Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 240409

Area Forecast Discussion
National Weather Service Phoenix AZ
909 PM MST TUE AUG 23 2016


A weather disturbance exiting the area will confine any lingering
thunderstorm activity to higher terrain areas east of Phoenix this
evening. Drier air will move in from the west Wednesday and Thursday
keeping storm chances limited to the higher terrain of south-central
Arizona. An increase of Monsoon moisture is anticipated late in the
work week with a return of storm chances to the lower deserts of
Arizona...but only slight chances. Drier air returns Sunday through
next Tuesday. Overall, temperatures will remain near, to slightly
below, seasonal normals.


Latest streamline analysis shows the axis of the mid-upper trough
along the AZ/NM border. Some showers and thunderstorms have developed
on the back side of the trough across northern Arizona. However,
latest runs of the HRRR and other CAMs continue to show the activity
dissipating this evening as a drier subsident flow overspreads the
desert southwest.

Fall-like pattern prevails in the wake of the aforementioned trough.
Surface dewpoints are generally in the 40s and 50s across the lower
deserts, which is drier than normal. Temperatures in Phoenix this
afternoon failed to even reach the 100 degree mark and overnight
readings will also be a few degrees below normal. Forecast remains in
good shape and only minor adjustments to the short-term temperatures
and dewpoints were needed this evening.


.PREVIOUS DISCUSSION... Main upper low continued to spin across
northeast Arizona early this afternoon, and the low center appeared
to be nearing the New Mexico border at 1 pm. Latest 300mb SPC
mesoanalysis graphic showed some upper divergence well off to the
east of Phoenix mainly northeast of Globe. MLCAPE was rather low
across the south central and southwest deserts as well, mostly at or
below 250 j/kg. With central desert temperatures still running below
normal and ranging between the mid 80s to low 90s, it indicates that
our boundary layer has become stabilized and worked over from the
rains earlier today. All told, a drier subsident northwest flow aloft
will continue to overspread the lower deserts behind the exiting low
this afternoon and evening and any lingering threat for convection
will remain focused over higher terrain areas east of Phoenix -
mainly east of Globe.

For the next couple of days, model guidance, including operational
runs of the NAM, GFS and ECMWF as well as GEFS guidance, continues
to call for low pressure troffing to set up along the west coast. As
this occurs, flow aloft into the southeast CA and Arizona maintains
a dry westerly component. Initially the steering flow aloft is
northwest - behind the exiting upper low - but over time shifts
towards the west and then southwest. Still, this keeps any
significant moisture confined to mainly far eastern Arizona and
across the higher terrain well to the east of Phoenix. Thus the
southeast California and southern Arizona deserts should experience
sunny or mostly sunny days through Thursday with generally clear
nights. High temperatures will stay near or slightly below seasonal
normals due to the influence of low pressure aloft.

Friday... GFS and ECMWF guidance have been consistent over the past
couple of days in calling for troffing to sharpen up a bit along the
central California coast on Friday. The result of this is flow
becoming more southerly in the low to mid levels and thus spreading
increasing monsoonal moisture back into central and eastern Arizona.
ECMWF has typically been more aggressive with this moisture input,
and the latest GFS has backed off and become drier. We will still
keep a slight chance of afternoon and evening storms across desert
areas east of the lower Colorado River Friday afternoon and evening
but we lowered POPs a few percent down closer to 10 percent over the
southwest deserts in nod to this slightly drier scenario.

.Saturday through Tuesday...
Ridging centered west of the Baja Peninsula expands into the region
over the weekend and into and Monday for more anticyclonic flow and
a drying trend. Only very slight chances remain over our easternmost
areas by Sunday evening. Temperatures slowly climb Saturday through
Tuesday but only peaking 2-3 degrees above normal.


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

As an upper trough exits to the east and drier air moves into the
region expect clear to mostly clear skies and light and variable
winds. By late morning to early afternoon winds will favor a
southwesterly direction with occasional breezes to 15 kts. by late

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday...

Low pressure moving inland into southern CA is expected to turn the
flow move southerly over the region, allowing some monsoon moisture
to return to the region on Friday and Saturday. This increase in
monsoon moisture will lead to an increase in convective activity
over South-Central AZ, with the best chance for wetting rains over
the higher terrain east of Phoenix, but with a few storms making
their way into the lower deserts as well. High pressure building
eastward into the region will then push the monsoon moisture back
off to the east, with mainly clear skies to prevail over the region
from Sunday onward into next Tuesday. Winds to remain on the light
side through the taf period, mainly following typical diurnal
trends. Minimum humidities in the 15-35 percent range on Friday and
Saturday to fall into the 10-25 percent range, with good overnight


Spotters should follow standard reporting procedures.




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