The Intense High Plains “Bomb” Cyclone of 12–14 March 2019

An intense “bomb” cyclone (Sanders and Gyakum 1980) formed over the southern and central High Plains on 12–13 March 2019. This storm deepened ~25 hPa in 18 h ending at 1800 UTC 13 March over southeastern Colorado. Surface cyclogenesis occurred initially over far southwestern Texas ahead of a leading upper-level cutoff cyclone near the Arizona–Mexico border. Dynamically driven rapid surface deepening commenced when a trailing upstream upper-level trough over California and Nevada deepened southeastward and interacted with a leading cutoff cyclone. Rapid surface deepening ceased after the trailing upper-level trough underwent cyclonic wave breaking (CWB) and developed a cyclonic “potential vorticity (PV) hook” on the dynamic tropopause. The surface cyclone remained quasi-stationary over southeastern Colorado during the rapid deepening phase, suggestive that orographic processes contributed to the rapid deepening process. All-time record minimum sea level pressure readings were established at several locations during this storm including Pueblo, CO (971.0 hPa) and Dodge City, KS (974.7 hPa). Widespread high winds and heavy, blowing snow during the storm caused widespread severe transportation impacts. The purpose of this presentation is to illustrate the structure and evolution of the noteworthy bomb cyclone of 12–13 March.

We hypothesize that the interaction of the aforementioned upstream trailing trough over California and Nevada with the leading downstream trough situated near the Arizona–cMexico border enabled the leading trough to: 1) acquire a negative tilt over the Southwest by 0600 UTC 13 March, 2) form a cutoff cyclone over southeastern Colorado by 1200 UTC 13 March and 3) reach maximum intensity on the Colorado-Kansas border at 1800 UTC 13 March. Large-scale deformation processes enabled a cyclonic vorticity maximum associated with the trailing upstream trough to wrap cyclonically around the aforementioned “PV hook” where it merged with a cyclonic vorticity maximum being drawn poleward ahead of the leading downstream trough. The merger of these two cyclonic vorticity maxima resulted in the formation of a strong 500-hPa cutoff cyclone on the Colorado-Kansas border. Deep subtropical moisture from the eastern Pacific was advected northward east of the Continental Divide, ascended into the upper troposphere ahead and over of the rapidly deepening Colorado cyclone, and was exported downstream toward the upper-level ridge. Diabatically enhanced deep moist ascent likely “turbocharged” the cyclogenesis process and contributed to rapid surface deepening in two ways: (1) enhanced strong ascent and low-level convergence facilitated rapid near-surface cyclonic vorticity generation, and (2) enhanced downstream upper-level ridge building likely increased the magnitude of differential cyclonic vorticity advection over the cyclone center.

We will put the bomb cyclone of 12–13 March into perspective by comparing it with a second bomb cyclone on 10–12 April 2019 that evolved more like a “typical” Colorado lee cyclone in that it initially moved southeastward away from the Rockies before turning northeastward and deepening rapidly in a strongly baroclinic environment. The rapid deepening phase of the April 2019 bomb cyclone coincided with a synoptic-scale CWB event over the Rockies. Strong cyclogenesis occurred near the tip of the “PV hook” in a very baroclinic but relatively dry environment in the lee of the southern and central Rockies. The absence of any severe weather reports west of the Mississippi River on 11 April coupled with only a smattering of wind and hail reports in Texas and Arkansas on 12 April suggests that the April 2019 bomb cyclone was “moisture starved” initially because widespread low-level northerly flow in place behind an antecedent trough resulted in considerable drying across the Plains. The presence of drier air likely precluded both a widespread severe weather outbreak and additional diabatically enhanced surface deepening east of the southern and central Rockies.