108
FXUS65 KPSR 182351
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
451 PM MST Thu Jul 18 2024

.UPDATE...Updated 00z 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
Friday and continue into this weekend across the lower deserts,
with forecast highs peaking up to 114 to 118 degrees for the
typically hotter locales.

 &&

.DISCUSSION...

THUNDERSTORMS: The Desert Southwest has entered a more active
period for monsoon thunderstorms, especially when it comes to
coverage. Thunderstorms are expected daily for the next week,
primarily over typical higher terrain areas of AZ and out into
SoCal. The forecast challenge will be figuring out which days are
more likely to see afternoon and evening thunderstorms survive
into the the lower deserts. Survivability and further initiation
is often heavily dependent on mesoscale interactions like deep
outflows, colliding outflows, and mid-level gravity waves, which
models are not always the most adept at resolving. That being
said, seasonal to above seasonal moisture is in place and does not
look like it will drop off significantly during the next week,
and daily instability will be sufficient to support showers and
storms across the region. Global ensemble models do indicate a
potential larger change to the synoptic pattern toward the end of
next week when a deeper Pacific trough pushes into the Northwest
CONUS and may flatten and weaken the monsoon high. It is uncertain
this far out if this change could have a major impact on the
storm activity, but it may at least lower temperatures down a bit.
Time will tell.

There does not look to be too much difference in the day-to-day
thunderstorm outlook today through Saturday besides subtle
differences in instability, moisture, and the position of the
monsoon high. The monsoon high is expected to gradually shift west
over the next few days, with mid-level steering flow going from
E-SE in eastern AZ today to more pronounced NE heading into the
weekend. Instability will be marginal the next few days, with
MLCAPE forecast to mostly fall in the 500-1000 J/kg range. So, not
expecting considerable severe thunderstorm threats and coverage,
but there is still potential for a few severe storms. DCAPE will
remain fairly high, with magnitudes around 1500 J/kg in the
afternoons and evenings. So, strong gusty downdraft winds are
anticipated and outflows traveling into the lower deserts during
the evenings and early overnight hours are likely. These evening
outflows will be capable of initiating storms over the lower
deserts, especially with any outflow collisions. Several runs of
the HRRR model have indicated potential for storms over the lower
deserts, including in Phoenix, this evening-tonight, but it will
still come down to the evolution and depth of the outflows as to
whether this can come to fruition. Similar conditions may happen
in the following days as well. NBM PoPs over the lower deserts
today through Saturday are generally around 15-25% in the
evenings. The flash flooding threat is still fairly marginal as
mean mixing ratios will mostly be near or just below 10 g/kg. Most
rainfall will be enough to wet the ground and create some
nuisance flooding, but there still potential for localized flash
flooding with any storm that anchors to a lifting mechanism or if
multiple storms hit the same area.

Sunday into early next week continue to look like even more
favorable days to thunderstorm activity with potential for even
greater coverage as the monsoon high continues to shift off to the
northwest of the Desert Southwest and mid-level N-NE winds
increase. This will be a more favorable setup for storms to move
off the Mogollon Rim toward the lower deserts of South-Central AZ,
so long as thunderstorm anvils do not blow in the same direction
as steering flow and inhibit surface instability. Global models
also indicate a slight increase in moisture, with PWATs pushing a
little closer to 2.0" and mean mixing ratios pushing up to around
the 11-12 g/kg magnitude. This along with continued above normal
heat will in turn result in greater instability (i.e. higher
CAPE). Ultimately, there will be potential for stronger storms,
possibly more severe storms, and better potential for flash
flooding, so long as the environment can be realized.

HEAT: An Excessive Heat Warning remains in effect for most lower
desert locations this Friday and Saturday, and continues through
Sunday for Southeast CA and along the Lower CO River Valley.
Widespread Major HeatRisk is forecast for for Friday and Saturday,
with afternoon highs in the 112-118F range. There will also be the
added element of humidity, especially in Southeast CA, where heat
indices are forecast to reach up to 116-120F. There is still
potential, with the increased thunderstorm activity around the
region, for some disruption in the heat, but it is still best to
plan for more extreme heat. High temperatures are expected to
lower slightly by early next week with the further increase in
thunderstorm activity anticipated and a slight decrease in the
height field. Even more "cooling" may come toward the end of next
week with the potential pattern change previously mentioned.

&&

.AVIATION...Updated at 2350Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Westerly winds with speeds generally around 10 kts with occasional
gusts 15-20 kts 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, with the 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. 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,
initially westerly/southwesterly will back towards the SE later
this evening. 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. 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 northeast 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...Benedict
AVIATION...Young/Berislavich
FIRE WEATHER...Whittock