Wednesday, December 15, 2010

Rare EF2 tornado in Oregon 12/14/10

Yesterday (Tuesday 12/14/10) saw an EF2 intensity tornado strike just southeast of Salem, Oregon shortly before noon PST (2000 UTC) at Aumsville (see photo above), with 2 injuries reported. It's been 17 years (1993) since a tornado that strong occurred in Oregon, and it was only the 4th tornado of F2/EF2 or stronger intensity to occur in the state since the early 1970's. Such events are so rare that they are typically unforecastable, as they usually occur in settings with small CAPE (<> 40 deg F) from western Washington into west central Oregon, and some spotty sun's heating on satellite. Although SPC mesoanalysis graphics showed total CAPE amounts not really registering (less than 250 J/kg, not shown), some low-level CAPE (SBCAPE below 3 km AGL) was indicated over the same area where dew points were maximized (see red areas in 4th graphic above at 1900 UTC). The small tornadic cell formed rapidly around 1910 UTC and began producing a five-mile track tornado around 1945 UTC (see radar inset on 4th graphic).

As is often the case in small CAPE settings, RUC soundings tended to under-estimate the environmental CAPE in this setting, showing surface temperatures around 45 deg F and surface dew points around 40 deg F. However, actual surface observations at Salem at 1900 UTC (not shown) before the tornadic cell formed were 47 F (temperature) and 42 F (dew point). When plugged into the RUC analysis sounding for Salem at 1900 UTC (last graphic above), these updated surface obs combined with cold air not far aloft (-10 deg C at 700 mb !) boosted total CAPE from around 100 J/kg to over 300 J/kg, with the "fattest" CAPE located in the 950-850 mb layer, very close to the ground. Even though total CAPE was unimpressive, this "packing" of CAPE down low in the sounding environment would likely help promote rapid vertical stretching in a local updraft, and given the accompanying sizable low-level and deep-layer wind shear, was able to support a tornadic storm as reported above. But picking out these subtle ingredients in a rapidly evolving environment is more than difficult, and everything apparently had to come together just right to generate a tornado in this case. Another similar setting might generate only storms with small hail. Adding to these problems, rotation in small cells (like this case) more than 25-35 miles from radar is usually not detectable for advance warning purposes.

Although it's not hard to go back and see contributing factors after a climatologically rare small CAPE event like this, detecting in real time these subtle ingredients that might make for a mesoscale "accident" in a location where tornadoes don't often occur is next to impossible.

- Jon Davies 12/15/10

1 comment:

An Atheist said...

Interesting how you can get an accidental tornado like this where they almost never happen, yet on a day just ripe for tornadic activity, you can come away empty-handed. It's also interesting to see how you don't need massive CAPE or deep-layer shear to get things going. It would be interesting to see what proportion of tornadoes occur under such limited conditions. Nice report! Thank you!