Sunday, December 28, 2008
It's unusual to get severe thunderstorms in the Kansas City (KC) area around Christmas, but it sure did happen yesterday morning (Saturday 12/27). The setting involved a very strong upper trough that brought unseasonably warm air northward ahead of it (record temps in the 60s F in KC on Friday), and an early morning bow echo type thunderstorm feature that interacted with a quasi-stationary outflow boundary in the south KC area.
The first graphic above shows the deep mid-level trough around midnight going into early morning on 12/27, along with SPC storm reports, and a WRF forecast of low-level CAPE suggesting an unstable surface-based environment extending northward into the KC area overnight. The 2nd graphic is a radar reflectivity image showing an ENE-WSW outflow boundary in place across the south KC metro area at 4:00 am, the result of thunderstorms that had moved across to the north of KC in the 2-3 hours prior. The bow echo thunderstorm feature is also visible approaching KC from the SW. Surface temperatures south of the ENE-WSW boundary were in the low-mid 60s F, while north of this same boundary, temperatures were at least 10-15 F colder as a result of storm outflow.
The 3rd graphic is a RUC analysis profile estimating the environment just south of the ENE-WSW boundary at Olathe KS (OJC) about 40 minutes before the damaging winds hit Johnson County (the SW side of the KC metro area). Although there was not a lot of CAPE (maybe about 200 J/kg), the profile was fairly surface-based with a deep low-level moist layer and around 50 J/kg of CAPE below 3 km. Also notice how strong the southerly low-level winds were at only 2000 ft above the surface (roughly 900 mb), with sustained speeds to 50 kts (nearly 60 mph)! It wouldn't take much storm downdraft to move these winds downward to the surface, particularly in a surface-based setting like that just south of the ENE-WSW boundary. In contrast, the 4th graphic above is a RUC profile at KC International Airport (MCI), roughly 20-25 miles north of the same boundary. Notice that with colder temps north of the boundary, this profile is "elevated", with no CAPE at all from lifted parcels in the lowest 100 mb. Even though the strongest part of the radar bow echo went across northward near the airport, the damaging winds were limited to the area along and just south of the ENE-WSW boundary. That's probably because that's where the warm surface-based environment was, where it was relatively easy to move strong winds downward from not far aloft, whereas farther north the cold near-surface setting probably kept damaging downdrafts from reaching the ground.
The last graphic above is both reflectivity and velocity showing the strongest winds coming into the south KC area around 4:40 am, reaching measured speeds around 80 mph. This was south of the strongest radar echo, but along the advancing outflow southeast of the bow echo feature. Again, that's where the warm surface-based setting was (south of the ENE-WSW boundary), better for allowing damaging winds to reach the surface.
What a weird week in the KC area... sub-zero (F) temperatures early on (Sun-Mon), then temps recovering to record high levels on Friday with thunderstorms, and back to ice and snow during the day on Saturday. On Friday evening, Shawna and I found ourselves driving in a heavy thunderstorm in north KC with sharp CG lighting, then getting up to ice covered trees on Saturday morning while more storms raged to our southeast. If you live in the Plains, just wait a few minutes... the weather will probably change :-).
- Jon Davies 12/28/08