Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 182235

Area Forecast Discussion
National Weather Service Phoenix AZ
335 PM MST Fri Aug 18 2017

Atmospheric moisture content begins to trend upward over the
weekend for the Desert Southwest. Accordingly, rain chances begin
over southeast and east-central Arizona Saturday and expand across
the region to the deserts of California by early next week.
Temperatures will remain near normal through next week.


The synoptic scale pattern for the Desert Southwest remains
largely unchanged from recent days with dry southwesterly flow
aloft in place due to troughing centered west of the California
coast. Meanwhile a belt of subtropical high pressure remains
centered over the Gulf Coast. However, within the subtropical
latitudes of North America and the Caribbean, are a variety of
cyclonic circulations associated with cutoff lows and MCS
remnants. The big picture synoptic pattern evolution remains the
same for the coming several days with the development of more
southerly steering flow over Arizona and southeast California
between the California trough/closed low and a consolidation of
subtropical anticylonic flow becoming centered further west. In
general, this will mean a return of richer moisture to the region
and in turn storm chances. However, there are differing model
solutions on just when, how much, and how far west the Monsoon
moisture will track. Some of the deterministic models such as the
GFS and ECMWF have trended toward a later onset (and subsequent
departure) of significant moisture for our forecast area. Whereas
ensemble based guidance, such as the NAEFS and calibrated GEFS,
are indicating an earlier onset and departure. The ensemble
guidance may be reflecting the potential impacts of one of those
aforementioned cyclonic circulations moving northward through
portions of Arizona over the weekend. While the deterministic
models depict that circulation feature, they are more reluctant
with the moisture advection though the NAM is an exception. Some
of the hi-res models depict debris showers for late Saturday night
and Sunday but they wouldn`t add up to much.

Overall, for the lower deserts, the nature of stronger storm
activity looks to be a scenario wherein outflows move in from
southeast Arizona (and thus dust storm potential) plus adjacent
higher terrain from central /east-central Arizona but with only
isolated thunderstorm activity directly over our forecast area.
Thinking this would be the case during the workweek next week. As
indicated above, there are definite uncertainties associated with
uncertainties in moisture availability. If the flow aloft
maintains some westerly component (versus southerly or easterly)
then a relatively dry scenario will prevail. The converse would be
true for more southerly and/or easterly component to the flow. As
for temperatures, expect they will remain generally near normal
with no major upswings.


South-Central Arizona including KPHX, KIWA, and KSDL...
Southeast California/Southwest Arizona including KIPL and KBLH...

Fairly light winds will favor normal diurnal sequences combined with
episodes of light and variable or calm winds. The skies will also
remain clear to mostly clear throughout the period.

Aviation discussion not updated for amended TAFs.


Monday through Friday...
Seasonably hot temperatures combined with moderate Monsoon
moisture will dominate the region resulting in a slight to
moderate chance of afternoon and evening T-storms through
Wednesday evening over the deserts and high country, and through
Friday over just the high terrain of Gila county. However, expect
the best storm chances over the higher elevations of Gila county
with the more modest chances across the lower deserts of Arizona.
A few of the stronger storms may produce only a slight chance of
wetting rain. Seasonably breezy southerly and southwesterly winds
near 10 mph with gusts of 15-20 mph can be expected from SE CA to
SW AZ by late afternoon and early evening each day through
Wednesday. Min RH values in the 15-19 percent range over the lower
deserts on Mon will gradually peak at the 17-23 percent range by
Thu. Expect overnight recoveries to be fair to good over southeast
CA and good to excellent elsewhere.


Spotters should follow standard reporting procedures.




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