San Luis, Arizona 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

490
FXUS65 KPSR 311201
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
501 AM MST Sat May 31 2025

.UPDATE...Updated 12z Aviation Discussion.

&&

.KEY MESSAGES...

- An influx of tropical moisture will spread over the region
  tonight into Sunday aiding in widespread showers and scattered
  thunderstorms on Sunday.

- A few strong thunderstorms with gusty winds, small hail, and
  localized heavy rainfall are expected on Sunday.

- Below normal temperatures with highs in the nineties across the
  lower deserts are expected from Sunday into the middle part of
  next week before temperatures rise above normal again by late
  next week.

&&

.DISCUSSION...
The cut-off low still remains situated just to the southwest of
the region with the low center positioned off the coast of
northern Baja. Mid and high level clouds continue to stream into
our region from the southeast, but near the cut-off low center
there is a lack of clouds due to the dry air that continues to get
entrained in from the southwest. The moisture associated with the
already decaying TC Alvin is beginning to shift farther northward
through Mexico, but the bulk of the moisture isn`t expected to
arrive into our region until late tonight and Sunday morning. For
today, we will again be quite warm with highs likely topping out
at or just above 100 degrees across the lower deserts. It is
possible we may see some thicker clouds inhibiting heating some
today, but overall the forecast temperature spread is small.

For tonight into Sunday morning, guidance continues to show strong
moisture advection streaming northwestward up the Gulf of
California reaching southwest Arizona and then spreading across
the rest of the lower deserts. Forecast PWATs are seen reaching
1-1.25" during the overnight hours before peaking at around 1.5"
mid day Sunday. PWATs this high are quite rare for this time of
year and would basically represent record levels for June 1st.
Low level mixing ratios are also forecast to reach 10-11 g/kg on
Sunday which would represent levels we typically expect during
the middle of the monsoon season.

Now that we are seeing hi-res model data going through much of
Sunday, we are getting a better idea of what to expect as far as
shower and thunderstorm coverage. We may see some scattered light
showers develop as early as this evening mainly across southeast
California and southwest Arizona, but little if any accumulation
is expected. Once the moisture advection really kicks in later
tonight and Sunday morning and the cut-off low begins to eject
northeastward toward our region, we should see shower activity
blossom by around sunrise across southwest Arizona into southeast
California. This main band of shower activity, with some embedded
thunderstorms likely at times, should eventually begin to shift
northeastward into south-central Arizona during the afternoon
hours. Rainfall rates within this band are not likely to be all
that high, but it seems reasonable to expect up to 0.1-0.2" per
hour. Also during the afternoon hours, we expect fairly good CAPE
at around 1000 J/kg or a bit higher to develop across portions of
southeast California and southwest Arizona. This area may see some
isolated to maybe scattered thunderstorms Sunday afternoon, some
of which could be strong. Hi-res CAMs are indicating this
convection is likely to originate along the spine of Baja before
moving northward into Imperial County and possibly even into
portions of Riverside County. Strong gusty winds, small hail, and
heavy rainfall are likely to all be possible with any of these
strong thunderstorms.

The shower and isolated thunderstorm activity is likely to
continue through Sunday evening into the overnight hours, but
coverage is expected to decrease with time. Forecast rainfall for
Sunday and Sunday night remains mostly unchanged across Arizona,
but with some increases across southeast California due to the
higher chances of convection. Average QPF from around 0.25" across
southeast California to upwards of 0.3-0.5" across much of
southern and central Arizona is expected. Some areas are likely to
miss out and not receive much, but other areas, especially those
impacted by thunderstorms, could see more than an inch of
rainfall. There is also the possibility of some very localized
heavy rainfall of up to 2" over a few hour period if any organized
convection manages to train over the same area. For now, guidance
is somewhat showing this as a possibility over western Imperial
County to just south of JTNP and over central Pima into maybe far
southern Maricopa and western Imperial Counties. As with any
potential convective events over 24 hours out, trying to nail down
the areas of heaviest rainfall is of fairly low confidence.

The cut-off low is still advertised to quickly weaken Sunday night
into Monday while moving out of our area later Monday morning.
Some lingering shower activity is likely through Monday morning
across portions of Arizona and maybe even through Monday
afternoon across the Arizona higher terrain. Drier air will filter
into the area from the southwest behind the low, but it will not
completely get scoured out. This leftover moisture is expected to
come into play for Tuesday as a second upper level low is forecast
to quickly swing through the area from the west northwest. The
NBM has been slow to introduce rain chances for this second
system, but confidence is starting to increase for at least some
slight chances for showers and a few isolated thunderstorms mainly
Tuesday afternoon. We have increased PoPs during this time to
mainly give higher terrain areas at least a mention of rain
chances. The lack of good moisture will be the main limiting
factor as the system actually looks like it will bring a brief
period of good forcing with cold air aloft.

Temperatures starting Sunday will cool down and bring some
temporary relief from the recent hot temperatures. Highs Sunday
may even struggle to top 90 degrees in many areas before we see
temperatures rise more into the mid 90s on Monday and Tuesday. The
below normal temperatures should continue through Wednesday
before highs finally top 100 degrees again by around next
Thursday. Guidance is still quite uncertain going into next
weekend as they are now showing a weak upper level low trying to
cut off from the main flow just off the California coast. At the
same time, a strong sub-tropical ridge will be attempting to shift
closer to our region from the southeast. It seems more likely this
ridge will win out bringing gradual increasing heights and
temperatures to our region by next weekend. NBM guidance is
showing highs getting closer to 110 degrees for next weekend into
early next week and this very well could be too low if the ridge
actually fully moves into our region.

&&

.AVIATION...Updated at 1200Z.

South Central Arizona including KPHX, KIWA, KSDL, KDVT:
Diurnal east to southeast winds early this morning will transition
earlier than usual to the west around 16-17z. Another surge of
southwesterly winds with some periodic gustiness 20-25 kts is
expected later this evening, delaying the easterly shift until
after 08z. There is some uncertainty of any easterly shift
occurring at KPHX, as indicated by a VRB group at 10z. SCT to BKN
mid and high clouds this morning will continue through the period,
with bases remaining above 12 kft through the evening hours, but
lowering going into the overnight period, where bases could lower
to around 10 kft by the end of the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KIPL will predominantly favor the SE to SW, while
directions at KBLH will favor the S-SW. Sustained wind speeds will
generally be aob 10 kt with occasional gusts into the teens.
Increasing mid and high clouds are expected through the period,
with bases generally aoa 10 kft through the period. Virga and
light showers will start moving up from the south tomorrow
afternoon. Confidence is too low to include any prevailing SHRA in
this TAF package, but introduced VCSH conditions at IPL late in
the period. Some of the outflows from any showers will be capable
of producing southerly wind gusts in excess of 20 kts.

&&

.FIRE WEATHER...
An unseasonably wet weather system will track through the area
Sunday bringing widespread wetting rain chances of 50-80% and
chances for thunderstorms (15-25% chance). Humidities will begin
to rise today, but only slightly with MinRHs between 10-15%. Winds
will also be a bit breezy this afternoon with gusts up to 20 mph.
Periods of showers and isolated thunderstorms are then expected
starting Sunday morning lasting through the overnight hours Sunday
night. The elevated humidities Sunday will begin to lower
starting Monday, but MinRHs should mostly stay above 15% into the
middle part of next week. A drier weather system is likely to move
through the region on Tuesday possibly bringing some isolated
showers and a few thunderstorms focused over higher terrain areas.
Hotter and seasonably dry conditions are likely to return later
next week.

&&

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

$$

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