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fxus65 kpsr 182000 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
100 PM MST Friday Jul 18 2008 


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


Update...updated aviation/fire weather sections. 


&& 


Synopsis... 
strong high pressure aloft moving north across the area means hot 
weather today. The high pressure system will move north over 
northern Arizona tonight and to Colorado by Saturday night and 
Sunday. Moisture from the east and south moving around the high will 
bring a significant increase in the chance of showers and 
thunderstorms...especially in south-central Arizona both late 
Saturday and Sunday. It will also be somewhat cooler Saturday and 
significantly cooler Sunday with much higher humidity. At least a 
slight chance of showers and thunderstorms is expected through next 
week with slowly rising temperatures beginning Monday. 


&& 


Discussion... 
a band of high cloudiness from the south was moving over southwest 
and south-central Arizona this morning while skies were sunny over 
the rest of the area. Temperatures were generally close to the same 
or a couple of degrees warmer compared with the same time yesterday. 


Temperatures may not quite reach excessive heat warning criteria 
thresholds but are expected to be close enough to continue the 
excessive heat warning today. And...even though relatively cooler 
air from the Gulf of California is expected to rush into our area 
Saturday morning...the air will also be much more humid with the 
heat index close to the same. 


It still looks as if we are in for a significant increase in showers 
and thunderstorms by Saturday night at least through Sunday night. 
More on this in the afternoon forecast package...stay tuned. 


Previous discussion... 
Friday...one more day of drier than normal conditions is expected. 
Middle-level ridge will drift slightly to the northeast towards the 
Mogollon Plateau. Mixing ratios below 750 mb will average 7.5 g/kg 
per the latest NAM-WRF. Even with a high temperature above 110 
degrees in Phoenix...this produces an elevated local and negligible 
cape. Main source of warm advection today will occur at 700 
mb...which will equate to about a degree increase at the surface. 
After a high of 110 degrees Thursday...will shoot for a high of 111 
degrees today under mostly sunny skies. Biggest concern for today 
will be the excessive heat and will continue the excessive heat 
warning (through Sat night)...which covers much of the forecast area. 


Sat-sun...models in excellent agreement that deeper moisture will 
advect northward out of Sonora as the ridge becomes established 
across The Four Corners. This will generate a more favorable 
southerly/easterly flow across the region. At this point...best 
chance for convection appears to be sun as precipitable waters  are projected to 
approach 2 inches. The increase in moisture is also reflected in the 
MOS temperatures...which indicate that we will fail to reach the 100 
degree mark. 


Extended...there are minor differences between the GFS and European model (ecmwf). 
GFS suggests a trough along the West Coast will shunt The Four 
Corners ridge and the deeper moisture eastward. However...the European model (ecmwf) 
suggests rain chances may actually be enhanced as an inverted trough 
moves westward through northern Mexico middle-late next week. Bottom 
line is that moisture should remain sufficient for near or even 
above average probability of precipitation. With moisture levels near or above 
average...temperatures are expected to remain below average through 
the period. 




&& 


Aviation... 
phx vicinity... 
fairly light winds through Friday night. Any thunderstorms today will be 
confined to the mountains North/East of phx. For Saturday...indications are that 
thunderstorm activity will a significant increase across the area. 
At this time it appears that impacts to phx area terminals will be 
roughly after 03z /8pm/ Saturday evening. 


Southeast California/SW Arizona... 
southerly flow will continue through Saturday afternoon. Expect a strong 
increase in southerly winds Saturday morning...with gusts to 30kt possible. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Sunday through Thursday...increasing moisture over the region will 
lead to an increasing threat for afternoon and evening showers and 
thunderstorms Sunday through the first part of next 
week...particularly from the central Arizona deserts eastward. 


&& 


Psr watches/warnings/advisories... 
Arizona...excessive heat warning from 10 am today to 8 PM MST Saturday 
for azz020>023-025>028. 


California...excessive heat warning from 10 am today to 8 PM PDT Saturday 
for caz031>033. 


&& 


$$ 


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


Discussion...Ellis/Hirsch 
aviation...iniguez 
fire weather...wanek