040
FXUS65 KPSR 042356
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
456 PM MST Wed Dec 4 2024

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
A mostly tranquil weather regime with above normal temperatures will
persist over the Southwest the next several days despite a weak low
pressure system affecting the region. Outside of a few isolated
light showers over higher terrain Thursday, dry conditions will also
prevail. Temperatures retreating closer to the seasonal average will
return during the first half of next week though dry weather should
continue.

&&

.DISCUSSION...
Large scale Rex blocking remains parked over the western Conus with
a pair of vorticity centers comprising the basal portion of this
block situated over the SW Conus (and one center directly over
central AZ this afternoon). While these types of blocks are
notoriously stubborn and slow to break down, WV imagery indicates a
Pacific fetch of shortwaves poised erode and retrograde the ridge
apex early next week while also allowing the southern low pressure
domain to be absorbed into downstream flow. During this transition,
little will change thermally over the lower and middle troposphere
with H5 heights hovering in a 572-576dm range through the weekend.
While not record breaking by any stretch, guidance spread is
extremely narrow yielding excellent forecast confidence that
temperatures will still reach 5F-10F above normal progressing into
the weekend.

While this pattern transition will not provide any widespread
inclement weather, there are some subtleties in the evolution that
may create minimal impacts over parts of the CWA. Despite a fairly
favorable synoptic ascent structure within the upper level cyclonic
cold core and envelop of vorticity forcing, antecedent atmospheric
moisture is significantly lacking with lower level mixing ratios
barely touching 3-4 g/kg. However, the position of the upper low is
creating a backed sfc-H7 wind profile which has been effective in
transporting modest moisture up the Rio Grande valley into far
eastern AZ. With this wind component also indicative of an upslope
regime, HREF members continue to advertise isolated high based
showers/storms Thursday and Friday afternoon. The largest
uncertainty is whether any of this activity will affect southern
Gila County or remain just outside the CWA bounds.

Over the weekend, models are in excellent agreement ejecting the
aforementioned upper low into the southern plains in response to an
upstream kicker system breaking over the ridge axis. This upstream
wave will crest the ridge, then amplify into the Great Basin filling
the height void left by the departing upper low. This pattern
evolution is common during blocking rectification, as well as La
Nina cool seasons where overland trajectories, positive tilt
orientation, and amplification downstream of the Southwest portends
a dry, cool outcome. While the overall ensemble mean and clustered
membership suggest the bulk of energy shearing east and bypassing
the region, there are still several members indicating a deeper
vorticty lobe pivoting through the trough base in NW flow through
Arizona on Monday/Tuesday.

Historical pattern recognition suggests that ensembles may not be
fully capturing the magnitude of the cold core and vorticity depth
as these NW flow events have frequently pulled more PV into the
trough base, producing showers over the Rim, and surviving into
valley locations aided by vorticity forced ascent and very cold air
aloft. While still a low probability event, mandated NBM grids only
advertise POPs less than 5%, and would not be surprised if these
values increase notably in future forecast iterations. In the same
vein, a deeper PV anomaly would create better height falls through
the larger portion of the forecast area forcing strong north winds
funneling down the lower Colorado River valley. Here again, the
required deterministic NBM output does not seem to fully capture the
potential magnitude of these winds next week. With this shortwave
passage, much cooler conditions will surge southward during the
first half of the week with readings retreating into a near normal
range. These near normal temperatures should prevail during the
middle of next week with some indication of moderation towards the
end of the week, though guidance spread becomes rather wide
indicating larger forecast uncertainty.

&&

.AVIATION...Updated at 2356Z.

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

Wind will remain light, at or below 7 kts, through the TAF period.
Wind directions will continue to follow typical diurnal trends.
However, the westerly shift Thursday may be delayed till as late
as 00Z at KPHX with a bit more of a northerly component to the
wind Thursday afternoon. SKC is expected over the Phoenix airspace
through Thursday afternoon. Diurnally driven convective showers
are expected to develop in the high terrain E/NE of Phoenix
Thursday afternoon with a light drift toward Phoenix. Mid level
clouds may drift over the Phoenix area in the evening, but
probability of -SHRA is too low to include in the TAF at this
time.

Southeast California/Southwest Arizona including KIPL and KBLH:

Across SE California, light W to N winds will be preferred.
Extended periods of nearly calm conditions or light variability
are likely as well. SKC will prevail.

&&

.FIRE WEATHER...
Overall tranquil weather conditions will continue to prevail through
the end of the week, but with some slight chances for a few very
light showers over the eastern Arizona higher terrain on Thursday.
Expect overall light winds each day somewhat following diurnal wind
patterns. However, some breeziness is likely mainly during the
overnight and morning hours across the higher terrain of eastern
Arizona and within the Lower CO River Valley. MinRHs will stay in
the teens across the western districts to 20-25% over the south-
central Arizona lower deserts and 30-40% over the Arizona higher
terrain. Dry and warm conditions will then prevail over the weekend
before temperatures begin to fall early next week. A dry weather
system passing through the region early next week will also bring a
period of breezy to locally windy conditions focused mostly over the
Lower CO River Valley.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...18/Kuhlman
AVIATION...Benedict
FIRE WEATHER...Kuhlman