Area forecast discussion 
National Weather Service Phoenix Arizona 
622 am MST Friday Sep 4 2015 


Update... 
updated aviation and fire weather sections. 


&& 


Synopsis... 
sufficient moisture will remain over much of Arizona today providing 
good shower and thunderstorm chances...particularly across the 
eastern half of the state. Somewhat drier air will move into the 
region over the weekend...reducing rainfall chances very slightly. 
While some thunderstorm chances will persist during the first half 
of next week...a substantially warmer and drier airmass will spread 
into the area for the latter half of the week. 


&& 


Discussion... 
early morning WV imagery shows a seasonably deep cold core low over 
the Pacific northwest...with the longitudinal trough axis extending all the 
way south to 20n. This scenario is creating an unusually strong 
meridional jet throughout the southwestern Continental U.S. With embedded shortwaves 
streaming north from the outer fringes of Tropical Storm Kevin (west 
of the southern tip of the Baja California peninsula). Meanwhile...infrared satellite 
loops showed numerous Gravity waves pulsing outwards from convection 
earlier Thursday...with resultant temporary subsidence in their wake 
over much of Arizona. Interestingly...00z/03z regional sounding data 
sampled a distinct layer of drier air (7-8 g/kg) in the 800 mb-700 mb levels 
hovering above more impressive shallow surface moisture. 
However...objective analysis and forecast soundings indicate mixing 
and advective processes modifying this boundary layer this morning 
closer to 12 g/kg. 


Very interesting and tricky forecast for the next 24-48 hours as 
numerous subtle features and processes will make or break the 
forecast. During the morning hours...an overwhelming amount of high 
resolution models initiate elevated convection through much of central 
Arizona in association with a speed maximum and jet divergence field. 
There is some evidence in model forecasts of Theta-E advection and 
weak isentropic ascent...albeit focused more north towards 
Flagstaff. Within the mesoanalysis fields...there is hints of a 
vorticity center and -10c 500 mb cold pocket propagating north with the 
jet segment...and indeed recent satellite images show a 
corresponding cirrus streak and altocumulus castellanus field blossoming through 
southwestern/central Arizona. Thus...feel model output does lend credence to 
showers this morning lifting from the Phoenix vicinity into northern 
Arizona. 


Amount and scope of afternoon thunderstorm development may hinge on 
any atmospheric contamination (or lack thereof) and potential 
limited insolation from morning showers and cloud cover. However 
given the stronger than normal winds aloft...any debris clouds and 
showers will likely be advected into northern Arizona quickly 
allowing for rapid airmass recovery and boundary layer warming. With 
the boundary layer conservatively holding 11 g/kg moisture 
levels...cooler temperatures and steep lapse rates aloft...and 
decent surface warming...MLCAPES around 1000 j/kg will be attainable. 
Deep layer bulk shear 25-30kt and adequate forward storm motion may 
be able to translate higher momentum air to the surface in more robust 
convection...however dcape values below 1000 j/kg may preclude a 
more substantial widespread severe threat. 


Convective potential for the weekend and early next week may be 
somewhat more muted than recent days as deep layer winds veer 
somewhat more westerly as troughing translates into the Great Basin 
and northern rockies. Still...boundary layer moisture will be difficult 
to completely scour out and some modest infusion of midlevel 
moisture peeling off the remnants of Kevin may be possible. 
However...there remains some disconnect with the more quality 
tropical moisture to the south...and the best WV transport may be 
deflected around the forecast area. In addition...with middle latitude 
troughing lifting to the north and heights aloft rising ahead of the 
remnant tropical system...models depict substantial midlevel warming 
and increased convective inhibition. Kept some modest near 
climatology probability of precipitation through Tuesday...however measurable rainfall at 
Lower Desert elevations may be difficult to come by. 


Some minor uncertainty still exists for the latter half of next week 
as subtle differences in the subtropical ridge position will dictate 
if and how quickly moisture is completely scoured south of the 
forecast area. Mesoscale processes and outflow from sonoran mesoscale convective system/S 
may allow for Gulf surges and boundary layer moisture 
advection...but definitively forecasting and timing such events 
would prove fruitless at this time. Regardless...500 mb height rises at or above 
590dm will result in temperatures peaking back above normal under 
gradually more compressional subsident flow. 


&& 


Aviation... 
south-central Arizona including kphx...kiwa...and ksdl... 
scattered-broken middle clouds...at or above fl100...can be expected this morning with 
possibly an embedded light shower. The southern end of an upper 
trough will help redevelop storm activity over Arizona again this 
afternoon. Confidence too low at this point to insert thunderstorms and rain in 
prevailing weather or in tempo groups in the tafs. 
Otherwise...surface winds will favor southeast and south for much of 
the day. 


Southeast California/southwest Arizona including kipl and kblh... 
anticipate familiar warm season surface wind patterns...favoring 
southerly directions. Any storm activity will be isolated and 
confined to areas east of the lower Colorado River valley. Otherwise 
anticipate only some scattered middle to high level clouds. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Sunday through Thursday... 
storm chances will continue over south-central Arizona through 
Tuesday before trending down significantly beginning Wednesday. 
Humidities will begin trending down from west to east next 
week...beginning as early as Monday. A gradual warming trend will 
continue next week climbing to several degrees above normal by next 
Thursday. 


&& 


Spotter information statement... 
spotters are encouraged to report conditions according to Standard 
operating procedures. 


&& 


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


$$ 


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


Discussion...MO 
aviation...aj 
fire weather...aj 



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