897 
fxus65 kpsr 301132 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
430 am MST Sat Aug 30 2008 


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


&& 


Synopsis... 
moist unstable air will remain over the region through the Holiday 
weekend...continuing the threat for showers and thunderstorms. The 
best chance for a valley wide rain event will be Sunday...as a 
vigorous weather system moves into the western United States. A 
drier airmass from the northwest will move into the area starting 
late Monday into Tuesday...significantly decreasing the threat for 
showers and thunderstorms. By the middle of the week...temperatures 
will return to near average for early September. 


&& 


Discussion... 
the anomalously strong middle-tropospheric east-northeast winds of 
30-40+kt...and decent cape seen in the 03 UTC kpsr sounding came 
together nicely to support the Severe Thunderstorm Watch Storm Prediction Center issued 
for portions of our zones through 2 am earlier this morning. The 
fairly organized complex of thunderstorms moved through central 
Arizona late last night before marching westward through La Paz 
County and crossing into eastern Riverside and southern San 
Bernardino counties between 2 and 4 am MST. Unlike 24 hours 
ago...this complex of thunderstorms moved more through north-central 
Maricopa County before exiting to the west. Reports of damaging 
winds in excess of 58 miles per hour...along with periods of brief heavy rain 
and localized street flooding were reported across portions of Cave 
Creek...Deer Valley...and Peoria...with reduction in visibility to 
less than 1/8 mile reported in Buckeye. Latest Sat/radar trends 
support latest forecast thinking to continue the isolated/scattered 
showers and thunderstorms mainly west of the Colorado River valley through 
the early morning hours. 


Later today...the West Coast trough axis extending from the Pacific northwest 
to central California will continue moving eastward...while the low to our 
south shifts northward a bit. The two systems will come together for 
a more southerly flow...particularly over the western County Warning Area...later 
today into the evening hours...while finally giving up the strong 
east-northeasterly winds that have supported thunderstorm activity 
over central Arizona the past few evenings. That said...the focus 
for showers and thunderstorms today will initially be over our 
western County Warning Area...before shifting back to the east after 00 UTC this 
evening. Still looks like a couple of easterly wiggles/disturbances 
will rotate around the large scale low today/tonight...reinforcing 
the deep layer moisture advection from the south while providing a 
continued threat for showers/thunderstorms. However...the best 
chance for convection looks to be Sunday...when the southern low 
sorta weakens and gets picked up by the more dynamical West Coast 
trough/low moving through the western Continental U.S.. the combination of the 
warm/moist air from the south and the cooler/drier air from to the 
north/northwest will make for a rather interesting situation Sunday 
into Sunday evening. Still looks like this will be our best chance 
for a state-wide rain event...with the main weather threats ranging 
from gusty winds in excess of 60 miles per hour...to 3/4 - one inch diameter 
hail...to localized (possibly even more widespread out west) rain 
showers. Since we'll have a ridiculous amount of moisture to work 
with...and the flow becomes much weaker...the threat for flash 
flooding will increase. Neighboring offices to our east (sgx) and 
our north (vef) have flash flood watches out...but for now...we'll 
hold off on issuing such. 


By Labor Day Monday...a much drier airmass from the northeast will 
spread into our western zones...then move across central 
Arizona...shunting our monsoon moisture to the south and east. The 
daily threat for showers and thunderstorms will thus be confined to 
the higher terrain well east of the greater Phoenix area... across 
zone 24/southern Gila County. It appears we'll stay in a 
drier/westerly pattern through the extended...allowing temperatures to warm 
back to near average by middle/latter part of the week. 


&& 


Aviation... 
will keep mention for thunderstorms in kiwa/kphx tafs through at least 
07z...with potential for strong gusty winds and reduced visibility 
due to heavy rain...as band of active storms extends east into 
northwest Gila County. Isolated storms and gusty winds...with some 
blowing dust or sand...will be possible over western taf sites later 
this evening as well. 


More significant weather...aviation wise...expected this weekend as 
a Pacific low pressure system is expected to approach Arizona and lead to 
significant thunderstorm activity over our lower deserts each day. 
Stay tuned. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
following the passage of a strong Pacific low pressure trough this 
weekend...a drier westerly to southwesterly flow aloft will move in 
and affect the area Monday through the middle of the week. 
Temperatures on Monday will be well below normal...then gradually 
rise to near seasonal values by the middle of next week. Breezy 
south to southwest winds are expected Monday...with decreasing wind 
the rest of the week. Otherwise no significant fire weather issues 
are expected during the period. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...wanek 
aviation...dg/cb 
fire weather...cumulonimbus