Weather Service Report

FXUS65 KPSR 291635

Area Forecast Discussion
National Weather Service Phoenix AZ
935 AM MST MON AUG 29 2016

A sharp warming trend is expected today across the region. Moisture
will return across eastern Arizona during the middle of the week
with higher humidity values building steadily westward across the
area. Afternoon thunderstorm activity initially confined to the
eastern Arizona mountains will likely affect the lower deserts mid to
late week. A drying trend is then likely through the weekend as a low
pressure system moves through the western states.


An upper low is centered over southwest Colorado/northwest Mexico this
morning and there is a positively tilted ridge across California,
the Great Basin, and western AZ. 12Z soundings from Arizona and Las
Vegas reflect this situation with a northerly component to the flow
all the way through the column as well as a lack of CAPE at those
sites. NAM and RAP initialize some pockets of CAPE over northern
Arizona (GFS doesn't). Deeper moisture lies to the east over New
Mexico and Chihuahua. That is where the storms will primarily be
today. Models depict moisture from there moving into Arizona tonight
(likely a result of the convection itself). For Tuesday and
Wednesday, there is model agreement on amplification of the flow
pattern over the CONUS and northeast Pacific which includes and
expansion of the previously mentioned ridge as well as a westward
expansion of the high centered near Kentucky. In the process, the
upper low near us weakens and merges into a larger upper low/inverted
trough centered near southern Sinaloa Mexico. This allows westward
moist advection and associated CAPE. Current forecasts reflect this
trend but some refinements will likely be needed. More later.


Latest water vapor imagery shows a well- defined closed low across
the Four Corners. Dry and subsident northwesterly flow persists
across the lower deserts with temperatures and dewpoints below normal
again early this morning.

Models are in good agreement showing the aforementioned closed low
filling rapidly today across NM. Northeasterly flow will subsequently
develop between an extension of the eastern Pacific ridge and a broad
area of low pressure in central Mexico. Height rises associated with
the building ridge and compressional (adiabatic) warming from the
increasing downslope flow will translate into a jump in temperatures
today close to normal readings.

Mean flow will continue to veer and become even more easterly
Tuesday, resulting in further increases in temperatures and low-
level mixing ratios. Return of the monsoon thunderstorms will likely
initially take place across eastern AZ with isolated storms possible
across eastern Gila County. Latest CAMs are not overly bullish with
their depiction of reflectivity, though conceptually a 20-30 percent
PoP is warranted across the higher terrain east of Phoenix.

The threat for storms will then increase across the lower deserts
beginning Wednesday and continuing through the end of the week as an
inverted trough lifts northward out of Mexico and into the Desert
Southwest. Uncertainty exists with regard to what areas will see
storms when, however the synoptic setup is favorable for activity
which is more typical of late August/early September.

Operational models continue to indicate that moisture will scour out
from west to east as two anomalous troughs move eastward through the
western CONUS this weekend. Timing differences still abound, though
the general trend will be towards drier conditions.


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

No weather related impacts to operations to occur for the TAF
period...and even beyond that as an unseasonably dry airmass settles
into the region. Storm chances will drop off for much of the state
with no chance for storms over the Phoenix areas and terminal
airspace. Typical AM easterly winds will linger later into the
afternoon than usual, with KPHX making a later-than-normal switch to
West possibly after 30/00z. 

Southeast California/Southwest Arizona including KIPL and KBLH:

Dry and quiet conditions in the forecast for all the SE CA terminals
for the TAF period and even slightly beyond. Lack of pressure
gradient will generate VRB winds for much of the period otherwise
wind headings to follow usual diurnal patterns. Clear skies will

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday...
During the midweek period, the best chances for convective activity
and wetting rains will remain over higher terrain areas north and
east of Phoenix; however, a couple showers could move into lower
elevation Wednesday and Thursday. The end of the week may feature a
return to more predominantly dry conditions, while very little chance
for rain exists from the Colorado River westward throughout the
entire week. Minimum humidity levels between 15 and 30 percent will
follow fair to good overnight recovery. Wind speeds do not appear to
be particularly strong through the week, with only some typical
afternoon gustiness.


Spotters should follow standard reporting procedures.




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