668
FXUS65 KPSR 040003
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
503 PM MST Wed Sep 3 2025

.UPDATE...Updated 00z Aviation Discussion...

&&

.KEY MESSAGES...

- Isolated showers and thunderstorms are expected through this
  evening with the best chances focused to the north and south.

- Thunderstorms with strong gusty winds along with heavy rainfall
  capable of producing flash flooding will be possible on
  Thursday with lingering chances into Friday.

- A Flood Watch has been issued for Thursday afternoon through
  Thursday night for most of south-central Arizona, southwest
  Arizona, and southeast California.

- Temperatures will cool over the next few days dropping to below
  normal starting Thursday and staying there through at least
  Saturday.


&&

.SHORT TERM /Today through Friday/...
The main forecast concern through the rest of the workweek will be
the threat for strong storms and the potential for heavy rainfall
capable of producing flash flooding Thursday into Thursday night as
the region see enhanced moisture from TC Lorena.

For today, satellite and radar imagery show isolated showers and
thunderstorms developing across parts of south-central Arizona and
over to the Kofas, while more widespread coverage remains focused
across the northern Arizona high terrain. Latest objective analysis
shows the best moisture and instability residing across western
portions of the CWA where PWATs are around 1.5-1.7" versus ~1-1.4"
across south-central Arizona. Thunderstorm coverage is expected to
remain overall limited this afternoon and evening as supported by
the latest HREF, which keeps the bulk of convection to the north
across the Arizona high terrain and down toward near the
international border. However, any strong outflows that materialize
today will be capable of generating new showers and thunderstorms
with the primary concerns being gusty outflows and perhaps localized
blowing dust. Could also see a few additional isolated showers and
thunderstorms try to develop into the Phoenix area later tonight,
but activity is not expected to be widespread.

Tomorrow is shaping up to be a very active day across the area as
moisture increases ahead of TC Lorena, which is currently noted
spinning off the west coast of the southern Baja Peninsula. Guidance
support PWAT values increasing upwards of 1.7-1.9" into our forecast
area, or around 150-200% of normal. Meanwhile, there are indications
of a weak trough that will be over our region with the axis looking
to set up near the Colorado River Valley. The combination of the
increased moisture, modest instability, and the upper level forcing
along with forcing from strong/colliding outflows should allow
for a good amount of shower and thunderstorm activity from late
Thursday afternoon through at least Thursday evening. Multiple hi-
res CAMs suggest a strong outflow originating from convection
across southeast Arizona will advance towards south-central
Arizona, firing off new showers and thunderstorms along the way.
Neighborhood probabilities for strong outflow winds in excess of
35 mph increases to around 50% for tomorrow afternoon and evening.
The trajectory of the outflow coming from the southeast could
present the possibility of dense blowing dust developing. However,
one of the greater concerns at this time looks to be the
potential for heavy rainfall leading to flash flooding given the
increase in anomalous moisture thanks to TC Lorena. The latest
HREF membership indicates the potential for some impressive
rainfall amounts in some areas, with upwards of a 50-70% chance of
3 hour rainfall totals exceeding 1" and up to a 10-30% chance for
exceeding 3", primarily focused across south-central Arizona and
into southwest Arizona. While current guidance isn`t overly
aggressive with developing convection across southeast California,
believe that moisture and thermodynamic profiles will be
favorable for shower and thunderstorm development which could lead
to instances of flash flooding. As a result, decided to loft a
Flood Watch covering the majority of the CWA.

What happens on Friday is more uncertain as ensemble guidance
remains uncertain with what eventually happens with TC Lorena`s
remnants. At this time, the more likely outcome, as noted by NHC, is
that Lorena will propagate toward the northeast into northern
Sonora, while a minority of members weaken the TC and keep it out in
the eastern Pacific. We will need to watch this to see if any major
changes occur with Lorena, but either way rain chances are likely to
continue for at least portions of Arizona on Friday. PoPs do go down
into Friday, but chances are still fairly high at 30-50%.

Temperatures over the next few days will be quickly trending
downward as moisture increases and shower and thunderstorms become
more widespread. Cloud cover will also increase dramatically by
Thursday with more clouds than sunshine across the bulk of the area.
Highs today are still expected to reach into the normal range, but
highs Thursday are likely to fall short of 100 degrees across much
of the lower deserts. Friday should be the coolest day of the week
with highs only reaching between 90-95 degrees across south-central
Arizona to the upper 90s across southeast California.

&&

.LONG TERM /Saturday through Tuesday/...
This weekend into much if not all of next week is likely to be
very quiet as a deep trough sets up off the West Coast providing
persistent dry westerly flow across the Desert Southwest. The
drier westerly flow should set up already on Saturday, but some
lingering moisture may still allow for some isolated convection
focused mostly across the eastern Arizona high terrain. By
Sunday, even the bulk of the lower level moisture should get
scoured out leaving virtually no chances for rain even across the
higher terrain. Forecast surface dew points are shown to still be
in the upper 50s and 60s this weekend, but then are likely to drop
into the upper 30s to mid 40s by next Tuesday. Temperatures will
also start to warm up as we dry out and this should push daytime
highs back into the normal range by early next week, but overnight
lows are likely to be quite comfortable in the mid 70s to lower
80s.

&&

.AVIATION...Updated at 0000Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The overall wind pattern through early Thursday afternoon will
generally follow the typical diurnal tendencies with speeds
generally aob 10 kts. Although the chances for any outflow
boundaries affecting the area terminals have decreased for this
evening, cannot rule it out completely, especially if thunderstorm
activity develops across southeast AZ. Otherwise, a much more
active thunderstorm period is expected late Thursday afternoon
through Thursday evening with significant operational impacts
likely. FEW-SCT mid to high clouds will prevail through Thursday
morning before increasing to SCT-BKN heading into Thursday
afternoon and evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KIPL will generally be light and variable through this
evening before a more defined southeasterly wind direction
materializes during the overnight period with winds at KBLH
fluctuating between the southwest to south. Overall wind speeds
should remain aob 10 kts through Thursday morning before
increasing Thursday afternoon with the potential for VCSH
materializing, especially at KBLH.

&&

.FIRE WEATHER...
Increased moisture levels will result in much better chances for
wetting rainfall and a reduced wildfire threat through Friday. The
best chances for rain are likely to be later Thursday into early
Friday, with flash flooding and gusty erratic outflow winds being
the primary concerns. Enhanced moisture availability will result
in MinRHs only falling to between 20-30% range today and 30-40% on
Thursday and Friday as temperatures drop to below normal. Outside
of thunderstorm impacts, winds will remain fairly light and
follow diurnal trends today before favoring a southerly direction
for Thursday and Friday. Strong drying conditions will spread
across the region this weekend ending rain chances by Sunday and
eventually lowering MinRHs back into the teens by around Monday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch from Thursday afternoon through Thursday evening for
     AZZ530>563.

CA...Flood Watch from Thursday afternoon through Thursday evening for
     CAZ562>570.

&&

$$

SHORT TERM...Smith/Kuhlman
LONG TERM...Kuhlman
AVIATION...Lojero
FIRE WEATHER...Smith/Kuhlman

NWS Phoenix Office