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Area Forecast Discussion
National Weather Service Phoenix AZ
847 AM MST WED JUL 27 2016

Despite lingering moisture across the region, strong high pressure
will keep thunderstorm activity isolated for the next couple days as
well as keeping temperatures unusually hot. As this high pressure
system weakens by the end of the week, better moisture profiles will
stream north throughout the state. In addition to bringing somewhat
cooler temperatures, increased thunderstorm activity will impact the
area through at least the beginning of next week.


Latest WV imagery shows a well-defined moisture discontinuity between
drier air across northern Arizona and moist air across southern AZ.
It is this boundary that will serve as a focusing mechanism for
convection this afternoon. Latest model soundings indicate that
northeasterly steering flow around the anticyclone to the north will
increase to around 20 kt this afternoon, which will be favorable for
storms to propagate out of the higher terrain and into the lower
deserts. Recent trends in the CAMs including the HRRR suggest that
convection is possible in the Phoenix area as well. Current setup
warrants at least a chance of storms and significant increases were
made to PoPs for tonight, up to around 30 percent in the Phoenix
area, but as high as 60 percent across eastern Gila County. There is
also a discernible threat for blowing dust, particularly across Pinal



Perhaps the most significant impact over the next 48 hours will be
excessively hot conditions manifesting through southeast California
and the Lower Colorado River Valley. In addition to strong heights
aloft, ensemble means depict the H7 thermal ridge nearing the 99th
percentile while best performing guidance suggests afternoon highs
peaking near 115F in the population centers of El Centro, Blythe, and
Parker today and Thursday. While this isn't the hottest weather of
the summer thus far, it certainly will be dangerous for those
outdoors during peak heating; and have issued an Excessive Heat
Warning highlighting these larger population centers.

All indications suggest Friday being a transition day where heights
and temperatures aloft begin to deteriorate while more substantial
deep moisture seeps north and west into a larger chunk of Arizona.
Forecast soundings still only depict around 9 g/kg boundary layer
mixing ratios, however the depth of this moisture may begin to reach
above the H7 layer and deeper more organized outflow boundaries may
become capable of initiating convection at lower elevations with
these type of sounding profiles. Based on SREF probabilities and
sounding appearance, not overly enthused about widespread rainfall
Friday though it certainly looks like events occurring on Friday
evening will set the stage for more activity over the weekend. 

At this time, its difficult to ascertain which specific day during a
Saturday through Tuesday time frame would be more convectively
active or provide the most significant impacts. However, all model
indications point towards at least one of these days yielding well
organized thunderstorms with the potential for isolated severe
weather and flooding. Forecast soundings show sfc-H7 mixing ratios
markedly improving to 11-12 g/kg while H5 temperature cool to around
-6C yielding MLCapes 1000-1500 J/kg which would be readily released
by any good inverted trough or deep combination of outflow
boundaries. Certainly, there will also likely be one of these days
where the environment become convectively overturned and
contaminated leading to gross inactivity, though conceptually any
one of these days justifies elevated POPs. 


South-Central Arizona Including KPHX, KIWA, and KSDL: 
Debris clouds from decaying storms over Sonora will largely stay
south of Phoenix metro. Storms later today will be favored over
southeast AZ. However, easterly steering flow will be strong enough
that any storms that might form over Gila County could propagate
into the metro area. Likelihood too low at this point to insert
strong winds or TSRA in the TAFs. Otherwise, winds will follow
familiar warm season patterns.

Southeast California/Southwest Arizona including KIPL and KBLH:
Debris clouds from decaying storms over Sonora will largely stay
south of the Mexico border. Otherwise, anticipate any storm activity
later today to be well east of the Lower Colorado River Valley. There
could be some enhanced south and southeasterly winds later this
morning from remnant outflow but otherwise, surface winds will follow
familiar warm season patterns. 

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday...
A more humid airmass is expected to develop this weekend as moisture
from Mexico and the Gulf of California pushes northward. This will
lead to a significant increase in thunderstorm chances. Wetting rains
will tend to be quite localized but at a minimum, there will be
higher humidities, more cloudiness, and cooler temperatures.


Spotter activation is not expected.


AZ...Excessive Heat Warning until 11 PM MST Thursday for AZZ020.

CA...Excessive Heat Warning until 11 PM PDT Thursday for CAZ031>033.



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