Area forecast discussion 
National Weather Service Phoenix Arizona 
441 am MST Friday Aug 26 2016 


Update...updated aviation and fire weather discussion. 




&& 


Synopsis... 
a series of weather disturbances will move across Arizona this 
weekend, and combined with a moderate surge of moisture from Mexico, 
a threat of afternoon and evening showers and thunderstorms are 
forecast mainly over central and eastern Arizona. Monsoon moisture 
will be pushed into eastern Arizona next week, with a threat of 
afternoon storms over the eastern mountains. 


&& 


Discussion... 


Today through Sunday... 


A very interesting mid/upper level flow pattern will develop over 
the region through Sunday. A wacky pattern if you ask ME. Unusual 
because a 500 mb trof is forecast to slowly develop and intensify 
over Arizona with origins from western canada? At the same time a surge 
of monsoon moisture has been consistently forecast to move into 
south central Arizona from Mexico this morning. 


Its 2 am and so far the precipitable water (pw) GPS sounders in 
southern Arizona have not detected a substantial rise in precipitable water. The Yuma 88d 
radar VAD wind profile was detecting 15-25 knot south winds below 4 
thsd ft at 2 am with surface dewpoints in the mid 70s at both Yuma 
and Imperial. 


Additionally, 00z plot data showed a couple of 500 mb disturbances 
moving across central Arizona from the west this morning. At 2 am these 
disturbances were initiating a few showers and thunderstorms in 
Phoenix near the Airport, and over areas north through east of 
Phoenix. The last of these disturbances will move across central Arizona 
around sunrise with a few showers and tstms continuing just east of 
Phoenix through the mid morning hours. 


What makes this pattern so interesting is, as the trof from western 
Canada slowly develops just west of Arizona today, despite an increase in 
low level monsoon moisture, mid/upper level southwest winds show 
little cyclonicity nor with lateral shear. In other words, winds 
aloft become laminar, with mid level warm advection, and not really 
supportive for diurnal convection. Therefore any storms today will 
be thermodynamically driven, first in the mountains with possible 
descending outflows onto the central deserts. Secondary storm 
development is possible on the central deserts with outflows. 
Southeast California and southwest Arizona will mostly storm free this afternoon 
and evening. 


Another remarkable thing about this developing mid/upper level trof 
in Arizona is, the vertical structure of this system tilts to the 
northeast. In other words, by Saturday evening the 700 mb low center 
is positioned over northern baja, the 500 mb circulation center just 
west of Needles CA, and the 300 mb center by Las Vegas. The entire 
trof wobbles eastward across Arizona in this negative tilt orientation 
through the weekend. Vertically tilt trofs like this are storm 
killers or produce stable weather, and perhaps this is why the 
models forecast very little precip over south central Arizona this 
weekend. Therefore the best chance of storms Saturday and Sunday 
will be thermodynamically driven, or heat of the day storms, and 
predominately over the mountains. Again, our forecast area in 
southeast California and southwest Arizona will be nearly storm free. 


Monday...models forecast the trof axis to be east of Phoenix by 
early afternoon. Dry and stable weather is inferred by this pattern. 


Tuesday...the upper level flow becomes more southwesterly, however 
models are forecasting a 300/250 mb deformation zone to develop, 
with its axis of dilatation from Yuma to Phoenix. This indicates a 
threat of afternoon and evening showers and thunderstorms on the 
south central deserts and mountains. 


Wednesday through Friday...a dry and more stable southwest flow 
pattern develops. Any afternoon storm threats will be relegated to 
the mountains east of Phoenix. Dry elsewhere. 




&& 


Aviation... 


South-central Arizona including kphx, kiwa, and ksdl: 


Scattered showers and isolated thunderstorms will continue for the 
next few hours over Phoenix terminals with some brief heavy rain and 
a few gusts possible. Later this afternoon and this evening, 
redevelopment of showers and storms is possible in south-central 
arizona; however, lower confidence in exact spatial and temporal 
coverage precludes mention in 12z taf. Outside of any thunderstorm 
wind gusts, easterly winds will continue for the remainder of the 
morning before switching to the west early this afternoon around 20z. 
Skies will remain scattered to broken aoa 10kft. 


Southeast California/southwest Arizona including kipl and kblh: 


No aviation weather impacts anticipated. Winds will generally retain 
a southerly component, occasionally gusting as high as 20 kt at kblh 
Friday afternoon. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 


Sunday through thursday: 


Chances for convective activity and wetting rains will remain mainly 
confined to the higher terrain north and east of Phoenix through 
Monday, with dry conditions prevailing across the lower elevations. 
A modest increase in convective activity is then expected Tuesday 
through Thursday across south-central Arizona. Minimum humidities will 
increase from 10 to 35 percent on Sunday and Monday to 15 to 45 
percent Tuesday through Thursday. Outside of any erratic 
thunderstorm wind gusts, winds will follow typical diurnal trends 
with some gusty southerly winds from time-to-time along the Colorado 
River valley. 


&& 


Spotter information statement... 
spotter activation is not expected Thursday. 


&& 


Psr watches/warnings/advisories... 
Arizona...none. 
California...none. 
&& 


$$ 


Visit US on facebook...twitter...and at weather.Gov/Phoenix 


Discussion...vasquez 
aviation...Hernandez 
fire weather...percha/Hernandez 






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