Area forecast discussion...updated 
National Weather Service Phoenix Arizona 
538 am MST Wednesday Jul 30 2014 


Update...updated aviation and fire weather discussions. 


&& 


Synopsis... 
there will be a break in the monsoon through Thursday...with 
decreased chances of showers and thunderstorms. 
Consequently...temperatures will continue their upward trend and 
highs in the 110-115 range will be common across the deserts through 
Thursday. A return to a more typical monsoon regime is expected late 
Friday into the weekend as showers and thunderstorms are once again 
expected across the region. 


&& 


Discussion... 
another quiet day convectively on Tuesday as there were only a few 
showers and isolated thunderstorms to contend with across the 
region. As a result...skies were mostly clear this morning with only 
some lingering clouds across northern Arizona. Meanwhile to the 
south...a weak complex of storms was dissipating across northern 
Sonora. Temperatures this morning as of 08z were very similar to 
values seen on Tuesday...generally in the upper 90s to middle 90s. Sky 
Harbor was still holding tough at 99. 


Temperatures really overachieved on Tuesday with most desert locales 
breaking the 110 degree mark. Looking back at the 00z sref and European model (ecmwf) 
analysis and comparing 850mb temperatures from Tuesday afternoon to 
Wednesday afternoon...we could actually manage another 2-3 degrees 
of warming this afternoon. Consensus guidance is still pointing at 
110 degrees...but based on what happened Tuesday i\m going to lean 
on the warm side and bump temperatures up a few more degrees. Inevitably 
our lows this morning will be in the upper 80s/lower 90s so we\ll 
already be starting off well above normal. One fly in the ointment 
is the lingering cloud cover across northern Sonora. With winds 
aloft expected to remain out of the southwest...it looks like the 
majority of that cirrus will be deflected into New Mexico with sunny 
skies prevailing across the deserts. As such...i\m going to go with 
an excessive heat warning for the lower deserts. Even with some 
degree of cooling expected on Thursday and the possibility of some 
increasing cloud cover from distant storms to the south...temperatures will 
remain dangerously warm through Thursday evening and the excessive 
heat warning will run until 8 PM Thursday. 


Monsoon moisture is expected to creep back into the area on Friday 
and by and large the forecast thinking has not changed for Friday 
through the weekend. Deeper moisture will lag a bit and is not 
expected to arrive in the deserts until this weekend. Probability of precipitation were 
increased a bit for Friday /still generally looking at slight 
chances/ but higher values seem warranted for Saturday and Sunday. 
Both the GFS and European indicate a trough moving through the area 
at 700-500mb Saturday afternoon/evening with middle level winds around 
20kts...MLCAPES around 1000 j/kg and mean bl mixing ratios around 
8-10 g/kg. If those values actually come to pass it could end up 
being quite active Saturday evening. Similar conditions are expected 
Sunday however it\s typically difficult to get back to back active 
days so my confidence in the weekend forecast is low...especially 
for Sunday. 


Looking down the Road to next week...southwest flow is expected to 
return to the area Monday/Tuesday...advecting somewhat drier air 
into the region from the west and once again resulting in decreased 
chances of precipitation along with increased temperatures. 


&& 


Aviation... 
south-central and southwest Arizona and southeast 
California...including kphx...kiwa...ksdl...kipl...and kblh... 


No aviation impacts through this evening as only few/scattered 
middle and high clouds develop mainly over high terrain removed from 
terminal sites. Largest forecast challenge will be timing of wind 
shifts...and much like the past 24 hours...typical directional 
changes may be delayed or absent. Even prolonged periods of variable 
winds will be possible...with peak winds generally at or below 12kt. 
Slightly better storm chances for Thursday afternoon/evening as humidity 
values start to climb. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Friday through Tuesday... 
the drier westerly flow aloft of midweek will be replaced by a more 
classical monsoonal southeast flow aloft...which will spread 
increasing moisture westward across the deserts Friday into the 
weekend. As this occurs the chances for afternoon and evening 
thunderstorms will increase from east to west each day. Humidity 
values will steadily increase with minimum values between 20 and 30 
percent expected across the deserts Saturday into the early part of 
next week. Correspondingly...high temperatures will fall 
off...dropping from around seasonal normals Friday to well below 
normal values on Sunday. Slight warming will return by next Tuesday 
as humidity values lower somewhat. Strong winds are not expected 
during the five day period...and they should typically favor south 
to southwest directions during the afternoon and evening hours. 


&& 


Psr watches/warnings/advisories... 


Arizona...excessive heat warning from 10 am this morning to 8 PM MST 
Thursday azz020>023-025>028. 


California...excessive heat warning from 10 am this morning to 8 PM PDT 
Thursday caz031>033. 


&& 


$$ 


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


Discussion...leins 
aviation...MO/cb 
fire weather...cumulonimbus 



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