344 
fxus65 kpsr 080354 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
855 PM MST sun Sep 7 2008 


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


Synopsis... 
a gradual moistening of the atmosphere is expected over the next 
several days...as low pressure aloft develops off to our west. This 
will result in an increasing threat of showers and thunderstorms 
across the region...especially over south-central Arizona. The peak 
days of thunderstorm activity are likely to be Wednesday into 
Thursday. By next weekend...the upper low should push off to our 
east and bring a drying trend to the region. Expect temperatures to 
be near or slightly below normal during much of the week. 


&& 


Discussion... 
isolated to scattered showers and thunderstorms...mostly high cloud 
based storms...developed across portions of Arizona on Sunday... 
with more widespread storms over Southeast Arizona and the Mogollon Rim. 
At 03z isolated showers and thunderstorms were noted just east of 
the Colorado River near Blythe and Yuma. 


A surprising amount of Mexican moisture flowed into Arizona Saturday 
night. In fact 00z Monday Arizona soundings showed precipitation water had 
doubled in Phoenix to nearly an inch...from 24 hours ago...with a 
50 percent increase at Flagstaff and 25 percent increase at Tucson. 
And...in conjunction with a weak Pacific 300/250 mb trough moving 
across Arizona...afternoon instability storms developed Sunday. 


As a series of weak Pacific troughs continue move into the western 
states...including Arizona the next few days...additional Mexican 
moisture is forecast to flow north. Weak upper level troughs or 
disturbances with the additional moisture will keep the threat of 
showers and thunderstorms alive the next several afternoons. 


We feel the models are having a hard time resolving a significant 
shortwave feature over northern California this evening (best seen 
in water vapor imagery). This feature may have some influence on a 
larger thunderstorm outbreak in parts of Arizona Tuesday or Wednesday...its still 
hard to tell. For now the current forecast trend of increasing 
threats for showers and thunderstorms the next few days looks OK. 
Perhaps tomorrow (mon) we will have some resolution on which day we 
may see a larger thunderstorm outbreak this week. 


Forecasts look OK for now. Minor updates planned. 




Excerpts from previous discussion... 
models in good agreement that the air mass will continue to undergo a 
gradual moistening as upper level troughiness takes shape along the 
West Coast the next few days and increases low level southerly winds. As 
surface to 700 mb mixing ratios increase to near 10 g/kg next few 
days...resulting increase in cape will yield more and better 
rain-producing thunderstorms by midweek. Presence of developing upper level 
trough to our west...and the proximity of an increasing upper level 
jet...will also add to the dynamical mix for storm production. 
Therefore...have increased probability of precipitation by Wednesday into the chance 
category over most of the south-central Arizona deserts...with mostly 
slight chances until then. With the added moisture and gradual 
lowering of thicknesses...also calling for somewhat cooler 
afternoons. 


After midweek...lots depend on how quickly the West Coast upper low 
moves eastward. The Euro has more energy in the northern branch system and 
keeps the southern branch trough very weak...while the GFS closed low 
opens up as pushes it eastward through Arizona by Saturday. Euro would argue for 
less weather Friday and Saturday...with the GFS calling for continued 
threat of storms. Will go middle of the Road for now and keep slight 
chances going Thursday and Friday over scntrl deserts eastward...then just zone 
24 on Saturday. Temperatures should begin slowly warming back up next 
weekend as heights begin to rise again and air mass dries out. 


&& 


Aviation... 
southeast California and southwest Arizona...including kblh...knyl...and kipl 
airfields. Isolated showers and thunderstorms will continue near 
the above airfields through 06z Monday. Gusty and variable winds 15 to 
25 knots are possible...along with scattered to broken clouds at or above 15 thousand 
mean sea level. After 06z Monday...expect scattered clouds based near 15 thousand mean sea level and 
light winds through 18z. After 18z Monday...and through 04z Tuesday... 
isolated afternoon and early evening showers and thunderstorms... 
along with gusty and variable surface winds between 15 and 25 knots 
are forecast. 




South central Arizona...including kphx and kiwa airfields. Scattered to broken 
clouds at or above 14 thousand mean sea level ... isolated sprinkles or light showers 
possible through 08z Monday. After 08z...scattered to broken clouds at or above 14 thousand 
mean sea level with light east wind under 10 knots. Between 20z Monday and 04z 
Tuesday...scattered clouds based near 10 thousand mean sea level with light wind. Also... 
isolated mountain thunderstorms with tops to 35 thousand are possible 60 
to 80 miles north through southeast of kphx. 




Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Tuesday through Friday...an area of low pressure will develop off 
the coast of Southern California...bringing a slight chance of 
showers and thunderstorms Tuesday for the entire forecast 
area...with a better chance in the higher terrain of central 
Arizona. Slight chances of thunderstorms will continue Wednesday and 
Thursday in the higher terrain areas of southern Gila County. 
Temperatures will remain near seasonal normals...while afternoon 
humidity values will range from the middle-teens in southeast 
California to the middle-twenties in south-central Arizona. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...vasquez/estle 
aviation...vasquez 
fire weather...Rogers