728 
fxus65 kpsr 041729 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
1030 am MST Friday Jul 4 2008 


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


Update...updated aviation 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... 
trend of increasing moisture/decreasing temperatures continues. 
Latest kpsr 12z sounding came in with a precipitable water of 1.6 inches. Modified 
sounding supports scattered convection this afternoon with SBCAPE 
between about 900 j/kg and 1400 j/kg...depending on how much 
low-level moisture gets mixed out. Latest forecast updates include a 
lower maximum temperature for today and an increased chance for precipitation 
this afternoon and this evening...even across the southwest deserts. 


Previous discussion... 
significant push of low-level moisture has taken place overnight as 
southerly outflow winds from large mesoscale convective system over northwest Sonora have made 
their way into the County Warning Area. Both kiwa/kyux vwp indicate moderate S/southwesterly 
winds in place across the region in the lowest 5k feet...with 40kts 
indicated on the kyux radar between 2 and 3k. Surface dewpoints now 
in the upper 50s to middle 60s in scntrl Arizona...rising well into the 70s 
out in western County Warning Area...with 78 degree f dewpoint in Calexico at 09z. 


In light of the significant moisture push...along with the fact that 
min temperatures most places have dipped below criteria and will likely 
remain below criteria through Saturday...have decided to cancel the 
excessive heat warning that was in effect for scntrl Arizona today...and 
the watch that was in effect for Saturday. It will still be near or 
slightly above normal temp-wise...but quite a ways under local 
criteria. 


Meanwhile...model soundings continue to look very unstable as one 
would expect with the added moisture now in place. And...with favorable 
northeasterly steering flows today...decided to hike up probability of precipitation a bit from what 
was inherited...still slight chance in the deserts...but chance probability of precipitation 
now in the higher terrain of zone 24. With less favorable steering flows 
and perhaps some working over of the air mass tonight...went with a bit 
lower probability of precipitation on Saturday than those today. Although gusty microburst 
winds will still be possible with any storm...threat of locally 
heavy rainfall is now increasing as well. In fact...storms last 
night in the SW greater phx area produced upwards of an inch of 
rain. 


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