After a rainy, cool and heavily clouded weekend, the new week begins with the western US S/W at 500 mb phasing and merging with a Pacific vorticity max over southeastern Arizona during next 24-hours. So a showery day on tap. Then by Thursday another broad and diffuse S/W moves across the west coast. This feature consolidates, strengthens and moves southeastward. This S/W's vorticity max passes very close to Tucson by Thursday evening and the evolution of this feature should be watched closely - currently the NAM forecasts its main precipitation impact to be in Arizona's mountains and New Mexico.
By next weekend a much stronger wave at 500 mb comes ashore and cuts off over the Southwest (according to the GFS forecast runs from last evening - 00 UTC 22 Feb 2010).
The GFS ensemble mean and spaghetti plot for the Northern Hemisphere valid next Sunday Feb 28th at 5 am are shown above. It is quite interesting that the ensemble spread of forecasts and uncertainty is greatest over the Western US; however, all 12 members forecast precipitation over southeastern Arizona for the 12-hours ending at 156-hours. It is also interesting that the operational GFS (white contours on right panel) is an outlier relative to the other members, moving the trough/cutoff eastward faster than the other forecasts.
Regardless, more rain on track for Tucson - noticed driving around over the weekend that our winter weeds have begun a period of wild, rapid growth. The same is true for the thousands of potholes lurking out there on Tucson's streets!
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