Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/30/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 505 PM MST Sun Aug 29 2021

.UPDATE...
Updated 00Z aviation discussion.

.SYNOPSIS...
The best storm chances will continue to favor higher terrain
areas through Monday, but some isolated showers or storms and
gusty winds could impact the lower deserts. Very warm conditions
will remain across the region through Monday, but with highs
lowering to near normal by Monday. Attention then turns toward an
influx of tropical moisture into the region starting Tuesday with
shower and storm chances increasing starting Tuesday afternoon. A
threat for heavy rainfall and localized flooding should then
affect portions of southern and central Arizona for Wednesday,
possibly lasting into Thursday. Below normal temperatures are also
expected for the latter half of the coming week.

&&

.DISCUSSION...
A persistent band of showers and storms, with moderate to heavy
rainfall, developed this morning near Yuma and lasted into the
early afternoon. The rain-cooled air and cloud cover through
these areas will likely keep temperatures from reaching excessive
levels this afternoon. Whereas, areas in southeast CA and
southwest AZ with more sunshine are likely to still reach highs up
to 110-113F. Temperatures through south-central AZ are expected
to be a few degrees warmer than yesterday with highs in the mid to
upper 100s.

In regards to storm chances through the rest of this afternoon,
objective analysis shows the greatest moisture and instability
being in the Lower Colorado River Valley, with MLCAPE up to
1000-1500 J/kg, and marginal instability across the rest of the
CWA. There is still a considerable amount of CIN analyzed across
the lower deserts, with ACARS soundings in Phoenix showing roughly
-150 to -200 J/kg. This should greatly limit convection over the
 lower deserts as long as it hold. With the cap over the lower
 deserts, most convection is expected to develop over higher
 terrain of northern and eastern AZ, as well JTNP in southeast CA
 and portions of La Paz county. Mid level lapse rates are steep,
 increasing to the north, with 8 C/km and above in northern AZ.
 So, there is potential for robust updrafts with strong storms and
 perhaps a few severe storms this afternoon. With high DCAPE, up
 to 1500 J/kg, damaging outflow winds will be the primary threat.
 As outflows translate to lower elevations blowing dust will be a
 threat (especially in areas that have not seen recent extensive
 flora growth). New storm development from outflows will be
 possible through the evening, dependent on the strength of the
 cap. Forecast soundings in Phoenix show the cap holding through
 the rest of this afternoon, however lingering elevated
 convection, above the boundary layer, late this evening cannot be
 ruled out.

Nora has been downgraded to a tropical storm this afternoon with a
center on the Mexico coastline near the mouth of the Gulf of
California. The latest track from the NHC keeps the central
circulation along the Mexico coastline with a gradual weakening.
The circulation from Nora is still not expected to be a threat to
our area, with it more likely becoming an open wave within the
steering flow before reaching the US border. Tropical moisture
associated with Nora advecting into the Desert Southwest and
leading to scattered to widespread rain remains the primary
threat. Steering flow begins shifting more southerly to
southeasterly in our region Monday as the anticyclone that has
been positioned over northern AZ slides east. Moisture increase
into the region will initially be gradual Monday. 12Z HREF is
suggesting Monday afternoon will be slightly more active than
today with a subtle increase in instability, but still with most
showers and storms favoring higher terrain. NBM PoPs climb to
20-30% across the lower deserts Monday afternoon as there will be
a slightly better chance for storms to survive to lower elevations
over today.

.PREVIOUS DISCUSSION...
Tropical moisture will continue to stream northward into our
region on Tuesday, likely creating a more unstable environment for
further chances for some stronger storms. The steering flow will
turn southerly on Tuesday and vertical ascent fields should still
be weak with an upper level jet max to our southwest. Given that
MUCAPE is likely to increase to above 1000 J/kg and there is no
obvious strong capping inversion, we should have some scattered
afternoon storms Tuesday with gusty winds, heavy rainfall, and
flooding all potential threats.

Models are still showing late Tuesday night through Wednesday
evening as being the prime time for an area of moderate to heavy
rainfall to impact southern and central Arizona, possibly centered
initially over the Phoenix area. The ingredients for heavy
rainfall should be in place as PWATs are forecast to increase to
around 2" across much of south-central Arizona and vertical ascent
may become quite strong over central Arizona. A band (not sure
how wide) of moderate to heavy rain is likely to develop by
Wednesday morning within the increasing southerly upslope low
level flow and within the right rear quadrant of the 60-70kt upper
level jet positioned across southern California into the Great
Basin. Warming aloft will eventually weaken instability on
Wednesday, so the likely threats should completely shift over to
heavy rain and flooding across south-central Arizona and then into
eastern Arizona later Wednesday into early Thursday as the band
is likely to slowly drift eastward. A Flash Flood Watch will
likely be needed for portions of the area during the middle part
of this week. WPC forecast QPF amounts show the heaviest rainfall
focused across Pima and Maricopa Counties, and areas to the north
of Phoenix with a large swatch of 1.25-1.75" of potential rainfall.
Some areas are likely to exceed those amounts, especially in the
more favored upslope foothills areas just north of Phoenix.
Although models have been consistent with showing the heaviest
rainfall falling over south-central Arizona, some east or westward
shift is still possible.

Considerable forecast uncertainty is then seen for Thursday into
Friday with some ensemble members ending rain chances fairly
quickly across the western deserts and maybe as far east as the
Phoenix area. However, other members show a continued chance of
showers and storms through next weekend with moisture remaining in
place over much of the region. For now, the NBM favors the drier
solution. Temperatures will for sure take a big hit into the
middle part of this week with highs likely falling to 10-15
degrees below normal in some areas through Thursday. Temperatures
are then likely to gradually warm up going into next weekend, but
below normal readings are still favored.

&&

.AVIATION...Updated at 0005Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Convective outflows this evening continue to be the main aviation
impact through the TAF period, with a low chance (10-15%) of
isolated thunderstorms developing along outflows, especially across
the northwestern and eastern parts of the Phoenix area. Current
thinking is that the main outflow will be from storms that are now
located over parts of Pima and SE Pinal Counties. Current timing of
this outflow into the Phoenix area is in the 02-03Z timeframe. There
is about a 40% chance of outflow winds in excess of 30 knots, so for
now we have included 20-25 knot gusts in the TAFs. There is also the
possibility of a 2nd, weaker outflow moving into the Phoenix area
from storms that are now located over the Rim, but confidence is too
low to include any sort of 2nd outflow in the TAF`s at this point.
Once the storms dissipate, expecting lighter southeasterly winds to
prevail the the rest of the night and into Monday morning. latest
high-res model output is now indicating a weak weather disturbance
will move northwestward across the region early Monday morning. This
disturbance could set off some showers, and even a TS or two, but
confidence is too low to include either VCSH or VCTS in the TAF`s at
this point.

A more typical switch to westerly winds is anticipated on Monday
afternoon. Monday evening appears it will be more activity
convectivewise, with at least a good chance of significant outflows
from the east of northeast moving to the Phoenix area along for a
better chance for TS at one or more of the TAF sites (20-30%). Cloud
bases should remain elevated above 10-12 kft aside from lower
ceilings down to 8 kft in storms.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KIPL to remain from a south-southeasterly direction thru
the TAF period, aside from a brief period of downsloping westerlies
this evening. At KBLH, isolated showers and thunderstorms may spread
from the northeast later this evening towards KBLH. Still low
confidence (<20% chance) that the terminal will be directly
impacted, but outflow winds from a generally easterly direction may
be possible. Otherwise, a quieter period of mostly southerly winds
is anticipated overnight into Monday morning.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Elevated moisture levels from an influx of tropical moisture will
bring very good rain chances across the region through at least
Thursday. Heavy rainfall and flooding may impact area burn scars,
particularly Wednesday into Thursday. Much cooler conditions will
also move into the region with highs dropping well below normal
starting Wednesday. The increased moisture and cooler temperatures
will keep minimum RHs mainly above 40% for much of the period
before some gradual drying occurs Friday into Saturday. Winds
should be relatively light with an afternoon upslope component and
overnight drainage expected for a good portion of next week, but
southerly winds are also likely to be seen a bit more than
typical.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST this evening for
     AZZ530>533-535-536.

CA...Excessive Heat Warning until 8 PM PDT this evening for
     CAZ560>570.

&&

$$

DISCUSSION...Benedict
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Percha/Rogers
FIRE WEATHER...Kuhlman