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Area forecast discussion 
National Weather Service Phoenix Arizona 
610 am MST Sat Nov 21 2009 


Update... 
updated aviation and fire weather sections. 


&& 


Synopsis... 
dry weather is expected to continue at least through Thanksgiving. 
The weak southern portions of Pacific low pressure systems will bring 
temperatures down somewhat with a relatively small amount of high 
clouds this weekend...but for the most part...mostly clear nights and 
sunny days are expected to continue through Thanksgiving. High 
pressure aloft between Hawaii and Baja California California is expected to 
build northeast over our area with temperatures rising to well above 
average for late November by Thanksgiving day. 


&& 


Discussion... 
progressive upper level low pressure system now moving east through 
the Pacific northwest will be passing by to our north today. Latest 00z 500mb 
plot data showed significant height falls...130-170m...occurring 
over northern Nevada and the northwestern states...but only 10m falls 
were seen at flg. Our airmass is very dry at this time...with precipitable water values 
below 0.10 inch. Surface dewpoints are also low...at 2 am they were 
running in the teens to upper 20s over the lower deserts. So...as 
this system passes by all it will bring is a bit of cooling and some 
high cloudiness...and possibly some breezes. Infrared imagery at 2m showed 
the high cloud band ahead of the trough/front moving into Southern California and 
western Arizona. Thickness falls over our County Warning Area today will be 
meager...around 30m...and maximum temperatures will fall a few degrees but 
should still remain at or slightly above seasonal normals. Phoenix 
normal high for today is 73...and the forecast high is 76 degrees. 


Dry northwest flow continues over the area Sunday into early next 
week...and another fast moving system passes by well to our north 
Sunday. That system will simply keep temperatures from climbing much 
if at all. Otherwise...except for some high clouds at times...mostly 
sunny or sunny days and clear nights will be on tap each day. 


Both the GFS and European are similar Tuesday through Thanksgiving 
day...keeping relatively strong high pressure aloft over the 
Desert Southwest...along with a split in the flow and a weak baggy 
trough well to the south of Arizona. This suggests a gradual warming 
trend during the period...with highs over the warmer deserts 
reaching to around 80 over the warmest deserts by Thursday...under 
mostly sunny skies. Weak southern branch of the flow should not play a 
role in our weather forecast. On Friday...forecasts start to diverge a 
bit...as the GFS takes the next shortwave east through the northern tier of 
states...while the European model (ecmwf) starts to dig the shortwave to the 
southeast...and towards Arizona. Tend to favor European model (ecmwf) a bit due to 
its track record over the GFS in past months...but at this time even its 
solution still appears dry with any quantitative precipitation forecast mostly staying north of 
Arizona on Friday. Mex MOS numbers remain single digits at day 7. 
Thus will keep dry forecast in place for next Friday. 


&& 


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


Light winds will continue to prevail through Sunday morning. Over 
the metropolitan Phoenix area...after the morning drainage winds 
diminish...afternoon winds will be light and variable...occasionally 
west. Northerly directions will be favored this afternoon over 
southeast California and the lower Colorado River valley. 
Otherwise...skies will be clear except for cirrus...decreasing from 
west to east during the day. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Monday through Friday... 
high pressure will build over the western states early next week... 
bringing warmer temperatures...low daytime humidity values and 
modest overnight recovery through Thanksgiving day. Winds will favor 
north and northeast...locally windy at times at the ridgetops. For 
the end of the week...there are indications that another trough will 
move through the Great Basin for a shift to southwesterly winds. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


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