Wednesday, December 15, 2010
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
Saturday, December 4, 2010
The Discovery television show "Storm Chasers" concluded another exciting season this past week. A repeating theme on the show is getting armored vehicles or probes into tornadoes (which makes for exciting storm chase footage) on the premise that this will help improve tornado warning lead times and save lives. But, in reality, that's really doubtful. Chuck Doswell's blog here discusses this issue very well. Measurements inside tornadoes may be useful toward understanding dynamics inside ongoing tornadoes (a valid goal), but will tell us next to nothing about which environments and storms will produce potentially deadly tornadoes. Chuck also points out that warnings for tornadoes most likely to cause deaths are already pretty good. Looking at four tornado days featured this season on "Storm Chasers" that each generated at least one violent EF4 tornado helps drive this point home rather well.
4/24/10 Yazoo City MS EF4 tornado (see photo by Dick McGowan above) - This tornado caused several deaths and was on the ground for 100+ miles in a setting that was well-recognized by NWS meteorologists as having high potential for tornadoes well in advance (for example, see experimental conditional sig tor probabiliities on 1st map graphic above). The specific warning for Yazoo City was issued approximately 30 minutes before the tornado hit town, and warnings for the same tornadic storm farther west had been ongoing for an hour. Can we really improve on those good lead times?
5/10/10 Tornadoes in central OK, including EF4 tornadoes just southeast of Oklahoma City - Again, the potential for this day was outlooked well in advance, with the coming together of environment ingredients quite evident (see 2nd graphic above). Tornado warnings in central Oklahoma were generally issued 30 minutes in advance of damage and deaths.
5/22/10 EF4 tornado near Bowdle in north-central ND - This setting was also quite evident in advance (see 3rd map graphic above), where a large tornado hit mainly in open country. A tornado warning for Bowdle was issued nearly 30 minutes in advance of the tornadic storm passing just northwest of town, and tornado warnings farther west had been ongoing for about an hour.
6/17/10 Tornadoes in MN, including the EF4 Wadena MN tornado - Another setting where tornado environment ingredients were very evident (see last graphic above) in advance, and most warnings were issued 30 minutes before sizable towns were struck.
Here are some representative values of individual bulk environmental ingredients associated with the above cases:
4//24/10 MLCAPE 1950 J/kg 0-1k SRH 670 m2/s2 0-6km shear 70 kts
5/10/10 MLCAPE 3340 J/kg 0-1k SRH 610 m2/s2 0-6km shear 62 kts
5/22/10 MLCAPE 3930 J/kg 0-1k SRH 345 m2/s2 0-6km shear 52 kts
6/17/10 MLCAPE 2525 J/kg 0-1k SRH 270 m2/s2 0-6km shear 54 kts
Any competent NWS meteorologist on days with environment ingredients like those above would be primed and ready to issue warnings immediately at the first indication on radar or credible information from spotters. The next level of improvement regarding saving lives will likely involve sociology to find ways to get people to hear and pay attention to those warnings already being issued.
The Discovery show is about 95% thrills and entertainment, and maybe 5% real science. But it makes interesting watching for legions of viewers, and I do commend the show producers this season for balancing the adrenaline quotient with some sober reality by showing the aftermath of tornadoes, including damage and injuries.
- Jon Davies 12/5/10