Dust Risk-Based Forecast | Maricopa County

Friday

Stagnation: Morning stagnation

Wind: Light and variable winds (gusty winds possible with outflows/storms)

Saturday

Stagnation: Morning and evening stagnation

Wind: Light and variable winds (gusty winds possible with outflows/storms)

Sunday

Stagnation: Morning and evening stagnation

Wind: West winds of 5-10 mph

Monday

Stagnation: Morning and evening stagnation

Wind: West winds of 5-10 mph

Tuesday

Stagnation: Morning and evening stagnation

Wind: West winds of 5-10 mph

Forecast Discussion:

While it certainly didn't look like there was unhealthy air quality in the Phoenix Valley yesterday (Wednesday), as there was no large dust storm or big brown cloud on the horizon, the air was indeed unhealthy. The reason you didn't see anything? Ozone is invisible, and that was the pollutant yesterday the resulted in 20 monitors within the Phoenix Valley exceeding the federal health standard, with one monitor (Falcon Field) coming in with values in the Unhealthy AQI category. A combination of mainly clear skies, light winds, local emissions, and regional smoke spread across the United States from fires burning in the Pacific Northwest and Canada resulted in ozone values quickly rising during the late morning and afternoon hours.

Today, things get a bit more complex for forecasting both the weather and air quality. High pressure is now centered over Kansas, with two disturbances noted on water vapor this morning rotating clockwise around the high. One is weak and moving through Arizona this morning, producing some rain showers across northern and eastern Arizona. For Phoenix, we are seeing cloud cover move overhead this morning as clouds move from east to west. A more significant disturbance is located in Northern Mexico, just south of Texas.

Clouds with the first disturbance are forecast to clear out of the Valley later this morning into the early afternoon hours, but the second system will help to increase thunderstorm chances later this afternoon into the evening hours as thunderstorms that form over the higher terrain east of the Valley will try to make that way into the region. Ahead of any thunderstorms, we could see strong outflow winds from the east-southeast move into the Valley. While not expecting very dense dust due to all the rain lately, we could undoubtedly see pockets of blowing dust with even a defined area of dust along the outflow boundary, but PM-10 is expected to remain in the low Moderate AQI category like we saw with the dust storm that moved through earlier this week.

Tomorrow, the disturbance in Northern Mexico will continue moving westward, allowing thunderstorm chances to remain elevated in the Valley through Saturday. While not expecting as widespread of storms over the next several days as we saw last weekend, there will still be the risk of flash flooding as the soils are already wet, and storms that form will be capable of producing heavy rain.

As for ozone, with light winds, breaks in the clouds, and outflow winds from thunderstorms being later in the afternoon/evening today, we are still forecasting ozone levels to quickly rise and exceed the federal health standard. As a result, an Ozone High Pollution Advisory will be in place for Phoenix today. Tomorrow, with added debris clouds in the morning and thunderstorms possibly moving in earlier in the afternoon, expecting ozone levels to lower back into the upper Moderate AQI category, with similar conditions on Saturday. Sunday through the first part of next week, we are forecasting a downward trend in storm activity, mainly clear skies and light winds. As a result, ozone levels are forecast to increase, so an Ozone High Pollution Watch will be issued for Phoenix on Sunday and Monday.

Make it a great Thursday!

- M. Pace
ADEQ Meteorologist