Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 181055

Area Forecast Discussion
National Weather Service Phoenix AZ
355 AM MST Fri Aug 18 2017

.UPDATE...Updated aviation and fire weather discussions


Relatively dry conditions will persist today before a gradual
increase in moisture begins over the weekend and continues into
early next week. There will be a slight chance of showers and
thunderstorms each day beginning Saturday, particularly across
eastern Arizona and over the higher terrain north of Phoenix.
Temperatures will also remain near normal through next week.


Latest water vapor imagery shows relatively dry air (NAEFS PWATS
~10 percentile) across the Desert Southwest with deeper moisture
situated further south across northern Mexico. Streamline analysis
reveals the moisture is associated with an upper-level
anticyclone and a low-level inverted trough. Further west near San
Diego, another weak mid-level trough is present.

Increasing southerly component to the mean flow between the
aforementioned trough to the west and a building ridge across NM
will result in continued height increases today. This will
translate into a few degrees of warming under mostly sunny skies.
For the Phoenix area, this will equate to slightly above normal
temperatures (106 degrees) and the warmest day since 108 degrees
occurred on 8/11.

Latest SREF depicts the trough to the west and the ridge across TX
becoming the predominant weather features through the weekend. In
between these two systems, the deeper moisture associated with the
vort max across northern Mexico will drift northward into Arizona.
Consensus among the hi-res guidance indicates that convection
will generally remain confined to eastern Arizona Saturday, though
there is a slight chance of activity clipping eastern Gila
County, per the latest NCEP WRFs. Model guidance suggests the low
pressure system across eastern Arizona will lift steadily
northward Sunday, leaving some residual moisture in its wake. Best
chance for storms will again be across the higher terrain.
Nevertheless, there is a discernible threat for isolated activity
across the lower deserts.

Pattern established during the weekend will persist through much
of the upcoming week with southwesterly flow between the trough to
the west and the ridge to the east. This will focus the deepest
moisture and the best chance for storms across eastern Arizona.
However, a slight chance of storms (along with threat of blowing
dust) will remain across the lower deserts each afternoon/evening.
Later in the week, latest GEFS mean suggests some additional
drying, particularly further west, though there is considerable
uncertainty this far out. Temperatures are also expected to
remain near normal through the period.


South-Central Arizona including KPHX, KIWA, and KSDL:

Clear skies will prevail for Friday with no aviation concerns. Winds
will remain light and follow typical diurnal tendencies.

Southeast California/Southwest Arizona including KIPL and KBLH:

Southerly winds will develop at KBLH today while KIPL transitions to
a southeast flow.  Winds may switch back to westerly again at KIPL
this evening although they will remain light.  Clear skies will

Aviation discussion not updated for amended TAFs.


Sunday through Thursday: Temperatures will remain near seasonal
normals as a moderate amount of monsoon moisture moves into our
area. This will bring a chance of thunderstorms to the higher
elevations north and east of Phoenix each day while the lower
elevations in south-central Arizona will only see a slight chance
of storms. At this point, SE California may remain dry as
thunderstorm chances are forecast to be low. Expect the usual
afternoon breezes for this time of year while relative humidity
drops into the 15-20% range each afternoon for the lower deserts.


Spotters should follow standard reporting procedures.




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