Wind Advisory 15, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
1050 AM MST Mon Feb 20 2023

.UPDATE...18Z Aviation Discussion.

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.SYNOPSIS...
Near normal temperatures will persist through Tuesday as a weak low
pressure system slowly advances across northern Mexico. As a result,
a few periodic light showers may be possible over the next couple
of days, though mainly across terrain features of southern
Arizona. A much stronger storm system will sweep through the
region Wednesday yielding a few showers, much cooler temperatures,
and a period of strong winds and potentially blowing dust. Below
normal temperatures and slight chance of showers should persist
through the latter half of the week.

&&

.DISCUSSION...
Early morning water vapor imagery showed a closed low continuing to
spin off the northern Baja coast while cyclonic flow continues to
stream thick mid and high clouds through southern Arizona and
northern Mexico. Across the CWA, IR imagery shows mostly clear skies
prevailing as much of the thick cloud cover remains to the south and
east. Meanwhile, a strong high amplitude upper level ridge remains
situated over the eastern Pacific, extending north toward Alaska.
This ridge will remain over the eastern Pacific while an Alaskan
shortwave is set to dig south and deepen substantially over the
western CONUS during the middle part of this week. This strong
weather system will likely bring a variety of widespread and
significant weather impacts across the region.

The quasi-stationary nature of the afomentioned closed low
continues to allow southerly flow to advect some subtropical
moisture northward into the state. The latest ACARS soundings
indicate most of this moist advection is focused between 700-800
mb with a dry profile in the upper troposphere. The greatest IVT
will remain situated to the south of the international border
across northern Mexico. Ensemble guidance keeps marginally better
PWATs around 0.60-0.70", which combined with favorable upper level
divergence and a few vorticity lobes rotating through the area
will act to produced periodic isolated light showers through
Tuesday. Isolated shower activity will be more likely Tuesday as
the cold core begins to eject east across northern Mexico. The
greatest rain chances will reside across southern Arizona Tuesday
given the closer proximity to the cold core. NBM PoPs put the
greatest chances across southern Gila County Tuesday
afternoon/evening with much lower chances across the Valley. Any
showers that develop will only produce light rainfall amounts.

A powerful weather system with a ~150 kt jet will descend along the
eastern periphery of a high amplitude eastern Pacific ridge toward
the Great Basin Tuesday and Wednesday. Global ensembles continue to
show 500 mb heights falling to around 540 dm across the forecast
area while the deepest core remains to the north over the Great
Basin. NAEFS indicates all-time minimum 500 mb heights for its
climatological period with the cold core of this system. ECMWF EFI
data continues to show widespread values greater than 0.9 across
the entire region with values pegged near 1.0 directly over the
CWA, indicating a very unusual high wind event. Along with all
the other evidence consistent among deterministic models and the
conceptual appearance of the system, confidence is excellent that
this system will produce widespread strong winds, and perhaps some
locations experiencing the strongest synoptically forced winds in
years. High Wind Watches have been issued for parts of SE
California as well as the mountains east of Phoenix where NBM
probabilities of greater than 50kt sfc gusts are unusually high
(>90%) over a rather expansive area. Damaging wind gusts will be
possible given the expected magnitude. Elsewhere through the
remainder of the forecast area, solid high end advisory wind
conditions seem most likely with NBM odds of 40kt gusts are
solidly above 50% for the majority of the area (sans parts of the
I-8 corridor in SW Maricopa/Yuma counties). It would not be out of
the question for some 50-60 mph gusts to affect foothill
communities outside of the Phoenix metro as a fine line of showers
could tap the higher momentum air just above the surface
Wednesday morning. Blowing dust will also be a concern as winds
ramp up across the region.

In addition to the winds, precipitation chances will increase,
primarily across the eastern half of the CWA Tuesday night through
Wednesday. NBM POPs peak Wednesday morning with the frontal passage
and the highest chances (>90%) over orographic/upslope favored areas
east of Phoenix. Further west into lower elevations, chances remain
modest in a 30-50% range as residence time of saturation along the
front, as well as a notable midtropospheric inversion restrict deep
ascent. NBM POPs quickly drop off west of Phoenix with chances less
than 10% across southwest Arizona and southeast California. The most
prolific QPF will remain focused across the southern Gila County
high terrain where the NBM indicates a 60-80% chance for amounts
greater than 0.25". Snow levels around 5500' will drop as colder air
advects southward with the frontal passage; however as with recent
systems, associated post frontal dry air will begin to cut off
dendrite production limiting snow amounts into lower elevations.

The majority of NAEFS members continue to indicate some form of mean
troughing maintained over the western Conus during the latter half
of the week. Ensemble trends have been towards a slower moving, more
closed system forming off the California coast towards the weekend
versus a progressive open wave (not surprising given the pattern
configuration and tendencies so far this winter). With the ensemble
membership all advertising a cyclonic flow regime, official NBM
output continues to spread modest POPS over the entire period given
the small varieties in output. In reality, precipitation would
likely only be focused in a few periods where elements of vorticity
and ascent sweep through the area. Based on recent models and
trends, this may be less likely during the end of the week and more
likely over the weekend once the main PV anomaly finally propagates
eastward. Nevertheless with excellent forecast confidence, the
higher amplitude pattern will almost certainly assure a prolonged
period of below normal temperatures from midweek through the
beginning of next week.

&&

.AVIATION...Updated at 1745Z.

South-Central Arizona including KPHX, KIWA, KSDL, KDVT:
No aviation concerns through the forecast period. Light easterly
winds AOB 6kt will continue over the next few hours before
shifting to the W by 21Z. Speeds will continue to be light through
the TAF period, with extended periods of variability from this
afternoon until tomorrow morning. FEW 8-10k ft cloud bases will be
present through tonight before becoming more SCT by tomorrow
morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation concerns through the current TAF period. Light and
variable winds will prevail at both terminals through much of the
afternoon hours before winds go more w-SW around 3-4Z. FEW 10-15k
ft cloud bases will persist through the forecast period.

 &&

.FIRE WEATHER...
Light winds will persist today before increasing Tuesday in
response to an approaching strong weather system. Minor chances
for periodic light showers will continue into Tuesday as a low
pressure system gradually shifts eastward into northern Mexico.
The best rain chances will remain focused across southern Gila
County with much lower chances across the lower deserts. Any
showers will only produce light rainfall amounts. A more
significant weather system will come into play for the middle part
of the week, bringing widespread strong, gusty winds, rain
chances, and cooler weather. Strong winds will begin to increase
across southeast California Tuesday afternoon/evening before
spreading into south-central Arizona Wednesday with a frontal
passage. Winds in excess of 40 mph are likely (>90%) for much of
the area, with gusts in excess of 58 mph possible across the
southern Gila County high terrain and Imperial County. Rainfall
with this system is expected to remain on the light side.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...High Wind Watch from late Tuesday night through Wednesday
     afternoon for AZZ541-545-547-552-557-558-561.

CA...High Wind Watch from Tuesday afternoon through Wednesday
     afternoon for CAZ562>567.

&&

DISCUSSION...Smith/18
AVIATION...Worley
FIRE WEATHER...Smith