Just got back from ChaserCon in Denver -- great to meet and talk with many storm chasers there. Of course, while I was gone, a rather unusual weather event took place near home... Some convective snow squalls generated sudden white-out conditions in the KC area, causing trafffic accident pile ups at a couple locations on the Kansas side just after lunch on 2/14/10. Thankfully, no one was seriously injured. With as many as 40 cars piled up on I-70 near Bonner Springs (see KC Star photo above), it could have been much worse.
Believe it or not, these convective snow squalls were caused by an intense "cold-core" type closed low in midlevels moving southeast (see the NAM 500 mb analyses in 2nd graphic above). A strong vorticity maximum, or center of atmospheric "spin", was assciated with the midlevel low and moved right over the KC area around midday (see the "X" and red coloring on the NAM 500 mb maps above), providing a strong focus of forcing/lifting. A tight area of very cold temperatures just above the ground (-14 deg C, see the SPC 850 mb map at midday, 3rd graphic above) moved also across KC with this system. Combined with sun's heating (there was some sunshine just ahead of this system), low-level lapse rates increased rapidly (see orange area near Kansas City on the SPC 3-hr lapse rate change image, 3rd graphic above). All these ingredients working together generated a local area of CAPE and instability over the KC area (see RUC analysis sounding at KCI airport at 1:00 p.m. CST, 4th graphic above). Yes, there can be CAPE on days with surface temperature below freezing; this combined with the intense lifting/forcing system aloft undoubtedly contributed to the intensity and suddeness of the snow squalls.
The final graphic above shows a regional composite of radar reflectivity in low-levels around 12:00 pm and 1:00 pm as the "twisted" or curved area of snow squalls (see white arrow) associated with the vorticity center aloft approached the west side of Kansas City. The upper system was so intense and focused that an associated subtle trough in the surface wind field could be seen on the surface map at the same time moving southeast over the KC area (see blue dashed line on surface map in final graphic), even though the surface cold front was 400 miles to the south and east.
The Kansas City area National Weather Service at Pleasant Hill MO has posted a good page about this weather event setting at http://test.crh.noaa.gov/eax/?n=winter-02142010.
This event is a reminder that the contribution of low-level CAPE with some "cold core" midlevel lows to extreme weather isn't limited to just the warm season and thunderstorms!
- Jon Davies 2/15/10
