This 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).
Following the subtropical disturbance that moved northward across Arizona last Wednesday, a burst of drier westerly wind squelched the monsoon moisture a little more than I had expected. Today, the upper-level high pressure has become centered over southern Utah, bringing sinking air motions and hot temperatures to Arizona.
As the high pressure expands northward over the next few days, expect deep easterly flow to develop across Arizona. This will gradually moisten the atmosphere and lead to an increasing chance of thunderstorms through Thursday and a continuing chance of thunderstorms into the weekend. The added moisture and cloud cover combined with less sinking motion over Arizona will bring a gradual cooling trend this week, bringing high temperatures to near normal levels.
For the first half of the week, thunderstorms will be high-based due to the high temperatures and low humidity, and will be capable of igniting fires until more wetting rains arrive. With the flow aloft becoming easterly, look for lines of thunderstorms moving into the Prescott area from the east each day (especially Wednesday and Thursday). Storms will generally propagate westward or west southwestward at about 10 – 15 mph.
By the weekend, a Pacific trough is forecast to approach the West Coast and turn the winds aloft to a more southerly direction again. This will help to bring deeper moisture across Arizona and enhance thunderstorm potential for the weekend before possibly drying things out again as the trough moves across the Great Basin early next week. Stay tuned!
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