Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
533 PM MST Sun Jan 1 2023

.UPDATE...Updated Aviation discussion.

&&

.SYNOPSIS...
A potent storm system is currently moving across the
area, bringing with it widespread rainfall along with embedded
thunderstorms as well as higher elevation snowfall. Conditions
across the lower deserts will improve by this evening while across
the higher terrain, generally above 5000 feet, periods of snow will
continue through Monday morning. Another weaker system will move
through on Tuesday, delivering additional precipitation but with
lower amounts. This will be followed by a drying trend and a gradual
warming of temperatures to near normal for the latter half of the
week.

&&

.DISCUSSION...
Water vapor satellite imagery shows a very potent upper-level trough
centered across the Great Basin Region. This trough is dragging a
strong cold front from west to east across the CWA. The front has
cleared Southeast California, with downsloping westerly winds now
becoming the main issue with gusts over 40-50 mph reported across
portions of the Imperial Valley. These strong wind gusts will
continue into the early evening hours before gradually subsiding
through the overnight.

Meanwhile, out ahead of the front, a band of showers with embedded
thunderstorms is progressing eastward through southwest Arizona
approaching south-central Arizona. Latest Hi-res model guidance
continues to indicate that this band should strengthen as it moves
through south-central Arizona through the afternoon. Moderate to
brief heavy rain as well as strong wind gusts in excess of 40 mph
will be likely. HREF continues to depict elevated probabilities of
50-70% of winds exceeding 35 mph across most of south-central
Arizona as the frontal band moves across. There is even a marginal
risk (5% chance) of some of these wind gusts reaching severe
criteria (>58 mph) due to the strong low and mid-level shear and
marginal instability in place. Additional showers, although more
scattered in nature, will likely develop behind the cold front
through this evening associated with the cold-core of the upper-
level trough. Some of these showers may contain embedded lightning
strikes as well as small hail.

In terms of rainfall amounts, given the progressive nature of the
front and the short residence time of the rainfall activity, average
rainfall amounts will generally range from 0.25-0.6" across the
south-central Arizona deserts, including Phoenix, to up to 1-1.5"
across the higher terrain north and east of Phoenix. Some minor
flooding cannot be ruled out in susceptible areas such as creeks
and washes.

Across the higher terrain, as the cold front marches through later
this afternoon and into this evening and colder air filters in
behind the front, snow levels will fall below 6000 feet. This will
cause the rain to change over to snow with activity persisting into
Monday morning. Snow accumulations of 3-6" are likely for elevations
above 5500 feet with up to a couple of inches possible down to
around 5000 feet.

More tranquil weather conditions are expected during the day Monday,
as the upper-level trough exits to the northeast with a period of
drying and lesser cloud cover. Temperatures will be cooler with
highs only expected to top out in the middle to upper 50s across the
lower deserts of south-central Arizona to lower 60s across the
western deserts. However, as we head into Monday night and through
Tuesday another weak shortwave trough will pass through Arizona
delivering more precipitation, mainly across the northern half of
Arizona. This feature will result in significantly less
precipitation compared to the system affecting the region today.
QPF amounts with this system generally range up to 0.10" across
the Phoenix metro to upwards of 0.25" across the higher terrain
north and east of Phoenix. Across elevations higher than 5000
feet, conditions will be cold enough for precipitation to fall in
the form of snow with only up to an inch or two of total
accumulation expected.

For the latter half of the week, ensembles are in strong agreement
of an upper-level trough amplifying just off the Pacific
northwest coast, resulting in an amplification of a ridge
downstream over our region. As a result, tranquil weather is
expected Wednesday and through at least the first part of Thursday
with overall dry conditions and temperatures warming back to near
normal. By Thursday evening into Friday, guidance is favoring a
weak shortwave tracking into region. This will favor increasing
clouds and the potential for some precipitation, especially across
portions of Riverside County in the vicinity of Joshua Tree
National Park in southeast California, where NBM PoPs have
increased to 40-60%. However, across the rest of the area, PoPs
are below 20% as it appears that the strongest ascent will remain
to the north. Ensembles are indicating yet another shortwave
potentially moving into the southwest late next weekend, however,
there are significant differences in the positioning and thus
there is high uncertainty on whether or not there will be
precipitation associated with the shortwave. Temperatures through
next weekend will continue to average near normal.

&&

.AVIATION...Updated at 0030Z.

South-Central Arizona including KPHX, KIWA, KSDL, KDVT:
A line of showers will continue moving eastward through the
Valley this early evening. In its wake, low-level moisture will
linger, resulting in persistent ceilings around 5000 ft through
Monday morning. There is also a 30 percent chance ceilings could
drop as low as 2500 ft around 12z.

The typical diurnal wind cycle has been disrupted by the line of
showers and consequently winds may be variable at times tonight,
along with periods of southerly flow. Winds are expected to
gradually acquire a light westerly component Monday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
The windy conditions will subside overnight, though occasional
breeziness is anticipated with gusts to 30 kt, particularly at
KIPL. Further east near KBLH, winds will be somewhat lighter
however ceilings will be a concern early in the forecast period.
Stratocu with bases around 6-8k ft will gradually dissipate
overnight.

&&

.FIRE WEATHER...
The series of Pacific storm systems impacting the region will
continue through at least the early part of the week resulting in
cooler temperatures and periodic precipitation mainly through
Tuesday. A strong weather system will impact the region today
bringing additional widespread wetting rainfall and isolated
thunderstorms with strong gusty winds, followed by accumulating
snow possible in higher elevations of eastern districts this
evening into Monday. Yet another weather system is then likely
for Monday night and Tuesday, but with overall lighter
precipitation expected. High pressure is then likely to take over
across at least the eastern districts, while only low end chances
for wetting rains remain across southeast California for the rest
of the week. Higher than typical moisture levels will remain over
the area through the week with daily minimum humidity levels
mostly staying above 40-50% across the lower deserts to 50-70%
over higher terrain areas. Aside from the gusty winds today, winds
will remain seasonally light for the majority of the time frame.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Blowing Dust Advisory until 8 PM MST this evening for AZZ532.

     Winter Weather Advisory until 11 AM MST Monday for AZZ557-558-
     563.

CA...Blowing Dust Advisory until 7 PM PST this evening for CAZ565.

     Wind Advisory until 7 PM PST this evening for CAZ563>567.

     Wind Advisory until 4 AM PST Monday for CAZ562.

&&

DISCUSSION...Lojero/Kuhlman
AVIATION...Hirsch
FIRE WEATHER...Kuhlman

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