649
FXUS65 KPSR 262016
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
115 PM MST Fri Jul 26 2024

.SYNOPSIS...
Thunderstorms will remain situated closer to higher terrain areas
north and east of Phoenix through the weekend as drier air begins
filtering into the region. Temperatures will also retreat somewhat
closer to the seasonal normal as high pressure weakens and shifts
into northern Mexico. This trend will continue through around the
middle of next week before a warmer and more humid airmass returns
to the region supporting increased thunderstorm chances.

&&

.DISCUSSION...
Subtropical high pressure over the SW Conus continues to weaken and
shift south in response to anomalous troughing and jet energy
punching into the central/southern California coast. A 594dm anti-
cyclone center was analyzed over far SW Arizona early this
afternoon, and excellent forecast confidence exists that this
feature will deteriorate further through the weekend eventually
reaching 591dm and shifting into northern Sonora. WV satellite
imagery already depicts substantial midtropospheric drying entering
the region which has resulted in eroding moisture towards the top of
the boundary layer. Although near surface moisture will be less apt
to erode, additional drying aloft and growing convective inhibition
through at least early next week will result in dwindling
thunderstorm chances, albeit not entirely precluding the threat for
isolated activity and gusty outflow winds at times.

Morning KPSR and KTWC 12Z sounding data sampled a nearly classic
convectively overturned environment along with the introduction of
reduced 8-9 g/kg mixing ratios in the H8-H7 layer. Not surprisingly
given this type of sounding, midday sfc temperatures have been slow
to respond yielding limited MLCape under larger CinH profiles across
lower elevations. While the preponderance of HREF membership
convincingly shows scattered deep convection continuing to develop
across the Rim and Whites with moist upslope flow into this evening,
more limited DCape may limit stronger outflow potential likely
incapable of dislodging convective potential over lower elevations.
This does not completely precluding showers and storms over lower
elevations, however the better opportunity may actually arrive late
night in the form of midlevel accas showers in association with a
shortwave descending south from the four corners interacting with
residual moisture and steep lapse rates.

As the ridge retreats into Sonora through the weekend, temperatures
will cool slightly, however still remain a couple degrees above
normal with nearly steady readings through the first half of next
week. Shower/storm chances will also continue to diminish
substantially during the first half of next week as westerly
midtropospheric drying becomes even more pronounced, and even the
more preferred areas of Gila County only justify modest POPs. Thus,
there is good forecast confidence of an extended period of more
quiet weather over the entire region through at least the middle of
the week.

During the latter half of the week, ensembles are in excellent
agreement of the southern plains subtropical ridging retrograding
back into the four corner while rapidly intensifying above 596dm
(with a handful of members suggesting a 600dm high pressure center).
In conjunction with an inverted trough ejecting north into southern
California, this pattern would bring the region back into the more
traditional SE deep layer monsoon flow. However, there is extremely
large uncertainty regarding the availability of better quality
moisture return with this transition as models indicate boundary
layer mixing ratios holding 8 g/kg or less into the end of the week
essentially precluding more than outflow boundaries into lower
elevations. Even official NBM output seems to be identifying this
contingency with abnormally low POPs for the beginning of August.
Eventually with such a flow pattern, deep moisture profile will
return though it may not be until next weekend before better
thunderstorm chances come to fruition.

&&

.AVIATION...Updated at 1728Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The main aviation concern through tonight will be the potential for
gusty northerly outflow winds early to mid evening. In the meantime,
expect winds turning westerly through around noon with higher
terrain showers and storms developing mid to late afternoon. The
convective potential today will be considerably less than yesterday
with only around a 10-20% chance of showers or an isolated
thunderstorm mainly in northern parts of Phoenix this evening.
Chances for outflow winds anywhere from the northeast to the
northwest look fairly good early this evening with gusts between 20-
30kt possible. Winds should eventually drift back to the east after
midnight with a chance of another modest outflow boundary coming
from the southeast.


Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather concerns will exist through the forecast period
with just a few passing high clouds expected. Winds at KIPL will
generally favor southeast this afternoon and then southwest starting
this evening, while KBLH will predominantly see southerly winds.


&&

.FIRE WEATHER...
Thunderstorm chances should remain confined primarily to the higher
terrain areas of eastern districts through the beginning and middle
of next week. Even over these locations, storm coverage and chances
for wetting rainfall will be much less than experienced the past
week. Aside from any thunderstorm outflows, winds will follow
typical upslope/downvalley patterns with intermittent afternoon
gusts in the 15-20 mph range. Minimum humidity values will trend
somewhat lower through early next week with lower elevations falling
into the teens while eastern district higher terrain near 25%. This
will follow widely varying overnight recovery ranging from poor 20%
in western districts to fair to good 40-60% in eastern districts.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ530-
     532.

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

&&

$$

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