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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 201309

Area Forecast Discussion
National Weather Service Phoenix AZ
609 AM MST Mon Aug 20 2018

Updated 12Z aviation and fire weather discussions.


Above normal temperatures are expected again today across the
region. Highs over the lower deserts will range between 105 and
110 degrees with the warmest readings over southeast California.
Substantial moisture returns to the region starting tonight with
shower and thunderstorm chances returning on Tuesday. Storm
chances remain across south central Arizona through the rest of
the week, but will mostly be focused across the higher terrain
north and east of Phoenix.


The upper level high center is currently situated over central
Arizona, but will drift southeast over the next day or two.
Last evening, several thunderstorm complexes developed across
northern Mexico just south of the high. As the high shifts
southeast, our flow will quickly turn southeasterly with moisture
levels beginning to increase from the current 1.00-1.40" PWATS.
Models continue to show this afternoon and evening will be very
active across northern Mexico and possibly just south of Tucson
this evening. An inverted trough is also shown moving northward
out of Mexico into southern Arizona sometime after midnight. A
strong moisture surge will accompany this inverted trough, partly
coming from the convection over northern Mexico and also from a
Gulf surge.

The forecast scenario for tonight and Tuesday is rather complex
with multiple factors coming into play. Current thinking is we
may see showers and thunderstorms overnight tonight across south-
central Arizona forming along the northern edge of the inverted
trough and/or from gravity waves produced from convection to the
south. Models are starting to show this possibility more in the
latest runs, but there is still quite a bit of discrepancy among
models. Even if storms don`t develop tonight over southern
Arizona, the moisture surge will be significant with PWATS rising
to between 1.75-2.00" by mid morning Tuesday. PoPs have been
increased some for tonight, but may end up having to be raised
even more if the 12Z runs show more model consensus.

The rest of Tuesday is even more uncertain as we could either see
a very active storm day with flooding concerns across south-
central and eastern Arizona, or we may end up mostly cloudy
lacking sufficient instability for widespread storm activity. At
this point models are generally showing the latter will happen
due to too much moisture and overall fairly cloudy conditions. If
tonight ends up being fairly quiet and cloud cover is less during
the day Tuesday, Tuesday afternoon and evening should be very
active with heavy rainfall producing storms mainly from Maricopa
County eastward. PoPs have been raised on Tuesday, but given the
uncertainty, chances mostly fall between 30-40%. Winds aloft are
initially strong Tuesday morning with 20-30kts out of the
south/southeast at 500mb and a 50kt upper level jet across
western Arizona providing ample divergence aloft over central
Arizona. We can`t rule out a few strong wind producing storms
early Tuesday morning, but this is not a favorable time of day for
strong to severe storms. Winds aloft gradually weaken during the
day Tuesday while the steering flow veers out of the
south/southwest. This should lead to mostly heavy rainfall
producing storms later in the day Tuesday with localized strong
gusty winds still possible within any microbursts. High
temperatures on Tuesday will drop several degrees from today`s
readings with temperatures topping out around 100 degrees in the
Phoenix area to around 105 across the western deserts.

The high center will continue shifting eastward on Wednesday with
our flow aloft turning solely out of the southwest. This will
bring drier air into the region, but we should still have
sufficient moisture for afternoon and evening storms mostly across
the high terrain north and east of Phoenix. Similar conditions are
seen for Thursday and Friday, but storm chances should diminish
each day with the lower deserts likely staying dry. Temperatures
will remain fairly steady through the rest of the week as 500mb
heights lower from 594dm on Wednesday to 588dm on Saturday. These
lower heights are due to a developing Pacific low well off the
California coast. Though it is around a week away, both the GFS
and European show a more significant Pacific trough digging
southeastward into the Pacific Northwest early next week. This
will definitely be something to watch for the middle part of next


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

Light and variable winds this morning with a brief period of
easterly winds this morning will become westerly at 10 knots by
the afternoon with gusts generally remaining below 15 knots.
Moisture will increase today to allow at least isolated to
scattered thunderstorms to develop over southern Arizona this
afternoon. These storms may be capable of pushing southerly
outflows (and crosswinds) into the Phoenix sites later tonight
after 03Z with the latest HRRR runs even showing a line of
thunderstorms pushing through. However, most hi-res and global
models show this convection dissipating well south of Phoenix
which seems most reasonable at this time.

The forecast for the overnight hours is rather uncertain, so for
now we have vicinity showers and southwest winds with broken to
overcast clouds above 10 kft after 06Z. However, there are many
possible scenarios overnight depending on how convection and
moisture return evolves this afternoon and evening, including
prevailing showers and thunderstorms for several hours. We do have
better confidence that showers and thunderstorms will occur at
some point late in the overnight hours or just after sunrise, so
we have added in a PROB30 group after 12Z. However, this may need
to be moved up or removed depending on how storms evolve tonight.

Southeast California/Southwest Arizona including KIPL and KBLH:

Southerly and southeasterly flow will predominate at KBLH and
KIPL, respectively. Winds this afternoon will be sustained to
15-20 knots with gusts to 25 knots possible at KBLH. A few hi-res
models show convection that develops over northern Mexico may
move into the vicinity of KIPL, but the vast majority of models
keep this activity out so the forecast remains thunderstorm free
for now. Aside from that, no aviation weather impacts expected.

Aviation Discussion not updated for amended TAFs.



Wednesday through Sunday:

Ample moisture during the middle of this week will keep chances
of thunderstorms elevated over the region, in addition to keeping
minimum relative humidity values above 20 percent over the lower
deserts of south central Arizona. Significant drying is expected
over southeast California and possibly southwest Arizona for
Thursday and beyond, with a reduction in convective activity
spreading further east later this week into the weekend aside
from the higher terrain north and east of Phoenix. Wind speeds
will be typical for mid-summer with the usual afternoon breezes
aside from stronger winds with thunderstorm outflows. Temperatures
will remain near to slightly below normal through the period.


Spotters should follow standard reporting procedures.




FIRE WEATHER...Hopper/Percha

Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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