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Area forecast discussion...updated 
National Weather Service Phoenix Arizona 
540 am MST Monday Nov 23 2009 


Update...updated aviation and fire weather discussions 


&& 


Synopsis... 
mostly clear nights and sunny days are expected to continue through 
at least Friday. A ridge of high pressure will slowly build over the 
western states this week leading to a gradual warming trend. 
Temperatures will rise to well above average for late November by 
Thanksgiving day...in the upper 70s to near 80 degrees on the low 
deserts including Phoenix...Yuma...and el centro. A large cooling 
trend is anticipated Saturday and Sunday as a low pressure system 
develops over the Desert Southwest. 


&& 


Discussion... 
water vapor imagery shows a low pressure system moving through the 
western states...the southern end of which is brushing Arizona and 
southeast California. This system will have little effect on our 
weather except for some more noticeable winds...with locally breezy 
conditions this afternoon over the lower Colorado River valley. A 
ridge of high pressure will slowly build in from the Pacific leading 
to a gradual warming trend. Anticipate temperatures to peak on 
Thursday. A strong north and northeasterly oriented pressure 
gradient will develop leading to locally breezy conditions at times 
Wednesday and Thursday. 


The models depict the ridge beginning to shift eastward Friday as a 
trough moves in. The GFS and the Canadian are faster with it than 
the European model (ecmwf). With time...the European model (ecmwf) and GFS show the flow pattern 
amplifying leading to a trough digging southward over the interior 
west Saturday...with the GFS faster than the European model (ecmwf). The operational 
Canadian shows a more progressive pattern however the Canadian 
ensemble mean supports the idea of amplification and a digging 
trough. On the other hand...the operational GFS is more aggressive 
with the troughing over Arizona and southeast California than most 
of the GFS ensemble members. Though European model (ecmwf) has a slower arrival of a 
digging trough over our forecast area it makes up for it by rapidly 
intensifying the upper trough into a closed low over Arizona on 
Sunday. The European model (ecmwf) has been showing showing a deep closed low over 
Arizona for this weekend for at least 6 runs now but it has had run 
to run inconsistencies in the evolution/track. So the GFS is now 
coming around to a solution more like the European model (ecmwf). 


The short of all of this is much cooler and breezy to windy 
conditions for this coming weekend. Conservatively...the maximum temperatures 
over the low deserts Sunday will be in the middle 60s...after peaking 
near 80 Thursday and Friday. Some additional cooling is anticipated 
Monday over the south-central deserts while our western areas 
actually start to warm up. Winds look like they will be considerably 
stronger Sunday than Saturday...mainly over southeast California and 
the lower Colorado River valley. Given the overland trajectory...the 
system looks to be moisture starved and thus have held off 
introducing any mention of precipitation...best chances look to be outside 
of our forecast area over the White Mountains. 


&& 


Aviation... 
southeast California...southwest and south central Arizona...including kipl... 
kblh...kphx...and kiwa. 


Except for occasional thin hi cirrus...at or above 20k feet...skies to be 
mostly clear next 24 hours all airfields. For the most part winds 
will be rather light and diurnal...except down the lower Colorado River 
valley today. Surface high developing to our north will tighten 
gradient down the river...and cause breezy north winds to develop at 
kblh after about 18z. Expect occasional gusts between 20-25kt starting 
early this afternoon and then tapering off shortly after sunset. 
Otherwise no aviation concerns. VFR conditions expected all 
airfields next 24 hours. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Wednesday through Sunday... 
high pressure aloft will dominate the weather pattern Wednesday and 
Thursday...with mostly clear or clear skies expected each day...and 
relatively light winds. High temperatures will continue well above 
seasonal normals...along with very dry air over the deserts. Many 
locales will see minimum relative humidity values in the single digits...with 
modest recovery at night. Forecast models in better agreement 
regarding a change in the weather developing over the weekend. A low 
pressure system will be settling in over the Desert Southwest... 
leading to a substantial cooling trend Friday through Sunday. High 
temperatures may fall around 15 degrees over this time frame with 
highs Sunday expected in the middle 60s over most of the lower deserts. 
This will be rather dry system for our area with no precipitation in 
the forecast at this time. However humidities will be on the 
increase...partly due to increasing low level moisture and also 
cooling temperatures. Breezy or locally windy weather is likely over 
the weekend as well. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...aj 
aviation...cumulonimbus 
fire weather...cumulonimbus