Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 301257

Area Forecast Discussion
National Weather Service Phoenix AZ
557 AM MST TUE AUG 30 2016

.UPDATE...To Aviation and Fire Weather Discussions...


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 Friday. Afternoon thunderstorm activity initially
confined to the eastern Arizona mountains today and Wednesday will
likely affect the lower deserts late in the week. A drying trend is
then likely through the weekend 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.


Latest WV imagery shows the weakening low pressure system just east
of the Four Corners. Dry and subsident northerly flow continues to
overspread AZ while an extension of the eastern Pacific anticyclone
stretches from southern CA into the intermountain West. The ridge
will yield a continuation of the above normal temperatures the next
few days, particularly across southeastern California. Although an
Excessive Heat Warning will not be needed, the Imperial Valley will
see high temperatures reaching 114 degrees this afternoon. Heat
impacts are likely in these areas, even for the general population,
if precautions are not taken.

More pronounced easterly flow will develop today, which will serve
two functions. First, compressional (adiabatic) warming from the
increasing downslope flow will contribute to the warm conditions.
Second, deeper moisture across NM will begin to be transported back
into AZ. Latest ensemble of CAMs points to the eastern Mogollon Rim
including eastern Gila County as a likely spot for isolated
convection this afternoon.

Latest streamline analysis also shows a cyclonic circulation across
central Mexico. Operational models continue to indicate that a piece
of this complex will fracture off in the form of an inverted trough,
which will lift northward into Sonora Wednesday and possibly as far
north as southern Arizona Thursday. Low-level mixing ratios will
continue to increase during this period across much of the area, with
the threat of convection expanding westward into the lower deserts,
including the Phoenix area.

By Friday, 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:

Clear AM skies will transition to partly clear with some aftn CU
fields developing over the lower deserts and the Phx area...somewhat
heralding the return of Monsoon moisture into the region. Easterly
winds will prevail for the day today from the sfc on up through FL180
(if not beyond) with some periodic sfc gustiness up to 20kts during
the late AM window. Aftn/evening storm activity will be visible over
the higher terrain to the east of the Phx area and storm outflow
winds are possible into the terminals later today, initial estimate
31/01z. Outflow winds would also be easterly in nature with some
increasing SCT clouds 15-18kft.

Southeast California/Southwest Arizona including KIPL and KBLH:
Mostly clear skies and light diurnally driven winds will continue for
TAF period. Transition to more pronounced southeasterly winds will
develop for KBLH during midday before returning to up-river/southerly
headings by the evening.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday...
Monsoon moisture will continue to return and linger through the
Southwest states Thursday, with storms chances mainly remaining east
of the Lower Colorado River Valley. Pacific troughing will then
approach from the west, transitioning 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.


Spotters should follow standard reporting procedures.




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