[ADEQ Forecasts] ADEQ Daily Air Quality Forecast for Friday, August 14, 2009

[forecasts@lists.azdeq.gov]

For more information, please visit our Web page:
http://www.azdeq.gov/ensemble.pdf

For transportation alternatives:
http://www.valleymetro.org


Health message for Thursday, August 13: Unusually sensitive people should consider limiting prolonged or heavy exertion outdoors.

Health message for Friday, August 14: No significant winds are expected.


Synopsis and Discussion
An early surprise... Thunderstorms to the south pushed north through out the evening, generating a lot of blowing dust a head of them.  More importantly, the outflow triggered some storms in the west Valley around 11 pm, from Peoria to the White Tank Mountains.  As the night progressed, storms weakened but rain spread across much of the Phoenix metropolitan area.  Totals generally range from a quarter to a half an inch.  At about 4am this morning, south Phoenix (including Phoenix Sky Harbor) got a nice downpour with amounts nearing an inch in some places.  As of 10 am, thick clouds continue to block the sun while keeping temperatures down (only 82°F at PHX vs. 96°F just 24 hrs earlier).  

There is a chance for more rain later this evening as clouds begin to break, adding additional energy to an already unstable atmosphere.  A disturbance currently over Santa Cruz County will move northeast through the afternoon, helping spawn thunderstorms in southeastern Arizona.  The race is on to get some storms from southern Arizona to move north into our region before dry air pushes in from the west.  The latest water vapor image shows the dry boundary has already breached the Colorado River between California and Arizona.  Once it reaches the Phoenix forecast area, the amount of available moisture for local precipitation will be greatly reduced.  Advancing at a rate of 7 mph, this would put the boundary near Phoenix by 4 am Friday morning, plenty of time to get some additional storms into the forecast area.  The associated trough of low pressure will continue to push through the western U.S. this weekend.  High pressure is expected to rebound quickly by Monday and Tuesday, returning the moist Monsoon flow to Arizona once again.  

Valley air quality has taken a positive hit thanks to the widespread rain.  As mentioned, there was some initial blowing dust ahead of the storms, impacting the local monitors in a big way.  Several sites registered PM-10 concentrations greater than 1,000 micrograms per cubic meter around 10pm.  This put their 24-hr averages close to exceeding the health standard.  Fortunately the subsequent rain washed the air clean so that morning concentrations started out very low.  The damp soil will take several days to dry out, keeping overall PM-10 and PM-2.5 concentrations well in the "Good" category through the weekend.  Because of the thick cloud base Thursday morning, necessary sunshine for ozone formation has been restricted.  This means actual ozone concentrations will be well below the original mid "Moderate" forecast.  Increased sunshine this weekend could help ozone rebound slightly, but sustained dry, westerly winds will push the invisible plume to the east and northeast of the Valley, resulting in maximum local concentrations in the mid to upper "Good" range.  

Check back tomorrow for final storm totals and more detailed look at your weekend's weather and air quality.  Until then, have a good day!

If you have any questions or comments regarding this forecast please contact:

The Arizona Department of Environmental Quality
Office of Communications
1110 W Washington St, Phoenix, AZ 85007
(602) 771-2215
ms15@azdeq.gov


 
 

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