Tuesday, June 21, 2011

Unusually strong cold-core tornadoes in KS and NE - 20 June 2011

I had intended not to write about so-called "cold-core" tornado settings (see this paper) for quite awhile, as most tornadoes associated with such settings are weak. Given the terrible death tolls from violent tornadoes in 2011, I feel the main focus should be on settings that produce stronger tornadoes and how we might reduce the number of deaths. But the prolific cold-core tornado event on Monday 6/20/11 in northwest KS and south-central/central NE was atypical; some of the tornadoes were strong in intensity (preliminary rating EF3, rare for tornadoes in a cold-core setting), so I felt a short write up might be justified here.

Many cold-core events start relatively early (late morning or early afternoon) due to the strong dynamics and cold air aloft. The 20 June setting was no exception. The first tornado report came near Quinter KS at late morning just northeast of the surface low, and large tornadoes were in progress in the Hill City/Norton area of Kansas by early afternoon (see Mark "Storm" Farnik's impressive photos above near Long Island KS after 2:00 PM CDT, see also David Mayhew's awesome photo above from the same storm; go to this link for an incredible sequence of his images). Tornadoes continued along a corridor into south central NE with this and another supercell, and other tornadoes occurred at late afternoon with yet another supercell in central NE near York. All these tornadic storms were northeast of the surface low near the warm front/dryline interface, a typical focus area for cold core tornadoes with the closed mid-level low and associated cold air aloft not far to the west.

What made this event so prolific with strong rather than weak tornadoes? I modified the RUC analysis sounding at Hill City at 1900 UTC (2 PM CDT, 3nd image above) for surface inflow parcels coming into the tornadic cell north of Hill City at that time on north or northeasterly winds (see surface observations northeast of the surface low on the surface map above at 1930 UTC). This resulted in an unusual amount of CAPE for a cold-core event, on the order of 2500 J/kg (most cold-core events are associated with CAPE values less than 1000 J/kg). In addition, 0-1 km storm-relative helicity (SRH) was sizable, in the range from 200 to 300 m2/s2 within a focused area ahead of the storm. This resulted in large energy-helicity index values (0-1 km EHI greater than 3.0, see 5th image above) in a corridor ahead of the tornadic storm, also unusual for a cold-core event. The sharp trough and tight closed mid-level low moving into the KS/NE area were unusually strong for mid to late June (see NAM forecast for midday above), a time when, unlike early spring cold-core events, large CAPE values can be present with surface dew points in the 60s F or higher. This large CAPE and SRH certainly provided support for larger and longer-lived tornadoes on 6/20/11 than typically associated with most tornadic cold-core events.

NOTE: Cold-core events aren't defined by whether the storms are "mini-supercells"; yesterday's case featured larger storms that were essentially classic supercells because of the larger/deeper CAPE, contrasting with early spring or fall cold-core systems that may have surface dew pointsonly in the 50s F. The important issue is the _pattern_, with the closed low aloft that organizes intersecting boundaries in a fairly narrow corridor ahead of the surface low and cold air aloft as the midlevel system moves east or northeastward, setting the stage for potential rapid tornado formation. This is a common repeating pattern with cold-core systems that produce tornadoes, and can be very useful operationally in anticipating such events.

An additional factor on 6/20/11 was probably steep low-level lapse rates in the vicinity of the tornadic storm at early afternoon over northwest Kansas. On the last SPC mesoanalysis graphic above, notice the axis of steep lapse rates extending through the surface "dry slot" over western KS into the environment of the northwest KS tornadic storm at 1900 UTC, along with plentiful 0-3 km CAPE. This combination could enhance stretching in low-levels that would already be rather strong due to the cold air aloft from the nearby midlevel low, even as cloud bases might rise some with this hotter/steep lapse rate surface air mixing in from the east from the surface dry slot (again, see surface map above). Indeed, many of the tornadoes formed quickly and were "dusty" (see the Mayhew and Farnik photos above), suggesting non-mesocyclone stretching processes similar to "landspouts", but combining with supercell processes involving strong SRH (see SRH field, also in the last graphic above). According to both Storm Farnik's and David Mayhew's descriptions, multiple dusty tornadoes were sometimes in progress simultaneously in close proximity, suggesting that enhanced low-level stretching processes were ongoing in the northwest KS storm environment northeast of the surface low.

Thankfully, there have been no reports of injuries with any of Monday's tornadoes so far, even though damage as high as EF3 in intensity was observed with at least one of the tornadoes in northwest KS.

Thanks to "Storm" Farnik for sharing his excellent photos and descriptions, and to David Mayhew for allowing me to use one of his images. Check out David's awesome and beautiful sequence of shots from this tornadic storm at:

Jon Davies - 6/21/11