Areal Flood Warning 15, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
436 AM MST Thu Aug 12 2021

.UPDATE...

Aviation discussion update.

&&

.SYNOPSIS...
High levels of moisture will remain entrenched across the region
into early next week, leading to persistent chances for showers
and thunderstorms. The primary threats with any storms remain
localized heavy rainfall that may cause flash flooding. Strong
winds and some blowing dust will also be possible for desert
locations. With increased cloud cover and showers, temperatures
will remain near to below normal.

&&

.DISCUSSION...
Overall a messy forecast across the region the next week as
abundantly high levels of moisture remain in place and various
lows of varying size influence our weather.

A very moist environment is in place across our CWA. Surface dew
points are in the mid 60s to low 70s. The 12.00Z KPSR sounding had
boundary layer mixing ratios near 15 g/kg, which is right at the
upper end of the climatology. The PWAT was 2.1", also at the upper
ends of climatology for the first half of August. Upper air data,
objective analysis, and satellite data depict the upper level
low/vort max we've been watching currently circulating around
Payson. Instability of 1000-1500 J/kg, mostly uncapped, is present
currently across Maricopa and Pinal counties, so would not be
surprised at all to see additional convective development at any
point through this morning. Across the western CWA, a little more
capping is in place but elevated instability is likely to be
present so showers/storms can not be entirely ruled out there.
Given the environment, of course locally heavy rain and flash
flooding is the primary concern.

The general pattern looks to remain a bit stagnant the next two
days (and perhaps into early next week with a larger view) with a
broad anti-cyclone over the Southwest US and the mid level vort
max trapped and meandering around. Today is a bit of a tricky day.
Most CAMs are interestingly quiet for much of the day, but there
are hints of significant activity possible. Much will depend of
course on how the morning plays out, but there is a continued low-
end threat for mountain convection to translate into much of
Maricopa/Pinal/La Paz counties tonight. Similar conditions are in
place for Friday, with some (certainly not all) CAMs developing
such a scenario again with possibly slightly stronger northwest
mid-levels winds and shear in place. Even outside the diurnal
cycle, temporally atypical convection will be possible. While the
main threat is conditionally isolated heavy rains, can't discount
more widespread strong winds and localized severe within wet
downbursts. WPC has kept much of our eastern CWA within a MRGL
category for flash flooding and SPC maintains a General TS in the
outlooks the next few days.

By early Saturday, the vort max overhead may be finally shearing
out... only to be replaced by the next one arriving from the east.
The continued presence of supporting weather features and the
very high boundary layer moisture, which isn't going anywhere
(GEFS mean boundary mixing ratios and PWAT remain AOA the 90th
percentile through Tuesday), means rain/storm chances will
continue right into early next week. Confidence in specifics
within this pattern will drop each day and be highly dependent on
day-to-day mesoscale features. Toward the middle/end of next week,
ensembles point to a trough developing over the Pacific Northwest
which will influence the upper level high currently over us.
Various solutions are possible of course - ranging from a shallow
and progressive trough across the northern Rockies which doesn't
influence us much to a deeper trough that leads to more widespread
rains. For now, the general forecast is for a gradual reduction
in moisture and PoPs, though confidence in the extended forecast
is rather low and carries a high spread.

As far as temperatures - they will likely remain near to below
normal through the next week. With the continued rain chances and
debris clouds, Phoenix has the potential to go the entire next
week without eclipsing the 100F mark. It has been fifty years
(1971) since Phoenix has had an August with 10+ days of high
temperatures below 100F (recently 2016 had eight and 2014 nine).
With more sun likely, El Centro and Yuma will still be popping
into the low 100s. Modestly cooler overnights will also help
mitigate heat impacts, though elevated humidity will still make it
challenging for those subjected to extended outdoor exposure.

&&

.AVIATION...Updated at 1136Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Area of showers/thunderstorms has generally pushed northeast of
metro terminals, but can not rule out additional activity through
the next few hours. Should additional activity develop, cigs and
vsby may briefly drop into MVFR. After ~15Z, anticipate the rest
of the morning and afternoon to be fairly quiet as the atmosphere
settles. Later this evening (~02-06Z), potential continues for
outflow winds from storms north of the metro to move through and
possible trigger additional showers/storms. Given progressive
nature of outflow, convective threat should subside after 06Z.
However, given very moist/unstable environment, there is a 10-20%
chance of redevelopment Thursday night into Friday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

Weak winds and fairly quiet weather through this afternoon
expected. This evening, storms northeast of KBLH should send a
strong outflow through the area. Gusts could exceed current levels
in the TAF. In the Imperial Valley, light winds into the afternoon
will be followed by stronger westerly downslope winds in the
evening.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Generally an active weather period will continue with persistent
chances for showers and thunderstorms, with the best chances
across southern Gila county decreasing as you go west. Humidity
values themselves will remain quite elevated during this period,
with overnight values rising into the 50-90% range and daytime
values falling only 25-45% range. Winds will favor typical
diurnal/terrain influences most days.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

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

DISCUSSION...Iniguez
AVIATION...Iniguez
FIRE WEATHER...Iniguez

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
823 AM MST THU AUG 12 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0615 AM     FLASH FLOOD      9 N SAGUARO LAKE        33.69N 111.54W
08/12/2021                   MARICOPA           AZ   MESONET

            RAID 10 FOOT RISE IN SYCAMORE CREEK.
            RAINFALL OVER THE BUSH FIRE BURN SCAR. DATA
            SOURCE USGS STREAM GAGE.

&&

MS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
848 AM MST THU AUG 12 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0530 PM     FLASH FLOOD      12 ENE GILA BEND        32.99N 112.53W
08/11/2021                   MARICOPA           AZ   DEPT OF HIGHWAYS

            **DELAYED REPORT** STATE ROUTE 238 CLOSED
            DUE TO FLASH FLOODING. LOCATION ESTIMATED
            BASED ON AREA OF HEAVIEST RAINFALL ON RADAR
            AND INFORMATION PROVIDED BY ADOT TWEET.

&&

MS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
903 AM MST THU AUG 12 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0955 PM     FLASH FLOOD      7 ESE BUCKEYE           33.33N 112.50W
08/11/2021                   MARICOPA           AZ   MESONET

            **DELAYED REPORT** ROAD CLOSURE DUE TO FLASH
            FLOODING AT NARRAMORE ROAD DIP CROSSING.
            TIMING FROM MARICOPA COUNTY FLOOD CONTROL
            STREAM GAGE LOCATED ALONG WATERMAN WASH.

&&

MS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
954 AM MST THU AUG 12 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0950 AM     FLASH FLOOD      6 N DEER VALLEY         33.76N 112.12W
08/12/2021                   MARICOPA           AZ   MESONET

            ROAD CLOSURE DUE TO HIGH FLOWS IN SKUNK
            CREEK. MARICOPA COUNTY FLOOD CONTROL STREAM
            GAGE LOCATED UPSTREAM NEAR CAREFREE HIGHWAY
            EXCEEDED ALARM STAGE. TIMING BASED ON TRAVEL
            TIME INFORMATION PROVIDED BY MARICOPA COUNTY
            FLOOD CONTROL.

&&

MS