724 
fxus65 kpsr 060028 aaa 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
525 PM MST Sat Jul 5 2008 


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


Update...updated aviation and public discussions for this evening. 


&& 


Synopsis... 
a couple of weak upper level disturbances will move across northern 
Arizona the next day or two. These systems will serve to act on the 
available moisture to give a slight chance of mainly afternoon and 
evening showers and thunderstorms...mainly over the higher terrain 
of south-central Arizona. The airmass will dry out a bit early next 
week as high pressure strengthens over the area...with afternoon 
temperatures expected to warm a few degrees...possibly approaching 
110 degrees by midweek. 


&& 


Discussion... 
this evening... 
decreasing convective trends noted North/East/southeast of the Lower 
Desert of south-central Arizona late this afternoon. Outflow 
boundary approaching Phoenix from the north has weakened and its 
forward motion has slowed...due to dissipation of storms that 
produced the outflow. Decent storms persist over far SW Yavapai 
County...so could still see a few storms over and west of Wickenburg 
in far northwest Maricopa County this evening. Weak convection over far 
southern Gila County generated a weak outflow boundary moving very 
slowly toward the southwest. Storms over and near Tucson have also 
weakened...so no outflow from the southeast is expected this 
evening. Given weak steering flow...unfavorable upper level 
streamline configuration...and diminished potential for outflow 
interaction over south-central Arizona desert...have decided to lower pop 
below 10 percent over most of south-central desert...including the 
greater Phoenix area...this evening. Will update grids/tabular/text 
forecasts shortly. Remainder of discussion is from dayshift... 


Another short-wave trough is prognosticated to move through the forecast 
area sun. Main effect of this system will be to shunt the deepest 
moisture and best chance for convection into eastern Arizona. Strong 
ridge will develop off the California coast early next week...generally 
producing a drier westerly flow across the region. Drying trend will 
be short-lived as the ridge builds eastward across the southwest 
states by Tuesday. Resulting light easterly flow around the west-east oriented 
ridge will slowly advect deeper moisture back into Arizona. 


Latest GFS and European model (ecmwf) continue to suggest that a wetter than average 
regime will set up by late in the week...continuing into next 
weekend. Western extension of the semi-permanent western Atlantic 
ridge will build even further westward into Mexico. Meanwhile...a 
trough will approach the Pacific northwest coast. A veering 
southerly flow between the two is expected to advect deep tropical 
moisture northward into southern Arizona and California. 


&& 


Aviation... 
storms over southern Yavapai County north of Phoenix metropolitan 
dissipated shortly after 4 PM...while a few cells continued over 
southwest Yavapai County. Leading edge of outflow air had reached 
far northern Maricopa County by 5 PM...but forward motion had slowed 
noticeably. Meanwhile...a few storm cells continued over southern 
Gila County...but outflow from those storms was rather weak and 
appeared likely to stay south of kiwa/kphx. Given short term storm 
trends...not much chance for storms over metropolitan Phoenix this evening. 
So...removed mention of gusty outflow winds and a cumulonimbus from kphx/kiwa 
tafs this evening. Clear skies out west...with south wind at 
kyum/kblh and southeast wind at kipl. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
with higher humidity and favorable winds aloft today...expect a 
slight increase in the chance for showers and thunderstorms in all 
zones. Anticipate strong and gusty winds...often erratic...with some 
of todays storms. 


High pressure is forecast to build over Southern California and 
Arizona by next Tuesday and Wednesday. This will lead to another 
warming trend with desert highs between 110 and 114. In addition 
expect minimum relative humidity values on the deserts to drop to less than 15 
percent both Tuesday and Wednesday. For next Thursday and Friday the 
winds aloft become favorable for another increase in the chances for 
showers and thunderstorms. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 
California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 
weather.Gov/Phoenix 


Discussion...Hirsch/dg 
aviation...dg 
fire weather...Sipple