Phoenix (PSR) — the National Weather Service forecast in plain English · Plaincast

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An incoming weather system will begin to cool temperatures and increase rain chances today and Saturday.

Sunday and Monday will be warm and dry before a series of weather disturbances will bring occasional rain chances and cooling temperatures back to the Desert Southwest by Tuesday.

Short Term

The short wave trough that will bring cooler and wetter weather to the Desert SW starting today is currently in the Eastern Pacific just off the southern CA coastline. This system will move quickly, passing through southern AZ and Northern Mexico over the course of today. precipitable water will increase to near 200% of normal, which, in conjunction with the energy from the short wave troughing feature rain chances will also increase, with chance of rain peaking (30-60%) this afternoon across south-central AZ, then into the eastern higher terrain heading into the evening hours. Most of the expected shower activity will remain relatively light, talking about a few hundredths to at most a tenth of an inch, a few isolated thunderstorms resulting in briefly heavier rainfall rates cannot be ruled out given that there will be a least a few hundredths J/KG of storm energy present.

Behind the short wave just discussed is another short wave that will quickly move in from the northwest. However this next system will remain across northern AZ. This will lead to rain chances being mostly confined to the north with the lower deserts forecasted to be dry, while the higher terrain areas could squeeze out an additional 0.05"-0.10" Saturday.

Outside of rain chances, these systems will also being in cooler temperatures as H5 heights will drop between 564-567 dam on Friday and 570-573 dam on Saturday. Afternoon highs will be in the upper 70s for Friday then as the first short wave passes temperatures will cool into the low 70s across south-central AZ while out west temperatures will remain in the mid to upper 70s. after the first system passes low temperatures will lag in response, resulting in morning lows in the mid to upper 50s starting Saturday morning.

Long Term

Sunday will be a down day for shower activity with a brief period of shortwave ridging and some slight dry air advection from the northwest. This will lead to considerably less clouds, virtually no rain chances and temperatures climbing back into the upper 70s by Monday. Models remain in good agreement that another northern Pacific waves will continue to be ejected southeastward toward the U.S. West Coast starting early next week. Currently models suggest the next potential system reaching the California coast early Monday before reaching our region Monday night into Tuesday, where additional rain chances will return to the forecast with the blend-of-models forecast chance of rain peaking between 50-80% in south-central AZ and the higher terrain areas, however this far out this may need to be adjusted based of how the track of this low progresses. By Wednesday models are pointing towards yet again another low system, however larger discrepancies between models are evident for this system. Due to this confidence is low for rain chances by mid to late next week as things are too inconsistent at this time.

Despite if these next systems bring rain to the region or not, a more significant cooling trend can be counted on by the middle of the week as these series of weather systems move in from the northwest. They will likely usher in a cooler air mass with afternoon highs trending more towards the upper 60s to low 70s, the higher or lower end of this range largely being dependent on the depth and position of the systems.