Tropical Storm Erin Dumbfounds Oklahomans

John writes in from Oklahoma City:

I was in Wisconsin for a dealer show over the weekend and, as a result, missed an amazing meteorological event. Saturday night the remnants of Tropical Storm Erin, which caused minimal damage along the Texas coast, redeveloped over western Oklahoma. We have seen remnants of tropical storms and hurricanes--both Atlantic and Pacific--drift through here over the years, but this one was quite different. Normally the result of such an amorphous mass of clouds is extremely heavy rain, at times reaching 12 inches locally. This time the storm redeveloped into an actual hurricane complete with an eye in the middle of a classical spinning hurricane system. There were 60+ mph winds over most of the central and western part of the state and locally rainfall up to 14" in a few hours. Several towns now have massive flooding as a result. Anyway, you ought to take a look at a doppler radar loop of the system. It's really quite amazing.

That’s a remarkable system! Amazing – it even seemed to retain the eye structure too!

When rotating tropical systems like hurricanes move out of the tropics and get caught in the zonal flow of the mid-latitudes, they often retain some rotation, and can get captured by mid-latitude systems that also have some rotation.

There is also the phenomenon in dynamic meteorology called ‘conservation of potential vorticity’, whereby rotating air squeezes flat as it passes over high terrain (like the Southwestern Plateaus and the Rocky Mountains), thus losing rotation, then gains the rotation back again once the air passes over lower terrain. Thus, storm systems lose punch over high terrain (like the desert plateaus of the American Southwest) but then gain strength in the lee of the high terrain. Certain places are renowned for this phenomenon: the Atlantic coast of the Carolinas (lee of the Appalachians); central China (lee of Tibet); off the eastern coast of Victoria, Australia (lee of the Snowy Mountains); but the granddaddy of all the strengthening places on the Earth is the Great Plains of the U.S. and Canada, where Oklahoma sits, and where the differences in temperature, humidity, and elevation (compared to the Rockies) are often stark.

Looking at some of the forecast loops for the last several days, it looks like this indeed happened. The center of Erin passed into west Texas near the NM border, drifted north and then got caught by a rapidly-strengthening, rapidly-moving short-wavelength trough passing eastwards over Rockies. Instead of winding down from the loss of tropical moisture, the system wound up again. Amazing!

Meteorologists sometimes see weird interactions between the mid-latitudes and the tropics. In September, 1996, a strong low pressure system moved from the Canadian Plains over the Great Lakes, just when water temperatures in the Great Lakes were reaching their annual maximum. The rotating low pressure system promptly set up a double eye wall and looked, for all the world to see, like a hurricane. The freak system, dubbed “Hurricane Huron,” (check their photo!) dumbfounded meteorologists. Such a hybrid weather system had never even entered the realm of anyone’s imagination.

Nature loves playing tricks!