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fxus65 kpsr 172043 
afdpsr

Area forecast discussion
National Weather Service Phoenix Arizona
143 PM MST sun Nov 17 2019

Synopsis...
warm and dry conditions will continue through Monday. Then, a
pattern change takes place as a couple of storm systems affect the
region Tuesday through Friday. There will be high chances for rain
during that time frame, especially east of the lower Colorado
River valley. Temperatures will trend much cooler, with highs
struggling to get out of the 60s.



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Discussion...
a closed-low off the coast of northern Baja California is now
well-defined, with southerly flow aloft streaming high cloudiness
northward towards eastern Arizona. A significant increase in
moisture has occurred along the eastern periphery of the low, with
GOES-16 precipitable water imagery sampling over 1 inch into
northern Baja California. In the meantime, upper ridging across
the Great Basin will maintain warm temperatures and dry conditions into
Monday, with high temperatures forecast to creep close to the
upper 80s.

A significant change to the large-scale pattern is forecast to
occur Tuesday as a compact but strong shortwave trough digs along
the Pacific coast. This will promote the transition of the closed
upper-low into an open wave while accelerating northeastward, and
will also draw in Richer moisture from the south. Isolated
showers and thunderstorms are forecast to develop Tuesday
afternoon, primarily from near the lower Colorado River valley
eastward. Enough diurnal destabilization amidst increasing
moisture should result in a few hundred j/kg of cape Tuesday
afternoon. Ensemble guidance probabilities for precipitation
increase markedly early Tuesday evening into Wednesday morning as
the aforementioned open wave moves northeastward, resulting in
more widespread precipitation focused across south-central
Arizona. May see periods of heavier rainfall in areas enhanced by
orographic ascent, especially north and east of Phoenix and into
parts of La Paz County. There may be a brief lull in activity at
some point Wednesday morning in the wake of the lead system as the
colder Pacific northwest trough deepens into a closed low over
California, and stronger forcing for ascent overspreads the
region. This should promote a second round of widespread activity
Wednesday afternoon and evening across much of Arizona, but
especially the northern half of the state. The ensemble spread
continues to tighten with the evolution of the low as it minors
out with eastward progression, though lingering moisture and
focused areas of forcing for ascent will maintain chances of
showers into Friday. Much cooler temperatures will also accompany
the increasing precipitation and arrival of the trough, with high
temperatures generally struggling to get out of the 60s across the
deserts Wednesday through Sunday.

Overall, storm-total precipitation amounts generally remain on
track, although the eps solutions have decreased slightly from
previous runs with a slightly more progressive evolution of the
upper-low becoming more likely. The official forecast for quantitative precipitation forecast
amounts, which was initialized from wpc, generally ranges between
1-2 inches across most of south-central Arizona and La Paz County
deserts. Isolated higher totals are likely. The highest rainfall
amounts will likely be found in typical terrain-favored areas
north and east of Phoenix, with some parts of southern Gila County
potentially exceeding 4 inches of rain. Given the likelihood of a
long-duration rain event with periods of intermittent heavy
rates, a Flash Flood Watch will probably be needed eventually for
at least parts of south-central Arizona for the Tuesday-Thursday
time period.

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Aviation...updated at 1725z.

South-central Arizona including kphx, kiwa, ksdl, and kdvt:
no significant aviation concerns through Monday afternoon. Trends
in wind directions should be nearly identical to Saturday with
light westerly to northerly winds less than 8kt developing
mid/late afternoon before rapidly returning to an easterly
component mid evening. Some increase in high clouds is expected
mid-day Monday, but should remain at or above 20kft.

Southeast California/southwest Arizona including kipl and kblh:
no major aviation concerns through late Monday morning under clear
skies. Winds will generally favor some variety of a northerly
component at kblh with some increase in speeds this afternoon.
Kipl will mostly favor a westerly direction though a shift towards
a more northerly direction is possible during the afternoon hours.
High clouds will increase Monday morning, especially closer to
kipl, but should remain at or above 20kft.

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Fire weather...
Tuesday through saturday:
a change to cool and relatively wet conditions is expected
beginning Tuesday. A prolonged period of rainfall potential
(relative to desert climatology) is expected as a couple of
weather systems affect the forecast area. There won't be non-stop
widespread rain during this whole period. But, when added up, a
lot of places (especially east of the lower Colorado River valley)
are expected to receive significant rainfall. By and large, it
will be beneficial rainfall but it is anticipated that some areas
will receive excessive amounts which will cause runoff. This is
most likely over the higher terrain and foothill areas. High
temperatures will be well below average beginning Wednesday. Snow
levels are expected to remain above 5000 ft and quite possibly
above 6000 ft.

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Spotter information statement...
spotters should follow Standard reporting procedures.

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Psr watches/warnings/advisories...
Arizona...none.
California...none.
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