Tuesday, February 28, 2012

Severe Threat Very Small Today

Yesterday I showed you the conditions coming together for a conditional threat of severe weather across East Texas. Today after looking at the latest data it appears the EML (elevated mixed layer) or cap will be to strong across East Texas for thunderstorm development. All the other conditions I mentioned in yesterday’s write up are still coming together so if A storm were able to develop, it would likely become severe rapidly producing large hail, gusty winds, and tornadoes. But that just doesn’t appear likely.When we take a look at the forecast sounding for this afternoon we notice a very large area of CIN (convective inhibition). This is a large amount of negative energy (blue shaded region), or sinking air, that would not allow upward motion of air parcels allowing for thunderstorm growth.
Now yesterday I mentioned this as well and was concerned what would happen if we saw a few more breaks in the clouds allowing for warmer afternoon temperatures, like we saw yesterday. Well when the forecast sounding is modified for a surface temperature around 76°F it looks a lot more conducive for thunderstorm development. Notice the area of CIN (blue shaded area) is much smaller than the actual forecast. Also the amount of CAPE, or buoyant energy (yellow shaded area), is much greater. This could allow for explosive thunderstorm development if we see surface temperatures warm into the upper 70s this afternoon.
Again in theory severe weather is possible this afternoon based on forecast soundings. That is why looking at the actual sounding is so important. So this morning I wanted to see what was happening downstream from East Texas because this air mass will be over us later today. By looking at the actual sounding from Del Rio area we can see the EML is very strong with a temperature of 15°C at 800mb. We you modify the forecast sounding for that 800mb temperatures the convective temperature, or the temperatures at where air rises freely allowing thunderstorms to develop, is well into the 80s which we will not see today. So based on all the information I see right now East Texas appears to be safe from severe weather this afternoon. We will see a line of showers and storms move in overnight but by that time the dynamics for severe weather and tornadoes will be off to our north and east. But if the temperatures are able to reach the upper 70s to near 80, there could be an isolated severe storm late today but right now that does not seem very likely at all.

Monday, February 27, 2012

Conditional Severe Weather Threat Tuesday


A strong area of low pressure at the surface will develop in western Kansas tomorrow afternoon and race off to the northeast into south central Nebraska during the evening hours. As this system develops strong southerly winds will bring warm moist air into East Texas which could set the stage for strong to severe thunderstorms late Tuesday into Wednesday morning. Instability is forecast to be marginal for severe storms but with a little more sunshine than predicted, things could get much worse.

Right now forecast models are indicating Mean Layer CAPE values to be at or slightly above 1000 J/kg. This is basically the beginning threshold for the possibility of severe thunderstorms. This will provide enough energy to accelerate surface air upward causing clouds, and eventually thunderstorms to develop.

Now that we see there is just enough energy to produce severe convection we need to see what is happening to the air as it rises to predict what kind of storm will develop. For supercell thunderstorms to develop we usually like to see effective shear, or the amount of shear the entire updraft will encounter, to be greater than 40 knots. Tomorrow afternoon much of East Texas is seeing effective shear greater than 70 knots. Now usually this would set off an alarm in my head but with the MLCAPE only being around 1000 J/kg, many of the updrafts will actually be sheared apart before a storm can develop. More on this later. So if a storm develops it would become a supercell.

The amount of storm relative helicity or the amount of spin available for the lower levels of the storm to ingest, are also showing high enough levels to produce low level storm rotation. We usually look for values at or higher than 200 and much of East Texas tomorrow afternoon is above 300. So if a storm can develops it would easily be able to produce low level rotation which could lead to tornadogenesis.

Now that we now the storms that develop tomorrow should be supercellular, what type of supercell is likely to develop? For this we need to take a look at the storm relative anvil winds to give us an idea of the titling available to the main updraft. High precipitation supercells develop in areas with numerous storms, which won’t happen tomorrow, or storm relative anvil winds less than 40 knots. Classic super cells tend to form when the storm relative anvil winds are between 40 and 60 knots with anything higher than 60 knots leading to low precipitation supercells. The storm relative anvil winds tomorrow are between 50 and 70 knots, so classic and low precipitation supercells would more than likely be the storm mode. This means the rain cooled down draft will be well displaced from the updraft helping to keep the storm strong and making it live longer.

So now that we know what our storm mode should be, it is time to see if the parameters are there to allow the low level rotation that should occur with any storm that develops could turn into a tornado. One area we look at is the storm’s inflow. Inflow of 20 knots or greater is usually a good indicator that tornadoes could develop. Tomorrow most of East Texas will see storm relative inflow greater than 25 knots. We also need to see the directional shear meaning winds changing direction as you move up through the atmosphere. Now in tomorrow’s set up the wind are fairly unidirectional, or blowing from the same direction, as you move up through the atmosphere. Normally this would limit the tornado threat. However when you factor in the storms motion, the low level storm relative winds become southeasterly setting the stage for directional shear.

Another factor we look for in tornado development is the CAPE in the lowest level of the atmosphere. This is the amount of energy to force air to rise. The stronger the lowest level CAPE is the better chance there is any surface vorticity can be stretched into the storm’s low level rotation which in turn can lead to the development of a tornado. Tomorrow we are seeing 0-3KM CAPE values approaching 80J/kg over much of East Texas leading to a VGP, vorticity generating potential of greater than 0.40. VGP values greater than 0.30 can lead to tornado development.

Finally we like to see what the lowest level shear values will be to determine tornado development. Usually 0-1km shear levels above 20 knots are favorable for tornado development. Tomorrow much of East Texas is seeing that level between 25 and 35 knots.

So all the factors are coming together for what seems to be a fairly active severe weather day. So why is there not a higher risk of severe weather tomorrow and the threat right now is only figured to be conditional? Remember earlier I mentioned many times when the bulk shear is as high as it will be tomorrow the storm’s updraft can be sheared apart. With the lack of forcing and no surface convergence, storms will develop from rising air caused by differential heating across the area. The image below is a Skew-T forecast for tomorrow evening at Tyler, TX. I have pointed out a very strong cap located around 700mb, or about 9,000’ in the atmosphere. This will put a lid on all thunderstorm development which is why there is not a greater risk of severe weather. If the storm system would move a little farther south then the forcing, or lift across East Texas would be greater allowing for a few thunderstorms to develop. What we really need to watch out for tomorrow is any additional clearing which could happen tomorrow afternoon. With the surface winds being out of the south southwest tomorrow, slightly drier low level winds could remove the lower cloud deck. This would allow temperatures to approach the convective temperature, which is around 77°F tomorrow afternoon. If this does happen then isolated thunderstorms will develop, like we see in the summer. But these storms will develop in an atmosphere that is very likely to develop not only severe storms, but tornadoes as well.

So we will have to monitor what happens throughout the afternoon tomorrow. A little more sunshine and it could get very active. If the clouds hang around then the likelihood for severe weather will diminish and very little rain will occur until the cold front moves into East Texas early Wednesday morning.

Tuesday, February 21, 2012

Severe weather awareness week: LIGHTNING


One of the most deadly of all weather phenomena is lightning. Lighting is basically a large spark of static electricity within the clouds or between the cloud and the Earth. Over the past 30 years lightning has killed an average of 55 people per year but that number continues to drop thanks to educating the public on lightning safety. In fact, last year had the lowest amount of lightning deaths on record with 26.

As a thunderstorm develops, air inside the storm moves up and down in the form of updrafts and downdrafts. Inside the currents of air are ice particles, whether small ice crystals or large hailstones. These ice crystals will collide with each other causing a buildup of static electricity, much like what happen when you rub your feet across the floor only to get shocked by the door knob as you reach for it. The thunderstorm is doing the same thing. As the ice particles continue to collide in the storm, some become positively charged and some become negatively charged. The positive charged ice crystals will move towards the top of the thunderstorm while the negative charged ice crystals will settle across the bottom or mid level of the storm. As the storm moves over the ground the surface charge becomes positive. Once the difference becomes great enough a discharge of static electricity will occur just like the shock that comes off your finger as you reach for the door knob. Only in this case the shock is a bolt of lightning that can reach a temperature of 50,000°F. As the air heats around the lightning bolt it will expend very rapidly causing a loud boom or thunder.

In some cases the positive charge at the top of the storm can become so great that a large lightning bolt will race from the top of the cloud towards the ground well out ahead of the storm. This is called positive lightning and occurs because the ground well ahead of the storm has not yet become positively charged. In some cases this lightning bolt can hit up to ten miles away from the storm. This is the most dangerous type of lightning as many times it will strike when you don’t know a storm is near you due to the lack of clouds and it usually has the strongest amount of electric current.

So next time you hear thunder it is time to move indoors. For every five seconds you can count between lightning and its thunder is a distance of 5 miles. So even the faintest sound of thunder means it is close enough to strike. Also, it is a great idea to wait 30 minutes after the last sound of thunder to make sure the storm has passed to a safe distance.

Monday, February 20, 2012

What's going on???

Not sure why I see Photobucket all over the blog this morning. Might need to update the template. Sorry for the inconvenience.

Severe weather awareness week: HAIL

Today starts severe weather awareness week in the state of Texas. Each day we will write a brief story on one type of severe weather to educate you on how this severe weather phenomenon occurs so you can take precautions to protect life and property. Today’s discussion will cover hail.

Hail can be a very dangerous and damaging form of severe weather. Here in East Texas we are hit every year with many hailstorms, some of which have produced a substantial amount of damage. Yours truly has replaced the roof on my home twice in 4 years and had a car totaled by hail. In fact, nearly 40 percent of all home insurance claims in the state of Texas are from hail damage.

So how does hail form and why are some stones much bigger than other stones? Hail forms when small ice particles are held above the freezing level by strong thunderstorm updrafts allowing for super cooled water droplets to freeze on these ice particles causing the hail stone to grow. As long as the updraft is strong enough, the hail stone will continue to grow. At times the hail stone will fall out of the main draft falling towards the ground only to be picked back up by the updraft before hitting the ground. This will cause the stone to take a roller coaster ride up and down inside the storm collecting more water droplets that will refreeze causing the stone to become even large. So the stronger the updraft, the larger the hail stone. Hail the size of peas will be supported by updraft speeds of 24 mph where as softball size hail need an updraft speed of 104 mph! The largest hailstone to hit the United State fell in Vivian, SD July 23 2010. This hailstone measured 8” in diameter and weighed nearly 2 pounds. A stone this size can reach downward speeds of over 120 mph! So it is easy to see how hail can be very deadly and damaging during severe thunderstorms.

Severe weather awareness week: HAIL

Today starts severe weather awareness week in the state of Texas. Each day we will write a brief story on one type of severe weather to educate you on how this severe weather phenomenon occurs so you can take precautions to protect life and property. Today’s discussion will cover hail.

Hail can be a very dangerous and damaging form of severe weather. Here in East Texas we are hit every year with many hailstorms, some of which have produced a substantial amount of damage. Yours truly has replaced the roof on my home twice in 4 years and had a car totaled by hail. In fact, nearly 40 percent of all home insurance claims in the state of Texas are from hail damage.

So how does hail form and why are some stones much bigger than other stones? Hail forms when small ice particles are held above the freezing level by strong thunderstorm updrafts allowing for super cooled water droplets to freeze on these ice particles causing the hail stone to grow. As long as the updraft is strong enough, the hail stone will continue to grow. At times the hail stone will fall out of the main draft falling towards the ground only to be picked back up by the updraft before hitting the ground. This will cause the stone to take a roller coaster ride up and down inside the storm collecting more water droplets that will refreeze causing the stone to become even large. So the stronger the updraft, the larger the hail stone. Hail the size of peas will be supported by updraft speeds of 24 mph where as softball size hail need an updraft speed of 104 mph! The largest hailstone to hit the United State fell in Vivian, SD July 23 2010. This hailstone measured 8” in diameter and weighed nearly 2 pounds. A stone this size can reach downward speeds of over 120 mph! So it is easy to see how hail can be very deadly and damaging during severe thunderstorms.

Friday, February 3, 2012

Severe storms possible today

A strong storm system across the High Plains today will drag a cold front into East Texas providing a chance of rain and thunderstorms today and tonight, some of which could be strong to severe. The severe storms we saw across East Texas last night have moved out of the area and this morning we are left with low clouds fog and drizzle. With a few breaks in the low clouds later this morning, peaks of sunshine will help warm us into the lower to middle 70s. This will provide the energy needed for thunderstorms to develop across the area this afternoon into the evening hours. All of East Texas is under a slight risk of severe storms this afternoon with the main threat being gusty winds. If we can see enough sun to raise temperatures in to the middle 70s area wide, then there would be enough energy in the atmosphere for a few hail storms as well.

As for the tornado threat, right now it looks pretty small across East Texas. It looks as though a broken line of storms could develop to our west this afternoon. These storms will move into the area during the late afternoon and evening hours. The overall low level shear across East Texas at this time is not that impressive but if any storm is able to remain isolated, it could tap into what shear is available and produce an isolated tornado. The greatest tornado threat today looks to remain to our northwest near the Wichita Falls area where low level shear will be much greater this afternoon.

A second line of thunderstorms looks to develop this evening along the cold front that will be moving into the area. These storms could also produce strong gusty winds and small hail as they move through late tonight into the early morning hours on Saturday. By Noon Saturday the front and its line of storms should be east of the area allowing for drier and slightly cooler air to move in. By the end of this event much of East Texas could receive another inch of much needed rainfall.

Wednesday, February 1, 2012

A Severe Storm or Two is Possible Today

There is a conditional threat of severe across parts of East Texas this afternoon, mainly north of interstate 20. A few breaks in the low level clouds late this morning is already allowing surface temperatures to approach 70 degrees. At the same time colder air in the mid levels of the atmosphere are spreading across the northern counties of East Texas. This imbalance will cause air to rise in a few areas leading to an isolated thunderstorm or two. Any storm that develops this afternoon will have the potential to produce hail, possibly up to 2 inches in diameter. The upper air disturbance that is moving through the area will exit East Texas later this evening and our thunderstorm chances will diminish. Again this is a conditional threat meaning there is a good chance no storms will develop at all. But if they do, we will probably see a few large hail reports this afternoon.