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fxus65 kpsr 211030 
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Area forecast discussion 
National Weather Service Phoenix Arizona 
330 am MST sun Mar 21 2010 


Update...updated aviation and fire weather discussions... 


&& 


Synopsis... 
high pressure aloft will result in dry and warmer weather today 
through Monday. A Pacific low pressure system from the west is 
expected to merge with another low pressure system from the 
northwest over our area with cooler weather Tuesday. It is expected 
to be much cooler in south-central Arizona including the greater 
Phoenix area Tuesday with a slight chance of showers at least into 
Tuesday night. There is a slight chance of lingering showers over 
the higher terrain east of the greater Phoenix area into Wednesday. 
Otherwise...dry weather is expected Wednesday through the rest of 
the week with warmer weather returning Thursday through the weekend. 


&& 


Discussion... 
upper level high pressure ridge moving into the desert SW...with the 
axis through western Nevada/scnrl California...giving a dry subsident northwesterly flow aloft 
to our area. 00z plots showed 60-90m 500 mb height rises...and some of 
this warm air aloft will mix to the surface today for a warming 
trend with maximum temperatures to climb slightly above seasonal normals. Infrared 
imagery at 2 am showed quite a bit of mainly high clouds approaching 
the California coast...and some of this will overspread the area today and 
tonight...leading to partly cloudy skies. 


On Monday...ridge will begin to flatten as an area of low pressure 
over the Pacific northwest moves southeast and into the Great Basin. At 
the same time...an upper low embedded in the southern branch of the 
westerlies approaches the far Southern California/northern Baja California coast. This low will 
continue to spread middle and high clouds into the state for partly 
sunny skies. Despite the upper ridge flattening...low level 
thicknesses continue to rise so Monday will continue to warm with 
maximum temperatures climbing into the 80s over most of our lower deserts. 


Latest GFS and European model (ecmwf) have been coming more and more into line 
regarding the weather pattern for the early part of the 
week...Tuesday into Wednesday. By Tuesday afternoon both models 
indicated that the short wave dropping down from the northwest will 
phase up with the upper low moving into the northern Baja California coast...and 
form a consolidated upper trough over Arizona...with the axis running 
from northestern Utah southward through south-central Arizona. As this 
occurs clouds will increase...along with the threat for showers 
over our eastern zones. Both models are now similar with the amount 
of moisture prognosticated...and neither forecast significant quantitative precipitation forecast with the 
system. Quantitative precipitation forecast totals for the event should be less than one quarter 
inch for the lower deserts...possibly approaching one half inch over 
the higher terrain east of Phoenix. Based on good model 
consistency...raised probability of precipitation over south-central Arizona into the slight 
chance category Tuesday afternoon and Tuesday night...with low end 
chance values for zone 24. 


Pretty much all guidance indicates that we will see more cooling 
with this system for our County Warning Area...especially south-central Arizona...on 
Tuesday. GFS forecast sounding for Phoenix showed 12 degrees of 
cooling from Monday...mex MOS numbers indicated a high of 72...and 
most other gridded guidance forecast Phoenix highs from upper 60s to 
middle 70s. Thus decided to lower the high for Phoenix from 78 down to 
73. This also blends well with our neighbors to the south. 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 for 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... 
kphx...kiwa...kipl...kblh...and surrounding airfields... 
high pressure remains in control the next 24 hours. Light diurnal 
winds and increasing high cirriform cloudiness will continue through 
12z Monday. 


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...cumulonimbus 
aviation...paddock 
fire weather...paddock