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(T. F. “Storm” Walsh III)
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Good day everyone!
Since it appears that no major changes in the weather should occur over the next 3-5 days, I wanted to try and touch on tornadoes safety and tornadoes in general, with some, what I would call, basics as far as formation.
When a TORNADO WARNING is issued for your area, the following link to NOAA’s ready.gov site could save your life:
TORNADO SAFETY TIPS:
The following is somewhat the “basics” as to how a tornado develops. We as meteorologist pretty much understand the mechanics, however it is still an “unknown”as to where a tornado may exactly touchdown.
Tornadoes, like hurricanes, are used by nature to distribute excess heat from the tropics (as far as hurricanes, to the temperate latitudes and poles), while Tornadoes try to achieve a balance between extreme varying conditions of heat/cold, moisture/dry air, over a much smaller area, thus being more intense.
Tornado formation is more than just warm, moist air, meeting cold dry air. Many different items come together in various levels of the atmosphere. High humidity throughout at least to the mid levels of the atmosphere, high dewpoints, enough potential energy, lift in the atmosphere, wind shear, helicity, weak cap strength, or in the case of a cap, a way for rising air to “break” the cap, etc. I am going to try and touch, very briefly on some of the items, in the simplest terms possible. I will provide links for definitions, so you can get a better working understanding of what goes on in the atmosphere for tornado formation.
A tornado, according to research begins as a horizontal “tube” of air near the surface, or below mid levels of the atmosphere. Wind shear, or the changing of direction and / or speed of the wind with height. This causes rotation or spinning of the column of air. If certain forces are strong enough to “lift” the column, it becomes vertical, and takes on the familiar funnel cloud. Here is a simple explanation:
You may have heard me mention things such as a low level and mid level “jet” or “jetstreak”…when conditions are right, we can see a similar situation as what the jetstream is, which is right around the 250-200 mb level, or near 35,000-40, 000 ft in the atmosphere. A jet is usually winds of 50-60 kts or more. A low level jet usually is found at the 800 mb level (5,000 ft) and mid level jet at the 500 mb level (approx. 18, 000 ft). A strong upper level “jetstreak” within the jetstream also aids in this process. When these winds come in from different directions, this is what we call directional shear. And since winds lower in the atmosphere generally are slower moving, we have speed shear as well. Shear is important to a severe thunderstorm, as for one, it provides the turning in the atmosphere for the familiar “mesocyclone”, form which tornadoes can develop. Shear also tilts the top of the thunderstorm cell, which doesn’t allow the cooler temperatures from precipitation cut off the energy, instead, the being tilted ahead of the system allows for warm, moist air to keep fueling the “supercell”. The difference in the wind direction at different levels along with faster wind speed as you go higher in the atmosphere, is what produces rotation, (vorticity, in this case Positive Vorticity or PVA) in the atmosphere.
Parameters such as CAPE (SBCAPE and MUCAPE) along with “Lift”, add to instability, and strength of the “updraft”.
Sometimes, even if all the parameters are “favorable” for severe thunderstorms, sometimes a “CAP” or Capping Inversion is present, not allowing for warm moist air to rise as far as it needs to to develop these thunderstorms. This may occur often near the “dryline”. However, certain conditions can occur to allow the cap to ‘break”… this is when we see the most violent severe weather.
Again, there are numerous factors that dictate the formation of severe thunderstorms and tornado development…I cannot mention them all…however, the following composite map shows some parameters in place for a severe weather setup:
An example of a composite chart; in this case it is a composite prognosis , depicting 12 h forecast positions of surface and upper air features at 0000 UTC 27 April 1991 based on initial data from 1200 UTC 26 April 1991. Black symbols are conventional surface frontal features, green lines denote 50 and 70% mean (surface to 500 mb) relative humidity, red streamlines are at 850 mb with a 50 kt maximum indicated, blue stream line is the 500 mb jet stream axis, with a 76 kt maximum indicated, the blue dashed frontal symbols depict 500 mb thermal trough axes, and the orange lines are isopleths of the forecast lifted index.
I will post links to the weather acronyms used, after the following.
When I perform my analysis, I look at a lot of the items mentioned. One thing that is important, is analyzing forecast soundings. Just some of the items I look at are CAPE, MUCAPE, LIFTED INDEX VALUES, STP, EHI, SWEAT, and VGP. Yeah, I know…more acronyms, but as I said, I’ll post them and links for what they mean. The F5 DATA software I use, retrieves all of this information for me in color, and represents the forecast sounding data projected by the RAP model, out to 12 hours only, the NAM out to 84 hours, and GFS which goes out to 2 weeks. However, these parameters are analyzed for the day of projected severe weather from the Storm Prediction Center, regarding the day 1 convective outlook. This is where I derive the outlines I draw. For the most part, the models can agree fairly good. However, when some differences do occur, I use a blend of SPC graphics, RAP model solution, and NAM model solution, to err on the side of safety. The following are samples of some of the indices and parameters, which will be included in the linked definitions:
SEVERE WEATHER ACRONYMS DEFINED
LIFTED INDEX: http://www.theweatherprediction.com/habyhints/300/
VGP: Vorticity Generation Potential. Another experimental field used to predict supercell tornadoes. Based on the work of Rasmussen and Wilhelmson (1982) it relates CAPE and shear. It is defined as the sqrt(CAPE) times the 0-3 km mean shear. The units are m/s2 (an acceleration) or m/s times 1/s which is the product of w times vorticity – a tilting term. It is possible to have high values of VGP when helicity (and the EHI) is low. The VGP climatology shows that supercell tornadoes rarely happen with VGP less than 0.3, and go from unlikely to likely as VGP goes from 0.5 to 0.6, and are likely above 0.6. It’s also possible that the larger the VGP the stronger the tornado. The VGP was developed by Erik Rasmussen for use over the central plains and may or may not be valid over other regions. Problems – Unknown. But, since it depends on the assumed CAPE and depth of the inflow wind layer, it might inherit their problems.
HABY HINTS SEVERE INDICES PAGE
Please visit the following link for radar and satellite loop images:
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS