Wednesday, November 14, 2012

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.

Monday, November 12, 2012

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.

Monday, November 5, 2012

Severe Weather This Weekend?

Long range models are still pointing to a very active weekend weather wise across the central portions of the United States. A deep trough will swing across the Rocky Mountains Saturday and move into the Plains on Sunday. This will cause a strong area of low pressure to develop and swing a cold front through the area. Now there are still some differences on the exact timing of the event, but confidence is rising with the threat of strong to severe storms across the mid section of the nation Saturday and Sunday.

Right now it appears two areas of low pressure will develop between Friday and Sunday across the high plains ahead of the aforementioned trough. This will cause strong southerly winds to set up all across the Plains States this weekend including East Texas. The second area of low pressure that develops near the Texas Panhandle Sunday will be the one that drags a cold front through the area with the possibility of strong to severe storms Sunday afternoon.

 Right now the limiting factor across the area in preventing severe storms will be the minimal amount of buoyant energy available for thunderstorm growth. Models are paining between 500 and 1000 J/kg of Mean Layer CAPE across the area and we would usually like that number to be greater than 1000 to get strong convection or storms to develop.
 But the surface based Lifted Index, or LIs, are all negative and approach -4, not extremely unstable but are significant when overall CAPE values are low.
 By looking at the profile of the atmosphere we notice the CAPE, energy for thunderstorm growth, is what we call short and fat.
 This means even though there is not a substantial amount of CAPE for thunderstorms the updrafts that do develop have the potential to be very strong. This would increase the threat of severe weather including hail and tornadoes.

 Now the wind shear available on Sunday in more than adequate for severe weather across the Plains. The overall bulk shear up to the mid level of the atmosphere is close to 50 knots in most areas and approach 70 knots near the Red River Valley.
We normally need to see around 40 knots to see supercell development, storms that are capable of producing very large hail and tornadoes. In addition, the 0-1km shear is on the order of 30 knots across most of the area.
Levels of 20 knots or greater are usually what we look for in tornado development. Third, the winds just of the surface will be between 50 and 60 knots so any thunderstorm downdraft could easily produce winds over 60 miles per hour.

So the winds energy is more than adequate for high winds and tornadoes if storms can remain scattered.

 The good news, if there is any, is it looks as though this situation will result in a fast moving squall line which will cut down on the tornado threat. The shear vectors will run parallel to the front causing storms to rapidly form into a line. This will increase the threat of widespread wind damage but limit the overall tornado threat. Based on what the models are advertising right now it looks as though storms would begin to develop around midday Sunday. As they initially form there would be a tornado threat due to all the parameters mentioned earlier. But the storms would rapidly form into a squall line before moving into East Texas meaning our main threat would be hail and high winds.

 Again we are still a long way out from this potential event and a lot could and probably will change between now and then. The timing is 24 hours slower than that of my last update and could easily change again. The forecast will be fine tuned as we approach this weekend and all the parts of the puzzle come together. Remember we are entering a period of the year that climatologically produces severe weather. Stay tuned!

Friday, November 2, 2012

Severe Weather Possible Next Weekend

This weekend there will be a chance of thunderstorms across East Texas as a weak cool front moves into the area.  As we transition from summer temperatures to cooler autumn temperatures, the clash of air masses occur more frequently across our area giving us a second severe weather season that typically peaks around the second week in November.  This weekend it appears there is a slight chance one or two of the storms could produce gusty winds and small hail but the overall threat of severe weather looks minimal at this time.

That could change though for next weekend.  Long range forecast models continue to hint at a major trough setting up over the Rockies and sending a very strong jet streak across the Central Plains sometime next weekend.  Now this is still a long way out and the position of the trough and jet streak will probably change but with the models consistently showing this feature it appears likely a major severe weather event could occur somewhere across the central part of the United States next weekend. 

At the writing of this article it appears the greatest threat for severe weather would be form Iowa southwest into North Central Texas.  As the strong jet streak crosses the Rocky Mountains, an area of low pressure will rapidly develop in Eastern Colorado and move northeast towards the Great Lakes by Sunday evening.  Out ahead of this low pressure, strong southeasterly winds will set up all across the Plains bringing in moisture from the Gulf of Mexico.  Above these southeasterly winds, winds will be from the southwest wrapping around the low pressure as it moves across the area.  This will give the atmosphere plenty of spin needed for severe weather.

Now in order to get the severe storms we will need enough instability for thunderstorms to develop.  Right now the models are forecasting just enough instability to get marginal strong to severe storms.  But there are other factors that lead me to believe the storms could be much stronger.  The area across the southern plains is forecast to be under an area if diffluent winds, or winds that spread apart aloft.   We call this upper air divergence and it causes the air to rise from the surface.  This will cause the updraft in a thunderstorm to be stronger than just factoring in the available instability. 

So will this be just a severe thunderstorm event with strong winds and hail or will this be a tornado outbreak?  Well this far out it is too hard to tell.  We need to wait to see how the mesoscale features develop between now and then.  There will be an awful lot of forcing with this system so it appears likely a strong line of storms will develop and sweep across the central parts of the country.  However, the low level shear is forecast right now to be very favorable for tornado development.  So if the line of storms can remain broken there would be an increased threat of tornadoes as it moves through.  But with the amount of forcing that is forecast it seems right now at least the more likely scenario would be an intense squall line with lots of wind damage and a few isolated tornadoes.   Again it is way too far out to even guess at the exact location of severe weather next weekend but it does appear likely there will be a threat somewhere across the central United States from Saturday into Sunday.  More details as we get closer to the event.