Three strong tornadoes in 2008 associated with boundary intersections and narrow instability close to 700 mb “cold-core” lows

This paper will briefly examine the synoptic and mesoscale settings for 3 strong tornado events in May and June of 2008 that were associated with narrow moisture/instability axes and boundary intersections in relatively close proximity to “cold-core” lows at 700 mb. These tornado events are:

- 1 May 2008 in northwest Iowa (the Rock Valley, Iowa tornado, rated EF2)

- 22 May 2008 in northern Colorado (the Windsor, Colorado tornado, rated EF3, with 1 death)

- 6 June 2008 in north-central Minnesota (tornadoes in Hubbard County, Minnesota, the strongest rated EF3)

These events involved varying degrees of forecast difficulty. The first (1 May 2008) was well forecast, with a tornado watch issued well in advance. The second (22 May 2008) was not well anticipated, with a tornado watch issued only while initial tornado reports were coming in. A tornado watch was not in effect during the third event (6 June 2008). These variations illustrate how difficult it is to forecast some tornado events that occur at the northwest edge of narrow instability axes with cold air aloft due to the presence of a midlevel closed low to the west. Sometimes the features and ingredients involved are subtle, and can be overlooked when larger instability is present in a broader axis some distance to the east and south.

Although each of the cases examined differed in the specific mix of local ingredients and time of day, all had some common pattern recognition features that can serve as a “heads up” for forecasters to do more detailed assessment in similar situations. These features were:

- A boundary intersection detectable from careful surface analysis near the northwest edge of the narrow instability axis.

- A closed 700 mb low within 200-300 miles to the west (2 of the cases examined suggest that 700 mb low positions may be more useful than 500 mb low positions in assessing the proximity of relevant cold air aloft in "cold-core" events).

- The presence of significant low-level CAPE (e.g., CAPE below 3 km AGL) around the boundary intersection area, due in part to cold air centered at roughly 3 to 4 km above ground related to the approaching midlevel low.

One might argue that the 1 May 2008 and 22 May 2008 cases were not true “cold-core” events (a broad categorization) because of the distance of the 500 mb low to the west, even though the 700 mb low was relatively close in all 3 cases. The intent of this paper is to avoid rigid categorizations (something that the atmosphere resists), and instead to focus on some common features that may be useful regarding situational awareness for forecasters in such settings.