Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 170416

Area Forecast Discussion
National Weather Service Phoenix AZ
916 PM MST Wed Aug 16 2017

.UPDATE...Updated aviation discussion.


Relatively dry conditions will persist across the Desert Southwest
through Saturday. After a brief period of below normal
temperatures, a warming trend will continue into the weekend. Moist
monsoonal flow will return to Arizona over the weekend bringing an
increase in thunderstorm chances. Afternoon and evening storms will
remain in the forecast through the middle of next week along with
near normal temperatures.


The afternoon WV imagery looks more like mid September versus mid
August as stronger westerly flow has descended equatorward through a
vast majority of the Conus. Within the Southwest, the
midtropospheric u-wind component is a good 2 normalized standard
deviations above normal which has scoured all quality moisture
necessary for good deep convection away from the state. 12Z regional
sounding data sampled PWATs general in a 0.50-0.75 inch range; and
KTWC measured 0.61 inches ranks well below the 10% percentile
historically for this time of year (and not terribly far from daily
record lows).

Last night, dewpoints around the Phoenix metro actually fell into the
upper 20s and dewpoints this low are rare in August; the last time
they were this low this time of year was in 2013. While current
dewpoints are still hovering the lower/middle 40s, some 30s will be
likely by sunset and the dry air along with the clear skies will
result in favorable radiational cooling conditions across much of
Arizona. Essentially a persistence type forecast tonight with lows
falling into the delightful 60s and 70s.

As negative height anomalies lift into the plains and Upper Midwest,
heights/thicknesses will continue to increase slowly and steadily
through Friday. While temperatures will remain slightly below normal
tomorrow, at or above normal temperatures will materialize
thereafter as the subtropical ridge migrates westward into Texas.
With H5 heights sitting steady around 590-592dm, temperatures will
remain well below record levels.

The approach of another weak trough in the eastern Pacific will
ultimately herald southerly flow and return of the monsoon moisture
to the region. However, there is uncertainty in the extended period
with regard to the precise timing of the moisture return. The latest
operational GFS suggests that moisture associated with a convective
complex in Sonora could be drawn northward as early as Saturday,
while the ECWMF is a bit slower. It`s also worth noting that the
operational GFS is an outlier when compared to other GEFS ensemble
members. Forecast is instead weighted towards the slower ECMWF,
which implies a return to isolated storms (and the threat of blowing
dust) beginning Sunday and continuing through the middle of next


South-Central Arizona including KPHX, KIWA, and KSDL:
Weak northwesterly winds are affecting central/northern portions
of the Phoenix Metro, while winds are nearly calm at KIWA this
evening. Expected drainage winds to become more prominent after
06-07Z at area terminals and persist into the mid-afternoon Thursday.
Clear skies amidst a dry air mass will keep aviation concerns
minimal through Thursday evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
West winds will prevail into early Thursday morning across KIPL
and most of southeast California, before weakening and becoming
more variable in direction at all sites through late Thursday.
Clear skies will result in minimal aviation concerns.

Aviation discussion not updated for amended TAFs.


Saturday through Wednesday:
Better moisture will finally begin to seep into the Arizona
districts this weekend where rain chances will start increasing
again. Higher terrain areas of Gila County will have the best chance
of storms over the weekend, with modest chances working into lower
elevations of Arizona early next week. With the increasing moisture,
minimum afternoon humidity levels will increase back to a 15-30%
range with good overnight recovery. Except near the influence of
thunderstorms, wind speeds will be rather light.


Spotters should follow standard reporting procedures.




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