Area Forecast Discussion
Issued by NWS Phoenix, AZ

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703
FXUS65 KPSR 201535
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
836 AM MST Fri Jan 20 2017

.SYNOPSIS...
A series of Pacific storm systems will move into the region over the
next 7 days. The strongest and wettest of them will affect the
region today through early Saturday. Snow accumulation will be above
5500 feet, with a Winter Storm Warning posted for the mountains east
of Phoenix. A break in the weather is expected Saturday night and
early Sunday, however another Pacific storm is expected in the
region later Sunday through next Tuesday. Dry but cool weather will
settle over the region for the middle of next week.

&&

.DISCUSSION...

A multitude of Pacific weather disturbances continue to race through
the region this morning. Last nights system produced some generous
rain across the south central deserts and mountains. Under a 0.10 of
an inch generally fell throughout the Phoenix area and surrounding
deserts, with even more in the upslope areas north through east of
Phoenix. Some higher elevations in southern Gila County, our area of
responsibility east of Phoenix, had upwards of 0.33 inches, with
around an inch in the northern parts of Gila County. Snow levels
this morning were around 5800 feet based on the Hill Top remote Raws
site (5700 feet) of 34 deg F at 8 am. Spotty light rain was also
observed in our southeast CA and Colorado River zones, however rain-
shadow effects from the coastal mtns will continue to diminish the
rain potential there, even with the next stronger system for late
this afternoon and evening.

Forecasts look on track for today, with much of the forecast area
under cloudy skies with occasional light rain, i.e. inbetween storm
systems. As stated above, the strongest weather system in the series
will race through the area later this afternoon and mainly tonight.

Morning weather balloon soundings showed another batch of serious
low level moisture has moved onto the southern CA coast, and ready
to surge upslope into south central AZ for periods of heavy
rain/snow tonight, especially from Phoenix north through east. And
with 850 mb southwest winds approaching nearly 60 mph, some southern
Gila County areas could realize between 1.50-2.00 inches of rain,
with heavy snow in the mountains. Snow levels may be problematic due
to abundant low level moisture creating artificially high snow
levels, however the higher elevations above 5500 feet should see
heavy accumulations. Do the math, 10 to 1 ratio of wet snow to rain
should give you an idea of the snow potential. A Winter Storm
Warning for southern Gila County continues through Saturday
afternoon.

Taking about strong low level winds tonight, falling rain and or
thunderstorms could tap this momentum for very gusty surface winds.
Therefore a wind advisory continues tonight for the south central
AZ deserts continues.

So far forecasts look ok. No short term updates needed.


.PREVIOUS DISCUSSION...550 AM MST...

A very wet and unsettled weather pattern will continue to affect
the desert southwest for the rest of the week and into early next
week as a series of wet Pacific disturbances race inland across
southeast CA and Arizona and drop quite a bit of precipitation. By
next Tuesday, rainfall totals in the greater Phoenix area may well
exceed one inch, with amounts around 3 inches possible in the higher
terrain areas northeast/east of Phoenix. The most significant storm
to affect the area will move in from the west today and then push
across Arizona tonight and into Saturday producing well over one
half in over the central AZ deserts and over 1.5 inches across
higher terrain areas east of Phoenix.

At 230 am this morning, the first in the series of wet Pacific
storms was already exiting off to the east, however radar indicated
lingering scattered light showers from Phoenix eastward. Skies over
the western deserts including far SE California were partly cloudy
and there was little or no rainfall detected in the area. As the day
progresses, the most powerful storm in the series will move inland
over the CA coast, leading to scattered to numerous showers across
the deserts of SE CA and southern AZ. This system will be strongly
negative tilt with pronounced difluence aloft. By early evening most
of the area will be under the favorable front left quadrant of an
upper level jet with winds at least 150kt at the core. Operational
guidance from the GFS and ECMWF as well as various ensemble
guidances such as GEFS, SREF and NAEFS have been very consistent in
calling for this approaching strong and wet storm. Most of the
ensemble guidance calls for rain chances around 100 percent tonight
from the lower CO river valley eastward; the ESRL situational
awareness table has most of Arizona with a 100 percent chance of
getting at least 0.04 inches tonight. Our forecast rain chances will
be 100 percent from the lower CO river valley east tonight with
scattered to numerous showers expected in SE CA. The rain chances
will steadily diminish from the west later tonight and during the
day Saturday and by Saturday afternoon the rain chances will mostly
be confined to central Phoenix eastward with clearing skies and
single digit POPs expected west of Maricopa county.

In addition to the rain, we can expect heavy snow mainly tonight
across the higher elevations of southern Gila County; snow levels
will fall to near 5500 feet tonight and over 1 foot of snow is
possible this evening into mid morning on Saturday above 5500 feet.
We will keep the Winter Storm Warning in effect above 5500 feet
until 11 am on Saturday. Actually, snow levels may be slightly
higher than previously thought but given how wet we should be and
how dynamic this system is, heavy snow is still very much expected
across the higher terrain areas east of Phoenix. We also can expect
strong and gusty winds this afternoon and overnight tonight as the
front sweeps east across the western deserts today and across south
central AZ tonight. Gusts over 40 mph are likely in the central
deserts associated with the frontal passage and a Wind Advisory
remains in effect from 8 pm this evening thru 8 am Saturday for most
of the central AZ deserts. It is also possible that for a few hours
this afternoon and early evening we may see southerly gusts to near
40 mph over portions of Joshua Tree National Park but due to
expected borderline conditions and lower confidence we will opt to
leave them out of the wind advisory for now.

A weak and flat short wave ridge will move into Arizona later
tonight and into the day on Sunday behind the exiting strong storm
and by early Sunday we can expect just a few lingering light showers
over southern Gila county as well as over Joshua Tree NP. As the day
progresses on Sunday, the ridge flattens and allows copious amounts
of clouds/moisture to spread through the ridge from the southwest;
the GFS/GEFS calls for a prominent atmospheric river to move into
far SE California later Sunday and Sunday night with over 800 units
of IVT. As such, we will see scattered showers develop across the
western deserts during the day Sunday although rain amounts should
not be excessive. Showers will continue to increase from the west
Sunday night and we can expect the last in the series of wet Pacific
storms to roll through Monday into Tuesday with the best chances for
significant rain and higher elevation snow across south-central AZ.
We are looking at rain chances from Phoenix east from 60 to over 80
percent Monday into Monday evening, with scattered showers west of
Maricopa county. This will be a colder storm than the one expected
tonight with the snow level falling to around 4000 feet and as such
we may see some significant snow down to at least 5000 feet or lower
before it is all over.

The final disturbance is expected to push east across Arizona
Tuesday night into Wednesday but it will steadily weaken as it moves
through with drier more subsidence west/northwest flow overspreading
the area and as such by Wednesday there will be just a few light
showers left over southern Gila county with skies becoming mostly
sunny over the lower AZ and SE CA deserts. Continued mostly sunny
skies are expected Thursday as dry subsident northwest flow
continues to spread across the area.

This wet and unsettled week will produce significantly below normal
temperatures pretty much every day. From today through next
Thursday, high temperatures across the south central deserts will
typically range from the upper 50s to around 60s with somewhat
warmer highs into the mid 60s possible over the western AZ and far
SE CA deserts. The continued clouds, showers and overall cooler air
mass lingering across the desert southwest will keep these temps
well below normal; the normal high in Phoenix over the next week
will be 68 degrees.

&&

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

Light showers will continue across the eastern portions of the
Phoenix metro through mid-morning with CIGs/VSBYs continuing periodic
dips down around 5kft and 5-6SM respectively. Morning winds will hold
east to southeasterly headings with winds 8kts or less. Midday
conditions should see CIGs AOA 6kft along with a break in shower
activity before the next and much stronger Pacific weather system
begins to move into the area. Winds will begin to turn southerly
ahead of an approaching surface front with showers increasing in
coverage through the afternoon.

Surface cold front currently timed into the Phoenix area near
21/05z, producing a burst/period of heavy rainfall, low CIGs and
VSBYs, and strong winds of 20-30kts with gusts as high as 40 kts.
Cannot rule out some embedded thunder development along with the
initial frontal band, but timing TS/CB into the TAF may have to wait
until the rain/storm line forms and can be timed into the terminals.
Burst of activity right along the front is expected, with rain and
low CIGs/VSBY/BR/FG continuing into Saturday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

Solid mid and high level clouds will continue to stream in from the
west, with elevated west and southwesterly winds persisting for KIPL
and KBLH into the morning. Next Pacific weather system will sweep a
strong front through the area in the afternoon/evening, increasing
shower coverage as well as surface winds with gusts approaching 35
kts. Even stronger winds to occur over area ridgetops with local
mountain waves possible near KIPL by the evening. Window for showers
and associated 4-6kft CIGs will run the overnight hours before waning
in coverage towards Saturday morning.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday: Another Pacific low pressure system will
move into the districts Sunday afternoon, producing additional
rainfall into Monday along with breezy to windy conditions. High
pressure will gradually rebuild across the area Tuesday and
Wednesday, resulting in a drying trend though temperatures will
remain below normal. Fire danger will remain low through the period
due to the persistent and anomalously moist conditions.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter reports of rain and snow will likely be needed later this
week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory from 8 PM this evening to 8 AM MST Saturday for
     AZZ022-023-026>028.

     Winter Storm Warning until 11 AM MST Saturday for AZZ024.

CA...None.
&&

$$

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DISCUSSION...Vasquez/CB
AVIATION...Nolte
FIRE WEATHER...Nolte/Hirsch



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