At last, monsoon moisture is beginning to stream north into Arizona as the mid-level high moves to a more favorable position near the Four Corners. Today will be a transition day, with temperatures remaining warm, mostly scattered cloud cover and a 10-15% chance of thunderstorms over the Prescott region (see the NWS attachments). Any storms today will be high based, with potential for brief gusty winds and new wildfire starts. Tomorrow and Wednesday will see the best chance for thunderstorms (around 50%) and more wetting rains. The chance for storms decreases to 30% for the rest of the week. Temperatures will be a little cooler with the increased cloud cover. Models are hinting at a warming and drying trend by the weekend.
Additional notes for the weather nuts out there:
Will I get a thunderstorm? Three necessary ingredients for thunderstorms are instability, low-level moisture and a lifting mechanism. Instability means that an air parcel that is lifted will continue to accelerate upward, possibly producing a thunderstorm. Instability is strongest when the surface is strongly heated by the sun and there is plenty of low-level moisture. The dew-point temperature is a convenient way to monitor surface moisture – the higher the dew-point, the more moisture there is. Around here, dew-points in excess of about 50F are conducive to thunderstorms. Lifting is typically caused by mountains, thermals, and sometimes outflows from nearby thunderstorms. Storms often form first near mountains, then move off in a direction determined by average winds in the thunderstorm layer.
If you see a storm, there are two ways to figure out which way it could move. The first is to look at weather radar loops. Go to http://weather.rap.ucar.edu/radar/, select FSX on the map, then under loop duration, select 1 hour. This will enable you to see precipitation echoes and which way they are moving. The second is to go to http://weather.rap.ucar.edu/upper/fgz.gif. This is a radiosonde balloon sounding from Flagstaff, our nearest sounding location. At the top of the graph there is a label that says CELL with something like 190/11 below it. This means that thunderstorm cells near Flagstaff are expected to move from 190 azimuth (i.e., toward the north) at 11 kts. Storms today and tomorrow are expected to move in a direction from southeast to northwest.
With all this information, you will be able to forecast monsoon thunderstorms like a pro! Enjoy!
And stay safe – remember, “if thunder roars, move indoors.” We don’t want any of you struck by lightning and turning into crispy critters.
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Official National Weather Service forecast