This past week, the monsoon has brought less rain to the Prescott area than in previous weeks due to the persistence of high pressure over northern Arizona, which has brought higher-than-normal temperatures and served to stabilize and dry the atmosphere a bit, making thunderstorms less likely each day. Nevertheless, with easterly winds aloft shearing with southwesterly winds, we saw a strong squall line on Monday that moved from the Mogollon Rim southwestward into the Phoenix metro area, prompting a flash flood warning and several severe thunderstorm warnings. Many reports of wind damage were received, with winds clocked at 70 mph at Sunset Point and 74 mph at Deer Valley Airport.
Today – tomorrow, the high pressure aloft will weaken and begin to shift eastward as a weak disturbance approaches from the west. The winds aloft have already shifted to a southerly direction as a result, and the upper atmosphere will cool a bit, making thunderstorms more likely today through Friday. Expect thunderstorms to drift toward the northeast or east at about 5 mph, and be capable of locally heavy rain and small hail. The storms will not have much organization to them as the vertical wind shear will be weak.
On Friday, as the disturbance swings across the state, forecasts are indicating that dry air from the west and northwest will stream across Arizona, lessening the chance of thunderstorms for the weekend. For next week, high pressure is expected to rebuild into the Four Corners region, then shift northwestward over the Great Basin. This will usher monsoon moisture back across the area for a chance of thunderstorms expected each day next week.
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