Weather

667 
fxus65 kpsr 070310 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
810 PM MST Sat Sep 6 2008 


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


Synopsis... 
a dry northwest flow aloft will keep the Desert Southwest generally 
clear and hot through Sunday. Early next week a low pressure system 
will encroach on the Desert Southwest resulting in increasing 
clouds...cooler temperatures...and increased chances for mountain 
thunderstorms mainly across Arizona. The low pressure system will 
remain a constant weather feature through the remainder of the week. 


&& 


Discussion... 
mostly clear skies covered the forecast area (southeast California to south 
central az) at 03z...but a few spotty middle clouds remained. Scattered 
clouds in southern Arizona Saturday afternoon were the leading edge of 
much deeper monsoon moisture in northwest Mexico. There were even 
one or two high cloud based thunderstorms noted Saturday afternoon 
just southeast of Tucson...and over the White Mountains. 


As mentioned above...moisture is much deeper in Mexico. Based on the 
Phoenix and Tucson balloon soundings...in order to develop 
meaningful c.A.P.E. For thunderstorms...700 mb dewpoints would need 
to increase to 2 degrees c ... 800 mb dewpoints around 4 c. Those 
values were not to far away in Mexico. 


As the previous discussion below mentions...a large Pacific trough is 
forecast to move into the entire western U.S. By Tuesday morning. 
This will enable backing low level south winds from Mexico...and a 
potential resurgence of monsoon moisture north into Arizona late 
Monday...but particularly Tuesday. Increasing jet stream winds and 
cooler middle level temperatures portend a very unstable atmosphere 
Tuesday for increasing thunderstorms...possibly severe in central 
and northern Arizona. 


Otherwise...dry weather will continue over the forecast area through 
Monday. Current forecasts reflect this...and also reflect an 
introduction of thunderstorms back into the forecast for south 
central Arizona Tuesday. No forecast updates needed. Previous 
discussion below still applies. 


Additionally...the excessive heat warning issued earlier today for 
southeast California expired at 03z. Forecast afternoon temperatures on 
Sunday are below warning criteria. 




Previous discussion... 
monsoon is already showing signs of returning...as cumulus (albeit 
low-topped) has developed along the Mogollon Rim and the higher 
terrain of southern Arizona. Environment generally remains too dry for 
thunderstorms this afternoon. This is reflected in local k-indices 
which are only peaking around 20 across much of the forecast area. 
However...latest NAM-WRF suggests deeper moisture currently over 
Chihuahua will lift northwestward into Southeast Arizona sun. This should help 
to prime the lower levels by Monday across south-central Arizona with 
dewpoints in the Lower-Middle 50s across the lower deserts. 


Models in generally good agreement that an area of low pressure will 
slide slowly southward down the California coast this week. This will set up 
a southwesterly flow...favoring the higher terrain (even across the 
southwest deserts) of the forecast area for shower and thunderstorm 
activity. Western extension of the semi-permanent Atlantic ridge is 
expected to drift westward as tropical cyclone Ike moves westward 
towards the Florida Straits and possibly into the Gulf of Mexico. Moist 
subtropical feed will continue across the Desert Southwest through 
midweek as the southwesterly flow strengthens between the eastern 
Pacific low (which may ultimately cut off) and a building ridge 
across Mexico. 


Best chance for precipitation in the valley appears to be late in 
the week as the low-level flow acquires more of an easterly 
component around the ridge...centered generally across The Four 
Corners or east of the area. Discernible threat for thunderstorms 
will also exist across the lower Colorado River valley (per the latest 
ecmwf) as the California low eventually slides eastward while dissipating 
next weekend. 


Made some minor adjustments to the digital database...mainly to 
increase probability of precipitation through the week. Temperature-wise...increase in 
moisture combined with decreasing thicknesses will result in cooler 
temperatures...though still remaining above average through at least Thursday or 
Friday. 


&& 


Aviation... 
southeast California to south central Arizona...including kphx...kipl...kiwa... 
knyl...and kblh airfields. Mostly clear skies and light winds are 
expected through at least 04z Monday. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Monday through Thursday...above normal temperatures can be expected 
Monday...before a gradual cooling trend will bring temperatures to 
near seasonal normals by Wednesday. The atmosphere will remain 
relatively dry...with afternoon relative humidity values in the teens Monday and 
Tuesday...rising a bit by Wednesday into the upper teens and lower 
twenties across the lower deserts. Winds will generally be light 
with only a few afternoon gusts. A weak low pressure system will 
develop off the coast of Southern California...returning a slight 
chance of thunderstorms to southern Gila County each day. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


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