[ADEQ Forecasts] ADEQ AIR QUALITY FORECAST FOR SATURDAY, AUGUST 30, 2008

[forecasts@lists.azdeq.gov]

http://www.azdeq.gov/ensemble.pdf

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

Health message for Friday, August 29:  Unusually Sensitive People should
consider limiting prolonged exertion outdoors.
 
Health message for Saturday, August 30:  Unusually Sensitive People
should consider limiting prolonged exertion outdoors.
 
Synopsis and Discussion
   Violent! Vicious!  It was easily the most destructive storm of the
summer here in Phoenix.  The line of storms that moved from northeast to
the southwest through metropolitan Phoenix, Tempe, Chandler, Mesa,
Gilbert, Laveen, etc. on Thursday evening will be the poster child of
the 2008 thunderstorm season.  Downed power lines, trees, traffic lights
lying in the street, roof damage and of course  flooding was among some
of the reported damage from the three plus-hour storm.  So what
happened?  Where did it come from?
   Despite high pressure not exactly in the favorable Four Corners area,
an area of low pressure formed over southeastern Arizona.  This drew
ample moisture north into the region while providing lots of dynamics in
the atmosphere.  The line of storms stayed generally to the south of
Phoenix much of the day and was not nearly as intense.  Slowly that line
edged north.  Streaming from Fountain Hills in the east Valley to Gila
Bend in southwestern Maricopa County, this line of storms at one point
stretched to 90 miles long.  Known as the "train effect", one cell after
another developed and moved southwest following the same path.
Microburst winds were measured in excess of 80 mph, resulting in the
widespread damage.  Rain gauges reported over an inch and a half or rain
in downtown Phoenix in less than 15 minutes.  It's hard to imagine that
1.89 inches was the highest recorded in Maricopa County Flood Control
District's network.  It's possible that power outages or the
sideways-moving rain resulted in lower estimates that actually occurred.
Never the less, this will be a storm to remember!
   Air Quality data was affected due to power issues Thursday night.
With limited information, it's difficult to tell exactly what happened.
Early indications show a possible Ozone exceedance at Fountain Hills
through 8pm, but without all the information, it can't be verified at
this time.  Rain came rather quickly with this storm, so it's unlikely
that dust was high for a long enough period to cause a 24-hr exceedance.
With another round of storms expected late Saturday through Sunday,
another inch for rain in Phoenix (already the 10th wettest monsoon on
record in Phoenix with a month to go) is possible.  This will likely
keep Ozone and Particulates in the "Good" range for at least a few days
beginning on Sunday.  Hopefully we'll have more for you in the way of
air quality data at that time.  Until then, have a great three-day
weekend!  -J.Paul

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