Area forecast discussion 
National Weather Service Phoenix Arizona 
420 am MST Tuesday Sep 2 2014 


Update...updated aviation and fire weather discussions. 


&& 


Synopsis... 
above normal temperatures will continue through at least Wednesday 
under mostly dry conditions. Moisture will make a gradual return into 
the region beginning Thursday...supporting increasing chances for 
afternoon and evening thunderstorms over the higher terrain of Arizona. By 
Friday...a deeper tropical moisture profile will continue to work 
into the region resulting in increased humidities...cloudiness and 
cooler temperatures. Slight storm chances return to the forecast over 
the Lower Desert elevations of Arizona and towards the lower Colorado 
River valley for the weekend. 


&& 


Discussion... 
another quiet evening continues into the early morning hours with 
clear skies and mostly light winds observed across the forecast area. 
Comparing dewpoint readings from those 24 hours ago it is hard to 
discern any sort of trend...with values higher or lower by only a few 
to up to 20 degrees of difference at varying surface sites. Looking 
higher through the whole atmospheric column...measured/observed precipitable waters  
last evening were holding below normal values - at 67 percent normal 
in Flagstaff with 0.45 inches and fairing a bit better down Tucson- 
way at 85 percent with 0.82 inches. Water vapor and 02/00z upper air 
analysis showed the high pressure center to our south with continued 
dry westerly flow moving inland off the Pacific. Fairly steady state 
forecast in store for today and Wednesday...with little changes 
expected in the broader mean wind flow and temperatures through the 
column over what we've seen the last few days. This will result in 
another round of above normal temperatures and mostly clear skies. 


Upper ridge center is expected to develop/relocate to our east by 
late Wednesday...gradually backing winds to more favorable 
monsoon headings. Early Thursday morning the GFS...and to a lesser 
extent the NAM...still tries bringing in some light precipitation over the 
south-central deserts. While the GFS has had some run-to-run 
consistency with this solution...still not sold we'll see wetting 
precipitation on the desert floors that early in the forecast period. 
Increasing cloud cover and maybe some virga shafts could be 
possible...but moisture remains limited below 500mb. When evaluating 
the column as a whole...1 inch and above precipitable waters  return to area 
overnight Wednesday across much of central and southern Arizona. Afternoon 
and evening thunderstorms will be possible across the central and 
eastern Arizona higher terrain...including southern Gila 
County...beginning Thursday afternoon. Most of the activity will 
remain North-East-south of the Phoenix area Thursday...with greater 
chances of seeing storm outflow intrusions and the ever-present 
threat for blowing dust along those boundaries across the Lower 
Desert elevations. 


Forecast for Friday is a bit trickier...as we see a decent return of 
moisture but not much by way of upper level support or energy to see 
organized storm development. Forecasts remain generally consistent 
with the beginnings of a deeper moisture surface moving in from the 
south...with 850mb dewpoints approaching 10-12c readings heading into 
the area later in the day Friday. Felt introducing at least 10 
percent pop/slight chance mentioning down through Maricopa 
County...including the phx metropolitan...was warranted. While organized 
storms are not expected...traveling outflow boundaries could support 
isolated pulse storm activity. During this same period...troughing 
will develop down from the Pacific northwest off the northern California 
coast...maintaining drier flow over most of southeast California...with a 
sharp moisture gradient along the lower Colorado River valley. The 
return of increasing humidities and clouds will help reign in our 
above normal temperatures...cooling temperatures to more normal 
readings for the late week. 


Saturday into early next week...tropical influence is looking more 
likely for the weekend and early next week as the current area of 
interest off the central Mexican coast is expected to develop within 
the next 2 to 5 days into a tropical storm while moving closer 
towards the Baja California peninsula. With continued adjustments to the 
tropical storm track along with the strength and residence time of 
the West Coast trough...felt a broad brush forecast approach for the 
weekend was warranted. Precipitation chances were increased given decent 
extended model consensus on moisture and cape/Omega fields for Sunday 
and Monday...with slight chances holding even during the overnight 
hours across much of the forecast area. Some discrepancies remain in 
moisture placement and influence of dry air intrusion from the trough 
to our northwest...with the latest Euro solution is more aggressive with the 
moisture expansion across southeast California. Bottom line...potential is 
there and confidence has been steadily increasing for a return of 
several stormy days and evenings that may last into in the middle 
part of next week. Longer range probabilities call for an extended 
window of above normal precipitation across the southwest region. With the 
uptick in storm activity and expectation of mostly to considerably 
cloudy skies...a cool down of welcome proportions will result for the 
weekend...with several best performing and consensus temperature 
forecasts cooling much of the forecast area below 100f by Monday and 
beyond. 


&& 


Aviation... 
south-central Arizona including kphx...kiwa...and ksdl...and 
southwest Arizona and southeast California including kipl and kblh... 


Through 04z Wednesday...very dry westerly flow aloft providing clear skies 
and light wind under 8 knots. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Thursday through Monday... 
monsoon moisture will return to the region this period along with an 
increasing threat of showers and thunderstorms. Heavy rain is a 
possibility in south central Arizona Sunday and Monday. Humidities 
will trend higher with a corresponding downward trend in afternoon 
temperatures. Minimum relative humidities will range from near 20 
percent Thursday...to 30 to 35 percent by Monday. Diurnal afternoon 
winds each day will range from 10 to 15 miles per hour. Recovery at night will 
be a function of nocturnal showers and thunderstorms. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


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Discussion...Nolte 
aviation...vasquez 
fire weather...vasquez/aj 



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