Weather

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fxus65 kpsr 281740 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
1040 am MST Thursday Aug 28 2008 


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


Update...updated aviation discussion. 


&& 


Synopsis... 
high pressure off the California coast will build across the Desert 
Southwest...resulting in a warming trend into the weekend. The 
threat for monsoon storms will persist...with the best chances south 
and east of Phoenix. The threat for thunderstorms will increase this 
weekend as a strong low pressure system moves through the region. A 
gradual warming and drying trend is expected next week. 


&& 


Discussion... 
it looks like our eastern County Warning Area may be setting up for an active afternoon 
and evening...convectively speaking. Upper level high has centered 
itself northwest of our County Warning Area...along the central California coast...and plot 
and radiosonde observation data show a strong northeasterly flow at middle/upper levels across the 
County Warning Area today. 500mb winds were running 25-30kts from the phx/tus 
soundings. Storms that form along the central/eastern rim should become 
more organized and long-lived as they propagated down into the lower 
central deserts this evening. Moisture and instability are plentiful 
over south-central Arizona...with precipitable water values up to 1.9 at Phoenix...and 
850mb dewpoints running around 15-16c. Surface dewpoints over the 
central deserts at 8 am ranged from middle 60s to low 70s. 
Finally...significant cape was seen on the morning tus/phx soundings 
and it should not take much lift to get strong storms to form later 
today. Main lift likely will be in the form of intersecting and 
colliding boundaries...as dynamics are somewhat weak. We do see some 
forecast diffluence aloft in the streamline fields this afternoon 
and evening...but otherwise no significant inverted troughs/vorticity maxes 
appear to be impacting the area. Current forecasts for today look OK 
and only small changes have been made...basically calling for slight 
chance for afternoon storms from the lower Colorado River eastward...and 
over parts of Riverside County. The better threat for storms will 
come late this afternoon and evening as peak heating really gets 
storms going over the higher terrain...then they will roll southwestward off 
the rim and into the lower deserts...feeding off the plentiful 
moisture. 


The only potential pitfall with this scenario is the potential for 
drying over northern Arizona to limit the convection over the rim. Should few 
or no storms form upstream...we will have problems getting storms to 
form spontaneously over the lower central deserts...and our probability of precipitation 
will end up overdone. Rest of the discussion is from the previous 
middle shift. 


Previous discussion... 
today-Fri...ridge will continue to build eastward across the Sierra 
Nevada extending into northern Arizona this afternoon. Deepest moisture 
and strongest dynamics will be suppressed south of the forecast 
area. However...sufficient moisture exists over the Tonto National 
Forest for isolated to scattered convection this afternoon. With an 
anomalously strong 500 mb flow (around 30 kt) between the ridge to 
the north and the persistent trough across northern 
Mexico...thunderstorm activity will likely propagate westward into 
the valley and desert floors late this afternoon and this evening. 
Pinpointing exact locations for thunderstorms will be quite 
challenging though...given that thick debris clouds this morning may 
delay or inhibit surface heating. More of the same expected for Friday 
as the ridge continues to slide eastward towards The Four Corners. 


Sat-sun...models in excellent agreement depicting a high amplitude 
trough moving through the Great Basin. This system will advect deep 
tropical moisture northward across the region beginning Sat. Best 
chance for precipitation appears to be sun...though the added 
moisture may be enough to produce a significant weather event Sat as 
well...even before the system really takes shape. Increase in 
moisture/cloud cover will result in much cooler temperatures and 
will be making significant changes to the existing values in the 
database. 


Extended...flow now looks to remain more westerly behind the 
departing trough...based on the latest GFS/ECMWF. This will confine 
highest precipitation chances to the higher terrain east of Phoenix. 
Gradual warming trend is expected...though temperatures should remain below 
normal through the period. 


&& 


Aviation... 
kphx/kiwa and surrounding airfields...the atmosphere remains plenty 
moist and unstable for a continued threat for mainly afternoon and 
evening showers and thunderstorms over the south-central Arizona deserts. 
Anomalously strong northeasterly steering flow indicates that level 
of activity tonight on the desert floor is dependent on what happens 
upstream...or to the northeast of the greater Phoenix area. The best 
time period for thunderstorms to affect the greater Phoenix area 
will be after 00z. Any thunderstorms that develop will have the 
potential for heavy rain...gusty winds...lower visibilities and 
ceilings. We will continue to monitor radar and satellite trends closely 
for the potential for thunderstorms tonight...and will amend/update 
tafs if necessary. 


In southwest Arizona/southeast California...there is a slight chance 
of showers and thunderstorms this afternoon and evening...mainly 
from the lower Colorado River eastward. Otherwise...expect 
south-southeast winds with scattered middle-level clouds...and a few 
cumulonimbi in southwest Arizona. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Friday through Tuesday... 
a drying and warming trend will continue through Friday. A West 
Coast weather system is expected to move through the region this 
weekend...increasing humidity and the chance for showers and 
thunderstorms. Cooler temperatures can also be expected over the 
weekend. A warming and drying trend will commence Monday/Tuesday of next 
week. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...cb/Hirsch 
aviation...Rogers/wanek 
fire weather...wanek 

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