Flash Flooding in California, September 9, 2022

Area Forecast Discussion
National Weather Service Phoenix AZ
510 AM MST Fri Sep 9 2022

.UPDATE...Updated 12z Aviation Discussion.

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.SYNOPSIS...
Copious amounts of moisture will affect the region today into
Saturday with moderate to heavy rainfall forecast across
southeast California today into Saturday. Showers and
thunderstorms with localized heavy rainfall may also impact
portions of southwest and south-central Arizona later today
through tonight. Storm chances will then again be fairly good on
Monday across south-central and eastern Arizona before drying
conditions mostly end rain chances by the middle of next week.
Temperatures will be substantially cooler today and Saturday with
readings eventually returning closer to normal by early next
week.

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.DISCUSSION...
The now Tropical Storm Kay continues to track north northwest
early this morning with the center just offshore of northern Baja.
The bulk of the convection with Kay is on the north side of the
circulation currently impacting areas across the northern Gulf and
northern Baja. There has been fairly steady northward progress
with this area of rain, as well as a persistent heavy rain band.
Winds have also picked up across Imperial County and southern
portions of Yuma County early this morning with gusts as high as
40 mph in El Centro.

Latest hi-res CAMs suggest the banded rainfall south of our area
will continue to push northward over the next several hours
eventually reaching Imperial County and possibly portions of Yuma
County around or just after sunrise. The HRRR shows considerable
weakening with this area of rain as it moves into southeast
California and far southwest Arizona, but other hi-res CAMs
suggest some bands of moderate to heavy rainfall will eventually
make it into the aforementioned areas by mid to late morning.
Rainfall rates of 0.25-0.5" per hour will be fairly likely as the
area of rain moves northward with any embedded convective bands
possibly bringing 1-2" per hour over a much smaller area.
Guidance continues to suggest the heaviest rainfall today will
fall across western Imperial County and central portions of
Riverside County where easterly upslope flow will be maximized.
Latest QPF amounts have come down some across eastern portions of
Imperial and Riverside Counties as well as much of southwest
Arizona where on average 0.5" is expected with localized amounts
over an inch possible. Average forecast QPF amounts across western
Imperial County and central portions of Riverside County are
still mostly in a 1-1.5" range with localized amounts of over 2"
expected.

Across south-central Arizona today, skies will remain fairly
cloudy with most of the area staying dry through this afternoon.
The 00Z HREF continued to show the potential for clusters of
showers and thunderstorms developing this evening into the
overnight hours across south-central Arizona, likely centered
across Pinal and Maricopa Counties. Forecast soundings look
supportive for a few stronger thunderstorms given the modest
shear, but MUCAPE will be somewhat limited at 500-1000 J/kg. The
main concern with this thunderstorm potential will be any training
of storms leading to localized rainfall amounts of over an inch,
but a southerly steering flow of around 15 kts may be enough to
keep rainfall amounts mostly in a 0.25-0.5" range for most
locations. WPC has highlighted this area in a slight risk on
their Day 1 ERO, but given the inconsistencies in the CAMs will
hold off on expansion of the Flood Watch through south-central
Arizona.

As Kay continues to weaken into Saturday, the NHC track has it
drifting westward, but it will still remain close enough to
southern California to continue to provide for scattered showers
and possibly a few thunderstorms on Saturday. Average forecast QPF
amounts on Saturday are now mostly under 0.5" across southeast
California, so it seems likely the Flood Watch will eventually be
cancelled early.

Drier air aloft will spread northward across southern and central
Arizona on Saturday and eventually through southeast California
by Sunday morning. This dry air will likely keep much of Arizona
mostly dry by the daytime hours Saturday with skies becoming
fairly sunny across southern Arizona. Low level moisture will
however remain very abundant and this will help to reintroduce
chances for showers and thunderstorms mainly across southeast
California and the Arizona high terrain Sunday afternoon. As Kay
makes the full transition into a extratropical low pressure system
on Sunday, it will begin to shift back to the east toward
southern California. Southerly flow is also forecast to increase
across south-central and eastern Arizona Sunday night into early
Monday leading to some additional moisture being advected into
our area. The better moisture along with increased forced ascent
as the remnants of Kay start to move through our region on Monday
is likely to bring good chances for showers and thunderstorms
across south-central and eastern Arizona. NBM PoPs have increased
to around 50% for Monday evening, while forecast soundings suggest
there may be a strong to severe thunderstorm threat as well.

Ensemble guidance shows the remnants of Kay quickly weakening by
Tuesday morning as it moves through our region. Tuesday should
also bring drier westerly flow across our region with any shower
and thunderstorm threat likely confined to the eastern Arizona
high terrain. By Wednesday, we should see enough dry air over the
region to mostly end rain chances.

Temperatures into the weekend are forecast to be below normal with
highs mostly in a 85-90 degree range across the western deserts
to the mid 90s in the Phoenix area. Through the first half of next
week, temperatures will warm a bit, but slightly below normal
temperatures are favored with highs mostly in a 95-100 degree
range.

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.AVIATION...Updated 1210Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Main aviation weather concern through the period will be the
potential for showers and thunderstorms this evening. In the
meantime, mostly light westerly winds will prevail through the
mid-morning hours with some periods of light and variable winds
likely as well. Winds, during the late morning through most of the
afternoon will predominate from south to southwest with speeds
generally remaining less than 10 kts. By the early evening hours,
between the 01-03z time frame, showers and thunderstorms
associated with the outer bands from Tropical Cyclone Kay will
threaten the area terminals. There is still uncertainty in terms
of coverage and whether the activity will directly affect the
terminals, therefore, at this time confidence is still low enough
to preclude from introducing anything higher than VCTS. Any
t-storms that do develop will be capable of producing gusty
winds/brief heavy downpours, resulting in MVFR or lower conditions
for a brief period of time. There is a 10-30% chance of winds
exceeding 30 kts with any of the thunderstorm outflow winds for
this evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
At KIPL, moderate SE winds will prevail through early this morning
with speeds between 10-15 kts with gusts around 20 kts as well as
light showers. Then, by mid to late morning through most of the
afternoon bands of moderate to locally heavy rain associated with
the circulation of Kay will move in, causing MVFR and lower
conditions due to reduced visibilities as well as ceilings. Winds
will be on the increase as well with frequent gusts near 30 kts,
with the potential for gusts near 40 kts at times this afternoon.
Lighter more scattered showers are expected by this evening,
improving conditions back to VFR once again. Winds will also be
diminishing to less than 20 kts while remaining from the SE. At
KBLH, southerly winds of around 10 kts this morning will increase
to around 15 kts with gusts between 20-30 kts by late morning
through the afternoon before diminishing this evening. Periods of
VCSH can be expected by late morning and continuing through the
afternoon. CIGs will remain above 10 kft for the most part, but
could lower briefly to less than 10 kft in shower activity.

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.FIRE WEATHER...
The return of higher humidities will continue through the weekend
as moisture from a tropical system affects the area. Widespread
wetting rains are expected today with more isolated to scattered
showers and possibly a few thunderstorms on Saturday. The
increased moisture will result in minimum afternoon readings
mostly above 40% through Saturday. Breezy southerly winds are
expected for much of today with gusts to around 20 mph across
south-central Arizona to 30-45 mph across portions of southeast
California and southwest Arizona. For Sunday, wetting rain chances
drop to below 10% for the majority of the area, but isolated
thunderstorms will be possible. Monday looks to be a more active
day with more widespread shower and thunderstorm chances across
central and eastern Arizona. A drying trend into the middle of
next week will mostly end rain chances while minimum afternoon
humidity levels drop to between 20-30%.

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory until 5 PM MST this afternoon for AZZ532-535-536.

     Flood Watch from 8 AM MST this morning through Saturday evening
     for AZZ530>533-535-536.

CA...Wind Advisory until 5 PM MST this afternoon for CAZ562-563-
     565>567.

     Flood Watch from 8 AM MST this morning through Saturday evening
     for CAZ560>570.

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DISCUSSION...Kuhlman
AVIATION...Lojero
FIRE WEATHER...Kuhlman