Was a very nice sunset yesterday (8 October 2012), with the Catalinas glowing in shades of purple to orange. Another cool morning here at house with low of 52F; down right along the Rillito Wash it was at least 5F cooler, so temperatures down there were in the upper 40s.
The closed low at 500 mb that's been parked off the West coast is now moving slowly south, as a new storm approaches from the north-central Pacific. The 1600 UTC water vapor image above indicates that the upper-tropospheric cyclone is located west of San Francisco. This morning's forecast for 500 mb from the NAM indicates the center of the circulation coming over southern California, north of San Diego, by Thursday evening. The 500 low then heads northeastward in the model forecasts, with a cold front and band of showers moving across Arizona Thursday night. The NAM forecasts some light rainfall amounts over southeast Arizona by Friday morning. The key to this event will be moisture availability and instability. The eastern Pacific blended PW product for 12 UTC this morning (below) indicates that values are increasing in a plume to the south and southwest of the 500 mb low. So, we need to keep track of how the low-level moisture inflow develops as the system moves into southern California tomorrow and Thursday.
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