Rainfall totals for the Prescott area thus far this monsoon are the highest in the state of Arizona, ranging from about 7” - 12” across the Quad City area. The heaviest rainfall has been from Williamson Valley and Ho Kay Gan to Yavapai Hills, extending from the northwest to northeast side of Prescott.
Late last week, winds aloft became northerly with high pressure to our west, which brought a layer of stable, warmer air across the upper atmosphere, but provided shearing over the very moist air that remained in place in the lower atmosphere. The result was the development of areas of strong thunderstorms Saturday evening in eastern Yavapai County and on the west side of the Quad City area. Lines of cells repeatedly developed northwest of Paulden and propagated southward across the west side of Chino Valley, Williamson Valley, and into Prescott, dissipating on the southwest side of town. The rainfall totals over this entire area on Saturday night ranged from 1-3”.
The upper-level high now located over Southern Arizona, has continued to usher dry westerly winds aloft over northern Arizona yesterday and today and produced sinking motions that stabilized the atmosphere a bit and have resisted thunderstorm development. Nevertheless, the lower atmosphere has remained very moist, with dew points near 60 degrees Fahrenheit. The high moisture content combined with temperatures rising to near 90 degrees, led to strong instability this afternoon. As a result, a severe thunderstorm developed, with severe thunderstorm and subsequent flash flood warnings issued for the Black Hills (Mingus Mtn.) where at least 2” of rain fell, and radar indicated the potential for up to golf ball sized hail and 60 mph wind gusts. I am not aware of any reports confirming that the thunderstorm produced these conditions at the ground, but it certainly was an impressive storm on radar imagery, with strong reflectivity and possibly some weak rotation.
For tomorrow and through the rest of the week, the high pressure aloft will shift eastward and eventually become centered over the Southern Plains of the U.S. This pattern will bring south-southwesterly to southwesterly winds across Arizona in the upper atmosphere. The south-southwesterly winds aloft will initially bring a disturbance northward into Arizona, with moist and cooler aloft late tonight and tomorrow, this trend will make thunderstorms more likely and more widespread across northern Arizona. The upper-level winds will be as strong as 50 – 60 mph, providing ample shear for strong to isolated severe thunderstorms. Also expect the possibility for flash flooding if cells redevelop over the same areas. Storm motion will generally be toward the northeast at 10-20 mph.
For the rest of the week, the southwesterly winds will be a bit drier and weaker, so less of a chance for severe storms but an ongoing chance of thunderstorms each day Wednesday through Friday. Another burst of stronger winds aloft and better shear for stronger or more organized storms could occur again this coming weekend.
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