Area forecast discussion 
National Weather Service Phoenix Arizona 
441 am MST Monday Jun 27 2016 


Update...updated aviation discussion... 


&& 


Synopsis... 
high pressure will move somewhat north and east of Arizona this week 
allowing warm temperatures to prevail, but also bring a couple of 
weather disturbances through the state. An increase in monsoon 
moisture will accompany these weather systems providing a threat of 
afternoon and evening showers and thunderstorms the remainder of 
week. 


&& 


Discussion... 
early this morning a well defined inverted trof could be seen moving 
westward across far southern/southwestern Arizona, and this feature 
was clearly evident in vapor imagery as well as recent plot data. 
Circulation around this feature was spreading increasing moisture 
westward and into the desert west of phoenix; latest blended total 
precipitable water imagery indicated pwat values over 1.2 inches 
over portions of the southwest deserts. The inverted trof created 
good upper difluence across the central and western deserts and 
coupled with a good uvv field, a field of mostly light showers could 
be seen spreading into the western deserts at 2 am. As this inverted 
trof slowly pushes west today we can expect a continued slight 
chance of showers or thunderstorms across much of the lower deserts 
to the west of Phoenix including portions of southeast California. Later 
this afternoon and evening across south central Arizona, the forcing 
becomes weaker behind the exiting inverted trof but there is weak 
southeast steering flow and sufficient deeper moisture and 
instability to allow for a 15-20 percent chance of mainly evening 
thunderstorms over the central deserts and a chance of storms over 
higher terrain east of Phoenix. This is supported by various 
mesoscale models such as the nmm6km. Clouds and moisture working 
across the lower deserts will keep high temps today mostly below the 
110 degree mark, even over the far western deserts. 


Model guidance, including operational runs of the GFS and ECMWF, as 
well as ensemble guidance such as the gefs and naefs, continue to 
call for a very monsoonal pattern to develop across the Desert 
Southwest starting Tuesday and continuing through the end of the 
work week. Initially the main upper high center forms near The Four 
Corners, setting up a deeper southeast steering flow over most of 
the area which will steadily transport increasing and deeper 
moisture westward over the lower deserts. Precipitable water values steadily climb 
and are forecast to exceed 1.5 inches across the main monsoon 
moisture corridor which sets up just east of the lower Colorado 
River valley and extends into far eastern Arizona. The steering flow 
is favorable to bring mountain storms into the lower deserts, and 
from time to time there will be disturbances rotating around the 
main upper high which will serve to enhance thunderstorm coverage 
and intensity. Model guidance typically has a hard time with the 
exact timing and track of these features and we generally cannot 
place a lot of Faith in them more than a day or two down the Road. 
Overall we are looking at a rather broad brushed forecast with 10-20 
percent chances of mainly afternoon and evening storms over the 
deserts mainly east of the lower Colorado River valley, and 30-40 percent 
chances across the higher terrain east of Phoenix. 


Later in the week the upper high starts to shift towards the 
southeast and steering flow becomes more southerly. Still, moisture 
remains high and in fact the GFS calls for precipitable water value in the central 
deserts to approach 2 inches later Thursday into Friday. This would 
suggest a much better chance for heavy rains to accompany any storms 
that develop. Of course, as moisture increases the temperatures will 
fall off and by midweek high temps across south central Arizona will 
approach and then fall below seasonal normals. It will stay a bit 
hotter further west where the atmosphere is a bit drier, with highs 
over 110 expected for much of the week. 


During the upcoming weekend, as low pressure sets up along the 
Pacific northwest coast, and as the high shifts further to the 
southeast, the upper streamline fields and steering flow turn 
towards the southwest over most of the area. This starts to bring in 
drier air from the southwest and we can expect rain chances to 
diminish from west to east with time. By Sunday afternoon, 
thunderstorm chances look to be confined mostly to higher terrain 
areas east and southeast of central Phoenix as skies become mostly 
sunny over the central and western deserts. 


&& 


Aviation... 
south-central Arizona including kphx, kiwa, and ksdl: 
much better chances for stronger east/northeast outflow winds are 
likely late this afternoon, but confidence in timing is still 
somewhat low. Actual storms may not survive to lower elevations, 
though mountain obscuration, blowing dust, and frequent lightning for 
arrivals/departures to the east may be problematic. Persistent 
easterly winds this evening should occur earlier this normal. Sct to 
bkn mid and high clouds should persist through Tuesday morning. 


Southeast California/southwest Arizona including kipl and kblh: 
cigs in a 10k-15k ft range will persist through this afternoon with 
some gradual thinning of cloud cover tonight. Virga showers this 
morning over the terminals are likely and should dissipate by around 
noon. While a south to southeast sfc wind will be preferred through 
this afternoon, winds will eventually become more southwesterly this 
evening. A few stronger gusts may also be possible for brief 
periods, especially this evening. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Wednesday through Sunday... 
temperatures will continue to "cool" with highs on Wednesday around 
105-112f dropping into the 100-110f range by Sunday. As monsoon 
moisture levels continue to increase over the region, isolated 
thunderstorms are possible each afternoon/evening over much of the 
Desert Southwest, with the highest chances over the higher terrain 
of central and eastern Arizona. Accompanying any storm that develops 
will be wetting rains, gusty erratic winds, and lightning. This 
increase in moisture will support a rise in minimum humidities, with 
values ranging in the 15-25 percent range each day with good 
overnight recoveries. Outside of gusty storm winds, speeds will 
range between 5 to 15 mph with a few afternoon upslope gusts up to 
20 mph. 


&& 


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


&& 


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


$$ 


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Discussion...cumulonimbus 
aviation...Kuhlman 
fire weather...Hernandez 






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