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.