559
FXUS65 KPSR 191823
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1123 AM MST Fri Jul 19 2024

.UPDATE...Updated 18z Aviation Discussion.

&&

.SYNOPSIS...
Thunderstorm chances will persist across the area through at least
the middle of next week with the highest probabilities over higher
terrain areas of Arizona. Far better odds for thunderstorm survival
into lower elevations will arrive over the weekend and early next
week. Otherwise, widespread excessive heat will impact much of the
region the next several days with temperatures better than 5 degrees
above the daily normal.

&&

.DISCUSSION...
Early morning WV imagery and objective analysis depict an expansive
subtropical anti-cyclone over the SW Conus with a 597dm H5 center
anchored directly over central Arizona. Several shortwaves were
rotating about the periphery of this high pressure bubble - most
notably, one lifting north into southern/western Nevada, another
diving southeast into northern New Mexico, and lastly, a far more
robust inverted trough over northern Sonora. Even closer to the anti-
cyclone center, there are very subtle perturbations such as an MCV
emanating from a convective complex over SE Pinal County.
Operational and emsemble members are in very good agreement showing
this feature remaining quasi-stationary through the next 48 hours
before weakening slightly and migrating northwest into central
Nevada.

With heights aloft nearing 2 normalized standard deviations above
the seasonal mean, H8-H7 thermal profiles will approach the 90th
percentile of climatology, or around +30C and +15C respectively.
Historically, values of this magnitude equate to lower elevations
surface temperatures around 115F, and forecast guidance spread is
very narrow around this expectation yielding rather widespread major
HeatRisk the next several days. It wouldn`t be impossible for
outflow moisture intrusions or extensive residual clouds to disrupt
the extreme heat, however no tangible evidence in model output
suggests this outcome. Thus, confidence is excellent that
temperatures 4F-8F above the daily normal will be common the next
few days, through still a few degrees short of record territory.

Thunderstorm evolution this afternoon/evening should be somewhat
similar to the past couple days with activity concentrated over
eastern Arizona higher terrain sending outflows downhill into lower
elevations. HRRR output continues to be the most bullish in
maintaining some form of deep convection into lower elevations,
however this model has displayed a high percentage of false alarm
forecasts the past few days. Given boundary layer mixing ratios are
still hovering at a marginally supportive 9-10 g/kg, it will likely
take multiple colliding outflows to invigorate new updrafts removed
from higher terrain; and with some amount of convective overturning
in SE Arizona, extensive storms wouldn`t be expected in this area
discounting strong outflow emanating from this direction. As a
result, the preponderance of evidence suggests limited chances of
storms surviving too far from the mountains, and even NBM POPs seem
a bit overaggressive.

Convective potential begins to change over the weekend as the
aforementioned subtropical high starts shifting to the northwest
allowing better divergence aloft combined with favorable steering
flow to direct midlevel ascent and deeper moisture intrusion into
the CWA. Although better quality moisture will likely still remain
removed from the forecast area Saturday with MLCape struggling to
exceed 500 J/kg, HREF output indicates a greater concentration of
storms in eastern Arizona thrusting outflow and deeper moisture
westward during the evening/overnight. While a few lower elevation
storms Saturday evening might be expected, this scenario looks to
set the stage for a greater impact event Sunday. Forecast BUFR
soundings suggest mixing ratios finally achieving a preferred 11-12
g/kg level yielding a "sweet spot" 1000-1500 J/kg MLCape juxtaposed
with DCape in excess of 1500 J/kg. Increasing N/NE midtropospheric
flow should direct storms initiating along the Rim into lower
elevations with minimal inhibition and favorable synoptic conditions
encouraging additional development. Strong, locally damaging winds
and areas of dense blowing dust have historically been associated
with similar scenarios. While assumed forward storm motion would be
progressive limiting flooding issues, it cannot be totally
discounted especially across recent wildfire incidents.

Forecast confidence deteriorates quite rapidly for the remainder of
next week as uncertainties in both the mesoscale and larger synoptic
patterns grow substantially. Daily convective trends will almost
certainly be modulated by previous days activity and ability for
atmospheric recovery after the influence of convective complexes.
However, there is convincing evidence that 10-12 g/kg mixing ratios
will be maintained through at least the middle of the week resulting
in a continuation of MLCape above 1000 J/kg and convective potential
after peak heating. Adding to the uncertainty towards the middle of
the week is a small handful of ensemble members indicating the
Southwest subtropical high re-strengthening (most aggressive members
near 600dm) and shifting back into northern Arizona resulting in
warming aloft and larger scale subsidence. However, while the
majority of ensembles also shift the high pressure center back
southeast, its magnitude is far weaker in response to deep troughing
approaching/entering the NW Conus. Given this model spread, mandated
NBM forecasts are only advertising temperatures in a slightly above
normal range with POPs essentially set near climatology, though the
range of possible outcomes is far wider than the official
deterministic NBM forecast.

&&

.AVIATION...Updated at 1810Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Westerly winds will continue into the evening, with speeds aob 10
kt. The main concern this evening will be any outflow boundaries
moving through the Phoenix Metro. It currently looks like most
likely outflow will come out of the north/northeast. KDVT and KSDL
continue to be the most likely terminals to be impacted by this
outflow between 00-03Z, with some gusts up around 25-30 kt
expected. Elsewhere, it is currently expected that the outflow
will die out before reaching the KPHX or KIWA terminals. However,
the HRRR and GFSLAMP are indicating that the outflow, may reach
KPHX between 02-05Z, with gusts up around 15-20 kt. Other hi-res
guidance does not show the outflow reaching KPHX and the HRRR has
been aggressive over the past couple of days, so confidence is low
in that model for the time being. VCSH/TSRA is not expected at
the terminals with the chance of rain in the Phoenix metro <20%.
However, if any terminals were to be impacted KDVT and KSDL would
be the most likely locations. Should multiple outflows move
through the Metro, if they were to converge, rain and thunderstorm
chances would increase. Skies will remain FEW-SCT and
occasionally BKN this evening as debris clouds from earlier
convection move overhead.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KBLH will continue out of the south through the TAF
period. With the exception of some gusts up around 20 kt over the
next hour, wind speeds will generally be aob 12 kt. At KIPL, winds
will continue out of the southeast through the afternoon before
going southwesterly this evening and then back southeasterly early
tomorrow morning. Wind speeds will generally be aob 10 kt through
the TAF period. Skies will be mostly clear with FEW-SCT mid and
high level clouds moving through the region this evening.

&&

.FIRE WEATHER...
Scattered afternoon thunderstorms will be common over higher terrain
areas of eastern districts through at least the middle of next week
with storms becoming more likely in lower elevations and western
districts early next week. The main threats with these storms will
be strong, gusty outflow winds which could exacerbate issues with
any ongoing wildfires or new starts. Winds outside the influence of
thunderstorms will generally exhibit typical afternoon upslope
tendencies with gusts commonly 20-25 mph. Minimum afternoon humidity
levels will mostly fall in a 15-30% range through the weekend, albeit
improving closer to a 20-35% range next week. Similarly, overnight
recovery should also improve from a 20-45% range to a 30-70% range.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Sunday for AZZ530.

     Excessive Heat Warning until 8 PM MST Saturday for AZZ531>546-
     548>555-559.

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

&&

$$

DISCUSSION...18
AVIATION...Berislavich
FIRE WEATHER...18