Severe Weather Weekend; November 17th, 2013

Needless to say, the weather this weekend proved beyond any shadow of doubt to be dangerous, destructive, and deadly. On Saturday, I spent some time and put together an animated GIF to demonstrate how we could see the potential for bad weather developing.

The animation is a collection of ten gifs I downloaded from weather.unisys.com. I then uploaded the gifs to gifmaker.me and sorted the images so the animation would begin with the surface pressure, 925mb (about 2,500ft) and run up through the atmosphere to the 100mb pressure level (about 50,000ft). As the animation runs, you can see the altitude change in the top-left corner. These images all represent the same day and time.

There are two aspects of the maps to keep your eyes on as our altitude changes. First, watch the isobars, the white lines of equal pressure. At the 925mb pressure level, the lines run almost north-south from the Great Plains east to the Atlantic coast. Watch as the animation runs how the isobars change from a north-south orientation to a very zonal west-east orientation.

The second aspect of the maps to note is the station symbols. In particular, we want to examine the wind speed/wind direction “flags.” Remember from Chapter 13, the flag points in the direction the wind is from, the source region. The number and type of “barbs” on the flag indicate wind speed. A full mark is roughly about 10knts, half a mark is about 5knts, and a solid triangle is 50knts. As the animation runs, notice at 925mb wind direction is generally out of the south-southwest at about 30knts, less in some places, more in others. As the animation runs to 50,000ft, notice how the wind direction changes almost to due west, and wind speeds increase to well over 100knots (two double triangles plus some barbs).

Let’s put this together.

From the surface to the top of the troposphere, we see wind speeds increase from about 30knts to well over 100knts. From the surface to the top of the troposphere, wind direction changes by almost 90º. What we are seeing on Saturday is the development of considerable wind shear.

Wind shear is simply the change in wind speed and direction over some distance, or some change in altitude. Wind shear is a hurricane killer, meaning that some small changes in wind shear can prevent hurricanes from gaining strength, or even forming. On the other hand, wind shear is an essential ingredient in the formation of tornadoes, as we have seen today. It is possible for very strong wind shear to blow out a storm for about the same reason as why wind shear interferes with hurricane development. The updraft which leads to rapid condensation and cloud formation is disrupted. A little wind shear is needed to encourage cloud formation, but too much can prevent storm development. Except in the cases of very strong storms, or very strong hurricanes.

Infrared-nov16

Visible-nov16

The above images show water vapor (upper, infrared) and visible wavelengths (lower) for the same time and day as the animation. The white areas are cloud cover; the darker grey areas are exposed surface due to clear, or mostly clear, air. We can see dry, clear air over Texas, Oklahoma, Kansas, and into portions of the upper Midwest. Over the South, we see clouds, indicating moisture and cooler temperatures aloft (because cool air means condensation.) If we refer back to the animation, we can see the surface winds pulling moist air from the south, which will eventually collide with the drier air from the west. The high amount of wind shear will drag the drier west air mass and the moist southern air mass into a collision with cold and moist air moving down from Canada.

Let’s add on.

We have strong wind shear, plus a dry air mass from the west, a moist air mass from the south, and a cooler and moisture air mass from Canada. We can see the west air mass and southern air mass colliding over our region over the next several hours (which is exactly what has happened), and we could predict not only dangerous weather for the Ohio Valley region, but furthermore, as the wind shear seems to favor pushing conditions to the east, we could also forecast some particularly bad weather for the U.S. Northeast, into the New England states, and probably into Quebec and Newfoundland.

Sometimes, looking at individual maps is not as informative as looking at a number of maps. In fact, regardless of the application, really, looking at a bunch of maps related to the same topic is generally more informative. Putting these maps together as an animation hopefully brings the patterns of weather development “alive” making conditions a little easier to interpret. Many times, animations show a storm from a weather satellite, or are created from weather radar, and illustrate horizontal changes in storm development. We don’t often get to see animations which demonstrate what happens vertically, as we move up through the atmosphere.

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