National Weather Service United States Department of Commerce

Heat Across The Southern U.S.; Storms And Heavy Rain in Central U.S.

The early-season heat across portions of the Southern U.S. will continue for much of this week with more potential record high temperatures. On Tuesday, expect 90s and triple digits from the southern Plains into parts of the Lower Mississippi Valley. Also on Tuesday, strong to severe thunderstorms and locally heavy rainfall will be possible from the central Plains into the Mid Mississippi Valley. Read More >

September 28th Tropical Funnels in the Texas Panhandle
Tropical funnels were spotted near several communites on the evening of the 28th. These funnel clouds were seen near Hereford, Clarendon, and Amarillo for brief periods. Tropical funnels (and the similar cold air funnel) are not as well understood as funnel clouds that form in supercells and produce classic tornadoes. Tropical funnels form in tropical environments, which Texas was experiencing due to moisture from Hurricane Miriam off  the west coast of Mexico on the 28th. Meteorologists have an idea of how these tropical funnel clouds form, but the exact process (much like classic tornadoes) is not completely understood. This lack of understanding is due in large part to how quickly these funnel clouds form and then dissipate, usually within just a few minutes. Funnel clouds are caused by vertical stretching of vorticity. Vorticity can be thought of as "spin" in the atmosphere, which is usually produced by wind shear. As this vorticity is streched vertically the area of rotation shrinks and the spinning air speeds up. The old analogy is an ice skater twirling who pulls in her arms and starts to spin faster. Funnel clouds from supercells have the same general process, however tropical funnels have this happen on a much smaller/slower scale which leads to these funnel clouds having shorter life-spans and much weaker wind speeds. One very important fact to note is that a tornado does not exist until the rotation is in contact with the base of the cloud AND the ground. If it is not in contact with the ground, then it is considered a funnel cloud.  These tropical funnels rarely make contact with the ground, (in part due to their short-lived nature) and if they do reach the ground they have been known to cause winds up to 70 mph. All around these storms are much less dangerous than tornadoes that form from supercells, but they should be noted as they can cause minor damage if they reach the ground.



The images below will show how radar data and pictures are different for classic tornadoes/funnel clouds from supercells and tropical funnels from the evening of the 28th. Classic tornado/funnel cloud images will always be on the left, and tropical funnels from the 28th will be on the right



Radar reflecitivity of Classic Supercell containing a tornado in Oklahoma  Radar reflecitivity from 4:30 pm on the 28th showing a storm that produced a tropical funnel
 Velocity signature of a supercell tornado (click the image for a larger version).  Radar velocity from 4:30 pm on the 28th showing the motion of the air in a storm that produced a tropical funnel


Diagram showing structure of a supercell that has the capability of producing a large damaging tornado. Tropical Funnel South of the Ariport. Note the lack of organized storm structure helpoing to differentiate this from a classic funnel from a supercell.                    (Courtesy of Krista Simpson)
Things to look for in a wall cloud. However not all wall clouds produce tornadoes, but they are a good indicator of where funnel clouds can form at any minute. Picture of a tropical funnel taken outside of Clarendon, TX. Again, note the lack of storm organization or wall cloud.  (Courtesy of Jeremy Felicia)
Picture of a classic tornado. Note the wall cloud above, and the fact that it is in contact with both the base of the wall cloud and the ground and rotating rapidly.     (Courtesy Jason Boggs)  Another image of the tropical funnels outside of Amarillo on the evening of September 28th. (Courtesy Daniell Railey Browning)