Severe Thunderstorm Warning 100, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
642 PM MST Sun Oct 21 2018

.UPDATE...
Updated 00Z aviation discussion.

&&

.SYNOPSIS...
Periods of isolated showers and thunderstorm for Phoenix and
scattered showers and storms to the south and east of Phoenix will
prevail through Tuesday, although only modest rainfall is
anticipated for most areas. Temperatures will also cool slightly
with readings closer to seasonal normals. Drier conditions will move
into the region by the middle of the week with temperatures
remaining near seasonal normals before a warming trend into next
weekend.

&&

.DISCUSSION....
Current radar is painting some scattered to isolated showers and
thunderstorms, some already severe, extending from eastern Pima
county northeastward to southern Gila county associated with the
latest shortwave rotating around the main upper level low pressure
system just off the west coast. Another day of gusty east-
southeasterly winds continue to blow across the central and
eastern parts of region ahead of this system associated with a
lingering low level southeasterly moisture jet.

For the later half of this afternoon, tonight, and on and off
through Tuesday expect fair chances of isolated showers and
thunderstorms through Phoenix with more scattered coverage expected
to the south, southeast and east of the Phoenix metro. Good
dynamics with southwesterly jet stream energy aloft and lingering
boundary layer moisture and moisture advection out of northern
Mexico, amidst increasing afternoon instability will support our
next round of isolated thunderstorms later this afternoon and into
tonight. The main threats expected with any of these larger
storms with be the potential for severe downburst winds and large
hail up to 1 inch. SPC mesoscale analysis also shows DCAPE values
from 1100- 1500j/kg for the afternoon and evening. There is also
the potential for locally heavy rainfall with some isolated areas
of fleeting, minor flooding, although rainfall totals are
expected to be mostly very modest and hit and miss.

After a period of stalling to our west, the low pressure system is
forecast to finally kick out to the east as an open shortwave on
Monday night and Tuesday giving the region another spike in the
early week, on and off, hit and miss storm chances. Although
rainfall amounts will continue to be mostly very modest. During this
time temperatures will also drop a bit into a more seasonable mid
80s.

By Thursday a ridge of high pressure builds from CA through the
Pacific northwest and a weak, saggy low develops off the coast of
Baja California. Then by the weekend the eastern nose of the
ridge will build into Arizona. As a result expect a warming trend
for the latter part of the week with highs again flirting with the
90 degree mark by Saturday. So overall the mid to latter half of
the week is shaping up to be fairly pleasant weatherize.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Showers and thunderstorms have generally stayed south and east of
the terminals with just one severe thunderstorm briefly moving
within the vicinity of KIWA between 00-01Z. New cells developing
southwest of CGZ will attempt to move north over the next two
hours, but the environment appears to be stabilizing as KPHX
aircraft soundings show a developing inversion between 8-10 kft.
Thus, we have included VCSH through 03-04Z expecting that these
cells will weaken before moving into the terminals, but shear in
excess of 30 knots may be able to overcome the stabilizing air
mass to allow thunderstorms to persist well into the evening
hours. If this occurs, we will amend the TAFs as needed.
Convective activity should be very isolated and much weaker
overnight before picking back up during the morning and afternoon
hours. Confidence on any one period for convection to affect the
terminals tomorrow is too low, so for now we left it out of the
TAF. Winds tomorrow should generally remain east-southeasterly at
10-15 knots with possibly a very late westerly switch after 00Z.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation impacts are expected through tomorrow afternoon as
light winds persist under mostly clear skies. Westerly headings at
KIPL will become more easterly tomorrow afternoon whereas KBLH
will be variable before becoming more S-SE tomorrow afternoon.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Dry conditions and slightly lower humidities will return during
the Wed-Sun period as winds aloft become more westerly. Winds
should remain at 15-20 mph or less area-wide. Minimum humidities
will be on the decrease through the weekend, with values in the
15-50 percent range Wed (lowest over SE CA) dropping into the
teens and 20s by the weekend.

&&

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

&&

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

DISCUSSION...Sawtelle
AVIATION...Hernandez
FIRE WEATHER...Hernandez

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
539 PM MST SUN OCT 21 2018

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

0520 PM     HAIL             8 ESE APACHE JUNCTION   33.38N 111.42W
10/21/2018  E1.50 INCH       PINAL              AZ   PUBLIC

            PUBLIC REPORTED PING PONG BALL SIZED HAIL IN
            GOLD CANYON VIA TWITTER.

&&

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
549 PM MST SUN OCT 21 2018

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

0516 PM     HAIL             6 SE APACHE JUNCTION    33.36N 111.47W
10/21/2018  E0.88 INCH       PINAL              AZ   PUBLIC

            PUBLIC REPORTED NICKEL SIZED HAIL IN GOLD
            CANYON VIA TWITTER.

&&

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
552 PM MST SUN OCT 21 2018

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

0520 PM     HAIL             6 SE APACHE JUNCTION    33.36N 111.46W
10/21/2018  E0.88 INCH       PINAL              AZ   TRAINED SPOTTER

            SPOTTER REPORTED NICKEL SIZED HAIL IN GOLD
            CANYON.

&&

INS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PHOENIX AZ
633 PM MST SUN OCT 21 2018

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

0452 PM     HAIL             9 NNW FLORENCE          33.15N 111.45W
10/21/2018  E1.00 INCH       PINAL              AZ   PUBLIC

            **DELAYED REPORT... PUBLIC REPORTED NICKEL
            TO QUARTER SIZED HAIL NEAR SAN TAN VALLEY
            VIA PICTURE ON TWITTER.

&&

INS

Special Weather Statement
National Weather Service Phoenix AZ
633 PM MST SUN OCT 21 2018

AZZ553-554-220215-
Northwest Pinal County AZ-West Pinal County AZ-
633 PM MST SUN OCT 21 2018

...SIGNIFICANT WEATHER ADVISORY FOR PINAL COUNTY UNTIL 715 PM MST/715
PM PDT/...

At 633 PM MST/633 PM PDT/, Doppler radar was tracking a strong
thunderstorm 8 miles west of Arizola, or 8 miles southwest of Casa
Grande, moving north at 20 mph.

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

Locations impacted include...
Casa Grande, Coolidge, Maricopa, Arizona City, Blackwater, Toltec,
Arizola, Stanfield and Ak-Chin Village.

This includes the following highways...
AZ Interstate 10 between mile markers 181 and 204.
AZ Interstate 8 between mile markers 155 and 178.
AZ Route 87 between mile markers 131 and 146.

LAT...LON 3305 11214 3308 11150 3284 11153 3273 11167
      3273 11174 3276 11174 3276 11177 3280 11178
      3279 11187 3277 11189 3276 11205 3273 11206
TIME...MOT...LOC 0133Z 188DEG 18KT 3281 11186

Hirsch