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Area forecast discussion...updated 
National Weather Service Phoenix Arizona 
435 PM MST Sat Jul 19 2008 


South-central and southwest Arizona...and southeast California 
forecast discussion 


Update... 
updated aviation discussion 


&& 


Synopsis... 
moisture is increasing over our area in a clockwise flow from the 
east and south around high pressure aloft that has moved to Colorado. 
This will result in a significant increase in the chance of showers 
and thunderstorms...some with locally heavy rain...tonight and 
Sunday. Strong gusty winds are also possible tonight. It will be 
cooler and more humid Sunday. At least a slight chance of mainly 
afternoon and evening showers and thunderstorms is expected this 
coming week with a recovery of temperatures Tuesday. 


&& 


Discussion... 
the expected surge of cooler and much more humid Gulf of California 
air was resulting in gusty winds from the southeast and south up the 
Colorado River valley and into southeast California today. This 
resulted in much more humidity across our entire area today. 


The strong high pressure system aloft that was responsible for the 
near excessive heat yesterday will generally be over Colorado 
tonight and Sunday. This has already resulted in a significant 
amount of moisture from the east as seen by the clouds on the 
visible satellite imagery today. This will also aid in bringing an 
abundant amount of moisture over over area through tonight and 
Sunday. 


The increasing moisture and low pressure systems in the flow from 
the east and south will bring a significant increase in the chance 
of showers and thunderstorms from east to west across the area 
tonight and Sunday with locally heavy rain. Strong gusty winds are 
also possible tonight. 


The showers and thunderstorms over the higher terrain surrounding 
the greater Phoenix area are expected to develop from the 
northeast...east and southeast into the greater Phoenix area tonight. 


The showers and thunderstorms will result in significantly cooler 
weather Sunday with high temperatures only near 100 in the southwest 
Arizona and southeast California desert areas including Yuma... 
el centro...and Imperial...and in the middle to upper 90s in the 
south-central desert areas including the greater Phoenix area. 


Have issued another Special Weather Statement...as we will have to 
be alert for urban and small stream flooding which is likely in some 
locations from tonight through Sunday into Sunday night along with a 
possibility of flash flooding. 


The chance of showers and thunderstorms is expected to linger at 
least through Monday with little temperature change...then possibly 
decreasing somewhat Tuesday with a recovery of temperatures at that 
time. 


Extended...models in generally good agreement that the ridge will 
remain anchored near The Four Corners. Persistent easterly flow will 
necessitate slight chance probability of precipitation for much of the Arizona counties through 
the week. Further aloft...an inverted trough currently located 
across southern Texas may enhance rain chances midweek. Temperatures expected 
to remain near or slightly below normal through the period. 


&& 


Aviation... 
kphx/kiwa and surrounding airfields... 
latest thinking based on current observations...trends...and model 
guidance is that thunderstorms will not develop across the phx area until 
roughly 04z this evening...though towering cumulus/cumulonimbus will affect air traffic 
routes this afternoon - mainly to the north and east. Increasing threat for 
significant thunderstorm activity somewhere over phx this evening...but will 
hold of on tempo/predominant mention in tafs of ts for now as much 
of what happens tonight will be dictated by how the afternoon 
unfolds. Regardless of whether or not a ts directly hits an 
airfield...there will be plenty of debris showers/clouds persisting 
into Sunday morning. 


By late morning Sunday...around 16-18z...significant moisture 
increase will likely produce lower cloud decks...with scattered ceilings 
near 5k feet and few/scattered decks a bit lower than that. Have mentioned 
thunderstorms in the vicinity after 18z in the kphx/kiwa tafs...but will not mention 
prob30/tempo as confidence is low in the ability to time exactly 
when storms may affect the airfields. 


Knyl/kblh/kipl and surrounding airfields... 
strong southerly winds expected through today into tomorrow. At this time 
appears the the juxtaposition of thunderstorm ingredients will be east of 
this area. May see some thunderstorms/showers Eek toward kblh/kyum late 
tonight/Sunday morning. Due to signficant moisture increase 
expected...have lowered ceilings to around 10k feet with scattered decks 
near 6k feet after 12z...and added vcsh/thunderstorms in the vicinity to the area tafs. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
significant moisture increase over the weekend will continue a daily 
threat for showers and thunderstorms...some with heavy 
rainfall...through the middle of next week. Near to slightly below 
average afternoon high temperatures...along with mild overnight 
lows...can be expected through the middle part of next week. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 
weather.Gov/Phoenix 


Discussion...Ellis/Hirsch 
aviation...cumulonimbus 
fire weather...wanek