Wednesday, January 20, 2010

Tornado setting in Los Angeles area 1/19/10

I haven't done a case study for awhile... Before today's tornadoes (Jan. 20) in Texas and Louisiana, it's been a quiet winter regarding severe weather in the southern states. But yesterday's little tornado episode on Jan. 19 (see 1st graphic above, a capture from video at Sunset Beach CA) near Los Angeles caught my eye, with some minor damage in the Huntington Beach area. Fortunately, the tornadoes/waterspouts around 2100 UTC (1 p.m. PST) were weak, as most California tornadoes are.

The 500 mb WRF model analysis showed a strong short wave of energy aloft with an intense vorticity max (atmospheric forcing and "spin") crossing southern California yesterday afternoon (see "X" and "orange" vorticity area on 2nd graphic above). At the surface (3rd graphic above), a small low pressure area (mesolow) was approaching the Los Angeles area from the southwest before 2100 UTC with an east-west boundary along the coast just south of L.A. (Huntington Beach is located by the black square in the same graphic). Dew points were in the low 50's F south of this boundary, and the tornado activity seemed to be focused along it where low-level shear and vorticity ("spin") were probably concentrated.

On radar from Santa Ana/SOX (4th graphic above), a "circular" pattern of small cells with red reflectivity was visible over the ocean to the southwest of the L.A. basin around 2000 UTC, moving northeastward (see black motion arrow in same graphic). This pattern of cells was associated with the mesolow mentioned above, and came onshore producing waterspouts and at least one tornado between 2030 UTC and 2100 UTC in the Huntington Beach area (black square in same graphic).

A modified RUC model sounding for 2100 UTC at Santa Ana (see last graphic above; just east/northeast of Huntington Beach) suggested that instability/CAPE was certainly not large (only around 200 J/kg). But the available CAPE was located extremely close to the ground, probably inducing strong stretching for storm updrafts in low-levels near the surface focus of the mesolow and boundary. In addition to CAPE, the wind profile on the same graphic suggested large low-level shear (storm-relative helicity, SRH > 300 m2/s2) available for tilting and stretching contributing to tornado formation near the focusing boundary.

In many ways, even without a closed 500mb low aloft, this setting has some common features with "cold-core" tornado settings in spring and fall in the Plains (e.g., strong forcing and sharp vorticity max aloft, surface low and boundary extending east of the low, surface dew points only low 50's F, but CAPE located very close to the ground). In fact, most California tornado settings, whether in the L.A. area or farther northward in the central valley of California, are some variation on this type of setting, with total CAPE often less than 500-1000 J/kg.

- Jon Davies 1/20/10

Update: 2/19/10 (1 month later) - L.A. area storm chaser Will Wilkens has called to my attention that photos from this tornadic event that I originaly posted here (and have now deleted) were "phonies". One was from the 5/22/08 Riverside CA landspout event, and another was a generic waterspout image. I've replaced those with a video capture above of the real tornado that Will pointed me to (Thanks!). I'll have to be more careful in the future... Various websites of TV or news viewer photos are increasingly posting stuff that's labeled wrong these days just to appear current, it seems. Thanks for the information, Will.