991
FXUS65 KPSR 140530
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1030 PM MST Sat Jul 13 2024

.UPDATE...Updated 06z Aviation Discussion.

&&

.SYNOPSIS...
Above normal temperatures will persist across the region through
next week, although weakening high pressure should allow readings to
fall below excessive levels. Increasing moisture will support
scattered thunderstorms the next couple days with the best chances
for more widespread storms reaching south-central lower elevations
occurring Sunday. Somewhat drier conditions will gradually spread
into the area early next week, however will not preclude mountain
storms and the potential for gusty outflow winds descending into
lower desert communities.

&&

.DISCUSSION...
Early afternoon objective analysis and WV imagery depicts an
expansive subtropical anti-cyclone with H5 heights near 597dm
shifting to the central Utah/Colorado border resulting in deep E/SE
flow across the forecast area. Concurrently, one subtle shortwave
was ejecting westward through SE California with another more
defined cyclonic circulation over the southern Gulf of California.
These 2 features were combining to place much of eastern AZ under
modest subsidence. If this were not enough to suppress widespread
convection later today, a large slug of dry air from the high plains
has rotated westward into eastern AZ where mixing ratios have
dropped under 6 g/kg. Not surprisingly, HREF membership develops
minimal deep convection over higher terrain focusing development
within a plume of better quality moisture (>10 g/kg mixing ratios)
from along the international border through the lower Colorado River
valley and far western Arizona/SE California.

Later this evening, confidence is moderate that a small northern
Sonora convective complex combined with inland thermal low pressure
will induce a modified Gulf surge. Forecast BUFR soundings are
consistent in depicting shallow sfc-H8 mixing ratios spiking near 14
g/kg, albeit with notable dry air lingering within the upper half of
the typical mixed boundary layer. As such, many models indicate
partial erosion of this low level moisture throughout the day
resulting in only 10-11 g/kg mixing ratios during peak heating and
maintaining notable convective inhibition across lower elevations.
This slight reduction in total available moisture should limit
MLCape under about 1200 J/kg, however impressive DCape around 2000
J/kg will encourage deep, long traveling outflow boundaries. With
strong evidence that low level CinH will exceed 50-75 J/kg, multiple
outflow boundaries will likely be needed to spark additional deep
convection over lower elevations. Recent HREF output (and
subsequently NBM forecasts) has reduced the forecast storm coverage
and probabilities given this evidence, however with such extreme
DCape forecast, still feel enough colliding boundaries will
materialize to invigorate isold/sct storms around the Phoenix metro.
Thus, have made minor increases to the mandated NBM POP
initialization though still lower than most previous forecasts.
Regardless, these storms will be capable of producing strong, gusty
winds, localizes dense blowing dust, and brief heavy downpours.

Through the middle of next week, there will be a tendency for modest
drying of the atmospheric column and more predominant subsidence as
the center of the afomentioned subtropical high propagates back
into central Arizona. This will not preclude diurnal mountain storms
with outflows directed towards lower elevations, and cannot
completely discount a isolated storm nearing population centers of
south-central AZ. However, with mixing ratios likely dropping near 8
g/kg, chances are significantly reduced; although there is reasonable
evidence of a few nocturnal/early morning elevated showers should
pockets of enhanced midtropospheric moisture be thrust into a
steeper lapse rate environment. Uncertainty regarding convective
coverage increases during the latter half of the week as ensemble
members diverge with respect to the position and magnitude of
central H5 heights. Ensemble mean forecasts suggest the anti-cyclone
center shifting more towards southern NV which would place the CWA
in a more favorable location for moisture advection. However, if
more moisture is scoured away from the region early in the week, it
may take several rounds of outflows and moisture deepening to
support more expansive storm coverage.

Another aspect tied to the position and magnitude of the sub-
tropical high is temperatures and the potential to return to
excessive levels. While there is good agreement that H5 heights will
relax near 594dm during the first part of next week, there is some
growing evidence this will only be temporary with 598dm heights
potentially returning to the central/northern part of the state. At
this time, NBM forecasts cap levels well below record territory such
that only expansive moderate HeatRisk exists with just very small
pockets of major category precluding any headlines. However, if the
75th percentile of the full ensemble spread comes to fruition, a few
days midweek could be flirting with excessive criteria.

&&

.AVIATION...Updated at 0530Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Primarily westerly flow is expected to continue through the
overnight hours. A period of enhanced winds are expected to move
through the metro TAF`s over the next several hours, with periodic
gusts up to 25 kts. Otherwise, the typical diurnal easterly wind
is not expected to develop, hence the indicated light and variable
directions for early tomorrow morning. Westerly winds will then
quickly develop tomorrow morning and persist through the rest of
the TAF period for the most part. Thunderstorm chances remain too
low (10-20%) at this time to introduce any VCSH/VCTS, but outflow
potential will increase towards the end of the TAF period. Current
thinking is a favor towards either a northerly or southeasterly
outflow, but currently only added TEMPO for a northerly component
outflow for SDL and DVT. Outside of outflow potential, westerly
winds will persist through the rest of the TAF period. Skies will
remain SCT, with BKN possible that will be dependent on convective
development across the region during the afternoon and evening
hours.

Southeast California/Southwest Arizona including KIPL and KBLH:
Typical diurnal wind tendencies will continue for the most part
through the TAF period at both sites. KIPL will favor a
southeasterly wind, while KBLH will favor a south to southwesterly
wind through the period. Thunderstorm activity remains very low
(5-10%) in the immediate TAF areas, but higher terrain convection
could result in outflows that could reach the terminals,
especially KIPL where a TEMPO for southwesterly gusty winds are
possible. Otherwise, periodic gustiness 20-25 kts are possible
during the afternoon and early evening hours at both sites.

&&

.FIRE WEATHER...
Temperatures will cool slightly early next week, though remain in an
above normal category as moisture increases over the region.
Thunderstorms and wetting rainfall chances will be primarily
concentrated over higher terrain of eastern districts through the
week. however chances for lower elevation storm chances peak on
Sunday. Gusty outflow winds will be possible on many days next week
exacerbating issues on any ongoing wildfires and new starts.
Otherwise, afternoon upslope gustiness of 20-25 mph will be
periodically common. An increase in moisture will keep minimum
afternoon humidity levels above 20% Sunday, however retreating to a
10-20% range by the middle of the week. Correspondingly, fair to
good overnight recovery in a 40-60% range will fall into a 20-40%
range midweek.

&&

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

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

&&

$$

DISCUSSION...18/Smith
AVIATION...Young
FIRE WEATHER...18