Area forecast discussion 
National Weather Service Phoenix Arizona 
200 PM MST Friday Sep 30 2016 


Synopsis... 
somewhat drier and calmer weather will return to the region though a 
few isolated showers and thunderstorms will linger into early this 
evening. The weekend will feature seasonably warm temperatures and 
mostly clear skies. Continued dry weather will persist next week 
though a sharp cooling trend will arrive early next week as a strong 
area of low pressure passes by to our north. 


&& 


Discussion... 
a weak trough stretches from the Grand Canyon southward through the 
Phoenix area. Meanwhile, surface dewpoints have remained steady, 
generally in the upper 50s across much of the area. Isolated showers 
and thunderstorms have developed across mainly the higher elevations 
of southern Yavapai, northwestern Maricopa and La Paz counties early 
this afternoon. Across south-central Arizona, skies are mostly 
clear, largely due to convective inhibition associated with a well- 
defined subsidence inversion around 500 mb. 


Latest cams are somewhat less bullish on the potential for shower and 
thunderstorm activity this afternoon and evening. Nevertheless, 
conditions remain unstable and favorable for isolated storms just 
about anywhere from Yuma County eastward, including the Phoenix area. 
However, southwesterly flow would conceptually favor the higher 
terrain east of Phoenix in the vicinity of the stronger deep-layer 
shear, and the highest pops (up to 40 percent) continue in these 
areas. Models remain in good agreement that any activity will fizzle 
out quickly this evening with the loss of insolation. 


&& 


Previous discussion... 
operational runs from the GFS and European model (ecmwf) as well as gefs ensemble 
members all call for the deep Pacific trof to drop southeastward and 
inland over the West Coast during the course of the weekend, keeping 
drier southwest flow in place over the area and resulting in sunny 
days and clear nights. Both GFS and European model (ecmwf) also suggest that as the 
low center moves inland, southerly lower level flow ahead of the main 
low will tap a bit of deeper tropical moisture and spread it into 
portions of southern Gila County. Cape appears to be minimal but 
there appears to be sufficient moisture and instability to justify 
adding a slight chance of thunderstorms to southern Gila County for 
Sunday afternoon and evening, mainly over areas from Globe eastward. 
Pops will stay low and just above 10 percent in that area. By early 
Monday, stronger west/southwest flow ahead of the approaching deep 
low will push moisture off to the east and end any threat of 
precipitation over our area. 


For Monday into Tuesday, guidance is consistent in calling for the 
deep upper trof to move progressively inland and pass through the 
Desert Southwest with the main upper low center to stay well north 
of Phoenix as it passes through Utah. We will still see significant 
height and thickness falls with the trof passage, and as such high 
temperatures will fall to well below seasonal normal levels both 
days. Desert highs will drop into the 80s with many of the lower 
deserts expected to see highs in the low to mid 80s at times. The 
coolest day in Phoenix will likely be Tuesday when the High Falls to 
just 84 degrees; the normal high for the date is 93 degrees. In 
addition to the cooling, the trof passage will lead to breezy or 
windy conditions Monday during the afternoon hours; winds should 
remain below Wind Advisory levels but may approach critical fire 
weather thresholds in some locations. 


For Wednesday into Thursday, behind the exiting upper low a drier 
northwest flow aloft will spread across the area bringing sunny 
days, clear nights along with a warming trend. High temperatures 
will stay below normal levels but climb back into the 90s over the 
lower deserts by Thursday. 


&& 


Aviation... 


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


There is a slight chance for storms in the area with the most likely 
spots North, East, and west of the valley. The possibility of 
outflow winds from a storm seems likely although indications suggest 
the winds will be light. If storms develop southeast of Phoenix, 
blowing dust cannot be ruled out and may create slantwise visibility 
issues. In any event, the impacts to aviation should be much less 
compared to the events earlier in the week. Otherwise, the terminals 
should experience a more traditional diurnal pattern with a switch 
to a westerly wind during the afternoon. 


Southeast California/southwest Arizona including kipl and kblh: 


Expect a more typical diurnal wind pattern today with a more 
westerly wind during the afternoon and maybe some gustiness late 
afternoon. There are some clouds in the area are expected but storms 
are expected to stay east of the terminals. Otherwise, there should 
be few weather impacts to aviation. 


&& 


Fire weather... 
Monday through friday: a low pressure system moving through the 
region will bring drier air and gusty conditions to the area. The 
minimum humidities will approach and possibly drop below 15% for much 
of the lower deserts. The minimum humidities for the higher terrain 
areas north and east of Phoenix will drop from 25-35 percent range on 
Sunday to 15-20 percent by Thursday. Fortunately, the overnight 
recoveries should rise into the 35-45 percent range for both the 
deserts and higher terrain. Additionally, there will be a slight 
chance for thunderstorms over the higher terrain in Gila County 
Sunday and Monday. Monday looks to be the gustiest day, particularly 
in Gila County and southeast California. 


&& 


Spotter information statement... 
spotter activation will not be needed. 


&& 


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


$$ 


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Discussion...Hirsch 
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