Flash Flood in Arizona, July 27, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
1038 AM MST Tue Jul 27 2021

.UPDATE...
Updated Aviation

&&

.SYNOPSIS...
Warmer and drier weather will temporarily return through Wednesday
with fairly minimal storm chances mainly confined to higher terrain
areas of Arizona. However, later in the week another wave of low
pressure is expected to affect the region bringing a return to more
widespread shower and thunderstorm activity as early as Thursday.
The more active monsoon activity should persist into the weekend
before a gradual decrease in shower and storm chances by early next
week.

&&

.DISCUSSION...
Yesterday was a much quieter day as the main weather disturbance
lifted northward through California and Nevada, but plenty of
boundary layer moisture remains in place. The drying aloft is
returning the moisture profile back to more typical for the monsoon
season, while a weak subsidence inversion is also evident over the
lower deserts. The generally clear skies are again expected today
with highs warming to a few degrees shy of normals.

The large scale synoptic pattern is still a bit atypical with a
leftover weak upper level low positioned west of Baja and the
subtropical high pressure ridge shifting to the east into the
Central Plains. This will provide broad southerly flow across our
region today with PWATs remaining near 1.4-1.6", or fairly typical
for late July. Additionally, a modest upper level low is seen over
the Gulf of Mexico which is forecast to impact our region later this
week. For today, the ample boundary layer moisture in place and
atmospheric recovery from strong heating should bring a return to
afternoon high terrain storm activity with the HREF mainly pointing
to scattered storms developing from the White Mountains
southwestward through Pima County. The south-central Arizona lower
deserts should remain fairly capped today, so storms are very
unlikely over the deserts. However, gusty outflow winds will be
possible coming out of Pima County late this afternoon/early evening
with the HREF probability of greater than 35 mph winds at 50-70% in
Pinal County and southern Maricopa County. This would normally be a
signal for potential blowing dust impacts, but the recent rains may
limit the dust potential.

Further drying should occur on Wednesday, likely limiting storm
development to far southeast Arizona and along the Mogollon Rim.
HREF members mostly point toward Wednesday as seeing less activity
than today. Temperatures will continue to warm during the middle
part of this week, especially across the western deserts with more
substantial loss of boundary layer moisture. Forecast NBM highs warm
to around 110 degrees across southeast California on Wednesday with
103-105 degrees in the Phoenix area. Readings on Thursday should
tick up another degree or two across the western deserts before the
next round of monsoon moisture and activity by Friday.

Model ensembles remain in good agreement showing another active
monsoon period later this week, but the timing is still somewhat
uncertain. With the subtropical ridge shifting over the Central
Plains, the large cyclonic circulation over the Gulf of Mexico is
seen slowly shifting westward over the next several days. Broad
synoptic scale ascent will begin entering our region from the east
as early as Thursday, but more so on Friday and Saturday. Storm
activity is likely to increase on Thursday, especially over the high
terrain of eastern and southeast Arizona. The lower deserts on
Thursday will still be under marginal moisture, but any substantial
colliding outflows into the south-central Arizona deserts may be
enough for storms to develop over the deserts Thursday evening. As
the easterly wave/inverted trough continues to track westward on
Friday into Saturday, deeper moisture will overspread the rest of
the lower deserts likely leading to fairly widespread chances for
showers and storms. This next disturbance is nowhere near the level
of what we just experienced, but more like a typical monsoon
easterly wave commonly seen during active monsoon seasons.

NBM PoPs reflect the expected increase in monsoon storm activity
later this week with PoPs of 30-40% over the Phoenix area late
Thursday and then widespread 30-50% PoPs over the lower deserts for
Friday and at least through Saturday. PoPs over the Arizona high
terrain also increase to 60-80% during the period. A bit lower
moisture levels compared to the last weather disturbance point
toward strong winds as more of a threat with this upcoming
disturbance versus the last one. Heavy rainfall and localized
flooding will also be a possibility, but probably to a lesser degree
than what we saw late last week and over the weekend. However,
existing wet soils from the recent heavy rains will make any
additional heavy rainfall more problematic than normal.

Model uncertainty increases into early next week with the EPS
favoring a period of drier conditions, while the GEFS holds onto
better moisture and thus continued monsoon storm chances. The NBM
reflects the decreasing storm chances with PoPs gradually lowering
Sunday through Tuesday. After a few days of below normal
temperatures starting Friday, temperatures are likely to begin an
upward climb by Sunday or Monday. NBM highs across the western
deserts do return to normal as early as Sunday, while continued
below normal temperatures look more likely for the Arizona deserts
through early next week.

&&

.AVIATION...Updated at 1738Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Mostly clear skies with light east winds will prevail into the
early/mid afternoon. The switch to light westerly winds may be
disjointed with some measure of variability during the mid
afternoon. Storms are forecast to develop in far southern AZ this
afternoon pushing an outflow boundary towards the Phoenix metro.
Confidence is moderate that this may only be a subtle wind shift
from westerly to SSW with minimal to no gusts. There's even some
chance (~50%) outflow winds never even enter visual ranges before
the typical nocturnal switch back to easterly around midnight. Light
easterly winds are then expected to continue into Wednesday morning,
then a rather typical switch to westerly winds can then be expected
during the early afternoon hours. Convective activity on Wednesday
afternoon/evening is expected to remain well to the north and of the
Phx area (along the Rim/White Mtns), with just an outside chance of
some weak outflows working their way SE-ward into the Phx area
during the evening hours on Wed.

Southeast California/Southwest Arizona including KIPL and KBLH:
Under occasional cirrus decks, wind will be light and primarily
favor southerly directions (outside of a period of light westerly
Sundowner winds at KIPL this evening). Extended periods of variable
directions should be expected as well.

&&

.FIRE WEATHER...
Thursday through Monday:
After a brief break from active monsoon activity during the middle
of the week, another weather disturbance is expected to track into
the region late in the week. This should bring more widespread
shower and storms by Friday, likely lasting into the weekend.
Thunderstorm chances in the eastern districts increase better than
50% with a returning threat of heavy rainfall as early as Thursday.
These heightened storm chances spread westward for Friday and
Saturday. A likely downtrend in monsoon activity is expected late in
the period. Temperatures will range from near to slightly below
normals the bulk of the period. Minimum afternoon humidity values
will range from 15-30% on Thursday before increasing to 25-45% into
the weekend. Overnight recovery will be mostly good to excellent in
a 40-80% range. Apart from thunderstorms, winds will favor typical
daily upslope/drainage patterns.

&&

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

&&

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

&&

DISCUSSION...Kuhlman
AVIATION...Percha/18
FIRE WEATHER...Kuhlman

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
837 AM MST TUE JUL 27 2021

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

0407 PM     TSTM WND DMG     2 NNE JTNP-KEYS VIEW    33.96N 116.17W
07/25/2021                   RIVERSIDE          CA   PARK/FOREST SRVC

            *** DELAYED REPORT *** DOWNED JOSHUA TREE ON
            KEYS VIEW RD. IN JTNP. TIME ESTIMATED USING
            RADAR.

&&

GL