Weather

773 
fxus65 kpsr 071700 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
1000 am MST sun Sep 7 2008 


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


Synopsis... 
moisture levels will begin to increase today across the Desert 
Southwest...and continue through Monday. This will lead to a slight 
chance for thunderstorms across the eastern Arizona mountains today 
and again Monday. Thunderstorm chances increase and spread to all 
areas Tuesday as a low pressure system moves across the region. 
Slight chances for precipitation will linger through the end of the 
week...mainly over the mountains...as the forecast becomes 
increasingly uncertain. Temperatures will generally be on a cooling 
trend throughout the week. 


&& 


Discussion... 
moisture...at least at the middle levels...was continuing to increase 
across most of the phx County Warning Area the past 24 hours...as evidenced by the 
latest visible satellite loops. Local 88d radars and lightning detection 
systems even showing an isolated thunderstorm or two embedded from time to time 
in these light showers. Have updated zones and gridded database to 
account for this present weather. 


Latest 12z model data showing very little change is needed to our 
current forecast thinking next few days. General troughiness is 
expected to develop near the West Coast...with of course differences 
in the details amongst the models. But...the general trend will be 
towards continuing an increase in moisture next two to three days as 
ageostrophic flow at the lower levels begins to tap and import 
moisture source to our S. And...atmosphere should remain unstable 
enough to support an increase in convection...especially by Tuesday 
across most of County Warning Area. As clouds and moisture increase...maximum temperatures 
should come down...and min temperatures should begin to warm up. All of 
this is handled well in current forecasts...so no further updates 
needed. 


Previous discussion... 
few surprise showers/thunderstorms popping up across our eastern County Warning Area this morning. 
This is likely due to a rather weak vorticity maximum moving across Arizona per 
water vapor imagery. It is also indicative that moisture levels are 
in fact increasing across Arizona...which has been anticipated for 
several days now. Little change in today/S forecast from previous 
shifts with slightly cooler maximum temperatures from Sat and low-end probability of precipitation over 
southern Gila County. 


Monday...slightly cooler temperatures again and an increase in probability of precipitation across 
our central and eastern mountains as moisture continues to increase. 
Showers/thunderstorms should be aided a bit by a weak shortwave lifting NE 
across our County Warning Area. One thing that may temper activity a bit is the fact 
we/ll be in the left entrance region of a southern branch jet streak. 


Tuesday...made a noticeable increase to probability of precipitation on Tuesday as models 
are indicating a more significant shortwave will move from SW to NE 
across our County Warning Area. Sref probability of precipitation peg-out Tuesday afternoon from Maricopa 
County east...which is usually a good and strong indicator of mountain 
thunderstorms. Storm motion will be to the NE at 10-20 kts so not very 
favorable to bring thunderstorms into the lower deserts. 


Wednesday a potent shortwave is prognosticated to drop out of the Pacific northwest 
and into the Great Basin. Our County Warning Area should be in the wake of the 
first...weaker...shortwave so probability of precipitation will be a bit lower Wednesday. 
Temperatures continue to cool. 


Thursday through Saturday forecast confidence drop at this point as 
models begin to diverge on the 500 mb pattern and offer little 
run-to-run consistency. The latest European model (ecmwf) and GFS ens mean are 
pointing toward continual troughing in our area as the potent 
shortwave grazes by to the north and a cut-off low remains off the 
Southern California coast. Given the uncertainty showed a gradual downward trend 
in probability of precipitation through the remainder of the forecast with minor upward 
fluctuations in temperatures. 


&& 


Aviation... 
scattered middle-level clouds are developing over central Arizona as 
a modest moisture increase is taking place. Expect these clouds to 
linger throughout the day...while mostly clear skies will be found 
in southeast California. There will be a slight chance of some 
afternoon thunderstorms...mainly over higher terrain areas to the 
east and southeast of the greater Phoenix area...but these are not 
expected to affect any airfields. Winds will be light with typical 
diurnal patterns through the next 24 hours. 


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...estle/iniguez 
aviation...Rogers 
fire weather...Rogers