Weak Tornado Briefly Touches Down in Peoria County

 

Weak Tornado Briefly Touches Down in Northern Peoria County

 

Photos taken by Brian Dallinger at 2:12 pm CDT (Mature Stage) and 2:14 pm CDT (Dissipating Stage). 

A brief weak tornado touched down in northern Peoria County on Tuesday Afternoon.  The tornado, of the landspout variety, was approximately 50 feet wide and was on the ground in a field from 2:10 pm until 2:14 pm, about 5.5 miles west-northwest of Princeville.  The tornado was moving to the west-northwest at 10 mph and was on the ground for approximately 1/2 mile.  No damage was reported.  The first report of the tornado to the National Weather Service occurred at 2:35 pm.  By that time, the conditions were no longer conducive for landspout formation so no Tornado Warning was issued.

The formation mechanism for landspouts is very different from that of the most destructive tornadoes. Landspouts develop as a thunderstorm, or even a towering cumulus cloud, rapidly develops above a surface boundary such as a wind shift. Surface boundaries have small scale circulations along their long axis that can play an important role in the development of landspout tornadoes. To help visualize these circulations imagine two hoses facing each other where the water flowing out of the hoses represents the wind on either side of a wind shift. If you were to set the hoses several feet apart and offset just a bit (i.e., not flowing directly into each other) you would see small circulations develop as the two streams of water which are flowing in opposite directions interact with each other. You could also mimic this effect if you lit a candle or incense and then moved your hand slowly downward alongside the smoke that is rising from the incense. As two air streams on opposite sides of a front or wind shift interact small eddies or circulations form along the interface of the two airstreams.

These surface circulations are critical to the development of landspout tornadoes. As a thunderstorm, or towering cumulus cloud, rapidly develops above one of these circulations the developing storm is able to stretch the low level circulation into a weak short-lived tornado on rare occasions. The process of tightening and intensifying the circulation below the storm’s updraft can be readily visualized. If you watch an ice skater spinning with his/her arms out they will rotate slowly. However, when they raise their arms above their head to make the axis of rotation more narrow they will spin much more quickly. Something very similar occurs when a landspout develops. In the case of a landspout as an innocuous low-level circulation is stretched vertically into the developing storm's updraft, the axis of rotation contracts substantially and correspondingly begins to rotate more rapidly.

Landspouts are most likely to occur when the following conditions are met:

  • A rapidly growing towering cumulus or thunderstorm is located directly over a surface boundary or wind shift
  • The boundary or wind shift is stationary or moving very slowly
  • The boundary or wind shift has a strong horizontal circulation, or circulations, along its axis which can be stretched into the developing updraft
  • Steep lapse rates (meaning that temperatures cool very rapidly with height)
  • An updraft that develops very rapidly

Typically land spouts develop very quickly after updraft initiation. They are extremely difficult to warn for as they often develop from innocuous towering cumulus and can occur very rapidly after the initiation of the updraft.  The conditions which cause them often last for only a few minutes, such as in this case, and are destroyed by nearby rainfall or the movement of the boundary away from the updraft.

Even at close ranges there is very little circulation evident in the towering cumulus or thunderstorm, even with the most powerful and sensitive of radars (such as the NWS WSR-88D), as the landspout tornado is not associated with a rotating storm, but instead is stretched from the surface to cloud base.

Below are several radar images from before, during, and after the tornado occurred.  The location of the tornado is shown as a red and white icon.



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