392
FXUS65 KPSR 181839
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1139 AM MST Thu Jul 18 2024

.UPDATE...Updated 18Z Aviation Discussion

&&

.SYNOPSIS...
Daily chances for showers and thunderstorms will exist through at
least the next 7 days. Better chances will continue to be focused
over the Arizona high terrain, but thunderstorm chances for the
lower deserts increase notably over the weekend and into early next
week. Widespread excessive heat conditions are expected to develop
by Friday across the lower deserts and persist into the weekend,
with forecast highs peaking around 115 degrees for the typically
hotter locales.

 &&

.DISCUSSION...
Latest wv satellite imagery and objective analysis reveals the
center of a subtropical high pressure system centered approximately
over Northeastern AZ, progressing slowly westward such that it will
be almost directly overhead later today. Convective activity
continues over portions of Northern Sonora/Southeast AZ early this
morning, and with southeasterly flow at mid levels, some elevated
moisture and debris clouds from earlier convection should make their
way into South-Central AZ over the next several hours. Though low
probability (<15%), there exists a potential for some elevated
convection leading to some isolated showers and a few strikes of
lightning early this morning over portions of South-Central AZ.
Otherwise, the main storm potential today will be this
afternoon/evening, as per usual. Scattered thunderstorms are
expected to develop this afternoon over the AZ high terrain (35-70%
chances), and there are isolated chances for the South-Central AZ
lower deserts and SE CA high terrain including Joshua Tree National
Park (10-20%). Subsidence aloft associated with the high pressure
overhead should maintain that any storms over the lower deserts will
struggle to form/survive through the next couple of days; the
expected hazards and storm coverage will be similar Friday to that
of today. Latest HREF probabilities for gusty outflow winds in
excess of 35 mph are slightly higher today than yesterday at between
10-50% for the lower deserts, and with this comes the potential for
some areas of blowing dust. Locally heavy rain can also be
anticipated with any convective activity today.

Over the weekend, the synoptic pattern becomes more favorable for
lower desert thunderstorm activity. By late Saturday, the
subtropical ridge is expected to settle over NV and subsequently,
northwest flow will start to predominate over the region. Though
PoPs on Saturday do not necessarily reflect wider coverage of storms
over the lower deserts, this may serve as a transition day to a more
active pattern, and PoPs may adjust upwards to reflect this in
subsequent forecasts. Forecast mixing ratios near the surface climb
upwards of 10 g/kg to as much as 12 g/kg over much of the region by
Sunday morning and through early next week. This tracks also with
the increase in ensemble mean PWATs, which climb upwards of 1.5"
late this weekend to as much as 1.7" and maintain these higher
values through early next week. Simultaneously, as the subtropical
ridge is centered over NV, a trough is expected to dig down into the
Plains and promote diffluence aloft late this weekend/early next
week over the Desert Southwest, which would act to remove a lot of
the inhibition previously in place. The combination of all these
factors will likely lead to impactful convection in the Sunday-
Tuesday time frame, and this bears watching as we get closer.

In addition to hazards associated with thunderstorm activity,
excessive heat conditions are expected to develop late this week as
the subtropical ridge tracks overhead and 500 mb heights strengthen
to 596-597 dam. Afternoon high temperatures will climb to 110-116
across the south-central AZ lower deserts and between 112-118
degrees across the western deserts on Friday and Saturday. This
increase in temperatures will also result in widespread Major
HeatRisk across the lower deserts. An Excessive Heat Warning has
been issued for South-Central and Southwestern AZ for Friday and
Saturday, and across Southeastern CA and the Lower Colorado River
Valley for Friday-Sunday. On Sunday, temperatures across the AZ
lower deserts are expected to cool down by about 3-5 degrees due to
increased low-level moisture and thus the overall HeatRisk drops
into the moderate category whereas across the Lower Colorado River
Valley through Southeast CA, temperatures are not expected to cool
off as much and thus areas of major HeatRisk will still persist. If
you plan to enjoy your weekend outside, be sure to pack plenty of
water and have a plan to cool off indoors in air conditioning
whenever possible to prevent heat-related illness.

&&

.AVIATION...Updated at 1835Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Westerly winds with speeds generally aob 10 kt will continue into
the evening. Forecast confidence remains low (20%) on if any
showers or storms will make it into the Valley and if they will
move over any terminals, but SHRA/TSRA should be in the vicinity
during this time frame. Forecast confidence is increasing that
there will be at least one outflow boundary with some gusty winds
moving through the Valley tonight. At this time, it is still
looking like the most likely direction of this outflow boundary
will be from the northeast. Wind gusts with this outflow will be
in the 20-30 kt range. There is a 30% chance of gusts exceeding 30
kt. If there are multiple outflow boundaries that make it into
the metro, there will be a greater chance for storm impacts if the
boundaries collide. FEW-SCT mid and high level clouds will
continue into the evening when they will become more BKN and
continue into the overnight hours.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds at KBLH will favor a southerly/southwesterly component
through the TAF period with speeds generally aob 12 kt. At KIPL,
winds will tend to favor a SE component until early this evening
when winds will go more westerly/southwesterly. This wind shift
will be caused by an outflow boundary from storms over the
mountains to the west. The probability of storms directly
impacting KIPL remains low (<15%), but it still bears watching in
case storms coming off the mountains try to wander towards the
terminal this evening. Winds will then go back southeasterly
during the overnight hours. FEW-SCT mid and high level clouds will
continue across the region.

&&

.FIRE WEATHER...
Daily scattered to widely scattered thunderstorm activity is
expected over the AZ high terrain through the end of the work week,
with only isolated chances elsewhere. The main hazards with any
thunderstorm activity will be strong, gusty outflow winds, which
could exacerbate issues with any ongoing wildfires or new starts.
Over the weekend, chances for thunderstorms over the South-Central
AZ lower deserts increase upwards of 35% as we enter an active
northwest flow monsoon pattern. Winds outside of the influence of
thunderstorms will exhibit diurnal tendencies with afternoon and
early evening gusts commonly between 20-25 mph. MinRHs through the
next several days will range from 15-25%, with fair to good
overnight recoveries between 30-60%. Moisture will increase over the
weekend, leading to overnight recoveries in excess of 40% across
much of the region.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM Friday to 8 PM MST Sunday for
     AZZ530.

     Excessive Heat Warning from 10 AM Friday to 8 PM MST Saturday
     for AZZ531>546-548>555-559.

CA...Excessive Heat Warning from 10 AM Friday to 8 PM PDT Sunday for
     CAZ560>570.

&&

$$

DISCUSSION...Whittock
AVIATION...Berislavich/18
FIRE WEATHER...Whittock