[ADEQ Forecasts] ADEQ Issues an Ozone Health Watch for the Phoenix Metro Area for Saturday, July 18, 2009

[forecasts@lists.azdeq.gov]

*AN OZONE HIGH POLLUTION ADVISORY REMAINS IN EFFECT FOR FRIDAY, JULY 17, 2009*
*AN OZONE HEALTH WATCH HAS BEEN ISSUED FOR SATURDAY, JULY 18, 2009*

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

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


Health message for Friday, July 17: Active children and adults, and people with respiratory disease, such as asthma, should limit prolonged or heavy exertion outdoors.  
Health message for Saturday, July 18: Unusually sensitive people should consider limiting prolonged or heavy exertion outdoors.

Synopsis and Discussion
Near record temperatures will continue through the weekend with the afternoon highs here in the Valley around 114-118°F.  Phoenix's record highs for Saturday, Sunday and Monday are 115°F, 116°F, and 114°F, respectively.  Decreasing moisture will only help promote higher temperatures the next few days.  

Subtle changes to the weather pattern will have an affect on our air quality this weekend.  Firstly, with lower moisture this weekend, thunderstorm activity will be greatly reduced state-wide.  Thus, local particulate concentrations should be able to drop back into the "Good" range once again.  Then there's ozone.  Early morning concentrations are identical if not slightly higher than 24 hours ago at few monitors.  This puts several sites on pace to exceed the health standard late this afternoon.  The target area continues to be the east Valley (such as Fountain Hills, Apache Junction, and Queen Valley), although in-town locations (North Phoenix, West Phoenix, Central Phoenix, Glendale...) may also see concentrations approach the standard on Friday.  

Lower level winds are expected to continue to be out of the southwest through Monday, keeping the surface fairly dry.  This will direct newly formed ozone towards the east and northeast each day.  As you go higher up in the atmosphere, winds will turn out of the southwest, then north, and finally northeast by Monday, all while the lower levels stay out of the southwest.  This sheering effect would be great for severe weather if we had the moisture (which we don't), because it promotes vertical movement.  It also has a negative effect on ozone formation, allowing some of the surface ozone to disperse into higher parts of the atmosphere.  The net result is lower surface concentrations.  In addition, the strength of the west-southwest winds will increase a bit Sunday, scattering the diminishing ozone plume across a much broader area.  Subsequently, the Ozone HPA will be allowed to expire on Friday, with an Ozone Health Watch being issued for Saturday.  Further improvement is expected on Sunday.  Check back on Sunday for a look at your coming work week's weather and air quality.

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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