000
FXUS65 KPSR 171033
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
333 AM MST Sun Jan 17 2021

.SYNOPSIS...
Above normal temperatures will continue through Monday,
especially across southeast California and far southwest Arizona.
An abrupt shift in the weather pattern will bring much cooler and
likely rainy conditions during the middle part of this week. Rain
chances will begin Tuesday areawide, but particularly across
southeast California and southwest Arizona where relatively heavy
rain will be possible. Rain showers should then spread east across
much of the forecast area Wednesday, likely lingering into
Thursday across eastern Arizona. A second weather system then
seems likely by next weekend bringing even cooler temperatures,
additional chances for rain, and high elevation snowfall.

&&

.DISCUSSION...
The strong high pressure ridge centered to our west continues to
bring unseasonably warm temperatures across the Southwestern U.S.
and above normal temperatures are seen lasting through Monday.
Slight cooling will be realized today into Monday as the ridge
begins to shift westward in response to a shortwave trough combo
overtopping the ridge later today. Highs today will top 75
degrees over the bulk of the deserts, but the coverage of 80
degree temperatures should be limited to Yuma to El Centro.
Monday will be a transition day with further cooling taking place
ahead of the incoming weather system, but highs will still be a
good 5-8 degrees above normal.

Some much needed rainfall still looks to be likely for much of the
region at some point this week as models show a shortwave trough
quickly diving southward just off the West Coast and another
shortwave taking an overland track from southern Canada through
the Great Basin. Eventually, these two shortwaves are forecast to
get wrapped up together forming a deep closed low just to our
southwest on Tuesday. Essentially all ensemble members agree to
this system evolution, but there are still differences in the
position of the closed low, especially within the EPS. Around 50%
of the EPS members show the low tracking farther westward off of
Baja, while the vast majority of the GEFS and Canadian members
show the low setting up in a more favorable position closer to our
region. Although some uncertainty remains due to the low
positioning, the most likely scenario would bring the low just off
the coast of northern Baja on Tuesday and remaining fairly
stationary through much of Wednesday before tracking east-
northeast on Thursday.

The following forecast details represent the most likely scenario
(representing ~70% of the latest ensemble members):

After the first offshore shortwave dives well south and west of
our area late Monday, the second shortwave is likely to track
through Nevada and southern California early Tuesday. As the
closed low forms just to our southwest, cooling aloft and any
minimal system moisture will begin to combine with moisture
advection from the south as southerly flow increases east of the
deepening low. As moisture levels increase and the low deepens,
showers will initially become likely across southeast and south-
central Arizona early Tuesday, but these should be mostly light.
Later Tuesday through Tuesday night, strong Theta-E advection into
southwest Arizona and possibly into southeast California will
likely combine increasing dynamics aloft for heavier rain showers
and embedded thunderstorms to develop. Hi-res guidance is already
pointing toward a potential moderate to heavy rain band setting
up somewhere across SW AZ and SE CA Tuesday night into Wednesday
morning. Although this potential band does look very possible, it
is far some certain and the positioning could very well shift.

For later Wednesday, the best area for rain will eventually begin
to shift eastward throughout the day, likely putting south-
central and eastern Arizona back into a more favored area of
rainfall. There is still some uncertainty with how long the low
will linger to our south and west, but it does seem likely it
should shift across northern Mexico on Thursday. Periods of rain
showers on Wednesday should then eventually shift to over eastern
Arizona during the day Thursday with most of the activity winding
down during the latter half of Thursday.

Impacts throughout the event will mainly be focused around areas
of moderate to heavy rainfall and potential localized flooding as
WPC currently has a large area of forecast QPF amounts of
0.5-0.75". There should also be areas that receive higher amounts
of rainfall, possibly in the 1-1.5" range. The current thinking
is the heaviest rainfall will be more favored over southwest
Arizona and the high terrain north of Phoenix. Winds may also be
an issue at times on Tuesday and possibly into Wednesday as wind
gusts may approach 25-30 mph at times, possibly higher in any
thunderstorms or isolated stronger cells. Snow levels throughout
the event now look to remain quite high (generally above 6500-7000
ft.), so snow should only be an issue for the highest peaks in
Arizona.

The following forecast details represent the least likely
scenario (representing ~30% of the latest ensemble members):

If half the EPS is correct and the low shifts farther to the west,
rainfall will be much more limited across the region with some
areas potentially seeing little if any rainfall Tuesday-Thursday.
However, this set of ensemble members would then shift the best
rain chances into Friday. Although this is the least likely
scenario, our forecast does somewhat account for this potential
by keeping PoPs lower for Tuesday-Thursday, while also keeping
PoPs in for Friday. Hopefully, these differences in the track will
resolve themselves over the next model run or two.

The unsettled weather pattern is likely to continue through next
weekend as one if not possibly two more weather systems look
likely. After the mid week system, models show a second deep
trough coming in from the northwest. Cluster analysis does show
fairly good agreement with this second trough digging southward
along the West Coast on Friday reaching the Southwestern U.S. on
Saturday. Although it is still quite far out, it is likely this
second system will be colder and as long as it deepens
sufficiently to our west it should bring another round of precip
late Friday into Saturday. A colder system would also yield lower
snow levels and possibly impactful snowfall over the Arizona high
terrain. Temperature guidance also reflects the colder airmass
with highs likely dropping to below normal starting Saturday. A
potential third trough is also seen out near Day 8-10.

&&

.AVIATION...Updated at 0547Z.

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

No aviation impacts are expected through Sunday under mostly
clear skies. Light winds, aob 8 kts, are anticipated with
occasional periods of calm winds. Winds will generally follow
diurnal trends, especially at KPHX and KIWA, but winds at KSDL and
KDVT will see more of a northerly to northeasterly directional
component tonight. With a northeasterly gradient wind aloft, KPHX
may also experience brief periods of light southwest winds and
north winds overnight through Sunday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

Northerly to northwesterly winds are favored at KBLH through the
next 24 hours with speeds generally aob 10 kts. KIPL winds will
tend to favor a westerly to northerly component through the TAF
period, but with extended periods of light and variable. Skies
will remain mostly clear.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Tuesday will mark the beginning of an unsettled weather pattern
across the region with the first weather system likely bringing
widespread wetting rains Tuesday-Thursday. Breezy to locally windy
conditions will also be possible over much of the area early in
the period, especially over higher terrain areas. A slight chance
of thunderstorms also seems possible across southeast California
and southwest Arizona during the latter half of Tuesday and again
across southwest and south-central Arizona on Wednesday. After a
brief break in the rain, likely late Thursday into Friday, a
second weather system may bring another round of rain and high
elevation snows into Saturday. The cooler conditions and increased
moisture during the period will lead to more favorable humidity
readings as minimum daily are likely to stay above 40% much of the
period.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

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

$$

DISCUSSION...Kuhlman
AVIATION...Benedict
FIRE WEATHER...Kuhlman

NWS PSR Office Area Forecast Discussion