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Area forecast discussion 
National Weather Service Phoenix Arizona
557 PM MST Wednesday Sep 2 2015

Update...updated short term forecast and aviation discussions...

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Synopsis...
another disturbance in the flow will continue to pull additional
moisture northward into the region...increasing thunderstorm chances
across south central Arizona tonight and Thursday. As the pattern
becomes modified over the weekend...reduced shower and thunderstorm
chances through Sunday before another upswing in monsoon activity
returns early next week.

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Discussion...
dry air has successfully pushed through much of south-central Arizona
this afternoon with the 00z kpsr radiosonde observation only showing 1.10 inches of
precipitable water. Morning cloudiness limited heating today and with
surface dew points dropping into the upper 40s over the lower
deserts...very little to no instability to work with. Have lowered
probability of precipitation for this evening...but still keeping at least a slight chance in
as storm activity over northern Mexico and Southeast Arizona may make
its way up here by midnight or so. The deep moisture should advect
northward into south-central Arizona by late Thursday morning with
no changes needed with the Thursday forecast.

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Previous discussion...
early afternoon convection is underway across southeastern Arizona
and north-central Arizona...as was fairly well advertised by the
local WRF high resolution model. While central Arizona had been
mostly clouded over through the morning hours...some pockets of
clearing and warming have allowed temperatures to reach right around
the century mark and also of note our dewpoint temperatures have
dropped substantially with Sky Harbor measuring nearly 15 degrees
drier as of 2130z compared with yesterday. Current area of
concentrated moisture /pw plume/ remains across the southern third
of the state. As was seen earlier today in the infrared satellite
imagery...an mesoscale convective vortex/vorticity maximum feature was seen over eastern Arizona and
a broad diffluent pattern still remains across the central and
southern half of Arizona.

Moisture streaming northward from the tropical feature well south of
Baja California continues to be advected into Sonora and southern Arizona. At
this time...no progress has been made in the strengthening of this
feature. However...with the upper level trough pushing onshore of
California and the high being positively tilted...a favorable south-
southwest flow will remain in place through the early part of next
week. Several small disturbances in the flow along with upper level
divergent flow should be the trigger needed to initiate convection
the next two evenings. Timing with convection has been tricky...as
with most monsoon forecasting...and evening to late night or
overnight storms are very possible. Location seems to favor Maricopa
County and places east with the southwest flow...and a much drier
airmass to the west should hinder any activity. Holding on to
slightly higher than climatology probability of precipitation through the next few evenings seems
reasonable.

As the upper trough moves farther east tomorrow and the flow becomes
slightly more confluent...wind shear remains fair with 40-50 kts
above 300mb..and slightly less precipitable water. Storm chances
still look favorable tomorrow and especially so if today does not
pan out to be as active.

Friday through Sunday...
somewhat less convective activity is expected during this time
period as somewhat drier air works its way eastward into south-
central Arizona. Still...at least slight chance probability of precipitation are warranted each
day across the lower deserts of south- central Arizona as precipitable water levels
remain in the 1.0-1.5 inch range and the vertical shear profile
remains favorable through the column.

Monday through Tuesday...
although forecast confidence levels are rather low this far out...a
increase in available moisture as the flow below 500mb becomes a bit
more southerly is expected to allow precipitation chances to rise a bit
during this time period. Somewhat warmer air aloft as ridging builds
back northward into the region will also allow temperatures to warm to
near...or slightly above normal values.

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Aviation...
south-central Arizona including kphx...kiwa...and ksdl...
drier air has worked into the area for the time being with all storm
activity expected to remain to the south and east of the terminals
through the evening hours. There is still a slight possibility of an
organized storm outflow could transition into Phoenix late this
evening...but confidence is low in this occurring. Increasing
moisture and dynamics tonight could lead to some pop-up shower or ts
activity after midnight. Rather vrb surface wind pattern continues over
the area...with erratic shifts from east to light west to vrb at kphx.
Expect this to continue this evening before typical easterly drainage
winds develop closer towards Thursday early am. Better chances for
showers and thunderstorms will occur from late Thursday afternoon
and Thursday evening with strong gusty winds and brief heavy rainfall
a possibility.

Southeast California/southwest Arizona including kipl and kblh...
high clouds will continue for the western terminals with westerly
winds developing for kipl in the evening and southerly headings
maintained for kblh. Any weather is expected to stay well east of
the terminals across Arizona this evening.

Aviation discussion not updated for amended tafs.

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Fire weather...
Friday through Tuesday...
moisture will increase across the area by the middle of the week
resulting in elevated storm chances from Phoenix eastward on
Thursday. Moisture values will wane a bit into the weekend although
daily storm chances will remain present through early next week.
Minimum relative humidity values will fall below 15 percent Thursday across
southeast California with values above 15 percent expected east of
the lower Colorado River. For Friday into the weekend...latest
guidance more insistent upon a return to deeper moisture...higher
humidity and better rain chances...particularly across south central
Arizona.

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Spotter information statement...
spotters are encouraged to report conditions according to Standard
operating procedures.

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Psr watches/warnings/advisories...
Arizona...none.
California...none.
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Discussion...Kuhlman
previous discussion...Dewey/percha
aviation...Nolte/Kuhlman
fire weather...percha/leins

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