Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
250 PM MST Thu Jul 29 2021

&&

.SYNOPSIS...
Storm chances will increase from east to west across Arizona today
with chances expanding into southeast California on Friday and
persisting through Saturday. Main storm threats will be strong
winds, blowing dust, and locally heavy rainfall. After near normal
temperatures today, readings will dip to below normal through the
upcoming weekend. Drying conditions starting Sunday will push
storm chances back to only the eastern Arizona high terrain, with
much of the area staying dry during the first half of next week.

&&

.DISCUSSION...
Monsoon activity will quickly increase today with multiple
weather disturbances poised to affect the region. Current radar
loops is already showing convective activity breaking out along
the Rim/White Mountains with additional scattered storms now
occurring across the lower deserts and over some the Gila County
Burn Scars. The main driving force behind thus increase in
monsoon storm activity is an westward advancing upper level
trough currently moving across northern Mexico. This upper level
trough is quite expansive in size and is analyzed as a closed low
at 300mb and mostly as an open trough at 500mb. As it continues to
track west northwest today into Friday it is forecast to weaken,
but much if not all of Arizona is seen coming under broad difluent
flow aloft as early as this afternoon.

latest HRRR high-res model runs are showing storms spreading
across SW AZ over the next few hours along with more activity
moving into southern Gila County. As far as the greater PHX area,
this morning's clouds/virga seems to be limiting activity across
that region at this hour, but latest SPC HREF model output does
indicate storms attempting to move into that region from the east
this evening. Storms will be capable of strong gusty winds with
multiple outflows expected to form and locally heavy rainfall.
With PWATs ranging from 1.3-1.6" and a modest 10 kt east
southeasterly steering flow, this should keep storms fairly
progressive likely minimizing the potential flood threat over the
lower deserts. The biggest threats this afternoon and evening
should be strong winds and localized blowing dust with the 12Z
HREF highlighting a large area of 70-90% probability of greater
than 35 mph winds across the majority of Arizona. Although the
HREF shows a greater chance for this to happen across southern
Maricopa and Pinal Counties, it could happen anywhere this evening
as several outflows are expected. One large westward advancing
outflow is likely to reach the Colorado River by late evening with
additional showers and weak storms likely across southwest
Arizona into southeast California during the early overnight hours
tonight.

Friday still looks to be another active monsoon storm day, but
with less activity across eastern Arizona as some drier air works
in from New Mexico. The weakening upper level trough slowly
lifting northwestward through the Desert Southwest should still
provide some upper level support, while a trough off the West
Coast also provides additional ascent across southern California.
Moisture levels Friday will improve over today in most locations
with PWATs upwards of 2" across the western deserts, to 1.5-1.75"
across much of south-central Arizona, but lowering closer to 1.25"
in eastern Arizona. MUCAPEs Friday should also be higher likely
ranging from 1000-1500 J/kg from Phoenix westward through
southeast California. The storm threats Friday and Friday night is
expected to transition more toward heavy rainfall and localized
flooding, but southeasterly steering flow of 10-15 kts should keep
storms from remaining stationary over one area for very long.
Strong winds will continue to be a threat, but mostly in the form
of storm downdrafts instead of outflows. Bulk shear also increases
by Friday afternoon/evening across southwest Arizona and parts of
southeast California, so a few longer lasting severe storms may
be possible, along with a greater threat for flash flooding across
that area, thus a Flash Flood Watch has been issued for that
region.

For Saturday, the inverted trough will become a non-factor as it
lifts north, but the lingering trough off the West is forecast to
send up another upper jet max out of the south. This jet should
again provide broad ascent over much of the area on Saturday, but
we begin losing moisture especially across the western deserts.
Storm chances for Saturday are still quite uncertain, but NBM PoPs
are mostly in a 30-40% range. The drier air working in from the
southwest should overtake much of Arizona on Sunday likely
limiting storm chances Sunday to the high terrain north and east
of Phoenix. The first half of next week also looks to be quite dry
and may only involve a few daily afternoon showers and storms
across far eastern Arizona. Boundary layer moisture should also
gradually lower with each day early next week as the subtropical
high reforms initially just to our southwest before slowly
drifting over southern California into Arizona by Tuesday or
Wednesday. Although NBM high temperatures increase back to around
normal readings early next week, would not be surprised if we will
have to worry about some areas near excessive heat. If we do dry
out the boundary layer next week and a 594dm 500mb ridge is
overhead, excessive heat will be a possibility.

&&

.AVIATION...Updated at 1854Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Very light showers and virga are moving through the metro area
late this morning. Minimal impacts are expected from these showers
besides some enhancement to easterly winds. As the showers push
east of terminals they may lead to a brief west to southwest wind
shift. Otherwise, winds may continue to favor an east to south
component through most of the period. Storm chances increase later
this afternoon and evening. Latest models are now keeping most of
the storms in the periphery of the metro, but for now will keep
the mention of VCSH and VCTS in the TAFs. Models are still
supporting a 50-70% chance for wind speeds in excess of 30 kts
this evening from outflows. The most favorable direction for
outflows is from the east to southeast, but cannot rule out
outflows from other directions like the southwest. Storms should
shift west of the metro before midnight, but some lingering
showers are possible overnight. Similar storm chances are expected
Friday with development in the afternoon after 20Z. Cloud bases
are expected to remain aoa 7 kft through the TAF period.

Southeast California/Southwest Arizona including KIPL and KBLH:
Thunderstorm chances at the TAF sites remain below 10% through mid
afternoon Thursday but will increase to 15-25% late Thursday
evening and early Friday morning. Showers and thunderstorms over
western Arizona are expected to weaken as they cross the lower
Colorado River, but still a chance they remain intact near KBLH.
At KIPL, the potential for showers or an isolated thunderstorm
over the Imperial Valley is more likely to occur Friday morning
and or Friday afternoon. Apart from thunderstorms, winds are
anticipated to remain light and favor downvalley nocturnal
patterns during the overnight hours (through the morning at KBLH)
before transitioning to south and southeast directions. An outflow
boundary may affect KBLH during the evening, likely favoring a
direction of southeast or east. Periods of high clouds are
expected today before thickening overnight.

&&

.FIRE WEATHER...
Sunday through Thursday:
Unsettled conditions are likely through Sunday with the last of
the weather disturbances moving through the region. This should
bring fairly widespread shower and storm chances on Saturday
before more limited storms on Sunday. Shower and storm chances for
Mon-Thu will be limited to a few storms during the daytime hours
over the eastern Arizona high terrain. Main threats on Sunday will
be strong thunderstorm winds, heavy rainfall and possible
localized flooding. Temperatures will range from near to slightly
below normals the bulk of the period. Minimum afternoon humidity
values will range from 25-45% through the weekend before falling
back to 15-30% early next week. 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...Flash Flood Watch from Friday afternoon through late Friday
     night for AZZ530>533-535-536-538-539-559.

CA...Flash Flood Watch from Friday afternoon through late Friday
     night for CAZ569-570.

&&

DISCUSSION...Kuhlman
AVIATION...Benedict/Rogers
FIRE WEATHER...Percha/Kuhlman

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