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Spokane Tornado May 21, 2004
Spokane
County Tornado- Friday 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:
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