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fxus65 kpsr 220505 
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Area forecast discussion 
National Weather Service Phoenix Arizona 
955 PM MST sun Mar 21 2010 


Synopsis... 
high pressure aloft will result in dry and warmer weather Monday with 
high cloudiness ahead of 2 Pacific low pressure systems. A Pacific 
low pressure system from the west in combination with another low 
pressure system from the northwest will result in cooler weather 
Tuesday. It will be much cooler in south-central Arizona including 
the greater Phoenix area with a slight chance of showers Tuesday into 
Tuesday night. Dry weather is expected Wednesday through the rest of 
the week with warmer weather again by Thursday through the weekend. 


&& 


Discussion... 
a considerable amount of high cloudiness from the west was moving 
across the entire area this evening. 


A ridge or axis of high pressure aloft was moving east and weakening 
over eastern Arizona this evening. The effects of this system are 
still expected to be felt in terms of warmer weather from southeast 
California across southwest and south-central Arizona Monday. 


A Pacific low pressure system was located west of northern Baja California 
California near 131 degrees west longitude late this evening. This 
system is moving east and is expected to continue spreading 
increasing high cloudiness out ahead of itself over our area into 
Monday. This system is then expected to move east across northern 
Baja California California Tuesday. At the same time...another low pressure 
system from the northwest...currently over British Columbia...is 
expected to move southeast to northwest Colorado by late Tuesday. 
These two low pressure systems are expected to result in cooler 
weather in our area Tuesday. It is expected to be much cooler in 
south-central Arizona including the greater Phoenix area with a 
slight chance of showers Tuesday into Tuesday night. 


Previous discussions... 
single digit dewpoints are resulting in rhs generally below 15 
percent across much of Arizona this afternoon. Further aloft...conditions 
are quite moist with high clouds moving eastward through a weakening 
ridge. 


Height falls will continue for Monday as two low pressure systems 
approach the Desert Southwest...one from the Great Basin and the 
other in the southern stream off the northern Baja California coast near 125w. 
Despite the steady height falls...warming below 850 mb will still 
support above average temperatures with maxes in the 80s in the 
lower deserts. 


Models have trended stronger with the low moving through northern 
Mexico...though moisture flux anomalies remain unimpressive. Phasing 
of the two systems now looks to take place well east of the forecast 
area...which will further limit the potential for precipitation Tuesday. 
This is most pronounced on the 12z European model (ecmwf) but also evident in the GFS 
and NAM-WRF. Probability of precipitation were not changed significantly and range from 
roughly 15 percent near Phoenix up to 30 percent across the higher 
terrain of southern Gila County Tuesday evening. 


System is fairly progressive...and by early Wednesday should be 
shifting east into New Mexico with downward vertical velocity overspreading our entire County Warning Area. 
As such...left just a lingering slight chance of showers over zone 
24 Wednesday...and dropped probability of precipitation to low single digits or zero 
elsewhere. Temperatures should rebound slightly Wednesday but should remain 
near or slightly below seasonal normals...especially from Phoenix 
eastward. 


High pressure is forecast to return Thursday into the weekend...for 
dry weather and a warming trend. Highs Friday and Saturday should 
exceed 80 over most desert locales...under sunny or mostly sunny 
skies. 


&& 


Aviation... 
expect cirrus to thicken overnight but otherwise remain above 18-20 
kft above sea level. Winds will generally remain light through 00z Tuesday favoring 
diurnal patterns. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Tuesday through Saturday... 
a Pacific low pressure system will track across the region Tuesday 
into Wednesday providing the forecast area with cooler maximum 
temperatures...breezy conditions...and a slight chance of 
precipitation for south-central Arizona. High pressure returns 
Thursday...along with warmer and drier conditions through Saturday. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...Ellis/Hirsch/cb 
aviation...aj 
fire weather...paddock