Tuesday, February 27, 2018

February Ends With Fast-Hitting Weather Event

Nice view of alto-cumulus over the Catalinas at 7:20 am MST this morning.

The 500 mb analysis for 12 UTC (above, from SPC) shows the closed low centered along the California coast north of Vandenberg AFB. Temperatures are very cold with the system, although it is not going to dig as far south as some earlier forecasts indicated. The 500 mb forecast below is from the 06 UTC run of the WRF-GFS at Atmo and is valid at 6:00 am tomorrow morning. By that time the model forecasts the system to have opened-up, warmed, and sheared to northeast, bringing a positively-tilted trough across Arizona during the night.

The 09 UTC SREF plumes from SPC forecast of QPF for TUS is shown below. The ensemble average (black) remains at less than 2/10s of an inch, with considerable spread.

The system is definitely moisture starved and the 06 UTC WRF-GFS forecast for Tucson now indicates only a Trace at the airport - a distinct drying trend from earlier WRF forecasts. The WRF now forecasts an event that mostly impacts higher elevations, and it's a tough call to guess how much will be in the low-elevation gauges by mid-day tomorrow.

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