Damaging Winds in California, September 30, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
354 PM MST Sun Sep 30 2018

.SYNOPSIS...
Scattered to numerous showers and thunderstorms at times can be
expected into Wednesday. This will cause localized flash flooding.
For most areas, the most significant rainfall is expected to occur
Monday night through Tuesday night. Meanwhile, high temperatures
will drop into the 80s beginning Monday. Subsequent weaker weather
systems will move through the western states during the latter
half of the week but with much lower chances of rain.

&&

.DISCUSSION...
Phase one of this multiday weather event started several hours early
with very strong storms over Yuma County already having occurred as
of this writing. At the time the storms first started, one of the
few models that had it forecasted was the UofA WRF-GFS which is
often on the aggressive end of the spectrum of the hi-res models.
However, other hi-res models were also indicating considerable
shower activity (if not strong convection) for later on over much
of southwest Arizona into western Maricopa County. This prompted a
revision of the Flash Flood Watch to start it right away in that
area. This is being driven not only by a preliminary push of
moisture but also one or more perturbations/PV anomalies ahead of
Rosa. The trend of more significant rainfall also affecting areas
east of there tonight has model support (plus ongoing convection
may provide additional lift). Thus, it was decided to start the
Watch early for all of our Arizona zones. Southeast CA (except for
the Lower Colorado River Valley) remains unchanged as it looks
like the more robust precip will remain just east of there until
later Monday. As for Rosa, the latest NHC Advisory has it as
barely a Cat 1. It will continue to weaken and by late Tuesday
morning will be only a tropical depression centered just southeast
of Yuma. 24 hrs after that, it will be only a remnant low
centered near Page.

After this afternoon and tonight, anticipate something of a lull
during the daytime Monday before the arrival of the rainfall
directly caused by the Rosa circulation on Monday night through
Tuesday night. This remains largely unchanged. One thing that has
changed is an increase in the storm total QPF due to activity
today/tonight but also model trends for late Monday night and
Tuesday morning. This is most noticeable over western Maricopa
County, Yuma County, and La Paz County, as well as far northern
Maricopa County.

On Wednesday, the large upper low to our west will start to move
inland, supplying enough forcing to result in scattered instability
showers over much of the Arizona deserts, with isolated showers into
far southeast California. The low will open up and race east across
the area Wednesday night into Thursday and as it passes, rain
chances will steadily diminish; expect just a few lingering showers
or storms over the higher terrain east of Phoenix by Thursday
afternoon.

On Friday a large and broad upper trof is forecast to develop across
the western CONUS and southern Arizona will be on the far southern
periphery of the low and under rather dry westerly flow aloft.
Expect cool but rather dry conditions area wide Friday with deserts
highs hovering near 90 and several degrees below seasonal normals.
There is some uncertainty as to how the large trof will develop over
the weekend, but we will likely see conditions becoming unsettled
with at least isolated showers developing Saturday especially north
and east of Phoenix. It will remain cool with central desert highs
mostly in the 80s.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

Showers and thunderstorms are already developing across southwest
Arizona, but much of this activity should stay west of the Phoenix
area this afternoon. We are anticipating additional showers and a
few thunderstorms to develop across southern Arizona later today
before moving into the Phoenix area during the evening and
overnight hours. Coverage of TS is still uncertain but some gusty
southerly or westerly winds and lowering cigs are probable at all
the Phoenix sites. Actual TS over any particular aerodrome is more
uncertain, as is blowing dust, but both are possible albeit not
included in this TAF package. The most likely time frame where
operations would be impacted by storms will be in the 03Z-09Z
range. Scattered showers and variable cigs in a 6K- 10K ft range
will persist well into Monday morning.

Outlook for Monday/Tuesday: A prolonged period of higher impact
weather will affect KPHX through Tuesday morning. Scattered showers
and periods of cigs 4K-6K ft will affect arrival rates for part of
Monday afternoon. Far worse conditions will arrive into the airspace
later Monday evening with widespread moderate/heavy rain, reduced
visibilities, and very low cigs. Many consecutive hours of MVFR
conditions are expected Tuesday morning with several hours of
IFR/LIFR possible within the heaviest rainfall. Though timing is
still uncertain, the worst conditions may occur around the morning
push seriously impacting operations.

Southeast California/Southwest Arizona including KIPL and KBLH:

Showers and isolated storms will move into SE California later
this afternoon and evening which could impacts either of the
airports for a period. Coverage is still a bit uncertain, but
it seems likely showers will be at least scattered, if not
widespread for a period this evening. Thunderstorm activity should
stay to the east of KIPL, but may impact KBLH. The strongest
storms would be capable of producing brief heavy rain, gusty
winds, and localized areas of blowing dust. Storm outflows that
occur will most likely be out of the S-SE to E-SE and sudden wind
shifts may be the greatest impact. Into Monday morning, E-SE winds
ahead of the approaching tropical system will be preferred and a
few showers could linger around the region.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday...
Lingering rain chances continue across the forecast area Wednesday
into Thursday morning...but with much lighter accumulations. On
Friday and Saturday, there will be only slight chances confined
to south-central AZ before chances expand westward into southeast
California Sunday. Below normal high temperatures will persist
this week. Unusually high humidity levels Wednesday will decline
through Friday before leveling off. Despite the drying trend,
humidities will remain above 15%. In fact, most lower deserts will
remain above 20%. Overnight recovery will remain at least good.
Winds will generally be light with more noticeable breeziness by
Sunday.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through late Tuesday night for AZZ530>563.

CA...Flash Flood Watch through late Tuesday night for CAZ569.

     Flash Flood Watch from Monday morning through late Tuesday night
     for CAZ560>568-570.

&&

DISCUSSION...CB
AVIATION...MO/Kuhlman
FIRE WEATHER...AJ/MO

Special Weather Statement
National Weather Service Phoenix AZ
238 PM PDT SUN SEP 30 2018

CAZ564-565-569-302215-
Chuckwalla Mountains CA-Imperial County Southeast CA-
Palo Verde Valley CA-
238 PM PDT SUN SEP 30 2018

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN IMPERIAL COUNTY
UNTIL 315 PM PDT...

At 237 PM PDT, Doppler radar was tracking a strong thunderstorm near
Glamis, or 26 miles east of Brawley, moving northeast at 20 mph.

Wind gusts up to 50 mph will be possible with this storm.

Locations impacted include...
Glamis.

This includes CA Route 78 between mile markers 36 and 64.

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 3288 11495 3299 11520 3332 11508 3321 11472
TIME...MOT...LOC 2137Z 203DEG 16KT 3302 11507

CK

Special Weather Statement
National Weather Service Phoenix AZ
323 PM MST SUN SEP 30 2018

AZZ532-535-536-302315-
Gila River Valley AZ-Southeast Yuma County AZ-Yuma AZ-
323 PM MST SUN SEP 30 2018

...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL YUMA COUNTY UNTIL
415 PM MST...

At 323 PM MST, Doppler radar was tracking a strong thunderstorm over
Fortuna Foothills, moving north at 15 mph.

Wind gusts up to 50 mph will be possible with this storm.

Locations impacted include...
Yuma, Fortuna Foothills, Blaisdell, Ligurta, Kinter and Araby.

This includes the following highways...
AZ Interstate 8 between mile markers 8 and 24.
AZ Route 95 between mile markers 31 and 44.
AZ Route 195 between mile markers 22 and 26.

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 3252 11433 3260 11456 3285 11443 3278 11417
TIME...MOT...LOC 2223Z 201DEG 14KT 3264 11444

CK

Special Weather Statement
National Weather Service Phoenix AZ
325 PM PDT SUN SEP 30 2018

CAZ564-565-567-569-302315-
Chuckwalla Mountains CA-Imperial County Southeast CA-
Palo Verde Valley CA-Imperial Valley CA-
325 PM PDT SUN SEP 30 2018

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN IMPERIAL COUNTY
UNTIL 415 PM PDT...

At 325 PM PDT, Doppler radar was tracking strong thunderstorms along
a line extending from 15 miles northeast of Holtville to 13 miles
southwest of Cibola. Movement was northwest at 10 mph.

Wind gusts up to 50 mph will be possible with these storms.

Locations impacted include...
Glamis.

This includes CA Route 78 between mile markers 28 and 69.

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.

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

&&

LAT...LON 3312 11541 3343 11490 3319 11469 3287 11521
TIME...MOT...LOC 2225Z 129DEG 7KT 3298 11519 3320 11481

CK

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
333 PM MST SUN SEP 30 2018

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

0158 PM     FLASH FLOOD      1 SSE FORTUNA FOOTHILLS 32.64N 114.40W
09/30/2018                   YUMA               AZ   TRAINED SPOTTER

            WASH RUNNING OVER THE STREETS AT 40TH AVE.
            AND DRIFTWOOD DR. AND 52ND ST. AND DRIFTWOOD
            DR. IMPASSABLE.

&&

JS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
337 PM MST SUN SEP 30 2018

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

0300 PM     HEAVY RAIN       1 SSE FORTUNA FOOTHILLS 32.64N 114.40W
09/30/2018  M1.84 INCH       YUMA               AZ   TRAINED SPOTTER

            FELL SINCE 1:00 PM.

&&

JS

Special Weather Statement
National Weather Service Phoenix AZ
400 PM MST SUN SEP 30 2018

AZZ539-559-302345-
Sonoran Desert Natl Monument AZ-Gila Bend AZ-
400 PM MST SUN SEP 30 2018

...SIGNIFICANT WEATHER ADVISORY FOR MARICOPA COUNTY UNTIL 445 PM
MST...

At 400 PM MST, Doppler radar was tracking a strong thunderstorm 12
miles north of Ajo, or 29 miles southwest of Gila Bend, moving
northwest at 25 mph.

Wind gusts up to 50 mph along with blowing dust and reduced
visibilities down to 1 mile will be possible with this storm.

This storm will remain over mainly rural areas of Maricopa County.
This includes AZ Route 85 between mile markers 19 and 32.

LAT...LON 3270 11320 3285 11293 3254 11275 3250 11282
      3251 11299
TIME...MOT...LOC 2300Z 144DEG 20KT 3257 11289

WATERS