Area forecast discussion 
National Weather Service Phoenix Arizona 
937 am MST Friday Jul 25 2014 


Synopsis... 
a noticeable increase in monsoon activity is expected to commence 
today and continue into the weekend. The high pressure center that 
brought the region very hot temperatures the last few days will 
weaken and shift further off to the east...allowing moisture that is 
now over Mexico to surge northward into the region. Sufficient 
monsoonal moisture will then remain around the region to support 
afternoon and evening storm chances through at least the middle of 
the coming week...with the best focus for storms likely over the 
higher terrain of south-central Arizona. Daytime temperatures will 
still hold above seasonal normals...but are expected to fall back 
from their near-record readings. 


&& 


Discussion... 
vastly different forecast set-up has taken shape when looking over 
the last 24 hours over the southwest deserts. This time Thursday 
morning we were observing a slight drying trend from the east with 
isolated convective activity resulting in the afternoon/evening across 
the western Arizona deserts. With the upper high now weakening and 
drifting eastward this morning...mean atmospheric flow has 
consolidated to broad southerly headings transporting a deeper 
moisture surface in from northern Mexico and the Gulf of California. 
Gulf surge conditions continue this morning along the Colorado River 
valley where Yuma has held persist gusty southerly winds with their 
dewpoints now climbing into the 70s. Dewpoints further "inland" have 
significantly increased into middle and upper 60 degree readings. 


Overnight loop of the water vapor imagery showed some subtle inverted 
wave feature traveling across northern old Mexico/southern New 
Mexico that is now moving Arizona this morning. There is some weak broad 
circulation to it...with only a few of the hi-res and the latest NAM 
showing it weakly around 700mb. In addition...upper jet support 
remains to our northwest across southern Nevada/central Utah...which 
could be an aide for storm development further in the day. Given the 
return of deep monsoon moisture...including at 850/700mb where 
readings are averaging 10c and 6c or so degrees across the 
state...and the dynamics/instability in place...confidence still 
remains high for an active monsoon day. We will likely see the 
blowing dust/outflow impacts in the afternoon hours first...with 
storm development along those boundaries as they move over the lower 
deserts. Anticipatory blowing dust advisory was issued by the 
previous shift and is in good shape. Outside of some minor sky and 
dewpoint grid tweaks...no major updates planned this am. 


&& 


Previous discussion /issued at 215 am MST/PDT/... 
a significant change to a more active monsoon regime 
now appears to be taking place across the region today. The very 
strong upper high center that brought US the very hot weather over 
the last few days is now shifting off to the east...over nm and West 
Texas. This is allowing a deep southerly flow to develop over the 
region this morning. Changes can already be seen at the 
surface...with Yuma now experiencing a southerly surge that has 
brought 20+mph winds and raised the dewpoint to 71f as of 1am this 
morning. The phx area also saw a surge on dewpoints as well late 
yesterday evening...from the upper 30s and low 40s up into the low 
60s when the winds shifted to a westerly direction. The latest GFS 
and Euro model output has greatly increase probability of precipitation across south-central 
Arizona as they now both push a weak disturbance northward across the 
region...with precipitable waters  rising as high as 2.0 inches by late this evening 
and MUCAPES shooting above 1000 j/kg. Given the fact that there will 
likely be even more ts activity to our south...over Pima and southern 
Pinal counties...it appears that it is quite likely that we will see 
at least some blowing dust from storm outflows. 


Saturday looks like it may be similar to today...with similar 
moisture and MUCAPE values being forecasted across the region. 
However...if today turns out to be quite active...the worked-over 
atmosphere may hold ts activity down somewhat on Saturday. The 
increasing moisture and cloudiness...along with lower 500mb 
heights...is expected to end our excessive heat...with highs around 
110f today and the 105-109f range on Saturday. Saturday could turn 
out to be even cooler if there is widespread rainfall today/this 
evening. 


Sunday through Thursday... 
it appears that a low-middle grade monsoon will likely prevail through 
the extended period as weak southerly flow aloft continues to pull 
moisture into the region...with the GFS showing a more active 
pattern as it keeps the upper high center off to our east...and the 
Euro showing a drier one as it attempts to reposition the upper 
high center westward back over the region. 


&& 


Aviation... 
south-central Arizona including kphx...kiwa...and ksdl...moisture 
increasing over the region today will mean a greater threat of 
thunderstorms this afternoon and evening. This activity has a good 
chance of impacting the kphx...kiwa...and ksdl terminals between 22z 
and 06z. Variable and gusty winds and cloud bases lowering at or above 5 kft 
are possible during this period. Otherwise expect scattered to broken middle and 
high clouds and prevailing south to southwest winds at around 10 kts. 


Southeast California and southwest Arizona including kipl and kblh... 
expect scattered middle and high clouds through today with bases at or above 15 kft. 
At kipl...winds will remain out of the southeast at around 10 kts. 
Along the Colorado River valley...including kblh...winds will 
increase through the day out of the south...with expected gusts to 
around 25kts after 16z. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Sunday through Thursday... 
slightly cooler afternoon temperatures are expected along with 
higher humidity. Minimum relative humidity levels will range from 15 
to 25 percent. A typical monsoon daily threat of afternoon and 
evening thunderstorms are expected with the highest probabilities in 
the mountains east of Phoenix. Typical afternoon and evening 
southwest winds are expected with good recovery at night. 


&& 


Psr watches/warnings/advisories... 


Arizona...blowing dust advisory from 2 PM this afternoon to 9 PM MST this 
evening azz022-023-027-028. 


California...none. 


&& 


$$ 


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


Discussion...Nolte 
previous discussion...percha 
aviation....McLane 
fire weather...vasquez 



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