Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 081855

Area Forecast Discussion
National Weather Service Phoenix AZ
1155 AM MST Mon Mar 8 2021

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. Drier and
somewhat warmer weather should return over the weekend.


Early this morning, a low amplitude dampened shortwave was quickly
propagating into southern CA with an attendant enhanced jet streak
pulling substantial upper level moisture into the region. With the
entirety of this moisture plume relegated above 600mb, virga will be
common through the morning though it wouldn`t be out of the realm of
possibility for a few sprinkles to reach the surface. The greater
impact of this shortwave will be the introduction of slow height
falls and cooling of the atmospheric column under increasing
southwest flow. Uncertainty in timing of thicker clouds moving in
and out of the area has resulted in larger than desired forecast
spread among models, however trends shows highs today somewhat
cooler than Sunday.

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 1855Z.

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

An exiting weak weather disturbance has left behind clear skies.
Expect a period of southerly winds with gusts to 20kts until
about 20Z before becoming south-southwest by 21Z and then
southwest-west by 22Z/23Z. As the westerly component takes hold,
anticipate gusts to 20kts to continue (lasting into the early
evening). Brief isolated gusts to 25kts are possible. Anticipate
the westerly winds to last longer than usual with the typical
change to downvalley/east directions being slow to develop - if at
all. Another weak weather disturbance will bring increasing high clouds
tonight with bases bottoming out around FL120 roughly between
12Z-16Z followed by rapid clearing. Couldn`t rule out sprinkles
with the thickest cloud cover but overall, anticipate just virga.
Expect stronger southwesterly winds Tuesday afternoon with gusts
of 25kts common.

Southeast California/Southwest Arizona including KIPL and KBLH:

An approaching weather disturbance will lead to breezy conditions
this afternoon and evening favoring westerly directions at KIPL
and southerly at KBLH. The breeziness will last longer at KIPL.
There will also be an increase in high cloudiness with bases
bottoming out around FL180 between 12Z-15Z followed by rapid
clearing. Stronger winds develop Tuesday (along with an earlier
onset) with gusts of 30-35kts common by 21Z at KIPL (25-30kts at
KBLH). There is some potential for brief visibility reductions
down to 6SM in blowing dust at KIPL but confidence in that
occurrence is too low to reflect in the TAFs.


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.




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