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000
FXUS65 KPSR 191829 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
1130 AM MST Sun Feb 19 2017

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
A strong Pacific weather system will quickly move northeastward
across the Four Corners today. Rainfall chances will continue
through this afternoon and evening across south-central Arizona,
before decreasing overnight as the storm system shifts into the
Great Plains. Much drier conditions are expected by early next
week as above-normal temperatures return to the area Tuesday and
Wednesday amidst a significant warming trend. A mainly dry cold
front will move through the region Thursday, with temperatures
returning to near-normal. A more potent storm system may affect
the region on Sunday.

&&

.DISCUSSION...
As of mid morning, the deep Pacific low is centered mostly over
northeast Arizona with a trailing trough axis stretched from north
to south bisecting Arizona. Still seeing quite a bit of
persistent shower activity on the eastern side of the trough axis
situated mainly from Phoenix eastward. These showers have shown
eastward progression over the past several hours and expect the
bulk of this rain to continue to slide to the east through early
afternoon. On the western side of the trough axis, will continue
to see isolated showers in response to very cold air aloft. This
isolated activity will gradually shift eastward through south-
central Arizona through mid afternoon with much of the shower
activity over eastern Arizona ending by early evening.

Have made some minor adjustments to the late morning and afternoon
PoPs, increasing values to the east of Phoenix and decreasing to
the west. Localized dense fog is still a problem across central
Imperial County, so have extended the Dense Fog Advisory through
10 am PST. Didn`t see any signs of dense fog, or much in the way
of any fog at all over Riverside County, so the advisory there was
allowed to expire at 8 am PST.

&&

.PREVIOUS DISCUSSION...
Primary vort-max associated with the large upper trough over the
Southwest was located over west-central Arizona as of 09Z, and is
beginning to accelerate northeastward. A band of moist ascent
attendant to this feature is continuing to support scattered to
numerous showers across south-central Arizona. Localized flooding
remains possible through about mid-morning across
northern/northeastern portions of Maricopa and southern Gila
counties, before precipitation is forecast to become more spotty
in nature. However, rainfall chances are still expected to persist
through the late afternoon across the Phoenix area, and into late
this evening over southern Gila County.

Today should mark the coolest day this week, before a substantial
warming trend begins on Monday and especially Tuesday, as midlevel
ridging builds over the Southwest. High temperatures are expected
to be just shy of 80 degrees across many lower desert locations on
both Tuesday and Wednesday. A significant, but mostly dry, cold
front will move through the area on Thursday, bringing high
temperatures back into the upper 60s to lower 70s for the
remainder of the week. This will be associated with a fast moving
storm system that will glance central Arizona.

Model guidance continues to advertise a more significant upper
trough developing next weekend near the Pacific coast, but with
significant timing/evolution differences still remaining. The
most probable solution at this point reflects the storm system
affecting southeast California as early as late Saturday evening,
and the remainder of western/central Arizona on Sunday. This storm
system does not appear to be as moisture-rich as more recent
events, but still could see an uptick in precipitation chances for
Sunday/Monday. However, given the uncertainty, only slight chance
PoPs are currently forecast. Below-normal temperatures are also
possible by Sunday, with high temperatures forecast to be in the
mid-upper 60s.

&&

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

Upper low slowly exiting to the east with scattered to numerous
light showers persisting across the central deserts. Expect light
rain/rain showers through early afternoon before dissipating and as
such will keep -RA in the TAFS thru 21z, after that may see VCSH
into the late afternoon as cold air aloft will lead to isolated
instability showers. Otherwise main concern will be low ceilings;
much guidance suggests CIGS in the 2-3k ballpark thru at least 21z
with a few lower CIGS down to around 1k feet. From mid aftn thru
early evening should still see CIGS 3-5k feet widespread. Do not
expect significant visibility issues this afternoon, maybe 5-6SM at
times in mist/drizzle but as showers decrease VIS values should
generally stay above 6SM. Any ceilings after 03z should genly be
above 6k feet. Winds not an issue, following mostly diurnal
tendencies aob 12kt.

Southeast California/Southwest Arizona including KIPL and KBLH:

Fog in the Imperial Valley should be over by 19z and no longer
affect the KIPL TAF. Otherwise expect few-sct lower cloud decks in
the 3-5k foot ballpark through the afternoon. May see some increase
in high clouds overnight tonight into Monday morning. At KBLH should
see few-sct low decks this afternoon mostly above 3k feet with
additional decks near 8k feet, clouds becoming few at best by 03z.
Again, may see SCT-BKN high cloud decks develop tonight into Monday
morning. Winds will be rather light, favoring the south at KBLH and
the west at KIPL with speeds mostly below 12kt, over the next 24
hours.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday...Dry conditions are expected for most
areas through Saturday. Above normal temperatures can be expected
Tuesday and Wednesday. A low pressure system passing mainly to
the north Wednesday night and Thursday will lead to cooler
temperatures and locally breezy conditions. Rain chances with the
midweek system will be limited to the higher terrain of south-
central AZ. Anticipate slight warming Friday and Saturday.
Minimum humidities will trend downward through the workweek (especially
on Thursday) with most lower elevations in the 15-20% range by
Friday. Overnight recovery though will remain good.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Dense Fog Advisory until 10 AM PST this morning for CAZ033.

&&

$$

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DISCUSSION...Kuhlman
PREVIOUS DISCUSSION...Rogers
AVIATION...CB
FIRE WEATHER...AJ

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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