Saturday, May 3, 2008

Bow echo tornadoes in Kansas City early a.m. on 5/2/08

The most damage of any of the storms that occurred last Thursday evening into early Friday morning (5/1/08-5/2/08) happened with a bow echo/derecho that moved through the Kansas City area around 2 a.m. CDT. After about a hour of sleep, Shawna and I awoke near Trimble to the loud roar of near-hurricane force winds and canon-like booms of thunder. Our TV and internet immediately went out, and our grill and patio furniture went flying off into the darkness.

Damage occurred scattered throughout the Kansas City area. Shawna's folks in North Kansas City had a large tree fall close to their house, damaging their neighbor's garage and car. A very interesting part of this event was a mesovortex that produced bow echo tornadoes near the apex of the bowing squall line in the Gladstone-Liberty area, causing a concentrated path of damage.

Bow echo tornadoes (also called mesovortex tornadoes or QLCS or quasi-linear convective system tornadoes) can occur within squall lines that have a bowing segment where air aloft is intruding and descending prominently from behind into the rear of the squall line (called a rear inflow jet or RIJ). Shallow circulations called mesovortices sometime form along the forward part of the squall line in front of this rear inflow jet and near or just north of the apex of the bow. These circulations can tighten up and produce tornadoes. Such tornadoes are typically short-lived and different from supercell tornadoes, but they are still potentially damaging. The first diagram above is a schematic showing how mesovortices and tornadoes from bow echoes can evolve.

Also above is an informal damage path survey above that I did on 5/2/08 in the Gladstone- Liberty area, showing a mesovortex that produced at least 3 tornadoes in sequence that I could find along an 8-mile narrow track of scattered wind damage. These tornadoes are difficult to pick out on radar, and if too much emphasis is given to detecting them, it can detract from the wider straight-line damaging wind threat across the bow echo in general. Unfortunately, these bow echo tornadoes on 5/2/08 were intense (EF-2 and EF-3 intensity, see this NWS link) and did significant damage to many homes. But fortunately, there were no serious injuries.

A close look at archived radar images suggests that the mesovortex formed rapidly near Gladstone and moved northeast over the next 10 minutes. At 0650z (1:50 a.m. CDT), a prominent rear inflow jet was evident and a subtle forward inflow notch was forming at the front and just north of the bow apex (locations marked on the 0650z reflectivity image above), suggesting the possible presence of a mesovortex. Subsequent images shown above at 0656z (1:56 a.m. CDT) and 0700z (2:00 a.m. CDT) showed this feature becoming more prominent and tracking northeast along the corridor where the mesovortex and tornado damage occurred from my informal survey. On radar velocity images (not shown), this feature was initially difficult to pick out as it formed near Gladstone, but then became somewhat more noticable as it passed northwest of Liberty and matured. Though not marked on the radar images above, another inflow notch suggesting a mesovortex (without a rear inflow jet behind it) could be seen along the bow farther southeast passing near the Independence area. This mesovortex was apparently nontornadic.

A similar mesovortex tornado occurred earlier west of Lawrence, KS just after 1 a.m. CDT from the same bow echo in an earlier stage, and another mesovortex tornado occurred later in southern Missouri (around 6 a.m. CDT) from a different bow echo segment that caused some damage in the town of Ava, southeast of Springfield. In both cases (not shown), the tornadoes were associated with a subtle inflow notch at the front of the squall line near or just north of the bow apex. Again, it should be emphasized that bow echo/mesovortex tornadoes are difficult to detect on radar (they are small and form quickly), and are generally impossible to see at all when located at distances more than 40-50 miles from the radar. This makes specific local tornado warnings by NWS meteorologists difficult to do, which is something the public needs to know and understand.

Jon Davies - 5/3/08

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