Another Fall Cutoff Low

I've been away for several days dealing with medical problems and am now back at work. Of note the past few days were the strong, high-based thunderstorms on Friday afternoon (October 15th). These produced light rains in the Tucson area but the rains came with very strong outflows - 52 mph at TUS, 68 mph at the Atmo rooftop anemometer, and I estimate 50 to 60 mph at TMC. There was one marginal severe storm report in the Tucson metro area.

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This week we're following another strong cutoff low at 500 mb. The water vapor imagery this morning (top graphic) shows that it is currently located south of San Francisco. The models forecast it slowly southward, before beginning to swing it northeastward toward central Arizona on Wednesday afternoon. The cutoff will bring heavy rains to southern California, especially tomorrow. This is the kind of cutoff that often brings strong storms and a threat of weak tornadoes to the LA Basin - so it will be interesting to see if 500 mb temps remain cold enough to produce thunderstorms in the LA area.

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The morning sounding at Guaymas, Mexico (middle graphic), shows that a northward push of quite moist low-level air is underway. The NAM model indicates a strong southerly fetch into Arizona the next couple of days, so lower-levels should moisten up (current surface dewpoints in sourthern Arizona - 40s to 60F at Yuma). The morning NAM foecasts also indicate that a backdoor front (currently moving southward down the High Plains) will move into southeastern Arizona by Wednesday morning, complicating the synoptic situation.

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The NAM model forecasts little in the way of rainfall with this system across southern Arizona through 84-hours. The 60-hour forecasted rainfall ending at 12 UTC on Thursday morning is shown in the bottom graphic. Given the cold 500 mb temperatures with this cutoff and the influx of low-level moisture from the south, it appears that deep convection will be likely in southeast Arizona. The NAM model forecasts for southern Arizona may well be out-to-lunch this morning. I should note in closing that the strong system in early October pushed the very moist mT air far to the south, well beyond the southern end of the GoC. Thus, it doesn't appear that this current system will be able to pull mT air this far north.