Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
233 PM MST Friday Aug 26 2016
weather disturbances moving through the Desert Southwest will keep
storm chances going tonight for most locations. Moisture trends
downward over the weekend for a corresponding downturn in storm
chances. Moisture will remain modest next week with storm chances
limited to the higher terrain of Arizona. Overall, temperatures will
be close to seasonal normals...slightly above west of the lower
Colorado River valley.
rest of today/tonight...
water vapor imagery shows troughing over much of the western Continental U.S.
With multiple short waves within it. The more distinct waves include
one moving through The Rockies and another centered near the or/ID/NV
borders. The latest rap initializes a much more subtle wave tracking
across southern Arizona. This corresponds to ongoing storms over the twc
County Warning Area including southern Pinal County. The rap also depicts some upper
level divergence over portions of Arizona which also corresponds to
some of the storms over Arizona including northern Arizona. Isolated
activity has also developed near Yuma and west of Blythe. Though the
steering flow is southwesterly (typically unfavorable for monsoon
season storm development) have at least slight chance pops the rest
of the afternoon and evening for much of the forecast area due to the
presence of moisture and perturbations moving through. The
approaching short wave to the northwest may begin to make its
presence felt late tonight and thus kept pops going overnight (east
of the lower Colorado River valley).
Saturday and Sunday...
a large short wave deepens over Arizona this weekend in response to
an amplifying ridge centered west of baja Mexico. In the process
however, dry advection takes place. This keeps pops limited to
south-central Arizona Saturday. Pops will be even lower Sunday with pops
limited to the higher terrain of south-central Arizona (only slight
chances). With drier air, Max temps nudge up a bit on Sunday (near or
slightly below normal).
models forecast the trof axis to be east of Phoenix by
early afternoon. Dry and stable weather is inferred by this pattern.
Tuesday...the upper level flow becomes more southwesterly, however
models are forecasting a 300/250 mb deformation zone to develop,
with its axis of dilatation from Yuma to Phoenix. This indicates a
threat of afternoon and evening showers and thunderstorms on the
south central deserts and mountains.
Wednesday through Friday...a dry and more stable southwest flow
pattern develops. Any afternoon storm threats will be relegated to
the mountains east of Phoenix. Dry elsewhere.
South-central Arizona including kphx, kiwa, and ksdl:
showers and thunderstorms that have been developing across south-
central Arizona this morning are expected to gradually diminish and move
off to the east this afternoon/evening as drier air filters westward
into the region. Although there is still an outside chance that one
or more of the terminals could be affected by showers/storms this
afternoon/evening, confidence/probabilities are too low at this
point to include them in the tafs. Winds are expected to become
light westerly late this afternoon (with periods of light/variable
winds possible as well), then become easterly a bit earlier than
usual this evening. Easterly winds during the day on Saturday will
then become westerly a little later than usual. Sct-bkn mid-level
cloud layers this afternoon/early this evening to become primarily
sct by late evening.
Southeast California/southwest Arizona including kipl and kblh:
no aviation weather impacts anticipated. Winds will generally retain
a southerly component at kblh through the taf period. Kipl to see a
return of light southwesterly sundowner winds this evening before
becoming southeasterly later tonight. Mainly clear skies to prevail
through the taf period at both terminals.
Aviation discussion not updated for amended tafs.
Monday through friday:
Chances for convective activity and wetting rains will remain mainly
confined to the higher terrain north and east of Phoenix Sunday and
Monday, with dry conditions prevailing across the lower elevations.
A modest increase in convective activity can be expected from
Tuesday Onward into Thursday across south-central Arizona. Increasing
westerly flow aloft will then push drier air into the region next
Friday, pushing convective activity back into extreme eastern
Arizona/western nm. Minimum humidities will increase from 15 to 35
percent on Sunday and Monday to 15 to 45 percent Tuesday through
Thursday, then fall back into the 15 to 35 percent range on Friday.
Outside of any erratic thunderstorm wind gusts, winds will follow
typical diurnal trends with some gusty southerly winds from time-to-
time along the Colorado River valley.
Spotter information statement...
spotters should follow Standard reporting procedures.
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