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Forecast Discussion for Phoenix, AZ

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000
FXUS65 KPSR 232336
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
436 PM MST Sat Jan 23 2021

.UPDATE...


&&

.SYNOPSIS...
An active weather pattern featuring a series of storm systems will
bring chances for rain, high elevation snowfall, and much colder
temperatures over the next several days. The first storm system
will move through the area today and tomorrow, followed by a
second colder system Monday and Tuesday. Below normal temperatures
are expected through at least next weekend with possible freezing
conditions Wednesday morning.

&&

.DISCUSSION...
The next several days will likely feature the most active weather
we`ve seen so far this winter. A series of low pressure systems will
move through the area through midweek, resulting in a variety of
conditions including rain, mountain snowfall, river rises, freezing
temperatures, and the potential for isolated thunderstorms, and even
some lower-elevation snow showers.

For the remainder of this afternoon, latest mesoanalysis indicates
an area of instability has developed across northern Maricopa
County. This is likely a result of the low-level moisture left
behind from the last storm system. Latest CAMs appear to have a good
handle on the situation, indicating isolated showers across
south-central Arizona will continue to lift northward through sunset.
Thereafter, vorticity-forced ascent will be on the increase overnight
ahead of a closed low off the California coast. Showers will develop
as far west as southeastern California, though consensus amongst
hi-res CAMs indicates heavier rain is more likely further east
across Arizona. Reflectivity depiction suggests some cells will be
capable of producing heavy rain and even a rumble or two of thunder.
Meanwhile, latest worst-case HREF indicates that the moist
environment will not be conducive for stronger winds. Precipitation
will likely taper off from west to east behind the trough axis Sunday
afternoon.

A break of a few hours is then expected in the wake of the departing
trough. This storm will again leave behind some moisture which will
quickly blossom into new shower activity ahead of the next Pacific
storm system. This low with its impressive negative height
anomalies will be notably colder. Given the stronger wind fields,
there is also slightly higher potential for thunderstorms,
with gusty winds and small hail being the main threats. Even outside
of the thunderstorm activity, the pressure gradient will support
wind gusts as high as 30 mph across portions of Arizona and up
to 50 mph in the southwestern corner of Imperial County. Across the
higher terrain north and east of Phoenix, conditions will continue
to deteriorate and a Winter Storm Watch has been issued for portions
of Maricopa, Pinal and Gila Counties above 4000 feet in elevation
beginning during the late morning Monday. Latest hi-res CAMs
indicate precipitation will likely taper off across the lower deserts
Monday evening, however rain and showers will persist across the
higher terrain through Tuesday.

Latest ECMWF ensemble mean indicates the center of the
mid-tropospheric low will migrate into northern Arizona Tuesday,
which is often favorable for snowfall further south. Critical
thicknesses will be low enough to support snow as low as 2000-2500 ft
in elevation. Wrap-around precipitation Tuesday morning could very
well take the form of snow across north Scottsdale and Cave Creek,
and possibly graupel a bit lower in elevation. Latest BUFR sounding
from the GFS shows an even stronger signal for snow with light
accumulations possible near Wickenburg. Across the higher terrain
north and east of Phoenix, total snow accumulations could amount to 1
to 2 feet above 5000 ft in elevation and perhaps 3 to 5 inches near
Globe around 4000 ft. If there is enough insolation across central
Arizona, hi-res ECMWF suggests afternoon redevelopment of low-topped
cells possibly containing hail.

Considering the uncertainties often inherent with moisture
and low pressure systems, overall ensemble spread is relatively low.
However, what does remain uncertain is exactly how much rain will
be observed in the lower deserts. Latest EMCWF ensemble mean
indicates around around an inch in the Phoenix area, while the WPC
deterministic amount is very close. The prolonged moist southwesterly
flow is conducive for even higher amounts in the foothills, where as
much 1 to 2 inches is possible. This could lead to runoff into Tonto
Creek near Tonto Basin, where increases are forecast.

&&

.PREVIOUS DISCUSSION...
After the showers, our attention will turn towards the cold.
Southeast California may hold onto the low 60s through the week,
avoiding the coldest portion of the approaching air mass, while
highs will drop into the mid to low 50s across south-central AZ as
early as tomorrow and not likely to warm much until late in the
week. The coldest period is likely to be Tuesday night/Wednesday
morning with numerous areas reaching the freezing mark, including
portions of the Valley. Hints of another weather system could mark
a quick return of colder temperatures, rain and higher elevation
snow by next weekend.

&&

.AVIATION...Updated at 2340Z.

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

Scattered showers at times will persist for much of the forecast
period, with a general uptick in coverage anticipated this
evening. Between showers, especially this afternoon, a general
ceiling near 5-6 kft will persist. During shower activity ceilings
will lower to 3-4 kft with some slight reductions in visibility as
well to 3-5 SM. Winds will generally be out of the west this
evening but will shift to southerly tonight with speeds of 5-10
kts common. An isolated thunderstorm will be possible tonight but
is not anticipated.

During the day Sunday ceilings will remain near 3-4 kft with
continued shower activity. The most likely period for showers will
be from around 12-18Z. Isolated thunderstorms are possible but
with low confidence they are not included in the TAF at this time.
Winds on Sunday will remain at 5-10 kts and generally
southwesterly.

Southeast California/Southwest Arizona including KIPL and KBLH:

Showers are increasing at KIPL at present, and will persist for
the next 3-4 hours. At KBLH a few showers are anticipated between
6-9Z this evening. At KIPL BKN coverage near 4-6 kft will develop
during the showers but then gradually diminish afterwards. Winds
will generally be light and out of the west but will increase to
15-25 kts Sunday afternoon. At KBLH light southwesterly flow will
dominate with FEW-SCT coverage developing near 3-4 kft and BKN
coverage near 5-6 kft.

&&

.FIRE WEATHER...
Monday through Friday:
No significant changes to the forecast...Multiple weather systems
will bring chances for precipitation across all fire districts
with the best chances east of the Colorado River. Some lower
desert areas could see 0.25-1.00 inches of rain, perhaps even
slightly more, by Wednesday. Isolated thunderstorms cannot be
ruled out. Snow is likely to fall for elevations above 5000 feet
through the period but lowering snow levels could allow for
accumulation as low as 2500-3000 feet by Monday/Tuesday. Breezy
conditions are also likely to affect the entire area at times.
Temperatures will also be significantly cooler with highs often in
the 50s while lows reach at least the 30s. The increased moisture
levels and cooler temperatures should keep RH values above 40%.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Winter Weather Advisory from 11 PM this evening to 11 PM MST
     Sunday for AZZ557-558-563.

     Winter Storm Watch from Monday morning through Tuesday afternoon
     for AZZ557-558-562-563.

CA...Wind Advisory from 10 AM to 10 PM PST Monday for CAZ563-566-567.

     Wind Advisory from 10 AM Monday to 4 AM PST Tuesday for CAZ562.

&&

$$

DISCUSSION...Hirsch
AVIATION...Hodges
FIRE WEATHER...AD

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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