Spokane County Tornado - May 21, 2004

On Friday afternoon at 415 pm a tornado touched down in western Spokane County near Fairchild Air Force Base.

A preliminary storm survey conducted by personnel at National Weather Service Spokane concluded that this tornado ranks as an F0 tornado on the Fujita Scale. The survey team observed shingles blow of roofs, some shallow rooted trees that were uprooted, and damage to two doors on a shed, all indicative of an F0 tornado with peak winds approaching 72 miles per hour.

Tornadoes in Washington are quite rare, with an average of only 1 tornado each year in the state. See http://www.erh.noaa.gov/er/cae/svrwx/tornadobystate.htm for details.

Here is a detailed explanation of the events:

After reviewing the radar data, it concludes that the tornado may have been produced by at least three separate processes acting in conjunction.The dominant one was the cyclonic line-end vortex on the northern end of the N/S "bowing" line segment. This vortex was within the pcpn band, not ahead of it as with classic supercell tornadoes (under updraft and rain-free cloud base). At the time of the tornado, a weak echo region was visible in this vortex.

Another process would be the merging of two separate storm clusters... the southern "bowing" line segment and the more E/W storm to the north (the storm the initial SVR was issued for). This merger could be analogous to a cold front-warm front occlusion...enhancing cyclonic vorticity and rapidly converting horizontal vorticity into vertical vorticity through vortex stretching by the updraft at the point of occlusion.

The third, and perhaps the most important (??), would be the pre-existing sfc boundary extending w/nw from the northern E/W storm. The tornado vortex developed rapidly along this feature, almost simultaneously through a deep layer, as it merged with both the southern "bowing" line segment and the northern "E/W" storm.

The overall conclusion was that this was NOT a classic supercell tornado. (ie., isolated supercell with hook, BWER etc.), but rather a tornado produced from the combination of processes that, by themselves, have been shown to produce tornadoes. This was a hybrid that did not fall neatly into any tornadogenesis conceptual model... ie., supercell hook, bow echo with line-end vortex, and immediately ahead and north of a bowing line segment, etc. The closest model for this case may be that of the bow echo / LEWP (line echo wave pattern).

Boundaries are key, whether they be of the outflow boundary type or the RFD/updraft/FFD interface type. The latest research (VORTEX) concludes that 70% of tornadoes are associated with boundaries. That's a lot!! The bad news, from a radar operator's perspective, is that these sub-synoptic-scale boundaries don't fall exclusively into any neat category. There is a continuous spectrum of scenarios where horizontal vorticity along a boundary can be tilted and stretched rapidly into the vertical... ultimately leading to a tornado vortex. This was a good example of a hybrid.

In addition, the region experience very heavy rainfall and areas with large hail. Urban street flooding was reported in many areas across the Spokane - Coeur d'Alene corridor. Hail, up to 8 inches deep was also reported. Here are some pictures from the tornado: