Sunday, December 30, 2012

Cold And Damp Storm Approaching

This morning the 500 mb trough stretches from Montana deep into the subtropics west of Baja. The 12 UTC NAM analysis (above) indicates several vorticity maxima within the trough. The one over northern California will serve to broaden the base of the trough and slow it down just a bit. The NAM 24-hour forecast predicts the primary vorticity center to be located between Tucson and Yuma at 12 UTC tomorrow morning. The NCAR RAL 250 mb analysis for 12 UTC this morning is shown below. Upper-level winds appear to be strongest to our east and this would typically result in the trough moving quickly by. However, the jet west of the trough is centered somewhere just off the west coast, so it's character is not quite well-defined.

The Atmo WRF-GFS model apparently ran at midnight last night. However, I can only find the tabular forecast for southeast Arizona - above. Note that the WRF-GFS forecasts a fairly light precipitation event, with forecast to have the most precipitation at 0.47". It appears that this system is not pulling subtropical moisture into the Southwest, as the models had tried to do earlier in the week. The NAM model now forecasts thicknesses over or near Tucson tomorrow morning down to 534 dm - meaning the snow level will likely drop into at least parts of the metro area late tonight/early tomorrow. Assuming of course that the model has accurate forecast. Some roads and bridges in eastern Pima County could be slick at sunrise tomorrow. Caution advised.

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