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fxus65 kpsr 042245 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
345 PM MST Friday Jul 4 2008 


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


Synopsis... 
high pressure aloft will move towards The Four Corners region today 
and then begin to drift slowly southward into central Arizona late 
in the weekend. This means that the threat of afternoon and evening 
showers and thunderstorms will continue today and Saturday...mainly 
over south-central Arizona...before diminishing somewhat on Sunday 
and Monday. A return to hotter weather is likely by the middle of 
next week. 


&& 


Discussion... 


..strong/severe storms possible this evening across the greater 
Phoenix area... 


Another active afternoon/evening is expected across the forecast 
area. Strong/severe cells are currently filling in across the rim 
and White Mountains. Atmosphere remains quite unstable across the 
Phoenix area with dewpoints hovering in the upper 50s to lower 60s. 
GFS appears to have the best handle on the low-level moisture and 
generates SBCAPE around 1500 j/kg late this afternoon...more than 
sufficient to support scattered convection across the valley. 
Dynamical lift will come in the form of the outflow boundaries 
generated from the upstream cells (both along the rim and southeast of 
phoenix)...which will be enhanced by the 15-20 knots easterly steering 
flow around The Four Corners anticyclone. Late evening/overnight 
activity also will be a possibility even across the western Arizona 
counties as upper level divergent flow develops ahead of an 
approaching trough across Southern California. 


Sat...yet another active day expected. Ridge will be suppressed 
southward as a short-wave trough moves through the northern 
rockies/plains. Timing of the passage of the trailing upper-level 
trough suggests that convection may fire on the early side. 
However...weak steering flow under the ridge axis may inhibit the 
propagation of storms off the higher terrain and into the valley. 
Meanwhile...high temperatures around 105-106 degrees can be expected 
in Phoenix...slightly below average for this time of year. 


Previous discussion... 
upper level high now near The Four Corners will drift southward late in 
the weekend...then reform off to our northwest. This should help to allow 
moisture to become more diffuse with time...and air mass should begin to 
warm back up next week. Have therefore kept probability of precipitation confined to higher 
terrain areas of zone 24 next week...and warmed maximum temperatures back over 
110 degree hottest deserts. May eventually Need More excessive heat 
products by middle of next week...stay tuned. 


&& 


Aviation... 
skies have cleared this morning across the area...but expecting 
another active day in the higher terrain surrounding the Phoenix 
area. Expect convection to develop by early afternoon to the 
northeast and southeast of kphx. Later in the evening...there is a 
chance of showers and thunderstorms across the greater Phoenix area. 
Did not include mention of thunderstorms as uncertainty is still 
high...but will amend later if trends indicate otherwise. Confidence 
is high enough that outflow from mountain thunderstorms will move 
across the Phoenix area later tonight...with gusty winds mentioned. 


In western Arizona and southeast California...south-southeast winds 
near 15 kts extend up to 5 thousand feet. A few gusts to 25 kts will 
be possible...especially along the lower Colorado River valley. 
Thunderstorms are not expected to affect kyum/kblh/kipl during the 
period. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
by Monday...look for a Pacific low pressure system to move across 
the western states...bringing slightly drier but hotter air to our 
area. However...enough moisture will remain for a slight chance of 
showers and thunderstorms in the higher elevations east of Phoenix. 
By Tuesday...the Phoenix area could see afternoon highs up near 
114...and in the Yuma area look for highs near 117 degrees. Along 
the lower Colorado River valley and Joshua Tree nm...minimum relative humidity 
values on Monday and Tuesday could dip to less than 15 percent. 


&& 


Psr watches/warnings/advisories... 


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


&& 


$$ 


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


Discussion...Hirsch/estle 
aviation...Rogers 
fire weather...Sipple