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).
Considerable cloudiness is streaming northwestward across Arizona today as high pressure aloft rebuilds into the Four Corners region. The low-level moisture is still a little bit lacking today, but I expect low-level moisture to surge across all of northern Arizona this weekend. Embedded in this southeasterly flow is a disturbance (or easterly wave) that is currently located over southern Texas and northern Mexico. As this disturbance moves across southern Arizona on Saturday and Sunday, it will bring with it greater moisture content, cooler air aloft (for greater instability), and lifting to help trigger thunderstorms. The disturbance will most likely affect southern Arizona more than northern Arizona, but the outflow from thunderstorms in southern Arizona helps to enhance low-level moisture in northern Arizona. All-in-all it appears that it will be a moderate monsoon pattern this weekend through at least the middle of next week.
Expect an increasing chance of thunderstorms through the weekend (up to about 50% chance on Sunday). Storms will generally propagate northwestward at about 10 mph on Friday and more westward at 10 – 15 mph on Saturday and Sunday. Vertical wind shear could become strong enough at times to support broken lines of thunderstorms triggering over mountains to our southeast or east and propagating across Yavapai County. Expect locally heavy rain, gusty wind, frequent lightning, and small hail in some locations.
The atmosphere won’t be quite as moist this weekend as it was last weekend, so don’t expect as much flash flood potential.
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