000
FXUS65 KPSR 162358
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
458 PM MST Sat Jan 16 2021

.UPDATE...Updated 00Z Aviation Discussion.


&&

.SYNOPSIS...
Above normal temperatures will continue through Monday, especially
across southeast California and far southwest Arizona. That will
mean highs across the lower deserts reaching the mid 70s to low 80s.
During the first half of next week a low pressure system will usher
in a  cooling trend for daytime highs along with periods of breezy
to windy conditions, especially on Tuesday and Wednesday.
Precipitation chances will begin Tuesday areawide but
particularly across southeast California and southwest Arizona
where relatively heavy rain may occur. Rain showers will again
spread east across much of the forecast area Wednesday into
Thursday.

&&

DISCUSSION...
An amplified ridge remains centered just off the west coast with a
shortwave trough dropping down along its eastern periphery into the
four-corners region today. This shortwave will have little effect on
the sensible weather, with just an increase in high clouds and
slightly breezier conditions along the Colorado River Valley. High
temperatures may drop a degree or two tomorrow but will remain well
above normal through Monday.

On Sunday, the beginnings of our next storm system will start to
take shape as two shortwave troughs move towards the region. The
dominant trough will follow a northerly trajectory as a PV anomaly
rounds the crest of the ridge and it dives south over Utah and
eventually off of the southern California coast. The second
feature will plow right through the high pressure area and lead
the primary trough, centered somewhat further to the southwest.
Eventually both features will merge somewhere off of the coast of
northern Baja Mexico. Even though uncertainty remains regarding
where this closed low will settle, the majority of guidance is in
good agreement on this progression at this point.

As the low closes off it will entrain a region of elevated
precipitable water which will quickly advect northward into southern
Arizona and California. IVT over southern Arizona may reach 500-600
kg/m/s, which according to NAEFS climatology is near the 95-98%
percentile for this time of year. The relatively warm and moist air
will be carried north by an anomalously strong low level jet of 40-
50 kts at 850 mb. The result will be healthy positive theta-e
advection and convergence in the lower atmosphere. This will occur
beneath the left exit region of an upper level jet streak, which
will overlay this area with upper level diffluence. Accordingly,
strong dynamic lifting will develop. As a result of these factors,
precipitation is expected to blossom on Tuesday. At the same time,
modest instability will develop along the leading edge of the theta-
e surge, which may allow for some isolated thunderstorms as well,
particularly over southwest Arizona.

In terms of expected precipitation locations, timing, and intensity,
uncertainty remains. However, here is what we currently see as the
most likely progression of events. A band of precipitation will form
along the leading edge of the theta-e surge on Tuesday over
southwest or south-central Arizona. This band will progress westward
and strengthen, particularly over southwest Arizona and southeast
California leading to locally heavy rainfall for portions of those
areas. NBM guidance shows a 60-70% chance of measurable rainfall
near Yuma, with about a 40% chance of exceeding 0.50" on Tuesday.
Further east in Phoenix, the odds of some precipitation is high
at around 50%, but the odds of exceeding 0.50" are less than 10%.
Alternative scenarios, as seen on individual ensemble members, are
that this precipitation band is either weaker or further to the
southwest, but we maintain that those are less likely solutions
at this point. Another factor to consider on Tuesday is the wind
potential. As some of the momentum of the low level jet mixes
down, wind gusts of 25-35 mph will be common, with an isolated
location hitting 40 mph not out of the question. Additional
gustiness may persist into Wednesday, but Tuesday will likely be
the windiest.

On Wednesday the closed low will begin to eject to the northeast,
filling in as it does so. The EPS continues to advertise a more
sluggish solution, with the low pressure lingering longer and
further south. The CMC ensemble has come mostly in line with the
further north and somewhat faster GEFS solution. Ultimately this
means that there is still uncertainty for this period, but the most
likely outcome is for Wednesday or perhaps Thursday to be the wetter
day for south-central Arizona. Regardless of when this happens, the
same dynamics previously mentioned will shift northeast towards the
Phoenix area, increasing the rain potential and shifting the flow to
southwesterly. A few ensemble members are indicating that a narrow
band of heavy rainfall could form during this period as low level
convergence increases, but as of now NBM guidance only shows a 20%
of exceeding 0.50" on Wednesday for Phoenix. This system is not all
that cold, but as the upper level low passes nearby a few wet
inches of snow may accumulate on the highest peaks of Gila County
by Thursday morning.

The forecast again becomes uncertain as we head towards next
Thursday and Friday. WPC cluster analysis does have good agreement
that a longwave trough will redevelop over the western US during
this period, but considering the lead time the details are unclear.
As a result, the forecast for next weekend carries a continued
chance of showers, but no particular day is favored at this time.

&&

.AVIATION...Updated at 1125Z.

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, is 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
overnight and Sunday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

Locally gusty north winds at KBLH will linger through this
afternoon under mostly clear skies. North gusts 20-25kt will be
common at KBLH before relaxing below 12kt after sunset. Winds will
not be as strong at KBLH Sunday. KIPL winds to remain light
westerly- northwesterly through the TAF period, with an extended
period of light/variable most of Sunday.

&&

.FIRE WEATHER...
Monday through Friday:
Above normal temperatures will continue Monday before a cooling
trend sets in Tuesday. Min RH will remain low on Monday, in the
12-17% range, with max RH in the 25-40% range. A likely weather
system by next Tuesday should drop temperatures to near or
slightly below normal readings while also bringing a fair chance
for precipitation and isolated thunderstorms beginning Tuesday and
possibly as late as Friday. Breezy to windy conditions will also
be possible on Tuesday through Wednesday morning. RH values are
also likely to improve significantly starting Tuesday, especially
over higher terrain areas. Min RH should improve to above 25-30
percent on Tuesday, and reach 30-40 percent by Wednesday through
the end of period.

&&

.Climate...

Record highs through Saturday:

 Date    Phoenix       Yuma     El Centro
 ----    -------       ----     ---------
Jan 16  83 in 1976  83 in 1961  86 in 1976

&&

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

&&

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

$$

DISCUSSION...Hodges
AVIATION...Benedict/Kuhlman
FIRE WEATHER...Sawtelle
CLIMATE...Kuhlman

NWS PSR Office Area Forecast Discussion