Area forecast discussion 
National Weather Service Phoenix Arizona 
920 am MST Wednesday Jul 1 2015 

after a rather active early week start...somewhat more stable air 
moving into the region will reduce thunderstorm activity somewhat 
over the region today. Building high pressure along with a slightly 
drier airmass is expected to gradually reduce convective activity 
further from Thursday into Saturday. Increasing southerly flow aloft 
along with a return of moisture will bring back a better chance for 
showers and thunderstorms on Sunday before another shift in the 
upper pattern from the southwest trends down thunderstorm potential 
early next week. 


scattered thunderstorm activity continues along and parallel to I-10 
from about Quartzsite all the way to the County Warning Area border at jtnp and near 
thermal/Palm Springs this morning. Regional satellite imagery this 
am shows the extensive debris and storm clouds over far western Arizona 
and nearly the southern half of California...with some thinning debris 
fields across south-central and Southeast Arizona. Ml/ul upper ridge 
circulation continues to migrate towards The Four Corners...with the 
300/250mb high centers now plotted over northern nm...transition 
flow aloft more southeasterly with its repositioning. 

Morning model streamline forecasts show remnant mesoscale convective vortex/300 mb inverted 
trough over southwest Arizona...with fairly confluent southeasterly flow 
upstream over northern Mexico. The 1.93 inch readings on this 
morning's kpsr balloon is at near-record levels...but likely 
contaminated/water weighted as plenty of debris cloudiness and a 
even rogue shower or two remained over the area. 500-300mb winds 
remain on the elevated side...with 35-40kts...but are forecast to 
gradually weaken as we get through the day. Forecast soundings drop 
convective available potential energy at or below 800 j/kg readings by the afternoon and there is 
some question as to how warm we'll get today to help support and 
even fire convection off the higher terrain. Hi-res and 
deterministic models keep any precipitation activity of interest gnly on 
the forecast area periphery through the evening hours. This solution 
seems reasonable and already reflected in the grids for today...with 
slight chances already in place for the afternoon/evening hours. 


Previous discussion /issued at 310 am MST/PDT/... 
today and tonight... 
after a rather active day on Tuesday...when a rather strong inverted 
trough moved westward across the region...along with a rather strong 
jet maximum...with winds in excess of 50 knots in the 200-300mb 
layer...and a very unstable airmass...with mixed layer convective available potential energy at or above 
1000 j/ appears that today will be somewhat quieter. The 
latest GFS model output is showing 200-300mb winds will drop into 
the 15-20 knot with mixed layer convective available potential energy falling into the 500-600 j/kg 
range across south-central Arizona...which represents a substantial 
decrease in instability and middle-high level shear when compared to 
Tuesday. In fact...after this morning...the hi-res arw and nmm 
models are actually showing a complete absence of convective 
activity across our entire County Warning Area today. However...given the fact that 
precipitable waters  will be still on the high the 1.40-1.60 inch 
range...with some modest instability remaining in the column...the 
current feeling is that there should be at least some 
shower/thunderstorm development this afternoon across the higher 
terrain of south-central Arizona today...with isolated storms pushing 
their way into the lower deserts later this afternoon/evening. The 
main impacts from these storms this afternoon will be locally gusty 
winds and heavy downpours...with the expected slower storm motion 
increasing the risk of localized flooding somewhat. Blowing dust 
should not be a major problem with the storm motion still more from 
a east-west direction rather then from a southeast-northwest one. 

Thursday through Saturday... 
a further decrease in shower/thunderstorm activity is expected 
during this the main high center aloft repositions 
itself right over our County Warning Area...and somewhat drier air filters into the 
region...with precipitable waters  falling into the 1.20-1.40 inch range. The 
expectations for the period at this point is that the vast majority 
of thunderstorm activity will be confined to the higher terrain 
north and east of Phoenix...with mainly just some outflow activity 
from these storms making their way into the lower deserts. Although 
there will be less cloudiness and moisture around during this 
time...high temperatures should not get too far above 
the main 500mb high center weakens somewhat...with heights staying 
mainly in the 590-292dm range...likely keeping highs mainly at or below 110 
across the south-central Arizona lower deserts. 

Sunday through Wednesday... 
both the GFS and Euro models are showing a temporary increase in 
southerly flow aloft ahead of an upper trough that is expected to 
approaching the West Coast...with 200-300mb winds rising at or above 40kts 
and precipitable waters  rising smartly into the 1.60-1.80 inch range. Thus...a 
significant increase in thunderstorm activity is quite likely on 
Sunday...with some storms once again becoming quite strong. 
Convective activity is then expected to take a downward trend from 
Monday into next Wednesday as the flow aloft becomes more light snow shower 
ly...and begins to import somewhat drier air into the region. 


south-central Arizona including kphx...kiwa...and ksdl... 
thunderstorm potential will remain the biggest aviation concern 
through late tonight. Isolated rain showers should remain on the west side of 
the metropolitan this morning...though be as close as Luke AFB/White Tanks. 
Regardless...ceilings should remain above 10k feet. Very low confidence 
forecast for this evening and overnight...with most model output 
suggesting no additional thunderstorm activity. However..with 
moisture in place...a rogue storm impacting one of the central 
Arizona aerodromes may be possible. 

Southeast California and southwest Arizona including kipl and kblh... 
scattered rain showers and isolated thunderstorms and rain will affect parts of southeastern California through 
much of the morning hours...and possibly into the afternoon. While 
ceilings will remain at or above 10k feet...abrupt wind shifts and very localized 
terrain obscuration will be possible. Showers should move well away 
from terminals by late afternoon...and very little to no model data 
suggests redevelopment through Thursday morning. 

Aviation discussion not updated for amended tafs. 


Fire weather... 
Thursday through Monday... 
a low grade monsoon pattern will be in place through the week... 
characterized by near or slightly above normal temperatures...and 
widely scattered afternoon and evening showers/thunderstorms that 
move from east to west off the mountains and into the lower deserts 
each day. For Thursday into Saturday the storms will primarily focus 
over the higher terrain with rather low chances of surviving into 
the lower deserts. Sunday into early next humidities rise 
there will be a better chance of thunderstorms and showers moving 
into the central deserts during the overnight hours. Daytime 
humidity values generally will be in the upper teens to middle 20 
percent range through Saturday with humidities rising several 
percent by Monday. Very good overnight recovery is expected each 
night. Winds each day will be light and favor southwesterly 
directions through Friday...becoming southerly by the weekend. 


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


Psr watches/warnings/advisories... 



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previous discussion...percha 
fire weather...cumulonimbus 

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