Flash Flood Warning 94, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
509 AM MST Thu Sep 26 2019

.UPDATE...
Updated 12Z aviation discussion.

&&

.SYNOPSIS...
Shower and thunderstorm chances will continue across the area today
into tonight, including over the greater Phoenix area as a strong
upper level disturbance slowly moves through the region. Heavy
rainfall and localized flooding will be the main hazards expected
today. This weather system will finally move east of the region
Friday with drying conditions expected this weekend and only slight
chances for rain across the higher terrain late in the weekend.
Temperatures will remain well below seasonal normals into the first
part of next week.

&&

.DISCUSSION...
At 3 am this morning, a large and nearly stationary Pacific upper
low continued to spin in place over the desert southwest, centered
near Yuma. A series of vort spokes continued to rotate around the
low; the enhanced upper divergence/PVA associated with each
disturbance kicking off areas of showers and storms as they pivoted
around the center. Hours ago strong convection moved through areas
such as far southeast California and Wickenburg and at 3 am
scattered showers were forming across portions of south central
Arizona including the Phoenix area - especially high terrain areas
northeast of Phoenix. Of course, moisture remains highly elevated
across the area with surface dewpoints in the low 60s to near 70,
850mb dewpoints above 12c and PWAT values over 1.5 inches. SPC
mesoscale analysis page showed MLCAPE across the deserts to be
moderate and mostly between 500 and 1000 j/kg.

Model guidance continued to keep the low centered near Yuma today,
moving it slowly east and into the far western deserts later in the
afternoon and early evening. Cool air aloft near the low center will
help to increase instability and aid in the formation of instability
showers and storms mainly in the afternoon, and additional areas of
UVV will move around the low and across the deserts associated with
vort spokes, aiding convection. POPs will remain in the scattered to
likely categories area-wide. As the low starts to move east and into
the central deserts later tonight, convection becomes more isolated
over the western deserts including far southeast California, and
mainly scattered over south-central Arizona. During the day Friday,
the low is forecast to finally open up and speed up its progression
as it starts to kick east ahead of the strongest westerlies that
will be dropping south into the area. By Friday afternoon, a drier
and more subsident northwest flow aloft will set up over most of the
deserts with only isolated showers lingering over the high terrain
of south central Arizona; skies over the central and western deserts
will transition to mostly sunny. Look for very cool high
temperatures today and Friday running nearly 10 degrees below normal
over the central deserts. Phoenix will hover near 86 degrees both
days compared to the normal of 96.

Over the weekend, the weather pattern will start to transition to a
much drier and more settled one, as a very large upper level trof
begins to develop across the western CONUS. For the most part,
Arizona will be on the south or southeast periphery of the large
trof and under a drier southwest flow aloft. Over the weekend, and
mainly during the day Sunday, a disturbance moving through the trof
will act on some lingering low level moisture to produce isolated
showers and possibly a thunderstorms over the higher terrain areas
to the northeast of Phoenix. Convection is not expected to develop
over the greater Phoenix area however. This disturbance moves away to
the northeast later Sunday and as we move into the first part of
next week, flow becomes ever drier for sunny days and clear nights.
Given the presence of the large trof we can expect high temperatures
to remain below seasonal normals throughout the 7 day forecast
period, with highs in the greater Phoenix area typically hovering in
the lower 90s.

&&

.AVIATION...Updated at 1209Z

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

Scattered showers and a few thunderstorms that are moving north
over the far West Valley near BXK should end around 13Z. A lull
in convective activity is expected for the rest of the morning
with SCT-BKN 6-8 kft ceilings being the main aviation concern
until southeast 10 knot winds gradually veer to southerly by late
morning, causing a prolonged period of crosswinds at KPHX and
KDVT. These winds will continue until later this afternoon when
isolated to scattered convection starts moving in from the west.
Hi-res models delay this convection until closer to 23-00Z, with
midlevel stability potentially limiting the depth of these cells
for the Phoenix terminals. Therefore, we have only included VCSH
for now until 02-03Z, but a shortwave currently moving through
the Gulf of California may generate enough ascent to allow for a
few thunderstorms. Light westerly or variable winds are expected
this evening before becoming easterly very late in the overnight
hours with morning showers most likely staying east of KIWA.

Southeast California/Southwest Arizona including KIPL and KBLH:

Shower and thunderstorm activity continues to move north just to
the northeast of KBLH with a lull in convective activity expected
this morning. The upper level low just southeast of the region
over the Gulf of California should finally eject to the east this
afternoon, helping trigger another round of thunderstorms over
the Colorado River Valley. Convection is not expected over KIPL
which should be under weak subsidence west of the low, but they
are expected at KBLH where we have included a VCTS period from
21-00Z for now. Gusty winds and blowing dust will be possible with
these storms, but storms should generally be weaker than they have
been over the past few days due to weaker instability. Light and
variable winds should prevail once convection wanes, with no
convection expected for the foreseeable future after tomorrow.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday:
A slight chance for showers and thunderstorms may exist Saturday
afternoon, but otherwise much drier conditions are expected into
the middle of next week with temperatures below normal. Daytime
RH values will generally remain above 15% with good overnight
recovery. Winds will be somewhat stronger on Sunday but will
otherwise be fairly light and favor diurnal trends.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should be prepared to submit reports through Thursday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through this afternoon for AZZ530>559-561-562.

CA...Flash Flood Watch through this afternoon for CAZ560-561-564-565-
     567>570.

&&

DISCUSSION...CB
AVIATION...Hopper
FIRE WEATHER...Rogers/Deems

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Special Weather Statement
National Weather Service Phoenix AZ
1126 AM MST Thu Sep 26 2019

AZZ540-550-553-559-261900-
South Mountain/Ahwatukee AZ-Sonoran Desert Natl Monument AZ-
Buckeye/Avondale AZ-Northwest Pinal County AZ-
1126 AM MST Thu Sep 26 2019

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA AND PINAL COUNTIES UNTIL
NOON MST...

At 1126 AM MST, Doppler radar was tracking a strong thunderstorm near
Estrella Mountain Park, or 10 miles south of Avondale, moving
northeast at 10 mph.

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,
Estrella Mountain Park, Rainbow Valley and Komatke.

This includes AZ Interstate 10 near mile marker 132.

LAT...LON 3317 11235 3327 11247 3347 11230 3331 11208
TIME...MOT...LOC 1826Z 223DEG 10KT 3327 11233

AD

Special Weather Statement
National Weather Service Phoenix AZ
1130 AM MST Thu Sep 26 2019

AZZ538-539-559-261915-
Sonoran Desert Natl Monument AZ-Tonopah Desert AZ-Gila Bend AZ-
1130 AM MST Thu Sep 26 2019

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

At 1130 AM MST, Doppler radar was tracking a strong thunderstorm near
Gila Bend Auxiliary Field, or 7 miles southwest of Gila Bend, moving
northeast at 10 mph.

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

Locations impacted include...
Gila Bend, Gila Bend Auxiliary Field and Bosque.

This includes the following highways...
AZ Interstate 8 between mile markers 107 and 125.
AZ Route 238 between mile markers 1 and 9.
AZ Route 85 between mile markers 1 and 12, and between mile markers
119 and 128.

LAT...LON 3276 11275 3286 11292 3312 11276 3296 11250
TIME...MOT...LOC 1830Z 216DEG 9KT 3287 11278

AD

Special Weather Statement
National Weather Service Phoenix AZ
1135 AM MST Thu Sep 26 2019

AZZ533-261915-
Central La Paz AZ-
1135 AM MST Thu Sep 26 2019

...SIGNIFICANT WEATHER ADVISORY FOR EAST CENTRAL LA PAZ COUNTY UNTIL
1215 PM MST...

At 1135 AM MST, Doppler radar was tracking a strong thunderstorm 8
miles southeast of Vicksburg Junction, or 41 miles west of Tonopah,
moving northwest at 5 mph.

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

Locations impacted include...
Salome, Vicksburg, Vicksburg Junction, Harcuvar and Wenden.

This includes the following highways...
AZ Interstate 10 between mile markers 42 and 58.
US Highway 60 between mile markers 43 and 60.
AZ Route 72 between mile markers 44 and 49.

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.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 3375 11349 3352 11357 3344 11370 3366 11385
      3394 11370
TIME...MOT...LOC 1835Z 143DEG 5KT 3363 11363

SDB

Special Weather Statement
National Weather Service Phoenix AZ
1142 AM PDT Thu Sep 26 2019

CAZ560-561-564-568-570-261930-
Chuckwalla Mountains CA-Chuckwalla Valley CA-Chiriaco Summit CA-
Joshua Tree NP West CA-Joshua Tree NP East CA-
1142 AM PDT Thu Sep 26 2019

...SIGNIFICANT WEATHER ADVISORY FOR SAN BERNARDINO AND RIVERSIDE
COUNTIES UNTIL 1230 PM PDT...

At 1140 AM PDT, Doppler radar was tracking multiple strong
thunderstorms over parts of Joshua Tree National Park to south of
Desert Center, moving southwest at 10 mph.

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

Locations impacted include...
Desert Center, Chiriaco Summit, Cottonwood Visitor and Lost
Horse-Keys Village.

This includes the following highways...
CA Interstate 10 between mile markers 82 and 108.
CA Route 177 near mile marker 1.

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.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 3410 11613 3405 11600 3403 11600 3404 11598
      3389 11563 3363 11522 3347 11540 3379 11608
      3392 11620 3393 11632 3395 11636 3397 11636
TIME...MOT...LOC 1840Z 023DEG 8KT 3375 11577

SDB

Special Weather Statement
National Weather Service Phoenix AZ
1217 PM MST Thu Sep 26 2019

AZZ531-533-262000-
Kofa AZ-Central La Paz AZ-
1217 PM MST Thu Sep 26 2019

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL LA PAZ AND NORTHERN YUMA
COUNTIES UNTIL 100 PM MST...

At 1217 PM MST, Doppler radar was tracking strong thunderstorms along
a line extending from 12 miles southwest of Alamo Lake to near
Brenda. Movement was west at 15 mph.

Pea size hail and winds in excess of 40 mph will be possible with
these storms.

Locations impacted include...
Quartzsite, Salome, Brenda, Vicksburg Junction, Wenden, Kofa Wildlife
Refuge, Vicksburg, Bouse and Harcuvar.

This includes the following highways...
AZ Interstate 10 between mile markers 10 and 57.
US Highway 60 between mile markers 31 and 64.
AZ Route 72 between mile markers 14 and 49.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

&&

LAT...LON 3424 11339 3336 11362 3354 11442 3401 11422
      3426 11400 3426 11386 3423 11384 3424 11373
      3421 11370 3423 11361 3426 11358
TIME...MOT...LOC 1917Z 097DEG 14KT 3411 11367 3367 11399

AD