Flash Flood Warning 14, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
533 AM MST Sat Aug 19 2023

.UPDATE...Updated Fire Weather Discussion.

&&

.SYNOPSIS...
Showers and thunderstorms will become increasingly more widespread
today across southeast California and southwest Arizona as highly
anomalous amounts of moisture associated with tropical cyclone
Hilary pushes into the region. This will lead to extreme rainfall
amounts this weekend across southeast California where
significant and possibly locally catastrophic flooding impacts
could occur. Below normal temperatures this weekend will return
closer to normal by the middle part of next week with drier
weather prevailing.

&&

.DISCUSSION...
A look at the radar this morning shows showers and thunderstorms
picking up across southeast California and southwest Arizona in what
will become a very active, impactful weekend. Jet dynamics and
gravity waves have aided in the development of this elevated
convection this morning with coverage increasing across the western
CWA as moisture continues to push into the region. Looking at the
current synoptic pattern, RAP analysis shows a closed low continuing
to remain situated near the southern California coast while
subtropical ridging remains centered over the southern Plains. This
pattern continues to promote deep southerly flow over the region,
leading to the continued advancement of deeper moisture into the
Southwest ahead of tropical cyclone Hilary. As of this morning,
Hilary remains at major hurricane status west of the southern Baja
Peninsula with maximum sustained winds of 130 mph. However, Hilary
will weaken as it progresses northward into cooler waters.

Thicker moisture will continue to stream northward into the region
through the day with PWATs jumping above 2", particularly across
southwest Arizona and southeast California. Combined with steep
lapse rates, CAPE >1,000 J/kg, and dynamic support, showers and
thunderstorms will blossom across the western CWA going through the
day today. Though DCAPE values won't be particularly high today
given the increasingly saturated atmospheric profile, mechanical
mixing of higher momentum air could produce some strong damaging
wind gusts and perhaps some blowing dust. The latest 00Z HREF keeps
the threat of such winds fairly low with chances of winds in excess
of 35 mph only climbing up to 30% today. As moisture continues to
thicken across the region going through the day today and lapse
rates weaken, convective impacts will transition to a hydrologic
event heading into this evening. Training showers and thunderstorms
producing heavy rainfall will be capable of yielding high rainfall
totals with HREF data indicating probabilities of 24 hour QPF
exceeding 10 year ARIs (Average Recurrence Intervals) increasing
upwards of 50-70% across southeast California. The latest Day 1 ERO
from WPC places most of southeast California under a Moderate risk
of excessive rainfall today/tonight. A Flood Watch goes into effect
this morning for the western half of the CWA and continues into
Monday.

Guidance on the track of Hilary remains very consistent in
propagating the central circulation along or just west of the Baja
Peninsula. Hurricane Warnings are in effect for parts of western
Baja while Tropical Storm Warnings extend up into southern
California. Regardless of any minor shifts in the track of Hilary,
very anomalous moisture will continue to advect into the region
ahead of the circulation center promoting heavy rainfall in the
Southwest. Confidence is high that this will be a very high impact
event across southeast California with significant and potentially
catastrophic flooding impacts possible.

Anomaly measures remain unbelievably extreme with NAEFS mean
climatological output showing maxed out percentiles for each
measured field for this time of year. Places will see potentially
record high PWATs as values increase upwards of 2-2.5" across the
western CWA. Poleward moisture advection persistent through the
tonight through Monday morning time frame should bring recurring
periods of moderate to heavy tropical rainfall to the western CWA.
Ensemble output continue to peg southeast California with the
heaviest rainfall where areas may see more than their normal annual
rainfall with this event. For example, the normal (1991-2020) annual
rainfall for El Centro is 2.38", which is currently forecast to
receive around 2-3". The greatest rainfall totals will coincide with
favored upslope regions in southern California, such as Joshua Tree
National Park where amounts could exceed 6". With such extreme
rainfall expected, very impactful flooding is expected with WPC
placing our far western CWA in a very rare High Risk of excessive
rainfall during the Sunday morning through Monday morning time
frame. Such rainfall is expected to lead to flooding concerns with
the New River in Imperial County, which is forecast to rise several
feet above flood stage according to the latest CNRFC forecast. In
addition to flooding concerns, isolated spin-ups can't be ruled out
with the Storm Prediction Center highlighting much of southeast
California and southwest Arizona in a 2% tornado risk during the
Sunday morning through Monday morning time frame.

Across the eastern CWA, much less rainfall is expected with any
heavy rainfall amounts remaining more localized according to the
HREF. Across south-central Arizona, WPC QPF continues to indicate
storm totals around 0.25-0.50" with locally higher amounts across
upslope regions. By Monday morning the remnants of Hilary should
completely lift north and shear into the western Conus trough with
a somewhat drier, subsident flow regime temporarily sweeping into
the area in its wake. However, enough residual moisture,
favorable southerly flow, and potentially the passage of an
easterly wave will renew convective potential into the middle of
next week.

&&

.AVIATION...Updated at 1200Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Wind trends and a modest chance of SHRA/TSRA highlight weather
challenges through Sunday morning under thickening and lowering
midlevel cigs. Forecast confidence in winds this morning is
moderate with a wide range if directions over the metro. In general,
a E/SE component would be preferred until a late morning switch to
westerly. However, some northerly winds could affect some terminals
as well until the late morning switch. Regardless, models still
suggest periods of higher gusts 20-25kt this afternoon, though with
thicker clouds, stronger gusts may only be intermittent.

Tentatively, have kept a VCSH mention valid during the early evening
hours, though high resolution modeling and forecast soundings only
offer modest support of shower and potentially an embedded storm.
Most likely, only lowering cloud decks and some variability in wind
directions will be the eventual outcome.

Southeast California/Southwest Arizona including KIPL and KBLH:
Periods of rain, occasional thunderstorms, and lowering cigs
resulting in MVFR and IFR conditions will all be weather hazards
today and Sunday. TSRA were quickly filling SE California this
morning and will continue to grow in coverage and intensity
throughout the daylight hours. There will likely be some breaks in
the storm activity, however confidence is low on timing these.
Occasional MVFR visibilities in heavy rainfall is possible, however
cigs should initially remaining above 6K ft. During daylight hours,
winds will be highly variable based on storm location, though trends
indicate directions more often from an east or north direction.

Models suggest numerous storms transitioning into steady moderate
rainfall by early evening with good confidence of cigs lowering
towards solid MVFR/IFR thresholds. The worst flight conditions
appear to be during a 02-08Z time frame. Late tonight into Sunday
morning, there may be a somewhat extended break from SHRA and IFR
conditions, however deteriorating conditions should quickly return
by Sunday afternoon.

&&

.FIRE WEATHER...
Temperatures will cool below normal for this weekend, especially
across southeast California where highs are forecast to mostly stay
in the 80s. This is in response to increasing clouds and
showers/thunderstorms due to tropical cyclone Hilary. Showers and
thunderstorms will become widespread today, primarily near and
west of the Lower Colorado River Valley where very heavy rainfall
is expected to result in extreme rainfall amounts through the
weekend. Can't rule out some strong winds associated with
thunderstorms today, but chances for these winds in excess of 35
mph look to remain low (up to 30%) across southeast California.
Winds will increase out of the southeast Sunday, especially across
the western districts where gusts in excess of 40 mph will be
possible as tropical cyclone Hilary reaches the southwest CONUS.
Drier and warmer conditions are expected to return by the middle
of next week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through Monday afternoon for AZZ530>536-538-539-554-
     559.

     Wind Advisory from 11 AM to 11 PM MST Sunday for AZZ530>533-535-
     536.

     Wind Advisory from 11 AM to 10 PM MST Sunday for AZZ539-559.

CA...Flood Watch through Monday afternoon for CAZ560>570.

     Wind Advisory from 11 AM Sunday to 2 AM PDT Monday for
     CAZ560>570.

&&

DISCUSSION...Smith/18
AVIATION...18
FIRE WEATHER...Smith

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