[ADEQ Forecasts] ADEQ Daily Air Quality Forecast for Thursday, August 13, 2009

Wed Aug 12 12:56:54 MST 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 Wednesday, August 12: Unusually sensitive people should consider limiting prolonged or heavy exertion outdoors.

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

Synopsis and Discussion
Timing is everything... High pressure has moved back into a favorable Monsoon position, a scenario that has been unusually absent for much of this season.  Moisture has begun to stream north from Mexico as evident by the rising surface dew points.  Early morning clouds have also been a good indication that the thunderstorm potential is increasing across the region.  Mountainous areas in southeastern and eastern Arizona can expected to see some strong thunderstorms later this afternoon.  The best chance for storms to move into the Phoenix forecast area will come late Thursday night into early Friday morning as an upper-level disturbance rotates northwest around the Four Corners high.  

As mentioned, however, timing is everything.  A large trough of low pressure will develop over the western U.S. in the next 24-48 hours.  Cool, dry air will spill south from Canada, strengthening the trough as it pushes east.  Models suggest that it will quickly shove the Four Corners high pressure system to the southeast once again, temporarily suspending the moist flow from Mexico.  If storms progress north from the Tucson area quick enough Thursday night before the trough imposes its authority, the Valley could be treated to quite a lightning show culminating in a wet night on Thursday.  On the other hand, if the cool, dry air moves in quicker that the storms from the south, we may just wake up to partly cloudy morning on Friday.  

How will all this affect air quality?  Ozone concentrations are currently rebounding from a temporary down day due to some cloudy skies on Tuesday.  Afternoon winds will be out of the west and northwest on Thursday under mostly sunny skies.  This could lead to some mid to upper "Moderate" concentrations in the east and southeast part of the forecast area.  Thus, the target area for highest ozone concentrations on Thursday will likely be around West Chandler, Queen Valley, and Apache Junction.  Morning cloud cover on Friday would shut down ozone generation for a good portion of the day, resulting in much lower levels.  PM-10 concentrations will be dependent on thunderstorm outflow winds from distant storms Wednesday and Thursday afternoons.  Any rain Thursday night and/or Friday morning would help precipitate any remaining suspended dust, effectively lowering overall PM-10 levels.  Thus, we will keep forecasted levels in the low to mid "Moderate" range during this period with further improvement expected over the weekend.  

Check back tomorrow for the latest on the rain potential and how it will affect local 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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