Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 090513

Area Forecast Discussion
National Weather Service Phoenix AZ
1013 PM MST Mon Mar 8 2021

.UPDATE...Updated Aviation Discussion.


A slow moving weather disturbance will gradually move into the
Southwest throughout the week providing a period of unsettled
weather for the region. Above normal temperatures today will
progressively cool to nearly 10 degrees below normal by the latter
half of the week. A few light showers will be possible Wednesday
through Friday though amounts will likely be unimpressive. Breezy
to windy conditions are also likely Tuesday and Wednesday. Drier
and somewhat warmer weather should return over the weekend.


Thick clouds and virga this morning across AZ resulted in a mild
start to the day. In fact, the morning low in Phoenix only fell
to 66 degrees, which if it stands as the low through midnight
tonight will set a new record warm low. This warmer start helped
temperatures warm up quickly through the day, into the low to mid
80s across the lower deserts, but highs are not expected to get
much warmer through the rest of the afternoon, as the atmosphere
has cooled slightly. High clouds are also on the increase again
and a subtle vorticity axis will roll across the area Tuesday
morning with potentially more virga over AZ.

Model consensus continues to improve for the rest of this week as
a deep low pressure system slowly sags toward the area. In
particular the latest NCEP clusters suggests the GEFS has come
more in line with the slower EPS timing and track of the low
progression across our area, while the CMC has seemingly diverged
and supports a quicker system. The slower progression is more in
line with our thinking. The low is expected to position somewhere
over AZ on Friday, which will likely line up with the best chances
for showers and possibly isolated storms in the warning area.
Prior to this, increased gradient wind flow ahead of the low is
expected to lead to breezy to windy conditions across the warning
area Tuesday and Wednesday. A wind advisory has been hoisted for
Tuesday for Imperial county CA and up into western Joshua Tree NP,
with wind gusts up to 35-45 mph possible. Winds could be just as
strong Wednesday. Temperatures have trended cooler with this
system, with Friday into Saturday expected to be the coolest
timeframe. Highs Friday may even struggle to reach the low-60s in
many places. The low should finally push off to the east into NM
by Sunday, but lingering moisture and dynamics could still support
some afternoon convection, mainly along the Rim, this weekend.

.Previous Discussion...
More pronounced height falls Tuesday should yield readings more
in a 5F-10F range cooler than today. Both HREF members and global
scale models suggest a midtropospheric boundary stalling through
SE AZ along a narrow axis of notable IVT, and becoming the focus
for ascent Tuesday afternoon. While this boundary could clip into
the far SE portions of the CWA, the elevated nature of the
moisture pool should limit activity to more virga and sprinkles.
Of greater impact will be increasingly gusty winds Tuesday
afternoon and evening as the boundary layer pressure gradient
tightens ahead of an approaching weather system. The momentum
transfer will be aided by seasonally deeper mixing depths with the
strongest winds in association with local terrain enhancement
(particularly in far SW Imperial County).

During the latter half of the week, a very slow moving positively
tilted trough will descend through California eventually becoming
nearly cutoff and wobbling over the four corners. Recent
iterations of GEFS membership (including the operational member)
have decidedly become slower in progressing this system through
the region looking more like prior CMC members showing the PV
anomaly losing influence from northern stream and just slowing
ejecting eastward. While this helps in confidence of timing
features through the area, the slower movement and orientation of
the trough may complicate determination of the amount of moisture
available along with convective potential. Trends in this slower
progression suggest better low level Pacific moisture becoming
entrained into the overall circulation acting upon a slow moving
cold core and favorable cyclonic jet region.

While the main PV anomaly will still be descending along the
central CA coast late Wednesday, there is good confidence a subtle
lead shortwave will be ejecting into SE CA with a strong zonal
jet core. By this time, the marine layer will likely have pushed
well inland setting the stage for mountain waves/rotors through
the western CWA potentially necessitating the need for wind
advisory products. Regardless, most moisture will be scoured out
on the windward side of the coast range though a few showers could
survive into the deserts. However with the cold core and main
energy lagging, anything making it over the terrain should be
decaying quickly.

With the slower forward movement, much of the forecast area may
end up under shortwave ridging and subsidence much of Thursday
awaiting the actual vorticty center. There is moderate confidence
from the preponderance of emsemble output the shortwave will
finally enter the forecast area late Thursday and Friday becoming
more detached from the northern stream and obtaining less positive
tilt. While IVT does not look overly impressive, there`s decent
evidence that some measure of boundary layer moisture becomes
entrained along a frontal boundary late Thursday afternoon into
Friday morning. GEFS QPF plumes still indicate amounts under 1/4
inch accumulations with this initial frontogentical band.
Automated NBM POPs and QPF seem a little low given the conceptual
look to this system, but may need better ensemble consistency
before seeing any increase.

Forecast uncertainty grows somewhat towards the very end of the
week though the improved model agreement yields some measure of
confidence. The cold core will almost certainly be spinning over
Arizona Friday with a least a couple vorticity centers
encapsulated in the larger circulation. Midlevel lapse rates will
steepen substantially while early spring insolation "heats" the
surface (in reality, temperatures Friday will struggle to reach
the low/mid 60s) and residual moisture lingers through the
environmental column possibly leading to modest instability and
convective activity. Even minus any deeper convective activity,
showers will still be preferred over higher terrain areas around
the Phoenix area.

There is moderate model agreement this quasi-cutoff system will
eject east into New Mexico Saturday slowly bringing stronger
subsidence into the forecast area. However, there are still a
number of CMC ensemble members showing an even slower movement to
the cold core, so its not inconceivable a few showers could linger
into the weekend. In all likelihood, any showers would be
sequestered to mountains in eastern AZ with a drying and warming
trend for the remainder of the area.


.AVIATION...Updated at 0510Z.

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

Westerly winds will continue for another few hours before the
overnight easterly winds predominate. Wind speeds tonight will
remain AOB 8 kts. By early morning around 12Z, thick cloud cover
will move over the terminal area as cloud bases drop down to 10-15k
feet. A few virga showers during this time are possible north and
east of KPHX. By mid to late Tuesday morning between 16-20Z,
ceilings will begin to lift above 20k feet and the wind directions
will become more southerly. Speeds will also increase into the
7-12 kt range with some occasional stronger gusts possible.
Headings will try to favor the southwest but extended periods of
southerly crosswinds are possible for E/W oriented runway
configurations. A strong southwesterly wind is expected to
predominate after 20Z with speeds reaching 12-16 kts and gusts of
25-30 kts. The strong southwesterly winds should subside a few
hours after sunset.

Southeast California/Southwest Arizona including KIPL and KBLH:

Wind speeds are the primary aviation concern for the entirety of
Tuesday. At KIPL, westerly winds are expected to prevail through
the TAF period. The overnight wind speeds tonight will be the
lightest at 8-12 kts with occasional stronger gusts. Beginning
around 16Z Tuesday morning, and lasting the rest of the day, wind
speeds will commonly reach 15-25 kts with gusts reaching 28-35
kts. Some stronger gusts around 40 kts are possible. At KBLH,
south-southwesterly winds will prevail through the TAF period.
Wind speeds will strengthen by late morning with speeds reaching
20-25 kts and gusts of 30-35 kts. Wind speeds should subside
somewhat around sunset. Otherwise, these wind speeds could create
areas of blowing dust that reduce surface visibility. Lofted dust
could also create slantwise visibility issues.


Wednesday through Sunday:
A cooler and more unsettled weather pattern will impact all
districts during the latter half of the week with warmer and drier
weather likely returning late in the weekend. Some localized areas
(mostly favoring south-central AZ) could experience wetting rains
with this weather system though heavy rainfall is unlikely. Breezy
conditions may result in an elevated fire danger Wednesday afternoon
although higher humidity levels should preclude any critical threat.
Wind speeds should relax somewhat during the end of the week as the
weather system moves through the region. Minimum afternoon humidity
levels will mostly fall into a 20-30% range, though probably falling
back into the teens late in the weekend. Overnight recovery will be
good in a 40-70% range.


Spotters should follow standard reporting procedures.


CA...Wind Advisory from 10 AM Tuesday to midnight PST Tuesday night
     for CAZ560-562>568.



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