723
FXUS65 KPSR 020017
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
517 PM MST Sun Jun 1 2025

.UPDATE...Updated 00Z Aviation Discussion.

&&

.DISCUSSION...

Key Messages:

- Showers and isolated thunderstorms will sweep across the area
through tonight with some lingering higher terrain shower activity
Monday

- Another round of isolated light showers possible Tuesday night and
Wednesday

- Temperatures falling below normal the next couple days, however
quickly rebounding near to slightly above normal the latter half of
the week

Early afternoon WV imagery reveals a compact, nearly cutoff upper
circulation center spinning along the northern Baja coast while an
elongated, positively tilted trough entering the Pacific NW will
act to dislodge and absorb this cutoff feature over the four corners
area in the next 48 hours. Concurrently, deep moisture profiles
originating from a remnant tropical system continue to be pulled
poleward in a robust meridional tropospheric wind profile with
objective analysis suggesting 9-10 g/kg boundary layer mixing ratios
hovering along the international border. Satellite and objective
analysis also indicates impressive jet level divergence and strong
ascent juxtaposed with this theta-e surge portending numerous rounds
of showers propagating through the forecast area.

While the anomalous moisture flux will support an unusually high POP
forecast for early June, limiting factors for more expansive,
heavier convective based rainfall include poor lapse rates owing to
the tropical nature of the incoming airmass and extensive thick
cloud cover muting insolation. Thus, embedded thunderstorms, while
prevalent, may not be as prolific through much of the area awaiting
the actual passage of the upper cold core this evening and
overnight realizing modest deep layer instability. Potentially, the
more robust thunderstorm activity may be possible in an area arcing
across SE California/SW Arizona late this afternoon and evening
where MLCape/MUCape 500-1000 J/kg have the best odds of being
achieved. Should this outcome materialize, a few stronger storms
would be possible with gusty outflow winds (20-40% chance of 35+ mph
gusts), patchy blowing dust, small hail, and heavy downpours.

Overall, the majority of the CWA should see some accumulating
rainfall with HREF membership only leaving out portions of Riverside
County on the NW side of the upper low where ascent mechanisms are
not nearly as robust. In general, HREF mean output indicates around
0.10" base total with better than 0.50" accumulations under isolated
thunderstorms and in preferred upslope locations. That said, it
wouldn`t be out of the question for some very localized areas to
receive 1.00"+ accumulation, but would likely require training
thunderstorm echoes with odds less than 20% and representing the
upper end of the ensemble spread. The upper low will have lifted
into northern Arizona by Monday morning with strong evidence of only
a few lingering showers rotating about the southern periphery of the
vorticity lobe. Any remaining showers or new development should
migrate almost exclusively to higher terrain locations Monday
afternoon. While automated NBM POPs and QPF are generally acceptable
forecast parameters over the next 18-24 hours, elevated values
persist far too long with model artifacts providing an unrealistic
outlook Monday, thus have trimmed this mandated NBM output to align
closer to HREF output and conceptual thinking.

Models remain consistent in breaking off a vorticity lobe from the
aforementioned Pacific NW trough into a quasi-cutoff feature
traversing the California coast, then ejecting across the Southwest
during the middle of the week. While some low level moisture may
remain available with this system passage, the majority of deep
tropical moisture will have been scoured away resulting in limited
rain chances. There also continues to be substantial ensemble spread
with respect to the depth and forward propagation speed to this
system yielding low forecast confidence to a scenario already
subject to greater uncertainty given the climatological rarity of
this synoptic setup. Have cut back NBM POP magnitudes and areal
expanse during this time frame given the preponderance of ensemble
output along with a recent precedent for egregiously deleterious NBM
POP forecasts.

While some measure of broad troughing will likely hover over the SW
Conus during the latter half of the week, moisture will finally be
completely scoured from the forecast area as subtropical ridging
begins to strengthen over south TX/northern Mexico. The primary
forecast uncertainty will be tied to the amount of retrogression
this ridge will obtain into the Southwest and the magnitude of
midlevel heights advecting into the region. Ensemble means suggest
H5 heights increasing somewhere in a 584-591dm range resulting in
temperatures rebounding back near the seasonal normal. Should
heights solidly above 588dm be realized, then forecasts towards the
upper end of the guidance envelop (several degrees above normal)
would come to fruition yielding a return to widespread moderate
HeatRisk by the end of the week.

&&

.AVIATION...Updated at 0015Z.

South Central Arizona including KPHX, KIWA, KSDL, KDVT:
Lower cigs, periods of showers, and isolated thunderstorms will be
the main weather concerns this evening. Winds have shifted
southerly/southeasterly across the terminals, which will persist
through the evening and into the overnight hours. SHRA will
continue through 02-03Z then VCSH through 07Z. Cigs should
generally remain aoa 6 kft. Brief reductions to MVFR cigs and/or
vsby is still possible (<30%) if showers become heavy enough over
the next couple hours. CAMs continue to show a potential area of
SHRA/TSRA developing NW of the terminals and drawing nearer before
moving off to the north and east early Monday morning. Confidence
remains too low at this time to include in the TAF for any site
other than KDVT. Winds will then do their usual diurnal switch
back westerly/southwesterly early tomorrow afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
The low pressure center is still over the northern Baja peninsula
and moving slower than originally anticipated. CAMs now show
storms trying to develop later this evening (~02-06Z), however,
confidence is low (<30%) of these showers/storms forming. So, have
removed any mention of VCSH/VCTS/SHRA from both TAFs as radar and
satellite imagery currently do not depict any signs of additional
showers or storms trying to develop. At KBLH, winds will exhibit a
southerly/southeasterly component through the TAF period, with
gusts into the upper teens and up around 20 kt until 09Z. At KIPL,
winds will exhibit a southeasterly component through the period
with a brief period of light and variable winds tomorrow morning.
Wind speeds will be in the low teens through the overnight hours,
otherwise will generally be aob 8 kt. FEW-SCT clouds with bases
aoa 7-8 kft with cigs of 10-12 kft will continue through the
overnight hours before scattering out and starting to lift.

&&

.FIRE WEATHER...
A very unusual wet, late spring weather disturbance will move
through the districts tonight and Monday yielding better than a 50%
chance of wetting rains. Chances for thunderstorms (10-30% chance)
will also linger into Monday which could produce isolated areas of
heavier rainfall. Additional light showers with lower wetting rain
potential will arrive Tuesday night and Wednesday. Otherwise,
afternoon minRHs will range from 25-40% Monday with temperatures
much below normal. As temperatures warm closer to normal by
Wednesday, minimum RH should fall into the teens, then closer to
single digits by the end of the week as temperatures warm further.
Excellent overnight recovery above 70% Monday will deteriorate into a
poor to fair range of 20-40% by the end of the week. Outside of
erratic, gusty winds near showers and storms early in the week,
winds should be fairly typical for late spring with the usual
afternoon upslope gustiness.

&&

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

$$

DISCUSSION...18/Young
AVIATION...Berislavich/Whittock
FIRE WEATHER...18