areas of the Heartland will experience hail today.  You may want to keep the car in the garage if possible even though damaging hail is not anticipated.  If the small hail is heavy enough it could chip some paint.

Monday, May 5, 2014

Hot Day to Fish Yesterday

Fished the Castor and ST. Francis Rivers yesterday as the temperature hit the 90 degree mark.  The Castor is amazing and clear.  I plan on floating this river next month.  Lots of fish, just none biting.  I saw my first smallmouth yesterday in the Castor River under Highway 72, just couldn't get hime to bite.  Still trying!

Sunday, May 4, 2014

Still Hunting For My First Smallmouth

I stalked Little Whitewater Creek yesterday in an area recommended by co-workers to try and catch my first small mouth.  I went with my 6wt fly rod and started with a crayfish imitation.  Second cast was a hook up.  A small but fun spotted bass.  I thought to myself, "this was going to be a great day."  Well, I did catch a number of small fish in the sunfish and shad variety, but no small mouth.  I did see many nice size spotted bass in this creek so I will be back.  my search continues............

Friday, May 2, 2014

Woodland Mills Tornadic Mini Supercell

(Photo source above Ben Yates FaceBook)

Sunday and Monday were very active weather days across the Heartland with severe weather and flooding. Monday afternoon parts of the Heartland were under consideration for a tornado watch but, one was never issued.  Conditions were favorable for tornadoes if deep convection(strong thunderstorms) could occur. All afternoon small thunderstorms developed showing weak signs of rotation as the atmosphere was extremely sheared.  Around 5PM a thunderstorm rapidly developed to the south of Union City, TN and would go on to produce a very strong EF2 tornado (top winds 130MPH) with out warning.  This type of storms is called a mini supercell, a small cousin of the usual large severe storms we see produce tornadoes across the Heartland.

The image above is what the storm appeared like on Doppler Radar as the tornado was on the ground.  At first glance it does not appear very strong at all. Our lightning detection was showing a few bolts to the north but at this time this storm was not very electrical, another sign of a weak storm.

 The three dimensional view the tornadic storm shows a small core not even reach 20,000' into the atmosphere.  The red colors which show heavy rain and hail are barely above 12,000' at this time. normally we see cores to 50,000' on storms that produce tornadoes.  So what made this small storm produce such a strong tornado for its size?  I had to know and decided to take a closer look.

The image above is a skew-t diagram from the 22:00Z (CDT +5 hours) hour RUC near Union City to get as close to the storm's environment as I could.  In the above forecast sounding there is enough CAPE (Convective Available Potential Energy) for surface based thundershowers to develop.  Overall it is pretty weak and this typically wouldn't throw out the "red flag" that something bad is about to happen.  So we need to look a little deeper.

The image above is the hodograph from the 22:00Z (5PM CDT) hour RUC near Union City, TN.  Here the large clockwise curve indicated an enormous amount of shear in the atmosphere, or spin to cause thunderstorms to rotate. Most of the shear is occurring in the same area of the atmosphere we see the positive CAPE.  So any thundershower that could develop in this environment would more than likely become a supercell, or mini supercell in this case. Not shown is the 0-1KM bulk shear, or the difference between the surface wind speed and the 1KM wind speed.  At the time of this storms the 0-1KM bulk shear was around 30 knots.  Any value above 20 knots is favorable for tornado development. 

We have already seen what the storm looked like on radar and in 3D.  Now let's see what the storm's velocity data was showing, or the winds inside the storm.  The image above shows an area of bright green next to an area of brighter red.  The bright green indicates winds moving towards the radar at 35 knots.  The red colors show winds moving away from the radar at 29 knots.  This rotation is indicating a mesocyclone, a rotating thunderstorm.  This storm showed a mesocyclone, although weak, for up to 15 minutes before the tornado was produced. 

So we now there was a mesocyclone so why was no warning issued?  Well to be honest, the mesocyclone was never that impressive on radar in terms of strength to produce a tornado. The chart above shows the comparison to rotational velocity [the absolute value of the inbound plus the absolute value of the outbound velocities divided by two or Vr= (|Vi|+|Vo|)/2] to the range from Doppler Radar.  The red dots on the chart show the time of the sampled mesocyclone and its strength, with the time moving from right to left.  Notice as the storm gets closer to the time it produces the tornado, the mesocyclone gets stronger but never more than a minimal mesocyclone strength. SO no warning was issued. But the above chart does not take into consideration the size of the mesocyclone, or the diameter.  Since mini supercells are much smaller in size than classic supercells, the diameter of their mesocyclone is usually much smaller as well.
Research performed by Kenneth Falk and William Parker from the Nation Weather Service in Shreveport, LA in 1998 showed that looking at the Rotational Shear (Sr=2(Vr)/D where D is the diameter of the mesocyclone in meters) which takes into consideration the diameter of the mesocyclone may be a better determination in whether a storm will produce a tornado.  The above chart shows the Sr*10^3 with values in orange showing a tornado is possible and in purple showing a tornado is probably.  Notice the depth of the mesocyclone is lower than 10,000' for most of the observations.  Normally we would see mesocylone depth greater than 10,000' but again with this being a mini supercell, everything is much smaller.  

The above chart is from Falk and Parker's study in which they examined 50 mesocyclones to determine when a tornado is more likely. Again the time line of the Woodland Mills storm goes from right to left.  With the rotational shear showing a tornado is probably and the near storm environment likely for tornado development, we will be looking to use this guideline more as we go through severe weather events.  Right now we do not have a way this for this information to be generated automatically but it is mathematically possible for us to perform these calculations during sever weather events.