The suppression of deep moist convection near the southern plains dryline

On 6 May 1995 forecasters for the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) were confident that severe convection would occur in the afternoon near the Southern Plains dryline in the Texas panhandle. However, only cumulus were observed before sunset.

The mid-level vertical motion and vertical vorticity fields in the National Centers for Environmental Prediction (NCEP) reanalysis suggest one likely candidate for the unexpected absence of convection. A shortwave migratory ridge moved over the area exactly during the time of maximum diurnal heating.

Observational data are consistent with the hypothesis that subsidence associated with the shortwave ridge was responsible for the suppression of convection. Operational and M-CLASS VORTEX soundings show the formation of a thermal inversion above the boundary layer during the day. Radar reflectivity data show that while the ridge was present over the general area no precipitation occured, whereas mesoscale convection was observed before and after the ridge passage. Water vapor satellite imagery shows the propagation of anticyclonically curved features ahead of a ``digging'' trough over the West Coast. Under the assumption of dry adiabatic subsidence of air parcels, the NCEP reanalysis data yield an observed warming of 3 K over 6 hoursduring the afternoon of 6 May 1995.

Operational models and simulations performed with the NCAR/Penn State mesoscale model MM5 provide some guidance regarding the presence of a pre-trough ridge, but largely fail to capture the mid-level warming and cap formation. A large number of permutations of various model parameters within MM5 showed no significant improvement in the simulation of the ridge. It is reasonable to assume that the conditions leading to the formation of the shortwave ridge were inadequately captured in the initial model conditions over northwest Mexico and the Pacific Ocean.