108
FXUS65 KPSR 201850
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1051 AM MST Sat Jul 20 2024

.UPDATE... Updated 18Z Aviation Discussion.

&&

.SYNOPSIS...
Thunderstorm chances will persist across the area through the
majority of next week with the highest probabilities over higher
terrain areas of Arizona. Better odds for thunderstorm survival into
lower elevations will arrive early next week, albeit with large
daily fluctuation in coverage. Otherwise, widespread excessive heat
will impact much of the region the next couple days with
temperatures better than 5 degrees above the daily normal.

&&

.DISCUSSION...
A 596dm H5 subtropical anti-cyclone has shifted into far NW
Arizona/southern Nevada early this morning with more pronounced E/NE
flow enveloping the majority of the forecast area. Very subtle
waves/boundaries can be seen on various satellite and radar channels
pivoting within the scope of the overall circulation, although the
more defined forcing mechanisms remain more removed from the high
pressure center. Forecast confidence remains excellent with respect
to the larger synoptic pattern over the next 72 hours of this anti-
cyclone center continuing to shift towards central Nevada while in a
weakening state. This scenario will provide more favorable
conditions for large scale ascent and thunderstorm sustenance into
lower elevations heading through the bulk of next week. However,
details in the mesoscale will be paramount in dictating the daily
evolution with little confidence in convective coverage and impacts
beyond about 72 hours.

With 596dm H5 heights persisting in the core of the subtropical high
and nearing 2 normalized standard deviations above normal, H8-H7
temperatures around the 90th percentile of climatology will allow
areas of major HeatRisk to continue this afternoon. As the anti-
cyclone migrates further northwest, H5 heights will gradually lower
Sunday resulting in temperatures a few degrees lower and a marked
decrease in excessive heat coverage. Nevertheless, confidence is
excellent that temperatures 4F-8F above the daily normal will be
common this weekend, through still a few degrees short of record
territory. Further weakening of midtropospheric heights and
increased boundary layer moisture influenced from more expansive
thunderstorm activity should keep temperatures more muted for most
of next week, albeit still hovering in a slightly above normal
range.

As mentioned above, the pattern will become progressively more
favorable for deep convection and longer lived storm complexes the
next several days as northeast steering flow combined with better
jet level divergence and boundary layer moisture become juxtaposed
over south-central Arizona. HREF membership continues to suggest
today as the transition period setting the stage for greater storm
coverage and impacts Sunday. Where as Friday exhibited lesser
convective activity in southeast Arizona, that doesn`t look to be
the case today as 12 g/kg mixing ratios hovering along the
international border get pulled north encountering subtle shortwave
energy rounding westward from New Mexico. Concentrated storm
coverage to the southeast should readily send at least one long
lasting outflow boundary north and west sweeping through Pinal and
eastern Maricopa counties. This setup certainly would support areas
of blowing dust in the typical locations along I-10 between Phoenix
and Tucson early this evening.

With low level mixing ratios still only around 9 g/kg within the CWA
this afternoon, renewed updrafts and storm development into lower
elevations will likely be dependent on a very deep outflow and/or
colliding boundaries given the CinH magnitude. However, there is
compelling evidence among ensemble members and forecast BUFR
soundings that another weak shortwave emanating from northeast
Arizona will tap elevated moisture and instability in the form of
nocturnal accas storms around the Phoenix metro. Regardless, all the
outflows and moisture flux should help set the stage for an even
more active period Sunday afternoon and evening as mixing ratios
finally reach a supportive 11-12 g/kg. Strong evidence still exists
that a thunderstorm complex with very gusty outflow winds will
descend from the Rim into lower deserts. While actual storm coverage
along and behind the outflow may only reach 50% (per NBM output),
the stronger winds should be the greater impact with the potential
for dense blowing dust on the western and southern extent of the
Phoenix metro and beyond.

Forecast confidence deteriorates rapidly for the remainder of next
week as uncertainties persist in both the mesoscale and larger
synoptic pattern. Daily convective trends will almost certainly be
modulated by previous days activity and ability for atmospheric
recovery after the influence of stronger storms. With the assumption
that Sunday will be rather convectively active, prospects for
another successive day of robust storm activity diminishes despite the
bullish NBM POPs. This type of outcome is even reflected in global
models and forecast soundings where more shallow diurnal mixing
keeps notable capping and CinH in excess of 100 J/kg. As a result,
have attempted to cut both temperatures and POPs from the required
NBM initialization, although the official forecast may still be too
aggressive. Nevertheless, there continues to be convincing evidence
that 10-12 g/kg mixing ratios will be maintained throughout the
majority of the week resulting in a continuation of MLCape above
1000 J/kg and convective potential after peak heating.

Synoptic uncertainty during the latter half of the week continues as
a small subset of ensemble members still attempt to re-invigorate
the magnitude of the subtropical high while shifting the H5 center
back into central Arizona. These solutions are still outliers with
the vast majority of NAEFS members only temporally strengthening
the high, then rapidly lowering heights as the anti-cyclone center
shifts back south in response to anomalous troughing and jet energy
punching into the Pacific NW and Great Basin. Given this model
spread, mandated NBM forecasts continue to advertise temperatures in
a slightly above normal range with POPs essentially trending near
climatology, though the range of outcomes is certainly wider than
the official deterministic forecast.

&&

.AVIATION...Updated at 1745Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Main aviation weather concern during the TAF period will be the
potential for gusty thunderstorm outflow winds impacting the
terminals this evening. In the meantime, winds will prevail out of
the west through this afternoon with occasional gusts between
15-20 kts. Thunderstorms are once again expected across the AZ
high terrain and will likely send a well-defined outflow boundary
this evening between the 02-04z time frame with winds shifting out
of the E-SE. There is around a 50% chance of gusts exceeding 30 kts.
In addition to the outflow impacting the terminals, there is a
20-30% of isolated showers and storms around the area terminals.
As of this package, confidence is not high enough to include TS in
the TAF and for now just continued the mention of VCSH. Once the
outflow winds move in, it is looking likely that the winds will
remain out of the east through the rest of the overnight and into
Sunday morning. FEW-SCT mid to high-level clouds will prevail
through this afternoon before becoming SCT-BKN this evening with
SHRA activity nearby.


Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation concerns are expected through the forecast period. At
KBLH, winds will generally fluctuate between the south and
southwest. At KIPL, winds will generally be out of the southeast,
with the exception for a period of westerly sundowners this evening.
Occasional wind gusts this afternoon into early this evening could
approach 20 kts. Skies should remain mostly clear through the period
with the exception of a few passing mid to high level clouds.

&&

.FIRE WEATHER...
Scattered afternoon and early evening thunderstorms will be common
over higher terrain areas of eastern districts through the majority
of next week with storms becoming more likely in lower elevations
and western districts beginning early in the week. While there will
be areas of beneficial wetting rainfall, the greatest detrimental
impact from 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 early next week, then improving closer to a 20-35% range
during the middle and latter half of the 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 this evening for
     AZZ531>546-548>555-559.

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

&&

$$

DISCUSSION...18
AVIATION...Lojero
FIRE WEATHER...18