Flash Flood Warning 47, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
115 PM MST Thu Sep 18 2025

.KEY MESSAGES...

- The potential for heavy rainfall resulting in flash flooding
continues through tonight with a Flash Flood Watch in effect for
southwest Arizona and southeast California.

- Lingering shower and thunderstorm chances will persist Friday with
activity mostly focused on Arizona terrain features.

- Near normal temperatures over the next couple of days should warm
into an above normal range this early next week with moderate
HeatRisk returning across the lower desert communities.

&&

.SHORT TERM /Today through Friday/...
Early afternoon WV imagery clearly depicts the remnant tropical
circulation spinning west of San Diego with 2 distinct vorticity
centers pivoting about the eastern periphery of the gyre. The
leading shortwave was lifting inland with a notable wing of
vorticity extending well eastward into Arizona while sweeping north,
and largely responsible for the elevated showers and storms across
south-central Arizona this morning. A trailing vorticity maximum was
evident SW of San Diego and will follow a similar trajectory lifting
into SE California this afternoon providing strong ascent mechanisms
in a highly difluent upper air pattern within the preferred left
front jet quadrant.

This favorable dynamic setup is juxtaposed with very anomalous
atmospheric moisture content as an expansive reservoir of 13-15 g/kg
mixing ratios have surged into the lower Colorado River valley and
adjacent areas. Objective analysis suggests total column PWATs
around 2", or around the 99th percentile of climatology though drier
midlevels sampled in satellite imagery was already punching into the
western CWA and will effectively begin to lower total moisture
content. However, this evolution will also allow for better
insolation and daytime instability while maintaining copious
boundary layer moisture such that MLCape 1000-1500 J/kg with minimal
CinH should be realized this afternoon. While exact positioning and
coverage of storms still remains somewhat uncertain, latest HREF
neighborhood probabilities suggests upwards of 50-70% chance of
storms with locally heavy rainfall across the entire western CWA.
Given the boundary layer moisture content below the freezing level
combined with ample instability should result in very robust,
efficient rainfall rates. Ensemble probability matched means
indicate a 30-50% of 1" accumulations with the expectation of some
localized areas exceeding 2" totals this afternoon and evening.

Unfortunately, convective evolution this evening and overnight
towards the eastern half of the CWA remains highly uncertain further
removed from the stronger forcing and more pronounced moisture axis.
Conceivably, convective complexes propagating eastward could
intercept some form of an outflow boundary emanating from higher
terrain convection over south-central Arizona forming renewed
convection during the evening and overnight. However, recent HRRR
iterations have largely been downplaying this scenario lifting
western CWA convection northward with the mean flow aloft while only
creating a limited outflow boundary emanating from higher terrain.
Nevertheless with dynamic structuring still favorable and anomalous
moisture still present over the region, any small perturbation could
spark showers and storms through the overnight and morning hours.

By Friday afternoon, better vorticity advective forcing will have
shifted well north of the area while some measure of anti-cylonic
ridging and warming aloft spreads into the forecast area. While
certainly not optimal considering a lack of forced ascent, boundary
layer mixing ratios of 12-13 g/kg will still be present resulting in
minimal inhibition and MLCape likely approaching 1000 J/kg. Deep SW
flow over any terrain escarpment will certainly be capable of
sparking deep convection through the areal coverage and ability for
convective elements to survive beyond immediate terrain areas is
highly uncertain. There is somewhat better agreement that more
pronounced subsidence will finally spread into the region Friday
night bringing a marked decrease in rainfall chances.

&&

.LONG TERM /Saturday through Wednesday/...
Model uncertainty will continue over the weekend into early next
week as models are having a difficult time resolving what will
happen with a cut-off low that is likely to form off the California
coast Friday into Saturday. Yesterday's model runs had mostly
dismissed this cut-off low as it called for it stay well to our
west, but the latest trends indicate it may get quite close to
southern California Sunday into Monday. We are still expecting our
region to largely be under the influence of the subtropical ridge
this weekend with overall dry conditions, but if the low does creep
close enough to our area we may see some moisture return Sunday into
Monday. The NBM shows 10-20% rain chances for later Sunday into
Monday, but this seems overly optimistic given the amount of
moisture that is likely to be available. We tend to believe the
ridge will win out keeping the cut-off low far enough to the west
resulting in the bulk of our area staying dry through at least early
next week.

Temperatures will be trending upward this weekend into early next
week as the ridge builds in from the south, but with the model
trends of keeping the cut-off low closer forecast highs are
noticeably lower than what we saw 24 hours ago. Highs are currently
expected top out at around 100 degrees this weekend before pushing
closer to 105 degrees by around next Tuesday. The upper end of
guidance does still show the potential for record highs for Phoenix
next Tuesday and Wednesday, but that will only happen if the cut-off
low stays far enough to our west.

&&

.AVIATION...Updated at 1808Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The primary weather concern during the TAF period will be the
potential for thunderstorms across the Phoenix area, though forecast
confidence is low. Confidence deteriorates later this afternoon
through this evening on how thunderstorm activity will evolve.
Latest hi-res guidance continue to show large discrepancies in the
overall coverage of thunderstorms this evening, yielding low
forecast confidence. Have maintained the PROB30 in the TAFs for now
when thunderstorm chances will be the highest. Any thunderstorms
that develop will be capable of producing gusty erratic outflows,
localized blowing dust, and locally heavy rainfall.

Guidance then suggests the potential for additional shower/storm
activity overnight tonight. SCT-BKN cloud bases will mostly remain
aoa 10 kft throughout the period, though could briefly drop lower in
thunderstorms. Outside of any thunderstorm impacts, winds will
generally follow diurnal trends with speeds mostly below 10 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:
The primary weather concern during the TAF period will be increasing
chances for thunderstorms going into this afternoon. Hi-res guidance
show thunderstorms developing across SE CA this afternoon that will
be capable of producing gusty erratic outflow winds, localized
blowing dust, and locally heavy rainfall. Chances for outflow wind
gusts in excess of 30 kts climb to 50% during this time. The primary
window for these thunderstorms looks to be between 21Z-02Z. SCT-BKN
clouds will mostly remain aoa 8-10 kft through this evening, briefly
dropping lower in thunderstorm activity, before clearing this
evening. Outside of thunderstorm impacts, winds will favor the W-NW
at KIPL and fluctuating between the S to NNW at KBLH with speeds
mostly below 10 kts.

&&

.FIRE WEATHER...
Abundant moisture with a weather system residing across western
portions of the region today will help to bring fairly widespread
showers and thunderstorms. Best chances will be across southeast
California and southwest Arizona where a Flood Watch will be in
effect for much of today. Rain chances will continue into Friday,
but mainly across the Arizona higher terrain. Humidities will be
elevated over the next couple of days with MinRHs mostly between 30-
40% before gradually lowering closer to 20% by Sunday. Outside of
any storms, winds will remain light over the next few days with
directions mostly following diurnal trends. High pressure should
settle back into the region over the weekend and into early next
week resulting in a warming trend and overall dry conditions.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through late tonight for AZZ530>533-536.

CA...Flood Watch through late tonight for CAZ560>570.

&&

SHORT TERM...18
LONG TERM...Kuhlman
AVIATION...Smith
FIRE WEATHER...Kuhlman

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