Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
430 AM MST Fri Jul 23 2021

.UPDATE...
Updated Aviation

&&

.SYNOPSIS...
A low pressure system will continue to drift slowly towards the Desert
Southwest over the next few days. With abundant Monsoon moisture in
place, thunderstorms will be likely, particularly across portions of
Arizona. The strongest storms will be capable of producing heavy
rainfall and areas of flash flooding. Temperatures will also be well
below normal through early next week. Warmer and drier conditions
are anticipated next week as the low pressure system lifts northward
into the Great Basin.

&&

.DISCUSSION...
Heavy rainfall did materialize across central Arizona last evening,
particularly across portions of the East Valley, north Phoenix,
Scottsdale and the higher terrain of Maricopa and Gila Counties.
This precipitation was largely forced by a well-defined area of
upper level divergence between the slow-moving closed low centered
across southeastern New Mexico and the Monsoon High near Las Vegas.

Roughly 60 percent of the Phoenix Metro area received measurable
rainfall. A few FCDMC gauges even recorded as much as 2+ inches in
downtown Scottsdale, which fell in a short period of time. Some
flood prone areas in north Phoenix also reported significant flash
flooding.

All of the ingredients are in place for continued periods of heavy
rainfall through at least Saturday. After a brief lull, scattered
storms have redeveloped across the Valley early this morning, most
likely due to additional lift provided by a rapidly propagating
gravity wave which originated in northeastern Arizona. The MCS
across central Arizona has become reinvigorated and areas of light
to moderate rainfall will continue into the early morning. On the
mesoscale, there is also likely an embedded MCV, which will promote
lingering precipitation past daybreak for some areas.

The evolution of storms for today has become a bit clearer, given
the stronger model consensus in the HREF. Forecast generally
follows this guidance which suggests precipitation will gradually
dissipate later this morning. Another round of storms is then
anticipated as early as late morning across Gila County. Convective
initiation across the lower deserts is more likely during the mid-
late afternoon as the cap erodes.

Models indicate forcing for ascent is mostly due to vorticity on
the synoptic scale, which is exceptionally rare this time of year.
Latest mesoanalysis confirms the presence of an anomalously strong
vort max across eastern Arizona. As the aforementioned closed low
weakens and opens up, additional vorticity advection is likely
across Arizona. Any CAPE that develops this afternoon is merely a
bonus. Given the larger scale forcing, it is prudent to rely heavily
on the HRRR, which tends to be more skillful than other models in
scenarios like this. This then points to the heavy rainfall
occurring in rounds: one early this morning, another this afternoon
and evening perhaps a bit further west, and then potentially another
round of heavy rainfall overnight. With several opportunities for
precipitation, the daily record precipitation of 0.41 inches in
Phoenix is in jeopardy of being broken.

With PWATs approaching daily records, rainfall rates of 1 to 2
inches per hour are expected. Isolated totals (through tonight) as
much as 2 to 4 inches are not out of the question. Rainfall amounts
of these magnitudes have the potential to result in very dangerous
flash flooding, particularly in urban areas, normally dry washes and
even along mainstem rivers such as the Agua Fria, Hassayampa and
Tonto Creek. A high risk of flash flooding and debris flow also
persists across the burn scars north and east of Phoenix.

Temperatures across the Valley are currently in the 70s and 80s,
reflecting the influence of the rain-cooled air. With abundant
cloudiness anticipated today, temperatures will struggle to reach
the 90s. NBM forecast was reasonable overall, but made a manual
adjustment downward from the NBM max temps, given the lack of
insolation and signals in the hourly data.

Model consensus regarding the low pressure system indicates a trend
towards slower westerly motion, which will prolong the threat of
heavy rainfall through Saturday and into Sunday. Consequently, PoPs
were increased considerably for the weekend. Ensembles remain in
good agreement warmer and drier conditions will return to the Desert
Southwest by Tuesday as the low pressure system gets pulled rapidly
northward into the Great Basin.

&&

.AVIATION...Updated at 1130Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Another brief round of SHRA/embedded TSRA is likely early this
morning before a presumed break in more active weather takes place
into the afternoon. Although NBM POPs have a 60% chance of storms
during midday, have cut off SHRA/TSRA after mid/late morning. Given
the early morning storms, confidence is lower that storms will
readily redevelop and move into the metro late afternoon/early
evening. Kept SHRA/TS mention valid, however refined and cut timing
of more impactful weather with the potential the more robust storms
move west of the metro.

Assuming scattered storms pass through the Phoenix area this
evening, another late evening break is possible before the next
round of SHRA/TS approach the area. HREF members show a near repeat
of numerous overnight storms, and NBM POPs peak around 70%. Timing
might be more uncertain, but anytime after midnight could see TSRA
with heavy rain and brief IFR conditions.

Southeast California/Southwest Arizona including KIPL and KBLH:
Southerly winds will be most common at KBLH while SE winds prevail at
KIPl through Saturday morning. There is the potential for Baja
storms to push an outflow boundary through KIPL again this
afternoon, however probabilities are too low to include in this TAF
package. Otherwise, periods of thicker mid/high clouds will
occasionally spread over terminals.

&&

.FIRE WEATHER...
Sunday through Thursday:
After a very wet period, isolated to scattered thunderstorms will
continue Sunday into next week with the potential for locally heavy
rainfall. Storm activity is anticipated to spread as far west as
southeast California, especially on Sunday and Monday with most
storms having good potential for wetting rain (0.10" or more).
Tuesday will see gradual drying conditions, but enough moisture will
linger for continued daily thunderstorm chances on Wednesday and
Thursday, especially across the higher terrain. Apart from
thunderstorms, winds will favor familiar diurnal patterns.

Min RH levels are expected to be in the 30-45% range across south-
central Arizona (45-60% higher terrain of south-central AZ) and 25-
35% across southeast California and southwest Arizona through
Monday. Max RH levels, outside of storms, will be 55-70% most places
(60-90% higher terrain of south-central AZ). Values will fall into a
more typical range for the summer by Tuesday, with min RH readings
ranging from 15-20 percent in southeast California to 20-35% in
south-central Arizona.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through Saturday evening for AZZ534-537>563.
CA...None.

&&

DISCUSSION...Hirsch
AVIATION...18
FIRE WEATHER...Rogers/Kuhlman

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Special Weather Statement
National Weather Service Phoenix AZ
1146 AM MST Fri Jul 23 2021

AZZ541-545-547-557-231915-
New River Mesa AZ-Mazatzal Mountains AZ-Cave Creek/New River AZ-
Rio Verde/Salt River AZ-
1146 AM MST Fri Jul 23 2021

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 1215 PM
MST...

At 1146 AM MST, Doppler radar was tracking a strong thunderstorm 8
miles east of Camp Creek, or 12 miles west of Sunflower, moving south
at 15 mph.

Pea size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Scottsdale, Desert Mountain, Camp Creek, Horseshoe Reservoir and Rio
Verde.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

If on or near Bartlett Lake, get out of the water and move indoors
or inside a vehicle. Remember, lightning can strike out to 10 miles
from the parent thunderstorm. If you can hear thunder, you are close
enough to be struck by lightning. Move to safe shelter now! Do not
be caught on the water in a thunderstorm.

&&

LAT...LON 3400 11175 3400 11157 3372 11157 3377 11189
      3396 11187 3401 11179
TIME...MOT...LOC 1846Z 007DEG 11KT 3387 11168

INS

Special Weather Statement
National Weather Service Phoenix AZ
1202 PM MST Fri Jul 23 2021

AZZ541-545-231930-
New River Mesa AZ-Cave Creek/New River AZ-
1202 PM MST Fri Jul 23 2021

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 1230 PM
MST...

At 1202 PM MST, Doppler radar was tracking a strong thunderstorm 7
miles east of New River, moving south at 15 mph.

Pea size hail and wind gusts up to 50 mph will be possible with this
storm.

Locations impacted include...
Scottsdale, New River, Cave Creek, Carefree and Anthem.

This includes AZ Interstate 17 between mile markers 225 and 237.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3402 11194 3402 11186 3377 11189 3382 11219
      3404 11212
TIME...MOT...LOC 1902Z 012DEG 11KT 3395 11200

INS