Tornadoes in the desert

Early in the morning on the day after Thanksgiving, a long-wave upper-air trough with an embedded short wave disturbance and associated diffluent flow forced thunderstorms in the Phoenix, AZ region. There was enough low-level moisture return to provide adequate CAPE within a well-sheared kinematic environment, providing storm organization and maintenance.

ProbTor (from NOAA/CIMSS ProbSevere) captured the evolution of these storms, two of which spawned three tornadoes in the Phoenix metro area. The twisters uprooted trees, and caused damage to powerlines and roofs.

Fig. 1: ProbSevere contours (ProbTor is the outer contour), MRMS MergedRef, and NWS severe weather warnings. 

The tornado that traveled north of downtown Phoenix was rated EF1. From the time series below, ProbWind and ProbTor were about 30% when the NWS issued a severe thunderstorm warning. Then, ProbTor spiked to about 65% as the 0-2km MRMS AzShear increased markedly. The paltry lightning activity and weak 3-6km MRMS AzShear in this storm show that the rotation was shallow in the troposphere and this was not a supercellular storm. The 0-2km AzShear, along with very strong effective bulk shear and 1-3km mean wind helped the ProbTor values increase rapidly.

Fig. 2: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the northern tornadic storm. NWS warnings and preliminary local storm reports are plotted on the lower axes.

Fig. 3: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the northern tornadic storm. NWS warnings and preliminary local storm reports are plotted on the lower axes.

Further south along the line of storms, ProbTor values behaved in a more cyclic manner, first hitting 40% before dropping to 15%, then increasing rapidly to 30% (at the time of the first tornado LSR) and 60% (at the time of the second tornado LSR). ProbTor values then decreased to 20% and rebounded to 55%. This cyclic nature followed the 0-2km AzShear somewhat closely.

Fig. 4: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the southern storm. NWS warnings and preliminary local storm reports are plotted on the lower axes. 

Fig. 5: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the southern storm. NWS warnings and preliminary local storm reports are plotted on the lower axes.