Damaging Wind in Arizona, August 9, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
445 AM MST Thu Aug 9 2018

.UPDATE...Updated Aviation and Fire Weather Discussions...

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.SYNOPSIS...
A shift towards a more favorable easterly flow aloft along with
an increase in available moisture is expected to lead to a more
active monsoon pattern through the upcoming weekend. Today looks
likely to be the most active day weatherwise, with the potential
for thunderstorms to approach severe limits, with very strong
winds, areas of blowing dust, and locally heavy rain possible.
This increase in moisture will keep temperatures below normal
through the weekend.

&&

.DISCUSSION...
Yesterday afternoon and evening, storms ended up developing along
the higher terrain of northern and eastern Arizona. Due to the
extensive cloud cover over southeastern Arizona yesterday, the
atmosphere wasn't able to destabilize enough and storm activity
remained rather limited. With a east-northeast steering flow,
activity over the higher terrain was able to propagate into the
lower deserts. Although the 00Z sounding for KPSR had a
substantial about of convective inhibition, storms along the
higher terrain were able to create a strong enough outflow
boundary that was able to overcome the convective inhibition and
develop additional thunderstorms over the lower deserts. Storms
and associated outflow boundaries continued their southwestward
progression, with activity making it all the way into southwest
Arizona. Strong and even some severe storms affected the central
and eastern half of the Phoenix metro and in these areas damaging
winds, dust, lightning, and flooding were the biggest hazards.
Areas that saw the most precipitation were Scottsdale and Fountain
Hills where rainfall totals ranged from approximately .5 to 1.5
inches. For the remainder of the morning, activity and clouds
will continue to dwindle away, with mostly clear skies existing
by late morning.

In the latest NHC public advisory, the center of Hurricane John
was located almost 300 miles west of the southern tip of Baja
California and was moving to the northwest. Although Hurricane
John continues to weaken and move away from the CONUS, it
continues to provide us with decent moisture surges. The center of
the 500 mb high will also push further north today, putting our
region under deep easterly flow and thus allowing return flow and
monsoon moisture to continue to overspread the region. At the 250
mb level, a weak westerly jet will still exist between a high over
the four corners and a low in northern Mexico, placing northern
Arizona under an area of upper-level divergence.

Convection this afternoon will once again develop over the higher
terrain of northern and eastern Arizona. With an easterly
steering flow, activity is forecast to push westward into the
lower deserts. The HREF supports this and brings strong outflow
boundaries into the lower deserts of Maricopa county between 00Z
and 03Z (5PM & 8PM MST) and then through southwest Arizona between
03Z and 05Z (8PM & 10PM MST) and then through southeast
California between 05Z and 07Z (10PM & 12AM MST). The HREF also
supports additional storm development along these outflow
boundaries all the way into southwest Arizona, with activity
dissipating as it moves into southeast California overnight. The
combination of a very moist airmass and sufficient day time
heating today will allow the atmosphere to destabilize with
forecast MLCAPE values this afternoon between 500 and 1500 J/kg.
Decent instability, combined with cooler temperatures aloft, steep
low-level lapse rates, decent shear, and divergence a loft, will
support stronger storms and thus SPC continues to have much of
Arizona under a MRGL risk for severe storms today. In addition, a
very weak steering flow will result in slow storm motion and thus
flash flooding will also be a threat. Lastly, due to the strong
and long-lived outflow boundary that is forecast, dust will
certainly be another threat today from Maricopa/Pinal county
westward.

Depending on how active things turn out today, Friday and
possibly Saturday will likely be a bit less active due to the
atmosphere being worked over by today's storm activity. However,
the activity is still expected to remain at above-average levels,
with the main threat being locally heavy rain/flash flooding due
to slower storm motion and ample available moisture.

A reduction in convective activity looks possible from Sunday
into early next week as somewhat drier air is imported into the
region, with temperatures perhaps warming up a bit as well.
flooding.

&&

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

Active monsoon conditions are again likely for today with good
indications of a significant line of storms moving through the
Phoenix metro early this evening. Confidence at this time is
medium to high that this line of storms will impact the terminals.
Gusty outflow winds, blowing dust, and a period of heavy rain is
very possible as indicated by the TAF sometime between 01-04Z.
Outside of the prime time for storms, winds will remain generally
less than 10 kts and follow diurnal trends.

Southeast California/Southwest Arizona including KIPL and KBLH:

South to southeasterly winds will persist with little aviation
concerns through this evening. Storms in Arizona this evening
will likely generate an outflow boundary that could possibly KBLH,
but not likely KIPL. This may bring a period of gusty winds and
blowing dust, but confidence is too low to include in any TAFs.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Modest moisture levels will affect the region during the period
resulting in day to day thunderstorm chances mostly over high
terrain areas. Minimum humidities will lower from 25 to 30
percent over the deserts on Saturday to 15 to 20 percent by
Monday. Apart from thunderstorms, winds will be light except for
typical afternoon and evening upslope breeziness. Near normal
temperatures this weekend will warm to a few degrees above normal
early next week.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter reports may be needed later this week.

&&

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

DISCUSSION...Hernandez
AVIATION...Kuhlman
FIRE WEATHER...Kuhlman

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
255 AM MST THU AUG 9 2018

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

1248 AM     FLASH FLOOD      2 SSE SCOTTSDALE        33.47N 111.91W
08/09/2018                   MARICOPA           AZ   TRAINED SPOTTER

            SWIFT WATER RESCUE. CAR WITH TWO PASSENGERS
            CAUGHT IN APPROXIMATELY ONE FOOT OF MOVING
            WATER ALONG INDIAN BEND WASH ON MURRAY LANE.
            TIME ESTIMATED.

&&

CK

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
711 AM MST THU AUG 9 2018

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

0730 PM     TSTM WND GST     2 E FOUNTAIN HILLS      33.60N 111.71W
08/08/2018  E62 MPH          MARICOPA           AZ   CO-OP OBSERVER

            **DELAYED REPORT** OBSERVER RECORDED 62 MPH
            WINDS.

&&

AD