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(T. F. “Storm” Walsh)
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Greetings to everyone!
Please be aware, even though I do not post every night, rest assured I am continuously monitoring various areas for any significant weather. I will be taking Sundays off (family time), unless we have active systems that may be posing a threat (i.e. Tropical, Winter Weather, Coastal Storms, etc.).
First, I wanted to start with an explanation of the term “cap” which I’m sure most of you are familiar with hearing this term. This term is short for “capping inversion. A “capping” inversion, or inversion, is where generally, in the mid to just below the mid atmosphere, air is warmer than the layer of air below it…or where the ELR (Environmental Lapse Rate) reverses and is warmer. Normally, as you go higher in the atmosphere, the surrounding environment grows colder. When a “cap”occurs, convection cannot grow or rise any further in the atmosphere. For thunderstorms to occur, convection has to continue to rise into the upper atmosphere, and the “cap” prevents this. When this happens, energy continues to “pile up” from the surface, to where the “cap” is located. The “average” ELR is approximately 6.49 ºC/km (3.56 °F or 1.98 °C/1,000 ft) from sea level to 11 km (or 6.8 mi). The average DAR (Dry Adiabatic Rate), or lapse rate of dry air (no condensation) is 9.8 °C/km or 5.5F /1,000 ft. When these averages are present, the atmosphere is considered stable. However, in my forecasts, you’ll see me mention mid level lapse rates. For instance, the lapse rates in my last forecast, where projected to be 7.5 to 8.0 Deg C. This is considered to be a steep lapse rate, which allows air to rise almost violently until it reaches the condensation level.
Here is an excellent article from Meteorologist Jeff Haby on the “cap”: http://www.theweatherprediction.com/weatherpapers/070/index.html
There are two distinct ways to break this cap. This is from Jeff’s article:
There are several ways, however, to erode the cap. This can best be illustrated by using a Skew-T chart. The first is with daytime heating. An air parcel will rise adiabatically from the surface if the air parcel temperature on a sounding is warmer than the surrounding environmental air. In the case of the elevated mixed layer, the environmental temperature will be warmer than the temperature of the air parcel. As the heating of the day occurs, the air parcel temperature will begin to rise. If it raises enough to surpass the environmental temperature, then the air parcel will be free to rise on its own.
Another way is synoptic scale forcing or lifting. This situation does not have to rely on surface thermal heating. If a mechanism, such as a front, should come through, lifting of the air can be forced up to where it will help erode the capping inversion. Temperature changes in the vertical and mesoscale features can also aid in the erosion of the cap.
As this occurs, the warm, moist air and energy that has been trapped shoots up through he atmosphere violently, which can lead to very severe to violent thunderstorm activity, including strong tornadoes.
You have probably heard the old, simple explanation for tornado formation is, “when warm air and cold air collide, tornadoes form” This is true to an extent, however there is much more that goes on in the various levels of the atmosphere that is a little more complex, than just hot and cold air mixing. In my studies, it is noted that scientists are still not completely sure what causes a funnel to drop from a supercell, however I am going to try and give you some of the “ingredients” and try to explain some of the “workings”.
To start, yes, you need the mix of warm or hot, humid air, which is usually drawn in from the south by high pressure, off the Gulf of Mexico, and cold, drier air from the northern regions, usually originating from Canada or the Arctic, usually brought down by an area of low pressure. Two types of air come from here…cP (Continental Polar) or cA (Continental Arctic). The low pressure area we see on weather maps “mixes” everything together, and is natures way of trying to reach “equilibrium” in temperature and pressure differences. Sometimes, these differences can be extreme, so nature tries to achieve the balance quicker over a smaller area. This is what the job of the tornado is.
Favorable conditions for tornado formation include high dewpoints (60F>), a steep lapse rate, a certain amount of wind shear (both speed and directional), helicity in the atmosphere, high humidity, etc. These are not all of the factors, but you kind of get the idea.
The following composite maps are from a forecast back in 1948, when two tornadoes hit 5 days apart at Tinker Air Force Based in OK. The symbols on the maps are explained briefly, as to what they represent:
Composite charts constructed for (a) 0600 UTC 21 Mar 1948 and (b) 0000 UTC 26 Mar 1948. Features are as on preceding charts with 500-mb features in blue, 850-mb features in red and green, with surface frontal analyses and 60°F dewpoint in black. The wavy green lines indicate moisture axes at 850 mb and the red dotted lines the axes of highest temperatures at 850 mb. Maximum observed wind speeds are indicated; many winds were missing at 500 mb.
With the great temperature differences at various levels of the atmosphere, we get the development of “jets”, or jetstreams (A jet stream forms high in the upper troposphere between two air masses of very different temperature. The greater the temperature difference between the air masses, the faster the wind blows in the jet stream). You’ll notice the two different “jets” in the composite above, the 500 mb depicted in blue, and the 850 mb in red. You will sometimes hear of a LLJ (Low Level jet), this refers to the 850 mb level jet created when severe weather conditions come together. And, if you notice their direction, you can kind of see what would cause rotation or “spin’ in the atmosphere.
When these conditions are present, along with some other factors or “indices”, supercell thunderstorms can develop. These usually occur near a cold front, in the “warm sector” of the advancing weather system. The warm sector is the area between the warm front and cold front of the surface cyclone.
The cold front is what forces (lifts) the warm, moist air upward. The colder is much heavier or “dense” and the slope of the front is steeper:
SUPERCELL THUNDERSTORMS AND MESOCYCLONES
The following are two good articles from Wikipedia:
The following video shows a Supercell/Mesocyclone in action:
Most of the very severe weather and tornadic activity may occur in supercells along what is called the Dryline. The dryline separates warm moist air to the east, from hot, dry air to the west. Hot, dry air is denser than moist warm air, thus the warm moist air is forced to rise. Here is a video and short explanation regarding the dryline from the Weather Channel: https://weather.com/science/weather-explainers/news/dryline-severe-weather-tornado-storms-plains
Finally, the following article is from Meteorologist Jeff Haby regarding the dryline:
The dryline depicted on a weather map:
In my research yesterday evening, I came across a decent article from the Markowski Research Group
in the Department of Meteorology & Atmospheric Science at Penn State University. The article is from Markowski and Richardson. Please take the time to read this…it is a very good article, at least from my perspective:
HOW TORNADOES FORM (LINK)
The following explains some of the indices you hear me mention in my severe weather forecasts:
ENVIRONMENTAL PARAMETERS AND INDICES FROM NWS LOUISVILLE, KY.:
You may direct any questions by contacting me personally, ANYTIME, at: email@example.com
Have a blessed weekend!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
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