Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 302111

Area Forecast Discussion
National Weather Service Phoenix AZ
211 PM MST TUE AUG 30 2016

Strong high pressure in the eastern Pacific will result in above
normal temperatures the next few days, particularly across
southeastern California. Meanwhile, moisture will increase across the
region through Thursday. Afternoon thunderstorm activity initially
confined to the eastern Arizona mountains today will likely affect
the lower deserts Wednesday and Thursday...mainly east of the Lower
Colorado River Valley. A drying trend starts Friday and continues
and into early next week as a low pressure system moves through the
western states. A return to below normal temperatures is also
expected early next week.


Rest of today/tonight...
Storm development over our area has begun a bit earlier than normal.
Ironically, so far this afternoon, storm activity overall has been
quite isolated. The development is toward the southwest aided by the
northeasterly steering flow. The RAP continues to show a rapid
downtrend of CAPE for today plus considerable CIN over the lower
elevations. This suggests that the activity will not be long lived
tonight over the higher terrain of central and southeast AZ and
storms will struggle to survive over the lower deserts. This is
supported not only by HRRR but also NCEP NMM/ARW, UofA WRFs, and
local NMM/ARW models. However, outflows can be expected for portions
of Maricopa and Pinal County. HRRR has trended stronger with the
outflow than previous runs. A weak wave associated with an inverted
trough over Mexico is advertised to track through southern Arizona
late tonight and Wednesday which is apt to produce some cloudiness
and could possibly produce some weak showers. Thus, expanded the area
of overnight slight chance.

Wednesday and Thursday...
The steering flow takes on more of a southerly component Wednesday
and Thursday due to the inverted trough tracking across northwest
Mexico as well as the expansion of a ridge centered over the south-
central CONUS. Of note, this is more true of the GFS than the NAM.
Within that flow, models indicate at least a couple of different
perturbations moving through. Meanwhile, moist advection takes place
from east to west leading to an expansion of storm chances - but
remaining primarily east of the Lower Colorado River Valley. Also,
the moisture availability is not tremendous with our area generally
remaining below 10 g/kg mixing ratio. With more of southeast Arizona
seeing storms Wednesday, anticipate more outflows and higher
likelihood of blowing dust. Thus inserted mention of patchy blowing
dust for portions of Maricopa and Pinal Counties. Models are in some
agreement on the steering flow becoming southwesterly Thursday but
with lingering moisture and CAPE. Temps slowly trend down but remain
above normal for southeast California and portions of southwest AZ.

Friday through Tuesday...
The models suggest a broad trough will move into the Pacific
Northwest. Resulting southwesterly flow across our region will shift
the focus for convection mainly to the higher terrain of northern and
eastern Arizona. Operational GFS/ECMWF are in better agreement today,
indicating a steady drying trend will commence Saturday continuing
through early next week as a reinforcing trough in the westerlies
scours out the remaining Monsoon moisture.


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

Thunderstorm chances this afternoon and evening will be confined to
the higher terrain east of Phoenix. However, guidance continues to
indicate an outflow boundary from the east-northeast approaching
Phoenix terminals. Thus, a change group indicating a more
northeasterly wind near 01Z with gustiness up to 25 kts is
represented in all Phoenix terminal TAFS. Prior to the arrival of any
outflow, easterly winds will weaken the rest of the afternoon. Some
debris clouds are possible with bases remaining AOA 10kft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Skies will remain clear and winds will remain light, following
typical diurnal trends.

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday...
Pacific troughing will import drier air into the region beginning
Friday. The more noticeable downward trend in dewpoints and
humidities will be into Sunday, with minimum RH values ranging in the
10 to 15 percent range by Monday. Overnight recovery will remain fair
to good. Afternoon and evening wind speeds will trend upward,
favoring typical upvalley patterns, but no widespread strong wind
events are anticipated. There are some indications that humidities
will start to increase Tuesday.


Spotters should follow standard reporting procedures.




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