Severe Thunderstorm Warning 64, 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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