Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
830 PM MST sun Jul 5 2015
A drier westerly flow is forecast to develop over the region the
next several days...with most of the monsoon moisture being pushed
into eastern Arizona by Monday or Tuesday. However...a slight chance
of afternoon and evening showers and thunderstorms will continue
over the eastern Arizona mountains. Afternoon temperatures are also
forecast near normal values through the upcoming week.
Discussion... during the late afternoon hours scattered to numerous
showers and thunderstorms developed across Southeast
Arizona...extending into portions of northwest Pinal County
including the communities of Casa Grande and Florence. Substantial
rain cooled outflow winds overspread much of the central deserts
during the evening hours...cooling temperatures down into the 90s and
stabilizing the boundary layer. As such there was very little
activity after 6 PM over south central Arizona...just considerable middle/hi
level debris clouds moving across the area. Meanwhile and further to
the west...an area of low pressure continued to develop along the California
coast...giving a drier southwest flow aloft to southeastern California and
the far western deserts. Between the drier air advecting in from the
west...and the much deeper monsoon moisture corridor running through
central and eastern Arizona...a dry line of sorts set up and
scattered intense thunderstorms developed across much of eastern La
Paz County. The storms produced very heavy rain and gusty wind and
resulted in a number of warnings...advisories and Special Weather Statement products being
issued. The storms dissipated following sunset and by 8 PM had
For the rest of the night we will continue to see a slight chance of
a shower or a storm mainly from Maricopa County eastward...with
mostly clear or clear conditions expected over the western deserts.
Forecasts have been updated for current weather trends and are in
tonight...regional satellite imagery reveals mostly sunny skies
across southeast California...the lower Colorado River valley and
southwest Arizona...while showers and thunderstorms have formed over
southwest Maricopa...northwest Pinal and portions of southern Gila counties
this afternoon. With the light steering flows aloft...storms are
slow moving and putting out brief heavy rain. Radar estimates on
average around 0.25in falling in most basins within the last
hour...but some isolated basins show upwards of an inch with some
storms...requiring the issuance of some urban small stream flood
This convection finally picked up due to a slow moving shortwave
that has made use of the greater instability in place over the
deserts... and hi-res models suggest that isolated convection will
continue to form south and east of Phoenix this afternoon/evening...
with the bulk of the storms confined across northern Arizona. Storms
will gradually move off to the northeast. As they travel...storms
will producing brief heavy rain that could lead to some localized
street flooding...and gusty winds up to 30 miles per hour with patchy blowing
dust. Debris clouds will continue over the area into the evening...
and even with rain cooled air...overnight temperatures will still be warm.
Monday through Friday...the 300-250mb anticyclone center is expected
to shift to just north of Mazatlan Mexico by tomorrow...then move
well offshore the remainder of the week. This pattern will bring a
break to our monsoon for several days...generating a drier and
stable westerly flow across southeast California and south-central
Arizona until Friday. Monsoon moisture will be slowly pushed into
eastern Arizona by Tuesday with only a slight chance of storms over the
mountains of southern Gila County/zone 24 the remainder of the week.
Daytime temperatures will still be hot...in the 100-106 range...but
will be moderated to some extent by decreased 500mb heights/in the
582dm range/ as the ridge of high pressure remains to our south with
an upper level low perched over the Great Basin.
Saturday and Sunday...westerly flow aloft will weaken...and the
300/250mb high centers are forecast shift back toward El Paso by
Saturday night. This indicates a return of monsoon moisture from
Mexico first into Southeast Arizona Saturday...then across much of Arizona on
Sunday. Slight chance of showers and thunderstorms however will
remain over southern Gila County this period.
South-central Arizona including kphx...kiwa...ksdl...
Rain-cooled outflow winds have significantly stabilized the boundary
layer and the threat of convection for the rest of tonight is rather
low. Still...cannot rule out a shower overnight but confidence is
too low to mention in the area tafs.
Winds have been very problematic this evening. During the late
afternoon strong southerly outflow winds moved into the area...
gusting over 30 miles per hour...and shifting the predominant west wind to a
south or southeast direction. Have expected winds to return back to
the west...as all guidance has suggested...after the outflow winds
dissipated but this has not happened and confidence is now low that
we will see a return to a westerly component later this
evening...especially at kphx. Cannot rule out winds later returning
to the west due to a long-traveling outflow wind generated from
storms in La Paz County...but will not put this potential outcome
into the tafs at this time. Thus...have removed west winds from the
Phoenix tafs for the next 6 hours.
Less of a threat for storms tomorrow afternoon/evening as flow aloft
becomes more southwesterly and storms that develop should remain
over the higher terrain east of Phoenix.
Southeast California and southwest Arizona including kipl and kblh...
Little to no aviation impacts expected next 24 hours as flow aloft
becomes more southwesterly. Skies to be generally clear at kipl and kblh
through Monday evening. Winds should continue to favor the south
next 24 hours at kblh...becoming locally gusty again Monday afternoon.
Less confidence with winds at kipl...they should favor the southeast
less than 15kt but may briefly swing to the west later this evening
and persist into the morning hours before becoming light/variable.
Aviation discussion not updated for amended tafs.
Wednesday through Sunday...
atmospheric flow will become more westerly/southwesterly throughout
this week...pushing deep moisture and thunderstorm chances well to
the east of Phoenix over higher terrain locations. Even the
typically active regions in southern Gila County may lack
thunderstorm development and coverage for much of the week.
Afternoon humidity level will retreat back into a 10-15 percent
range across lower elevations with fair to good overnight recovery.
Somewhat higher humidity values will be experienced in mountain
locations. Generally light and diurnally driven winds speeds and
directions will predominate all week...with somewhat more afternoon
upslope gustiness than is typical during monsoon season.
Spotter information statement...
spotters are encouraged to report any significant weather conditions
according to Standard operating procedures.
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