Last night another tragic bout of severe weather struck the U.S., Midwest, with a strong tornado leveling a large section of Joplin, Missouri (see video below). It may well happen again today. Looking at the climatology of U.S. tornadoes (below), you see that although the High Plains are struck most frequently, the southeast, and the upper Midwest have significant activity.
Why does this part of the U.S. get hit so often by severe thunderstorms?
It is a fascinating, but sobering, fact that the U.S. High Plains, Midwest and the adjacent southeast U.S. have the highest frequency of severe convection in the entire world.
It turns out that nearly every geographical and meteorological aspect conducive to severe convection comes together here.
Ingredient 1: Strong Instability.
To get big thunderstorms you need an atmosphere ready to convect, to mix in the vertical as a result of vertical instability. Such instability is aided by lots of low-level moisture and a large change of temperature with height. A good measure of such instability is a quantity called CAPE---Convective Available Potential Energy. Throw that term around at a party and people will take notice! Just think of it as a measure of the amount of energy available to drive the thunderstorms. Examining a plot of the average springtime CAPE (below) a fascinating fact is apparent (in image, darker values are higher)--no other midlatitude area in the world has as much CAPE as the High Plains of the U.S.! The highest values tend to be in the tropics, and no where does high values extend so far to the north as east of the Rockies.
The super-deluxe CAPE has a lot to do with the very warm Gulf of Mexico, which not only warms the air, but adds lots of moisture. This warm, moist air then moves northward across the eastern U.S., where it is heated further at low levels. We have the fuel! But truly severe convection needs more.
Ingredient 2: Large Vertical Wind Shear.
Vertical wind shear is a measure of how wind changes with height, either in direction or speed. A problem with typical thunderstorms is that they are essentially suicidal. The cold downdraft air they produce (due to cooling by evaporation and falling precipitation dragging the air downward) spreads out and eventually kills the warm, updraft air thunderstorms need to survive. A few years ago we learned that there was a way around this thunderstorm killer--- you guessed it, wind shear. Although it is too involved to go into now, large wind shear allows thunderstorms to organize in a way so that they can survive many hours, long enough to become severe. But wait, there's more! Large wind shear can also help produce rotation in strong thunderstorms, producing something call a mesocyclone, which in turn can spin up into a tornado!
Now guess what area has a large amount of wind shear during the spring, when CAPE is high?---the High Plains and the southeast! A Bermuda High off the SE U.S. causes southerly flow to move northward off the Gulf of Mexico (figure), while aloft the air is from the west.
The result: a consistent large shear from southerly at low levels to westerly aloft over the region. Suicide avoided and mesocyclones enabled.
Ingredient 3: Low Level Moisture
In some sense this is part of Ingredient 1 as well. Strong thunderstorms need moist air at low levels, since the condensation of water vapor is an important energy source for them. When water condenses it releases a large amount of latent heat---heat that it gained when the sun evaporated the water in the first place. The air coming off the Gulf of Mexico is very humid because the Gulf is so warm (the amount of water vapor air can contain depends on its temperature). A measure of this water vapor content is dewpoint (the temperature at which air becomes saturated when cooled), and dewpoints in the region can climb into the 60s and even 70s. Here is the Northwest we have low dewpoints even during the summer. Why? The cool waters of the Pacific.
Here is the dewpoint forecast map for later this afternoon--one in which severe convection is expected in Missouri and neighboring states. VERY high dewpoints are shown over the Gulf and northward into the U.S. A lot of fuel.
Ingredient 4: Lift
Even if you have lots of instability, moisture and shear, the atmosphere needs a little push to get convection going. Sort of like the starter on your auto engine. Lift comes in two forms. One is associated with large-scale weather features (like upper level troughs) and the other includes surface-based features such as fronts and dry lines.
Upper-level troughs are disturbances moving in the upper level westerly flow predominant in the midlatitudes, and the area in question is far enough north to troughts them during the spring. Here is what a trough looks like on an upper level trough (see figure). It is region of generally lower pressure or heights aloft.
And then there are low-level features that produce lift and focus convection...and the High Plains has a collection of these! The most important is the dry line--the boundary between dry air coming off the Rockies and moist air coming up from the Gulf. (see image)
Thunderstorms love to develop on the lift associated with this feature. If you ever watch movies about storm chasers they are always talking about dry lines.
There are other ways the High Plains and Midwest are well-positioned for severe convection. For example, to get truly big storms it is good to have a cap, a shallow stable region aloft that keeps the convection at bay until the convective energy gets enormous as the surface heats during the day. No cap and convection releases too fast and is not that strong. Too strong a cap and nothing happens. A modest cap---you get the big stuff. Guess what area often has the right kind of cap, often associated with air coming off the Rockies to the west. You guessed it.
In summary, the High Plains, Midwest, and parts of the SE of the U.S. are "endowed" with large amounts of the key ingredients for severe convection--particularly during the spring. The results often range from tornadoes and hail, to intense precipitation and straight-line winds. One of the grand challenges of my field is figuring how to forecast these events...and it is probably the most difficult problem in meteorology....but that will have to wait another blog. West Coast weather is easy in comparison.
Here is an impressive video of the Joplin tornado:
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