Surprise.......SLEET!!!!!

A weak disturbance moved through Heartland this evening and produced a little surprise for a few, sleet.  This system moved into very dry air and was forecast to die out before making it into our area. The image below is from the forecast sounding for Cape Girardeau, a profile of the atmosphere.  The red line indicates the air temperature as you move up the atmosphere, the green line shows the dew point.  The larger the difference between the two, the drier the air.  This sounding showed very dry air. The dark blue line is the freezing line.  You can see as you move up in the atmosphere the air temperatures warmed above freezing.  More on this later.

 As the precipitation moved into the area it encounters that warm level of above freezing temperatures. This of course causes the snow to melt.  Now with the new Dual Pol Doppler Radar we can see when the snow begins to melt.  The sleet shower was moving between Poplar Bluff and Dexter at the time of the image below.  Notice most of the colors here showing a dark red color.  This indicates the precipitation falling is all of the same type.  Since the radar beam was hitting the shower at an altitude of 7,500 feet here, it was below freezing and all snow.

The next image sampled the shower from an altitude around 5,800 feet. Here we have a mix of yellow and dark red.  The yellow colors indicate different types of precipitation in the shower.  Since the precipitation was all snow at 7,500 feet and seeing that warm nose of air from the sounding, we know snow is melting so this radar image shows rain and snow mixed. The warm nose was thick enough to allow for the snow flakes to melt  so when the precipitation fell out of the cloud, it was rain with a tiny bit if ice left in the center.

In addition to being warm, the air is very dry.  so as these drops of rain fall through the dry air they begin to evaporate.  As this happens a process called evaporational cooling takes place as the drops cool to the wet bulb temperatures.  Because the air was so dry the wet bulb was below freezing.  So the drops will refreeze on the way to the surface, even if the air temperatures is above freezing. But in this case the air temperature briefly drops below freezing again as well so there was no problem in seeing drops freeze into sleet. Weather is always fun!

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. 

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.

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!

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............

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.

Severe storms possible Wednesday

A strong area of low pressure will develop along a cold front and move across East Texas Wednesday evening.  As it moves towards East Texas scattered strong storms will develop Wednesday night.  Right now the best timing on the front’s arrival is around Midnight Wednesday Night Thursday morning.   Behind this front, strong northwesterly winds will usher in some fairly chilly weather for the end of the week.

Right now the greatest threat of severe weather appears to be strong gusty as the mid level flow across East Texas will be between 70 and 90 miles per hour.  The taller storms will be able to tap some of this wind and bring it down to the surface.  So it does appear likely the stronger storms will be able to produce winds of up to 80 miles per hour across East Texas late Wednesday night in to Thursday morning.  So start to plan now to remove loose objects from the yard and maybe the inflatable yard Christmas decorations just in case.

The overall threat of tornadoes right now appears rather low.  There will be a lot of low level shear favorable for tornado development but with surface winds remain out of the southwest ahead of the front, the amount of low level spin for tornado development appears less favorable.  There maybe one or two brief weak tornadoes but at this time the winds damage threat appears to be the greatest across East Texas.  Stay tuned for the latest developments. 

Very Active Weather Pattern The Next Few Days

A strong polar front is diving south towards East Texas this weekend and will provide some active weather that includes a chance of rain, storms, and maybe even a flurry or two across parts of East Texas.    For all the snow lovers out there, don't get too excited because it probably won't happen but, the chance is not zero.  Here is what looks like may happen this weekend as of Friday morning.

Right now a weak cool front is located across north central Texas with an area of low pressure developing west of the Hill Country.  This front will drift towards the I-20 corridor before washing out Saturday night.  Now the forcing along this front providing lift is fairly weak but, there looks as though enough instability will develop across East Texas to where a thunderstorm or two could develop Saturday afternoon.  The atmosphere will be conducive for a severe thunderstorm or two as surface based instability will be sufficient for robust updrafts which could produce hail.  Surface based Lifted Index will be negative, approaching -5, allowing for strong updrafts to develop.

The winds throughout the atmosphere on Saturday appear as though they will be out of the southwest limiting the tornado threat.  The shear will be strong enough for an isolated supercell or two mainly along and north of I-30 Saturday afternoon so if there were to be an isolated storm that becomes severe this would be the most likely location.

On sunday a much stronger front will move towards the area allowing for a line of scatter thunderstorms to develop on move southeast across East Texas.  Like Saturday, the surface based instability will be more than enough for robust updraft development as the surface based lifted index will range between -4 and -6 across East Texas.

Now unlike Saturday, the shear across east texas will be more than enough for supercell development.  0-6km bulk shear will be around 50 knots which will allow for the atmosphere to provide enough shear for rotating updrafts increasing the threat of large hail.

The low level shear however will not be supportive for tornado development.  Again the winds throughout the atmosphere will be mainly out of the southwest limiting the low level spin available for low level mesocyclone development.  The 0-1km bulk shear will be around 10 knots which is not the optimum level for tornado development.  Although not unheard of the overall tornado threat will these perimeters is fairly low.

So we will probably see a line of strong thunderstorms move across East Texas late in the day in tot eh evening hours with a threat of strong gusty winds and hail.

After the front moves through MUCH colder air will move into the area.  There are now indications as well that an over running precipitation event could unfold on Monday.  This is because winds aloft will still be out of the southwest forcing warm air to lift over the colder air at the surface providing clouds and the possibility of precipitation for the morning hours.  It still looks as though skies will clear by Monday afternoon but even with the sun breaking out, northwest winds ushering in the coldest air of the season will keep high temperatures in the 40s for most of East Texas, possibly upper 30s across the I-30 corridor if the skies do not clear soon enough.

As for the chance of flurries across the I-30 corridor, the farther west you live, the better chance of seeing this occur, all be it the chance is VERY low!  It looks as though most of the precipitation that develops behind the front will evaporate before reaching the ground but across north central Texas, from Dallas , northwest, there will probably be a few flurries Monday morning.  If these flurries can hold together we may see a few make there way into the Greenville and possibly Sulphur Springs areas by 9 or 10 AM Monday.  This is a very similar set up to a cold front that moved through last December giving a few area of East Texas some flurries, our only winter weather event of the winter season.  If this does happen, and right now this looks to be a big if, there will be no travel problems what so ever, just an added chill to get you in the Holiday Spirit.

Sleet Last Night?

 That’s right.  A few areas of East Texas saw a mixture of hail, sleet, and graupel (snow pellets) late last night and early this morning.  So what happened?  Well a fast moving disturbance brought a lot of high clouds to the area last night and in a couple of areas these clouds developed into small thundershowers.  The image below shows the profile of the atmosphere early this morning.  Here we can see the cloud base was just over 10,000’ in elevation.  When air was lifted from this level it encountered elevated instability leading to the development of elevated showers and thundershowers.  Since most of the cloud’s temperature was below freezing, snow and snow pellets were forming instead of rain.  In the strongest updrafts the graupel, snow pellets, were held aloft allowing water vapor to freeze encasing them in ice until they grew too heavy for the updraft to hold allowing hail, some up to dime size, to reach the surface.

Outside the heavy core mainly snow and graupel feel from the cloud base where above freezing temperatures were encountered below 10,000’.  Here the ice crystals began to melt but not completely as the dry air began the process of evaporation.  As the precipitation evaporated it cooled rapidly allowing for the water droplet to refreeze into sleet.

In the lightest areas of precipitation most of the precipitation evaporated completely before reaching the ground.  The image above shows a radar shot of the shower across the Lake Fork area.  In addition to the precipitation the elevated instability allowed for the collision of ice crystals causing a large amount of static charge producing numerous cloud to ground lightning strike while over the Lake Fork area.

Henderson Mini Supercell

Sunday afternoon a small but intense thunderstorm developed across Cherokee and Rusk Counties that on radar showed what appeared to be weak but persistent rotation for over one hour as it moved northeast paralleling highway 79 south of Henderson then highway 43 north of Henderson. After looking at the radar data it appears we had a mini supercell develop across East Texas. The storm was never more than 5 miles across and stayed for the most part under 30,000’ in elevation. This is typically not the type of storm that would produce severe weather but in Sunday’s environment, this little storm was able to ingest a lot of low level shear and spin like a top as it moved across the landscape.

At 3:10PM Sunday afternoon the thunderstorm in question was located near New Summerfield in eastern Cherokee County. At this time the storm’s top was just under 20,000’ in elevation and had a reflectivity core of with a maximum of 54.5dbz under 10,000’. This indicated basically a heavy rain shower, more than likely not even producing any lightning. But at the same time the velocity data did show rotation up to 8000’ in elevation.

Now the rotation was by no means strong but was the beginning of a long lasting rotation that would last in this storm for over 1 hour.

At 3:44PM the storm in question showed an increase in the inflow velocity a sign that it could be attempting to produce a tornado. Again the rotation was not very strong but at this point it had lasted for over 30 minutes and was showing an increase in strength. At this time the core was 62dbz at 10,000’ but the top of the storm was just barely reaching 30,000’. It was probably producing a few lightning strikes but again it was nothing you would normally consider severe.

At 3:48PM is when the storm finally peaked showing its mini supercell characteristics. At this time there was rotation, again not very strong, but it was the tightest it has been the entire life span. It is also around this time when a very suspicious photo was sent to us showing what appears to be a well organized mini mesocyclone with a funnel cloud lowering towards the ground. As of this writing it is unknown if the funnel ever touched down to produce a tornado due to no damage reports being submitted.

As mentioned before, the rotation on radar was not particularly strong and thus really not considered to be a tornado threat. But when you look at the strength of the rotation based on the diameter of the mesocyclone, it is not out of the question that a mini supercellular tornado occurred. A study by the National Weather Service Shreveport in 1997 developed a chart for warning tornadoes during mini supercellular events. A nomogram was developed in 1997 as a guideline for issuing tornado warning based on the rotational velocity (the absolute values of the inbound and outbound velocities divided by two) in a mesocyclone. Based on the velocity data on Sunday the storm in questioned rotational velocity was between 15 and 19, well below the threshold guideline for issuing a tornado warning.

But what the study found in the case of mini supercells was the rotational shear (rotational velocity times 2 divided by the diameter of the mesocyclone) was a better indication of tornado development. When using the rotational shear we come up with a value of 12.3 near the time of the suspicious photo based on a mesocyclone diameter of only 1.59 nautical miles on radar at 3:48PM. This places the rotational shear in the tornado possible area of the rotational shear guideline chart.

Since the only photo we have has the bottom of the funnel block by trees, and we have had no damage reports from the area, we will never know if a tornado was produced. Hopefully a few more photos will surface that are not obscured by trees to give us a better view of what occurred.