MCVs

On Friday evening, a cluster of thunderstorms coalesced and started rotating over southwest Georgia. If you have a well-performing Java plug-in to your browser you can see the evolution here during the next few days while the images are still in the UCAR/NCAR/RAP archive -- move the "Set Animation Speed" slider to the right for maximum speed. [Although not as exciting and with the evidence of rotation not as readily apparent, a still image is here.]

When a thunderstorm system known as an MCS (mesoscale convective system) or MCC (mesoscale convective complex) develops a pronounced mid-level spin, that's called a mesoscale convective vortex, or MCV (sometimes also referred to as an MVC, or mesoscale vorticity center). Keeping track of all those acronyms? (Meteorologists love acronyms!)

The MCV that formed a couple evenings ago was not the most vivid example of one. That title might have to go to the one below over the Czech Republic in late June 2006 (which is a crop of this image). Even though it's over land and not a tropical cyclone, it resembles a small one of those, doesn't it? That's because of the character of the cloud signature, which looks like it contains spiral bands and an eye!

Here's not only an example of how closely MCVs can resemble tropical cyclones on satellite imagery, in this case both are present at the same time and not far away from each other! One is a thunderstorm system in the Great Plains, the other is Hurricane Danny on the Gulf Coast. Jack Beven of the National Hurricane Center has a write-up here to go along with the satellite image below, which is a crop of this image from July 1997.

There can be structural similarities between MCVs and tropical cyclones, namely anticyclonic flow at upper levels and a warm-core nature aloft. That along with tropical cyclones vs. subtropical cyclones vs. polar lows (sometimes called Arctic hurricanes) just goes to show that there are fuzzy areas in meteorology despite distinct definitions of particular phenomena.

A mesoscale convective vortex typically doesn't extend down to the surface as in a tropical cyclone, at least not at first, and even if it works its way down, it can't develop into a tropical cyclone while over, say, Oklahoma. Sometimes, though, a mid-level vortex within convection that exists (usually originating) over warm tropical or subtropical water can lead to hurricane development.

HURRICANE ALICIA

Friday night's MCV moved offshore on Saturday, and for awhile looked interesting on satellite imagery as some bands of convection as well as lower clouds arced around the "center." During the day today (Sunday) showers and thunderstorms persisted along with some spin on satellite imagery but the mid-level center appeared to be inland in southwest Louisiana and there was only a weak low pressure trough at the surface. Now what's left of the whole thing is moving into eastern Texas with just some widely separated showers and heavy thunderstoms. This is in stark contrast to August 1983.

Every time since then, when a big batch of thunderstorms moves off the coast (esp. the Gulf Coast) during hurricane season and starts persisting, meteorologists evoke memories of Hurricane Alicia even though most of the time those thunderstorm systems that originate over land in the U.S. don't develop into anything further.

I couldn't find any satellite imagery of the precursor disturbance to the hurricane right as it moved off the coast into the Gulf (these images are from shortly thereafter), and I don't know if it was an MCV per se or not, but Hurricane Alicia, in August 1983, is famous for forming from a non-tropical thunderstorm system which came off the Gulf Coast, persisted, and spun up all the way to a Category 3 hurricane as it moved west over the northern Gulf toward Texas.

There are differences between the weather situation this weekend and the one preceding Alicia. As shown by these surface maps, in that case in '83 there was a full-fledged cold front which made it all the way to the Gulf Coast (dewpoints down in the 50s in Atlanta), whereas yesterday morning the cold front barely made it through the Carolinas and into northeast Georgia; the thunderstorm system referred to above developed way out ahead of the front.

Hurricane Alicia was relatively small in size (tiny compared to Katrina) and did not cause a calamity from surge, but is nonetheless memorable for reasons beyond how it formed. Alicia produced its share of death and destruction, and an image left in many people's minds is that of all the windows which shattered in downtown Houston ... a feat repeated in South Florida last year by Hurricane Wilma.