995
FXUS65 KPSR 250540
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1040 PM MST Fri Jan 24 2025

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
Temperatures will hover not far from the seasonal normal through
the weekend, however a slow moving weather disturbance will
result in gradual cooling early next week. This system will also
yield periodic chances for lower elevation rain showers and higher
terrain snowfall across the region, although amounts should be
primarily light. By the latter half of next week, drier weather
and moderating temperatures should return to the area.

&&

.DISCUSSION...
Significant changes in the Conus flow pattern are now just beginning
to materialize as a series of low amplitude shortwaves start to
consolidate over northern/central California. These features will
eventually fill into a midtropospheric height void and deepen into a
substantial negative height anomaly propagating through the SW Conus
early next week. This evolution represents the downwind edge of an
East Pacific omega block with H5 heights falling better than 2
normalized standard deviations below normal. Models remain in
excellent agreement through the next 72 hours with a nearly cutoff
circulation spinning along the West coast, however given this is a
blocking/cutoff situation, there is expectedly rapidly growing model
spread and uncertainty beyond the Monday morning time frame. Based
on overall ensemble trends, historical model performance, and
experience forecasting blocking/cutoff evolution, strongly prefer
solutions towards a slower progression and eventually more southern
track.

Gradual height falls will spread into the region tomorrow under
increasingly cyclonic flow, however low level thermal profiles will
change little yielding excellent confidence of a near persistence
forecast (other than a thickening mid/high cloud deck). Even through
Sunday with the primary vorticity center remaining offshore and well
removed from the CWA, only limited reductions in thermal profiles
across SE California will cause temperatures to cool slightly. The
delayed approach of this circulation will allow a more prolonged
period of southerly sfc-H7 winds transporting modest Pacific
moisture inland during the day. However, forced ascent should still
be lagging closer to the coastline with very little evidence of
rainfall leaking into the leeward side of the coastal range. Once
again, NBM POPs appear completely out of line with the synoptic
pattern and barely matching with any ensemble solution, so have made
major cuts to the initialization.

During the day Monday, the leading vorticty center should progress
inland shearing the cutoff structure eastward while allowing some
filling of midlevel heights. Despite the system weakening Monday,
the supporting upper level jet centered along the Arizona/Mexico
border will remain fairly strong providing broad vertical ascent
across much of the region. Low and mid level moisture should
continue to increase with the best juxtaposition of moisture/forcing
likely occurring across SE California Sunday night/Monday morning,
and south-central Arizona Monday afternoon/evening. However, several
limiting factors may exist arguing against more widespread
precipitation development including more marginal moisture transport
with forecast PWATs are only shown rising to near normal with low
level mixing ratios likely not eclipsing 4 g/kg. Upslope areas will
stand the best chances for accumulating rainfall with lesser chances
in lower desert communities. In fact, forecast BUFR soundings
suggest only a narrow depth of saturated ascent within a brief
temporal range along a decaying frontal boundary around the Phoenix
metro Monday afternoon. Thus, have continued the trend of
substantial reductions to mandated NBM POP initialization across the
board.

Guidance mostly agrees on slowly pushing the initial low center into
northern Arizona Tuesday with any precipitation likely focused
outside the CWA, but some locations in western and southern Arizona
may see some scattered showers associated with another distinct
vorticity lobe rotating and stretching through the western
circumference of the larger circulation. This is where model guidance
and forecast confidence begins to quickly deteriorate depending on
more detailed location, orientation, and movement of vorticity
forcing and residual moisture profiles. As stated earlier with a
preference towards a slower and further south outcome, the best
chances for precipitation may actually not arrive until Wednesday
afternoon when south-central Arizona becomes aligned in favorable
ascent and quasi-deformation region where showers developing over
higher terrain and Mogollon Rim are more easily transported into
lower elevation under the cold core.

All told, storm total QPF for the entire event is quite low with as
little as a Trace-0.3" over the lower deserts and adjacent foothills
areas and upwards of 0.25-0.5" over the higher terrain northeast of
Phoenix. This is a notable decrease in forecast amounts from
previous model iterations where, for example, GEFS plumes were
clustered in a 0.10-0.30" range for the Phoenix metro yesterday,
then have been clustered right around 0.10" in subsequent runs with
a notable increase in membership producing absolutely nothing. This
also impacts potential snowfall amounts, not only from a pure QPF
standpoint, but more so from a discontinuity in moisture profiles
and extended period of non-saturation within the dendritic growth
layer. As such, forecast amounts have decreased substantially with
little more than dusting amounts now most likely.

Temperatures will take a good hit during at least the first half of
next week as the upper level low slowly moves into the region. NBM
forecast highs have finally responded to what seems far more
reasonable for an event like this with highs dropping into the upper
50s to low 60s Tuesday and Wednesday. Eventually, drier air will at
least temporarily filter into the region towards the end of the
week, though given the larger scale retrogressive pattern and
realignment of Pacific ridging, some form of troughing could easily
return to the western Conus just beyond this forecast period.
Regardless, depending on how long it takes for the upper low to exit
the region, highs may struggle to return back near normal for at
least a few days. The western CWA could see highs return to normal
as early as Thursday, but for much of Arizona it may not happen
until Friday or Saturday, but this could be just a brief reprieve.

&&

.AVIATION...Updated at 0538Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Easterly winds aob 10 kts will continue through tomorrow morning
before switching to the W-SW tomorrow afternoon with a few gusts
into the teens. Otherwise, BKN-OVC highs clouds will continue
throughout the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
Light W-NW winds will be favored at the terminals through the
overnight hours, with speeds less than 8 kts. Winds transition to
a southerly component Saturday morning, with a few gusts into the
teens possible in the afternoon at KBLH. Otherwise, BKN-OVC high
clouds will continue throughout the next 24 hours.

&&

.FIRE WEATHER...
Near normal temperatures and dry conditions will persist through
at least Saturday before a weather system gradually moves into the
region early next week. After MinRHs between 5-10% today, humidities
will improve over the weekend with MinRHs likely topping 25% by
Sunday. Winds will be breezy again this morning over the Arizona
higher terrain before overall light winds take hold areawide later
today into Saturday. Precipitation chances are then expected to
spread from west to east starting Sunday, but wetting rain chances
will remain mostly below 30% through early next week.

&&

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

$$

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

NWS PSR Office Area Forecast Discussion