Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 07/14/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 448 PM MST Tue Jul 13 2021

.UPDATE...Updated aviation discussion.


&&

.SYNOPSIS...
Heavy rainfall potential increases this evening into tomorrow
morning across much of Arizona. Temperatures will cool tomorrow
across most of Arizona with readings remaining close to the seasonal
average through early next week. Southeast California will remain
warmer with readings slightly above normal. Later this week, low
chances for rain will mostly be confined to Phoenix and higher
terrain areas to the east, but then gradually spreading back
westward by the end of the weekend.

&&

.DISCUSSION...
A very convoluted pattern exists over the Southwest this afternoon
resulting from several convective complexes last night. These
systems left 3 notable MCV`s meandering across the region: 1) the
complex over Tucson decayed into an MCV now crossing southwest into
the Baja, 2) a very compact circulation moving south near Lake
Havasu area, and 3) a "comma head" type MCV lifting north into
southern Utah. Meanwhile, the 500mb ridge aloft continues to
constrict and weaken with the aforementioned MCV creating a very
chaotic looking flow aloft. One issue with this messy flow pattern
is determining mesoscale areas of ascent and subsidence that
eventually affects convective initiation, so overall forecast
confidence has deteriorated somewhat.

KPSR 12Z sounding data once again sampled a pronounced elevated
mixed layer and strong inhibition around the 700mb layer. However,
several recent ACARS soundings indicate much of the capping
inversion has already been eroded with 10-11 g/kg mixing ratios
extended through the boundary layer and a midlevel moisture
intrusion. It appears CinH is already falling below 100 J/kg around
the Phoenix area with forecast soundings showing further reduction
below 50 J/kg by early evening. KTWC 12Z sounding data was heavily
influenced by prior convection, however the profile wan`t completely
overturned and contaminated so airmass recovery is possible.
However, there has been a distinct trend among CAMs to delay or
displace concentrated storms further south into Mexico versus
Pima/Pinal counties. If this scenario comes to fruition, outflow
from the south may be late or absent to arrive at lower elevations
to the north. This may present the inability to achieve the
preferred boundary collision that may be needed for additional
convective formation around the Phoenix metro.

Nevertheless, there are still signals that at least isolated
thunderstorms will form/descend into lower elevations this evening,
though the more robust coverage and impacts are being advertised by
models which have generally been overaggressive this monsoon. While
the wind threat is a bit more diminished now, it is not gone by any
means. Both MLCape and DCape values are currently hovering in a 1000-
1500 J/kg range, so any storms will have the potential for strong
downburst winds. And here`s the conundrum: a single weaker outflow
boundary may kick off no storms and leave the Phoenix area dry this
evening, but a couple towering storms near the metro could produce
several colliding boundaries and many clusters of storms affecting
large chunks of population. Both HREF and NBM POPs fall in a 30-40%
range through much of south-central AZ. For SW Arizona and SE
California, rainfall chances are far more muted.

The highest confidence outcome is looking towards the overnight
hours where a very well defined shortwave in central NV kicks off a
cluster of storms closer to KLAS. There is excellent HREF agreement
that an organized cluster of storms migrates south through Mohave
County and towards the northern CWA with the main question hinging
on whether storms will maintain intensity for the entire journey
into Maricopa County. It`s also difficult to say exactly how any
cluster will interact with other storms or boundaries, but the
potential is there for a localized heavy rain threat.
Thus, a Flash Flood Watch remains in effect through Wednesday
morning. General amounts of 0.10-0.75" seem reasonable with upper
end amounts of 2-4" possible.

&&

.PREVIOUS DISCUSSION...
After an anticipated active early Wednesday, expect Wednesday
afternoon/evening will be a "down" day though a few storms are
possible in the northerly flow from Phoenix eastward. Thursday and
Friday seem to have similar setups with not much changing in
thermodynamic and kinematic parameters, with activity being a bit
depressed but not entirely shut down and continued focus from
Phoenix east. Saturday into early next week, the flow will develop
more of an easterly component as a strong mid-level ridge develops
over the central Rockies. This could yield more "traditional"
convective patterns with the Mogollon Rim/sky islands activating in
the afternoon with progression into the adjacent deserts during the
evening hours. However, given the long fetch of easterly flow, there
could easily be various perturbations within it that will cause
somewhat abnormal periods of forcing.

Temperatures may dip notably Wednesday across parts of south-central
where early morning storms and cloud cover should hurt daytime
heating. With no cooling rain expected, our western CWA will
continue to exhibit hot temperatures, within a few degrees of 110F
each day. Later into the week, the eastern CWA should see afternoons
with just slightly cooler than normal conditions and likely a
reprieve from the 90F overnight lows.

&&

.AVIATION...Updated at 2350z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Thunderstorm trends are favoring areas to the north of Phoenix
this afternoon and evening, with the most likely scenario being an
outflow boundary slowly pushing southward towards the Phoenix area
after 01Z. Phoenix-area TAFs are now advertising a wind shift out
of a northerly direction. Potential for isolated thunderstorm
still remains, but still uncertain as to how exactly this will
evolve. VCTS mention remains in TAF. Blowing dust potential
appears low. Another round of storms is still anticipated mid-
Tuesday morning (12-16Z time period) with scattered storms moving
in from the northwest. Impacts from these will be lowering CIGs
and periods of brief heavy rain.

Southeast California/Southwest Arizona including KIPL and KBLH:
Showers/storms are not anticipated to be major concerns at either
site through the TAF period. At KBLH, mostly southerly winds are
anticipated with late morning/afternoon gusts potentially reaching
25 kts. At KIPL, southeasterly winds are expected tho prevail
through the TAF period, with afternoon gusts peaking around 25
kts.

&&

.FIRE WEATHER...
Friday through Tuesday:
Temperatures will hover near seasonal averages as monsoon moisture
remains in place over the region. Storm coverage during the end of
the week may not be as widespread as earlier in the week, but
activity may increase again early next week. Scattered showers and
storms chance will be greatest over higher terrain of eastern
districts (30-60%) with lesser chances (<20%) in western districts.
Even if storm coverage is lower, lightning and locally heavy
rainfall will continue to be possible. Significant flooding may be
experienced on previously burned areas of Gila County. Minimum
afternoon humidity levels will mostly fall into a 20-40% range in
eastern districts and 15-25% range further west. Overnight recovery
will generally be 40-90% in eastern areas, but only 25-40% in SE
California. Aside from thunderstorm influenced gusty winds, speeds
will typically be fairly light under 20 mph.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through Wednesday morning for AZZ533-534-
     537>563.

CA...None.
&&

$$

DISCUSSION...18/Iniguez
AVIATION...Rogers/Percha
FIRE WEATHER...Iniguez/18