Sunday, April 22, 2012

Tornadoes after dark most dangerous during April 14 outbreak

Much has been written and many photos posted from last week's major outbreak of tornadoes on Saturday April 14, 2012.   The most damaging and deadly tornadoes turned out to be after dark, including the EF3 tornado that hit the southeast side of Wichita KS ($140 million damage), and the EF3 tornado that struck the Woodward OK area around midnight killing 6 people.

Above, grabs from video by my wife Shawna near and after dark in the Moundridge-Goessel and Marion areas of central KS show tornadoes visible only by lightning, including a large one near Marion Lake.  Associated maps and radar images above also show the tornado paths from that supercell, as well as the more prolific supercell that struck Wichita around 10:20 pm and produced other tornadoes before and after, also after dark.

Tornadoes after dark are particularly dangerous because they are difficult to see, and many people are asleep and not aware of what's approaching.  My paper with Andy Fischer about nighttime tornado environments (see here) shows that tornadoes after dark in the Plains occur when both CAPE and especially low-level shear are large after sunset.  Just as important, storms after dark need to remain relatively "surface-based" (lifted parcels of air near the ground that feed updrafts are able to move upward with little resistance).  This occurs when surface temperatures don't cool much and surface dew points remain steady or even increase, keeping convective inhibition (CIN) relatively small, rather than the more typical scenario where storms become "elevated" above a cool stable layer near the ground when surface temperatures cool significantly after sunset.

CAPE below 3 km  (CAPE closer to the ground) can also be an indicator of a more surface-based environment.  Notice that in the 2 graphics immediately above, along with large CAPE-shear combinations (suggested by both the Sig Tor parameter and the energy-helicity index), there were significant amounts of low-level CAPE indicated after dark on both the SPC mesoscale analysis in real-time, and on model forecasts in advance.  This suggested a continuation of the April 14 tornado event well after dark, which was the case, particularly with the Wichita KS and Woodward OK tornadoes.

Obviously, with this big an outbreak, it is very fortunate that more people weren't killed.  I can't emphasize enough the excellent forecasts and outlooks several days in advance, as well as pin-point warnings and media coverage during the event.

When I get time this coming week, I'll do a post about the setting that generated tornadoes in Minnesota yesterday (4/21/12) with early spring northwest flow aloft and 40's F dew points at the surface (very unusual!).

- Jon Davies  4/22/12

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