Monday, January 12, 2009

EF-1 tornado in Alabama on 1/10/09: a subtle small CAPE setting

A weak tornado occurred shortly after 2300 UTC (5 pm CST) on Saturday, Jan. 10, about 30 miles north of Mobile in southwest Alabama. It wasn't very impressive (1-2 mile path, EF-1 winds around 100 mph), and thankfully no one was hurt, but it did do some damage (see NWS photos above, also click here). Given the subtle setting, it was kind of interesting from an environment standpoint.

Radar reflectivity at 2304 UTC from Mobile above shows the tornadic cell, and NWS Mobile did a good job with tornado warnings that were issued across southern Washington county for more than 45 minutes before the tornado struck the town of McIntosh.

The RUC analysis profile above for Mobile (MOB) at 2300 UTC is somewhat unusual, with small total CAPE (only around 400 J/kg!), and most of the CAPE below 600 mb, very low to the ground. Settings like this can enhance stretching near the ground, and with decent SRH (> 200 m2/s2 in the lowest 1 km), they can occasionally support tornadoes, even with small total CAPE.

The SPC graphics above show a large positive tilt 500 mb trough was heading southeastward, with a cold front (not shown) ahead over central Tennessee into central Mississippi. The Alabama tornadic cell occurred well in advance of this front in a moisture axis of 60s F dew points. In the parameter graphics, the SPC total MLCAPE field over southwest Alabama wasn't impressive (> 250 J/kg), but low-level CAPE was large (near 100 J/kg below 3 km) along with low-level SRH (200-300 m2/s2) overlapping the southwest Alabama moisture axis. These ingredients, combined with the low-to-the-ground vertical CAPE distribution in the RUC analysis above, were just enough to help generate a tornado with the cell north of Mobile.

This case is a reminder that storm environments with CAPE "squeezed" low to the ground certainly aren't limited to cold-core events (for example, see here).

- Jon Davies 1/12/09


Bob said...

Hi, Jon. I have been in Orange Beach (just se of Mobile) since New Year's. On the 10th, I was cyberchasing this cell on GRL3. There was a very nice couplet on 0.5 (only tilt I was monitoring) as the meso approached Mcintosh. IIRC, inbound on one scan was ~35kt and outbound was ~65. This was 2 or 3 scans before the storm reached Mcintosh. It diminished (on radar) as the cell got to Mcintosh, but, obviously, the storm hadn't yet occluded.

BR never looked impressive by May "standards", but that is pretty meaningless... It did have a decent hook to it for quite awhile.

Anyway, sorry I didn't grab any screen shots, but I hope you find that info interesting. Cool analysis you did!

Bob Schafer

Jon Davies said...


Appreciate the information about the SR velocity appearance on this storm. I didn't get a chance to save or look at detailed radar data. Storms in subtle CAPE settings like this one are fascinating... a very different setting than for most tornadic storms in the plains!

Thanks for your input and comments...