442
FXUS65 KPSR 221142
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
442 AM MST Wed Jan 22 2025

.UPDATE...Updated 12z Aviation Discussion.

&&

.SYNOPSIS...
Temperatures will warm into the normal range starting this
afternoon, but we can expect below normal overnight lows from the
mid 30s to the lower 40s through Friday. A low pressure system is
then forecast to move into the region over the weekend bringing
decent chances for lower desert rainfall and higher elevation
snows starting Sunday. This unsettled weather with daily chances
for light precipitation is likely to continue through early next
week with high temperatures dropping into the upper 50s to the
lower 60s.

&&

.DISCUSSION...
Early morning temperatures have dipped into the 30s for a good
portion of the Arizona lower deserts with some areas even near
freezing as of 3 AM, despite some higher level clouds now
streaming across the area. The very dry air in place with surface
dew points around -10 degrees has really helped nocturnal cooling
this morning, along with overall light winds. Upper level ridging
is also pushing into the region and the warming aloft will be
partially realized starting this afternoon with highs topping out
in the lower 70s across the western deserts to the upper 60s in
the Phoenix area.

The deep polar trough over the central CONUS will remain in place
into Thursday with another shortwave trough diving southward
across the Rockies reaching near the Four Corners area Thursday
morning. This disturbance will once again help to bring some
increased winds across our area with gusts up to 30 mph focused
more across the Lower CO River Valley and across the Arizona
higher terrain later this afternoon through Thursday. The ridge
centered to our west will help to keep temperatures somewhat mild
into Thursday, but we are still in store for another cold morning
on Thursday with the typical rural lower deserts seeing lows down
to 30 degrees. The Phoenix area and locations closer to the
foothills will stay a bit warmer Thursday morning with lows mainly
in the upper 30s due to the elevated winds.

Friday and Saturday look to be transition days as our region will
largely stay under the weakening mid level ridge. This will help
to keep our daily highs near normal, but due to the clear skies,
decreasing winds, and very dry air overnight lows will still be
running 5-10 degrees below normal. The large scale pattern will
become much more interesting starting Friday with an upper level
trough developing just to our west along the southern jet branch
and another trough quickly diving southward out of western Canada.
These two features and their associated jet branches are then
forecast to phase together this weekend helping to support the
development of a deep low pressure system over California Saturday
into Sunday. The upper level trough will remain positively tilted
during this timeframe so system dynamics will not be that
impressive, but this looks to be the best weather system for our
region so far this winter.

The main issue for precipitation potential for this upcoming
weather system will be the lack of good moisture as it will not be
tapping into an expansive moisture source. Initially, there will
be some mid and upper level moisture and clouds later Friday into
much of Saturday, but this moisture will get pushed to the east
by Saturday evening. As the system continues to strengthen over
the weekend, we will eventually start to see some lower level
moisture getting pulled off the Pacific and advecting
northeastward into southern California and Arizona later on
Saturday into Sunday. There are still some subtle model
differences with the timing and the exact position of the upper
level low as it moves into our region this weekend, but both the
GEFS and EPS are quite similar with the low moving over southern
California and western Arizona later on Sunday. The positioning of
the low is forecast to be far enough to the west to put much of
Arizona into an upslope southwesterly flow by Sunday with moisture
levels increasing enough for precipitation development to begin.
Southern California is likely to see some precipitation potential
from the core of the low moving over the area, but as stated
earlier moisture will be a limiting factor. Ensemble mean PWATs
are only forecast to reach to 100-120% of normal starting Sunday,
while low level mixing ratios only rise to 3.5-5.0 g/kg. Despite
the rather meager moisture, the system dynamics and cold air
should be enough to bring scattered shower activity on Sunday with
the best chances for measurable precipitation focused over the
foothills areas of south-central Arizona into the mountains north
and east of Phoenix. The NBM shows PoPs of 30-40% over much of
the area by Sunday afternoon, but this could be underdone given
the potential system dynamics advertised.

Guidance then shows the upper level low basically stalling out
over our region on Monday before slowly drifting eastward on
Tuesday. If this holds true, much of the area will continue to
see light precipitation chances on Monday and even on Tuesday
across the eastern half of Arizona. Preliminary QPF amounts for
the entire event look to be on the order of a tenth of an inch or
less across a majority of the lower deserts to 0.1-0.25" over the
lower deserts adjacent to the south-central Arizona foothills. The
more favored upslope higher terrain areas north and east of
Phoenix could see QPF amounts of 0.5-0.75".

This weather system will also be quite cold with 850mb
temperatures dropping to between 0 to 4C and snow levels initially
at most 5000-5500 feet on Sunday before likely falling closer to
4000 feet Monday into Tuesday. This should result in periods of
light snow over the higher terrain, especially during the latter
half of the event. Temperatures across the area will drop
starting Sunday with highs falling to 60-65 degrees. It would also
not be surprising to see a day or two with highs in the mid to
upper 50s across the lower deserts early next week.

&&

.AVIATION...Updated at 0455Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major aviation weather concerns under periods of high cirrus
decks are expected through the TAF period. The overall wind
pattern will exhibit the typical diurnal tendencies with speeds
aob 7 kts through this evening. A more northeastward component
wind will be common going into the overnight period, with this
wind directions conducive for KPHX developing a light
southwesterly component wind after midnight.

Southeast California/Southwest Arizona including KIPL and KBLH:
Some gusty northerly winds at KBLH this afternoon and evening will
be the main aviation weather issue through the TAF period. Wind
speeds through this morning will remain light aob 7 kts with wind
directions fluctuating between the west-northwest to north. There
will also be extended periods of light variability to even calm
conditions. Winds will pick this afternoon at KBLH out of the
north with peak gusts between 20-25 kts going into the overnight
hours. Periods of high cirrus decks fluctuating between SCT to
BKN will be common through the period.

&&

.FIRE WEATHER...
Near normal temperatures and dry conditions will persist through
at least Friday before a more unsettled weather pattern settles
into the region over the weekend, lasting through early next
week. Expect minimum afternoon humidity levels in the 5-10% range
with poor overnight recoveries only to 15-25%. Light winds
initially today will become breezy across the Lower CO River
Valley this afternoon and then additionally over the Arizona
higher terrain beginning tonight. Overall light winds are then
expected on Friday. A weather system is then forecast to move into
the region this weekend, likely bringing chances for wetting rain
across the south-central Arizona lower deserts and snow over the
Arizona higher terrain Sunday and likely lingering through next
Tuesday. Increased moisture levels and cooler temperatures will
help to push MinRHs to between 25-35% over the lower deserts
starting Sunday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Freeze Warning until 9 AM MST this morning for AZZ530-531-
     533>540-542>544-546-548-550>555-559.

CA...Freeze Warning until 8 AM PST this morning for CAZ569.

&&

$$

DISCUSSION...Kuhlman
AVIATION...Young/Lojero
FIRE WEATHER...Kuhlman