Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 241008

Area Forecast Discussion
National Weather Service Phoenix AZ
310 AM MST Sun JUL 24 2016

An increase in moisture and cloud cover over the region is expected
to take some edge off the heat, and provide somewhat better chances
of showers and thunderstorms through the middle part of the upcoming
workweek. The best opportunity for rainfall will occur over the
higher terrain north and east of Phoenix, with lesser chances heading
west into southeast California. A better chance for showers and
thunderstorms is then forecast for the end of the week and into the
upcoming weekend, as deeper moisture begins to move into the region
from the south and southeast.



Today through Thursday...

After a relatively quiet today today, a return to more southerly
flow aloft is expected to bring a bit more moisture into the region,
starting tonight and continuing into Tuesday, with pwats across
South-Central AZ rising from around 1.10-1.20 inches up to around
1.50 inches. The first impact from this increase in available
moisture will likely be an increase in cloudiness and perhaps even
some light showers and a few thunderstorms late tonight and early
Monday as debris from thunderstorms that are expected to develop
over northern Sonora, Mexico and southeast AZ this afternoon/evening
move into the region. Available moisture levels are then expected to
remain pretty much steady through Tuesday, before decreasing
slightly on Wednesday and Thursday as ridging aloft rebuilds over
the region.

Although moisture levels will be decent (around average for this
time of year) through Tuesday, convective activity will likely be
limited by rather warm air aloft (500mb temps in the -5c to -6c
range), the fact that most available moisture will be at 600mb or
higher, and very light winds through the entire column. The impact
of the warm mid-level temps, lack of moisture below 600mb, and light
winds aloft can be seen in the latest NCEP SREF 3h-MUCAPE and 3h-
MLCAPE plumes, with the mean values remaining mostly aob 500 J/KG
across the Phoenix area through Thursday. Thus, it will be hard to
trigger any convective activity across the lower deserts, unless
significant lift can be provided by strong outflows from storms over
the higher terrain, or from weak mid-level disturbances, which are
hard to forecast/time. Thus, have currently gone with mostly slight-
chance pops across the lower deserts through Tuesday, with even
lower pops on Wednesday and Thursday, as somewhat drier air moves
back into the region. On the other hand, chances for locally gusty
winds/blowing dust appear to be better, as outflows from storms over
the higher terrain of South-Central AZ and SE AZ (which are much
more likely to develop than over our lower deserts) move into the
lower elevations each afternoon/evening. Along with the low-end
pops, have kept highs above normal through Thursday, with many lower
desert locations likely once again reaching/exceeding 110 degrees on
Wed/Thu as drier air/warmer air aloft moves back into the region.

Friday through Sunday...

There is still indications that this could be a more active period
weatherwise as the main 500mb high center weakens a bit and shifts
northward to near the AZ/UT border, allowing stronger easterlies to
develop over the region, and more moisture to work its way into the
region from the south and southeast. The GFS is showing PWATS as
high as 2.00 inches moving into the region by Saturday. This
increase in moisture, along with stronger winds aloft, and an
easterly wave moving across northern Mexico/extreme southern AZ
would lead to a rather substantial increase in convective activity,
if these model runs prove to be correct.


South-Central Arizona Including KPHX, KIWA, and KSDL:
Moderate confidence exists regarding limited aviation impacts
through Sunday afternoon before potential stronger outflow
boundaries affect aerodromes tomorrow evening. Some increase in
mid/high clouds will be likely from decaying storms over eastern AZ
tonight. Solid westerly winds will weaken later in the evening with
some uncertainty if/when a true shift to an easterly direction will
occur. Weak outflow boundaries from distant storms to the east may
eventually be the impetus for a wind shift overnight, though a quick
return to west winds would be common late Sunday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
Strong south to southeast prevailing winds will be the most likely
outcome through Sunday afternoon with lofted dust/haze impacting
slantwise visibilities tonight. After some weakening of speeds
overnight, occasional gustiness will once again be possible Sunday.
Some mid/high clouds will be possible along with dust trapped in the
lower atmosphere.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday...
Monsoon moisture working its way back into the region will bring
chances for afternoon and evening thunderstorms each day through
Thursday. The best chances for wetting rains will be over the higher
terrain east of Phoenix, will lesser chances over the lower deserts
of South-Central and southwest Arizona, and only slight chances over
southeast California. Some increase in convective activity is
expected on Friday, as deeper moisture begins to move into the
region. Temperatures to remain slightly above normal through the
entire forecast period. Minimum humidities in the 15-30 percent range
Monday-Thursday to rise into the 20-35 percent range on Friday.


Spotter activation is not expected.





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