Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 230400

Area Forecast Discussion
National Weather Service Phoenix AZ
900 PM MST SAT OCT 22 2016

Beginning on Sunday, a large Pacific low pressure system will move
into the western states. This low pressure system will draw a
considerable amount of subtropical moisture into the region with
threat of showers and thunderstorms starting Sunday afternoon and
continuing through Monday night. The best chance of showers will
occur Monday over south central Arizona, with afternoon
thunderstorms possible. Drier and warmer conditions will return over
the middle of next week.



Note: A change in the weather pattern is still forecast later Sunday
and Monday. However the most abrupt weather change and best chance
of shwrs/tstms is forecast Monday. The highest precip probabilities
are Monday...particularly over portions of southwest and south
central AZ including Phoenix.

The very large Pacific low pressure system responsible for the
weather change noted above is still organizing and developing off
the Pacific northwest coast this evening. And, no noticeable change
in upper level southwest winds aloft over AZ were noted in the past
12 hours. Therefore dry southwest flow aloft will continue over our
region tonight and early Sunday until we get a noticeable increase
in mid/high level cloudiness over southeast CA and southwest AZ
later Sunday afternoon. Again, since low levels will remain mostly
dry over our forecast area, southeast CA to south central AZ, only a
slight chance of light showers and or sprinkles are expected with
the most dense clouds in southeast CA later Sunday and Sunday

As full blown cyclogenesis develops with the large Pacific low
pressure system along the Pacific northwest coast Sunday night,
jetstream winds increase over AZ, and with a few embedded
shortwaves, the slight chance of showers will spread east into south
central AZ by late Sunday night. However the exciting weather period
will develop Monday.

Noticeable this Saturday evening, precip water (PW) imagery showed a
significant increase in PW over portions of the central and northern
Gulf of CA. The Guaymas Mexico weather balloon sounding has been
missing for a few days so any indication of estimated atmospheric
moisture levels will come from satellites. At 630 pm mst, satellites
measured a jump in precip water near Guaymas Mexico from 1 inch
yesterday evening, to a broad area of 1.50 inches.

Therefore the weather scenario for Monday is still the same, i.e. as
the Pacific low pressure system to the north intensifies, increasing
low level southerly winds and moisture will be directed into
portions of southwest and south central AZ.  Low level moisture
levels in southwest and south central AZ will likely be in
monsoonish category, significant for this time of year. With an
approaching shortwave, and with increasing upper level dynamics and
vertical wind shear, a good chance of showers and thunderstorms are
forecast Monday. Some thunderstorms could become organized and
marginally severe with a threat of strong surface winds and hail.
SPC in their day 3 outlook puts parts of southwest and south central
AZ in a marginally severe thunderstorm category.

We made a few minor changes on areal coverage for the slight chance
of showers for later Sunday and Sunday evening, since moist upper
level dynamics and deep moisture will arrive on Monday. Otherwise
current forecasts look ok for now.


Tonight through Monday...

A turn towards a period of more active weather continues to be
forecast by the global models, as the strong upper-level ridging
that has brought an extended period of dry and very warm weather to
our cwa shifts off the the east as a deep low-wave trof moves toward
the west coast. Although most of the cold air and energy associated
with this trof is expected to pass well to our north, deep
southwesterly flow ahead of the upper trof is expected to pull
increasing amounts of tropical moisture into the region over the
next few days. GFS model output continues to show pwats rising into
the 1.00-1.50 inch range on Sunday and Monday, which is 2-3 standard
deviations above climo, and well above the 90th percentile compared
to climo values. It is likely that a weak tropical disturbance that
is now located well to the southwest of the tip of Baja is/will be
contributing to the available moisture. It still looks like that
initially the atmosphere will moisten from the top down tonight and
Sunday, with increasing chances of sprinkles or light showers,
especially from Phoenix west. Given the lack of dynamics/extensive
mid/high cloud cover that is likely to develop, chances for
thunderstorms look to be low, even though a few hundred joules of
CAPE does show up on Sunday afternoon. Still, an isolated storm
cannot be ruled out, especially over SE CA on Sunday afternoon.

Monday appears that it will be a more active day, as the upper trof
moves close enough to our region to increase the winds through the
mid-levels of the atmosphere and to provide some cooling aloft. The
latest GFS meteogram is showing that perhaps a more organized line
of showers and thunderstorms will develop during the day on Monday
across SW AZ, then move eastward across south-central AZ during the
afternoon/evening hours on Monday. The combination of 20-30 knots of
bulk shear and a few hundred joules of CAPE will likely allow at
least some thunderstorm activity to develop, with a couple of storms
perhaps approaching severe limits. SPC current does have a portion
of southern AZ in a MRGL risk area for Monday. As far as rainfall is
concerned, most locations will likely see 1/4 inch or less, due to
the spotty/scattered nature of the showers/storms, but stronger
storms could produce greater amounts at a few locations. The
combination of showers/clouds/somewhat cooler air aloft should push
temperatures downward, with highs down in the 80s likely on Monday
at many lower desert locations.

Monday night through Friday...

Scattered to numerous showers and along with a few embedded storms
will persist into the evening Monday but as the short wave quickly
lifts off to the northeast, a clearing and drying trend will take
place from the west and by Tuesday morning there will be just a
lingering slight chance of showers over the higher terrain east of
Phoenix with mostly sunny skies developing over the central and
western deserts. High pressure will build over the area from the
west later Tuesday and continue through the day on Wednesday for dry
conditions along with warming temperatures. Highs over the lower
deserts Wednesday precip will climb back into the low to mid 90s
with a high of 92 forecast for Phoenix.

For Thursday into Friday, operational models really diverge and the
GEFS spaghetti ensemble members area all over the map showing a very
chaotic pattern. It looks to be a somewhat unsettled pattern as the
upper ridge breaks down a bit as short wave energy moves into the
ridge and then lifts off towards the northeast. The ECMWF is MUCH
drier and more or less gives the lower deserts a brush by whereas
the GFS brings a VERY wet and dynamic system into the state. NAEFS
POPs align closely to the ECMWF forecast and will be followed; we
will add a slight chance of showers mainly to the deserts west of
Phoenix - especially over far southeast California - with POPs
generally 15 percent or lower.


South-Central Arizona including KPHX, KIWA, and KSDL:
Dry southwest winds aloft will continue through Sunday afternoon.
Mostly clear skies with south to southeast winds under 7 knots are
forecast through 22z Sunday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Dry southwest winds aloft will continue through Sunday afternoon.
Light south winds under 7 knots are forecast, with increasing clouds
aoa 14 thsd agl and becoming broken after 18z Sunday.

Aviation discussion not updated for amended TAFs.


.FIRE WEATHER... Tuesday through Saturday... Only a lingering slight
chance of showers and storms are possible Tuesday over the higher
terrain of south-central Arizona.
Temperatures will be cooler but because the brunt of the system
stays west and norprecip th, there won`t be a lot of cold air moving
High pressure strengthens again Wednesday into Thursday for warmer
temperatures and lower humidities but overnight recovery remains
good. Another Pacific system brings a slight chance of showers again
on Friday to southeast California and southwest AZ. Winds throughout
the forecast period will be light overall except on Monday if
thunderstorms develop which will create the potential for strong
erratic winds in localized areas. &&

Spotters should follow standard reporting procedures.




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