The figure above indicates the forecasted high in orange (with error bar denoting the range of ensemble forecast members), average high for this time of year in orange (dashed line), forecasted low in blue (with error bar), average low this time of year in blue (dashed line), and forecasted wind speed in purple. Note that the wind speed forecast is sustained wind (not gusts), and it is based on only one model (not an ensemble).
A weak low pressure system off the West Coast of the U.S. and a high pressure system over Northern Mexico are directing deep south to southwesterly flow across Arizona. A dissipating tropical disturbance embedded within this flow field is currently moving into southeast Arizona, with heavy rain currently being reported in Tucson, Douglas, and Nogales. The tropical disturbance will continue to propagate northeastward, bringing heavy rains to eastern Arizona and New Mexico and considerable cloudiness will stream across much of the state.
Very moist air is surging northward today, with surface dew point temperatures already in the 70s in the Phoenix area this morning. Dew points are expected to climb into the upper 50s to lower 60s in the Prescott area this afternoon or this evening. Meanwhile, the low pressure system off the West Coast will move northeast the next couple of days and help to lift the moist air. As a result, there will be a decent possibility of rain showers or thunderstorms this afternoon through tomorrow night.
The vertical wind shear will be conducive to multicellular thunderstorms, leading to the development of lines of showers or thunderstorms with some echo training possible (where thunderstorm cells can develop in succession and propagate over the same areas) tonight to tomorrow morning. Cell motion will typically be toward the southwest at 20 mph tonight, turning toward the north or northeast at 10 mph on Tuesday. Flash flooding is a slight possibility in Yavapai County, and the National Weather Service has issued a flash flood watch valid for the next couple of days including Yavapai County. The forecasts are indicating an average of about 1 inch of total rain accumulation by Tuesday night for Prescott.
Due to the presence of warm air in the upper atmosphere today and tonight, the layer of instability will be below about 20,000’ initially. As a result, only shallow or weak thunderstorm cells will be possible, despite the very favorable wind shear with height. On Tuesday the upper atmosphere will cool as the upper-level low from off the California Coast approaches Arizona, leading to deeper instability, but the vertical wind shear will become weaker. As a result, severe thunderstorms are not expected in Yavapai County, but thunderstorms are a possibility (especially Tuesday).
Expect drier air to move back across Arizona Wednesday through the coming weekend behind the upper-air trough, with only a slight chance of lingering showers/thunderstorms on Wednesday.
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Official National Weather Service forecast