Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
1020 PM MST Sun Jul 18 2021

.UPDATE...
Updated Aviation

&&

.SYNOPSIS...
Abundant monsoon moisture will continue to bring daily chances for
at least isolated to scattered showers and thunderstorms across
south-central and eastern Arizona with a tendency to favor the
higher terrain of eastern Arizona, especially for the first half of
the week. Isolated storms are also possible over southwest Arizona
and southeast California. Near to below normal highs expected for
the first half of the week, then below normal by late in the week.
More scattered to widespread storms are possible for the second half
of next week and into the weekend. Storms will be capable of
producing both locally heavy rain, strong winds, and blowing dust.

&&

.DISCUSSION...
The H5 RAP and SPC mesoscale analysis shows the UL high centered
near the NV-CO border, barely just N of 4-Corners area.  Fairly deep
and moist S-SE-E flow aloft was present from below H7 to H5 to H3. A
large zone of elevated H7-H5 mean RH>=70 (& 80-90)% extended from N
MX, SE-S-Cent AZ into W-NW AZ.  SPC mesoscale analysis showed
elevated PW ranging from 1.7-1.8” S and W of Phoenix to near 1.5”
near Phoenix which was ~0.4” too low compared to the 12Z Phx
sounding which had over 1.85”.  The 12Z Phx sounding also showed
small caps in the mid levels and fcst sfc CAPE of 1389 j/kg.  The
Phx 12Z also featured above normal mean W of 12.5 g/kg, up from 11.6
gm/kg shown on the 00Z sounding.  SPC confirmed fairly widespread
elevated CAPE (& CIN) from the upper 100s to low thousands j/kg with
more elevated forecast values into the 2-3K j/kg forecast for the
afternoon along the outskirts and surrounding the CWA. 8 mid to late
morning ACARS showed mean MUCAPE up to 1276 j/kg.

Sunday GOES imagery showed abundant mid-upper blow-off clouds
associated with a weak PVA disturbance and and an area of associated
modest thundershowers SE of Phx earlier today. These thundershowers
moved into N Pinal and the SSE CWA including far SE Phx outskirts
this morning with spotty modest rainfall. By midday METSAT showed
cloudy-mostly cloudy conditions and an MCV in the W-central CWA and
W half of the Phx valley with an area of light rain returns, while
mostly clear skies, and stronger heating, dominated the E half of
Phoenix and much of the E CWA. The expanding UL ridge was driving
small disturbances and supporting the very moist and unstable
conditions across the high country, while igniting early afternoon
Mogollon Rim convection/thunderstorms.

The HREF and later runs of the HRRR support E AZ isolated to
occasionally scattered convection moving into S. Gila Cty for the
late afternoon and early evening for a few hours before decaying as
they edge west. Then a second late night round of convection is
depicted that could give Phx or the N-E Valley a glancing blow of
late night storms tonight. It is also very bullish on a very
pronounced E-NE outflow moving through Phoenix late tonight and even
possibly holding together through La Paz county early this morning.

For the first half of the week the UL high moves very little.  As a
result very moist and unstable conditions and diurnal convection
will persist through the region, especially for the higher terrain
with its orographic and upslope advantages. As a result the
preferred NBM POPs remain significant through the period near 30-40%
around Phoenix with slightly lower POPs even supported out W across
SW AZ and SE CA. Storms will be capable of producing both locally
heavy rain, strong winds, and blowing dust.

Clusters and ensembles are still signaling possible upticks in
moisture for the second half of the week and into the weekend. This
may be the result of a larger easterly wave disturbance moving in
from Texas into N-NW MX. Higher end WPC QPF has emerged along with
increased chances for scattered flash flooding. Max temperatures
appear to become quite moderated in and around Phoenix as below
normal highs look to favor the upper 90s by late this week and
possibly even the weekend.

&&

.AVIATION...Updated at 0520Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
E/SE winds should be reinforced tonight from remnant outflows of SE
AZ storms, then eventually switch to W/SW by early/mid afternoon
Monday. SCT mid deck should prevail through Monday morning with
anvil cigs slowly clearing overnight. Isold showers will move west
of visual ranges by midnight though NBM output still shows a 30%
chance of showers through Monday morning. Did not include any
mention in this TAF package. Thunderstorm chances and impacts
appear somewhat less Monday evening though NBM output still shows a
40% chance of storms. HREF output indicates little more than a 10%
chance of outflow winds greater than 30kt. Did not include any TS
mention in these TAFs though have hinted at a SE wind shift/outflow
mid/late Monday evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
Under occasional passing mid/high clouds, southerly winds will be
favored at KBLH with E/SE directions at KIPL. Speeds will remain
fairly steady around 10kt with a few higher gusts possible. Some
enhanced winds due to long traveling outflows may be possible Monday
morning along with localized haze resulting from thick moisture
increases around KIPL. NBM output shows 30% chance of morning
showers at KBLH and 20% at KIPL, as well as a 20-30% chance of
storms Monday evening, but have not included any mention in this TAF
package.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Temperatures will generally be near to slightly below seasonal
averages through the period as monsoonal moisture remains in place
over the region. Storm activity is anticipated to spread as far west
as southeast California with most storms having good potential for
wetting rain (0.10" or more). Threats from storms will continue to
involve locally heavy rainfall and flash flooding along with strong
outflow winds and localized downbursts within storms. Significant
flooding may be experienced on previously burned areas of Gila
County, particularly during the second half of the week. Min RH
levels are expected to remain in the 20-30% range most areas (30-50%
higher terrain of south- central AZ). Max RH levels will be 35-60%
most places (60-90% higher terrain of south- central AZ). Apart from
thunderstorms, winds will favor familiar diurnal patterns.

&&

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

&&

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

&&

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

Special Weather Statement
National Weather Service Phoenix AZ
1241 AM MST Mon Jul 19 2021

AZZ540-544-550-553-559-190830-
South Mountain/Ahwatukee AZ-Sonoran Desert Natl Monument AZ-
Buckeye/Avondale AZ-Northwest Pinal County AZ-
North Phoenix/Glendale AZ-
1241 AM MST Mon Jul 19 2021

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA AND PINAL COUNTIES UNTIL
130 AM MST...

At 1241 AM MST, Doppler radar was tracking a strong thunderstorm near
Estrella Mountain Park, or 10 miles south of Avondale. This storm was
nearly stationary.

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

Locations impacted include...
Avondale, Goodyear, Tolleson, Cashion, Phoenix International Raceway,
Liberty, Estrella Mountain Park, Estrella Mountain Ranch, Rainbow
Valley and Komatke.

This includes the following highways...
AZ Interstate 10 between mile markers 131 and 135.
AZ Route 101 between mile markers 2 and 3.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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 3323 11211 3313 11233 3335 11257 3349 11225
TIME...MOT...LOC 0741Z 146DEG 4KT 3328 11230

SDB