Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/25/21

Area Forecast Discussion

National Weather Service Phoenix AZ

545 PM MST Fri Sep 24 2021

.UPDATE...Updated Aviation Discussion.

.SYNOPSIS...
A slow moving low pressure system will bring unsettled
weather conditions across the region through this weekend. By late
Saturday into Sunday, showers and isolated thunderstorms will
become likely across south-central and eastern Arizona. Lingering
chances for rain will be seen across the eastern Arizona high
terrain through early next week before another weather system
possibly brings another round of rain chances to the lower deserts
sometime mid next week. Temperatures are expected to dip to
slightly below normal levels beginning this weekend, likely
lasting through the first half of next week.

.DISCUSSION...
Latest water vapor imagery and streamline analysis reveal deep
moisture across portions of the Desert Southwest along with the
presence of a well-defined cyclonic circulation. Several vort
maxes are evident across Arizona and latest models indicate these
features will coalesce into a stronger and more unified low
pressure system.

Thunderstorm activity started early this morning across portions
of the Phoenix Metro area, with one spot in North Mesa recording
almost an inch in less than an hour. Additional storms developed
across the Imperial Valley and that area has been the primary
focus for today. PWATs near 1.5 inches are anomalously high in
these areas. The deep moisture combined with exceptionally cold
conditions aloft are creating widespread instability.

CAMs were initially struggling to capture the activity across the
Imperial Valley, however the recent HREF and its members appear
to have a much better handle on the initialization and short-term
forecast. The consensus indicates showers and storms will
gradually dissipate late this afternoon, but not before producing
localized heavy rain and even flash flooding. Further south, a
nearly symmetrical MCS has developed across northern Mexico.
Although this complex is not expected to directly affect Yuma,
stratiform rain and embedded thunderstorms are expected to
continue this afternoon.

Further east across central Arizona, clouds are thinning from
this mornings MCS. However, afternoon ACARS flights out of
Phoenix indicate significant inhibition in the lower levels,
mainly associated with insufficient surface temperatures only in
the 80s. Latest satellite imagery suggests isolated cells across
the Mogollon Rim will persist through the afternoon, though they
will have a difficult time propagating into the lower deserts due
to weak steering flow.

Latest models are in much better agreement than they were a few
days ago, though there is still some uncertainty. It now appears
more likely conditions will generally remain inactive as the
aforementioned low retrogrades through Saturday. Consensus from
the CAMs indicates shower and thunderstorms will increase in
coverage Saturday afternoon and evening, before continuing into
Sunday. Given the anomalously high PWATs, localized flash flooding
will remain a threat. NBM guidance was followed for the official
forecast. This includes a high temperature only in the mid 80s
Sunday as the closed low migrates eastward across Arizona.

.PREVIOUS DISCUSSION...The unsettled weather is likely to continue into next week as model ensembles agree upon a deep Pacific low tracking through
the Western U.S. sometime during the middle of next week. For now
it seems this may bring more rain chances to portions of our
region with the current favored day being Wednesday.

.AVIATION...Updated at 0045Z.

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

Current light westerly winds are expected to transition back to the
east overnight while winds at KSDL and KDVT will mostly remain light
and variable during this time. Chances for additional showers and
thunderstorms look very remote through at least midday Saturday as
somewhat drier air has (for the time being) moved in from the east.
SCT-BKN clouds mostly aoa 10 kft are expected to persist into midday
Saturday. Convective activity is expected to increase across
southeast/eastern AZ on Saturday afternoon, with an outside chance
that an outflow from distant storms could move into the Phoenix
metro area by late afternoon/early evening on Saturday, but
confidence is too low at this point to introduce any sort of wind
shift in the Phoenix area TAFs. A better chance for convective
activity appears to be in the offing late Saturday night, with even
better chances by Sunday morning as an upper-level weather
disturbance passes just to the south of the Phoenix area.

Southeast California/Southwest Arizona including KIPL and KBLH:

Convective activity that was affecting much of SE CA earlier today
has now dissipated/moved off well to the south. Chances for
additional showers and thunderstorms look very remote through at
least midday Saturday as somewhat drier air has (for the time being)
moved in from the east. SCT-BKN clouds mostly aoa 10 kft are
expected to persist into midday Saturday. Winds are expected to
favoring a southerly direction at KBLH and a south-southeasterly
direction at KIPL, with winds at times becoming variable. SCT-BKN
clouds mostly aoa 10 kft are expected through Saturday afternoon.
Chances for convective activity return by late Saturday night/early
Sunday as another weak weather disturbance rotates northward into
the region.

.FIRE WEATHER...
Sunday through Thursday:
An unsettled weather pattern will be in place early in the period
as a slow moving upper level low affects the region. This low will
bring good chances for wetting rains and possible thunderstorms on
Sunday, especially across Arizona. Lingering chances for showers
and thunderstorms may impact the Arizona high terrain through
early next week before another weather system possibly brings more
widespread rain chances during the middle part of next week.
Moisture levels will remain quite elevated for much of the period
with min RHs mostly above 30% each day and overnight RHs
generally between 60-75%. With the exception of locally gusty
winds near thunderstorms, winds will be fairly light each day,
with typical afternoon breezes that will favor local
diurnal/terrain influences.

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

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Flash Flood Watch until 7 PM PDT this evening for CAZ560>570.

DISCUSSION...Hirsch
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Percha
FIRE WEATHER...Kuhlman