Tropical Depression Norman?

If you've been in Oklahoma at all during the latter half of June, then you already know that it has been a very wet month. As of June 29, 2007, Will Rogers World Airport has recorded measurable rainfall for 18 consecutive days--a new record. In the 4 weeks between June 1 and June 28, the Norman mesonet site has recorded 9.46 inches of rainfall, well above average for the month. Flooding in Texas associated with the most recent storm system has been blamed for at least 11 deaths and has made news around the world, including a front page article on CNN which reports that the flooding has prompted a state of emergency in all 77 of Oklahoma's counties.

Bearing this in mind, it likely comes as no surprise that the recent weather pattern has been unusual. What may surprise you is that the storm system that has been affecting the southern plains has many features commonly associated with a tropical cyclone. First of all, looking at the satellite image above we can see that there are several bands of clouds and rain spiraling cyclonically into the center of the storm system (located, at the time of the image, slightly west of Wichita Falls, TX) in the lower levels of the atmosphere, and above that, bands of cirrus clouds (especially prominent over Nebraska) spiraling out of the system anticyclonically in the upper levels of the atmosphere. This pattern, which can often be noted in tropical cyclones, is characteristic of warm-core low-pressure systems--most continental systems are cold-core. You may have noticed that despite all the heavy rain we've been receiving, there has been very little lightning and thunder--this is a result of the warm microphysical processes typical of warm-core systems. The system also lacks any clearly defined frontal structure, and is persisting under a large ridge, once again characteristics more commonly found in tropical cyclones than in mid-latitude cyclones.

Officially, this system will never be classified as a tropical cyclone, no matter how much it may resemble one. According to the AMS Glossary, tropical cyclones only originate over the tropical oceans and are maintained by extraction of latent heat from the ocean, so don't expect the National Hurricane Center to give our storm system a name anytime soon. There is good news, though--forecast models predict that the blocking pattern that has been allowing this storm to remain parked over the southern plains may break down even before Independence Day, giving Oklahoma and Texas a well-needed chance to dry out.
If you'd like to learn more about tropical cyclones, we recommend browsing the National Hurricane Center website.



Story is © Nate Snook, 2007