The system that will affect much of southeastern Arizona this afternoon has been digging south-southwestward across the Great Basin during the past 24-hours. I have looked at the NWS NAM and the Atmo WRF-GFS model forecasts that were initialized at 06 UTC (midnight). There are several interesting aspects to the forecasts. First, the 500 mb vorticity maximum is forecast by the NAM (above forecast is valid at 5 pm MST) is forecast to move across southern Arizona during the afternoon, so forcing for upward motion will be significant. This is another cold and relatively dry system, and temperatures aloft are again very cold. The NAM forecast of 1000-500 mb thickness (below) valid at midnight tonight brings thicknesses less than 5340 m across southeastern Arizona. Meaning the snow levels will drop very low late this afternoon - perhaps into the metro area, if precipitation lingers late enough into the evening.
The midnight run of the WRF-GFS has precipitation moving out of eastern Pima county by around 9 pm, so there may be some flurries around the area. The surface plot forecast for 2 pm is shown above, indicating a pretty raw afternoon here in Tucson. The WRF forecast for total precipitation through midnight tonight (below) is quite different than those for the last two event. The model is squeezing out more precipitation over the lower elevations of eastern Pima County, indicating that key ingredients come together nearly overhead. An interesting forecast, we'll have to keep an eye on the situation and take a look at the new forecasts from the morning data.
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