703
FXUS65 KPSR 112035
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
135 PM MST Sun Aug 11 2024

.SYNOPSIS...
Ample monsoon moisture over much of the area will continue into
Monday, but shower and storm chances will generally favor higher
terrain areas in Arizona. A significant downtrend in monsoon
activity is likely by the middle of the week across the entire
region as drier air filters into the region. Temperatures through
the upcoming week will average near to slightly above normal.

&&

.DISCUSSION...
Little has changed with the overall synoptic pattern over the past
couple of days with the subtropical high still centered over
Texas stretching into eastern Arizona and the weak upper level
cyclonic circulation positioned over the Baja peninsula.
Additionally, a leftover MCV is currently slowly progressing to
the west northwest across far southern Arizona. This feature has
brought considerable cloudiness for much of the morning and as a
result kept instability lower than forecast while not allowing
the CAP to weaken very quickly. Moisture levels early this
afternoon are still quite high, especially from south-central
Arizona through the Lower CO River Valley with PWATs from 1.5-1.8"
on average and low level mixing ratios around 12 g/kg. The main
forecast question for the rest of today is whether or not the
clearing skies early this afternoon will be enough to spark off
much convection. Current objective analysis still only shows
temperatures mostly in the mid to upper 90s as of 1 PM and it
seems likely we will fall just short of our forecast highs of
105-109 degrees across the lower deserts. The last several runs of
the HRRR show very limited convective development through the
rest of the afternoon, and this may very well be what happens as
the CIN may be too much to recover from due to the morning
cloud cover. The most likely areas of shower and storm
development are likely to be across the higher terrain well east
of Phoenix and across western Maricopa County. Given the lack of
strong instability and relatively low DCAPEs today, the threat for
any stronger storms should be limited.

Heading into Monday, the overall pattern will remain nearly
unchanged as PWATs will continue to remain elevated at 1.5"+ and
additional lobes of vorticity emanating from the Baja low will
remain possible. Thunderstorms will once again materialize during
the afternoon and evening, with the best chances confined across
the higher terrain areas of eastern and southeastern AZ, including
southern Gila County as well as the foothills east of Phoenix.
For the lower deserts of south-central AZ, including Phoenix, any
convective potential will be conditional as to what happens later
today. If later today does not turn out to be convectively active
such that the atmosphere is not overworked, then the chances for
convection across the lower deserts of south-central AZ Monday
evening would increase given that there will be plenty of
instability available as MLCAPE values will likely exceed
1000-1500 J/KG based on the latest forecast soundings.

A significant pattern change is still anticipated by the middle
to latter half of the week as a polar trough traverses from the
Pacific Northwest through the Intermountain West Region. As this
pattern evolves, the flow aloft will be shifting out of the
southwest, which will usher in significantly drier air into the
region. This would likely shut off thunderstorm activity across
the region for a couple of days, mainly Wednesday and Thursday
when ensembles indicate PWATs dropping below 1.2" across the lower
deserts and below one inch across the higher terrain areas. By
Friday into next weekend, as the polar trough moves into the
northeastern CONUS, allowing the subtropical ridge to build over
the southern Plains, a return to south to southeasterly flow is
likely, which will once again cause moisture levels to increase
with storm chances returning.

Temperatures heading into the first of the week will be slightly
cooling down to near normal levels as highs across most of the
lower deserts retreat close to 105 degrees. Then, by mid to late
week as upper-level height fields rise, temperatures will slightly
warm but most likely remain below 110 degrees. Overall, not too
bad of a temperature forecast expected this upcoming week with the
overall HeatRisk expected to be mostly in the moderate category
across most of the region.

&&

.AVIATION...Updated at 1837Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

There is moderate confidence in the thunderstorm outlook for this
afternoon and evening with closest storm activity favored just to
the southwest of the Phoenix area with around a 20% chance at the
terminals. At this time, there is no mention of VCTS or VCSH in
the TAFs due to the uncertainty. Confidence on wind directions
through the TAF period is lower. Current light variability, with
a south to southeast component, may not come around to the west
until as late as 21-22Z. Initial outflow winds are favored from
the southwest late this afternoon and evening, with potential for
gusts in excess of 20 kts. A later outflow from the southeast is
possible, but less certain. There is then potential for winds to
remain westerly most of the night with low confidence on a diurnal
east shift. FEW cumulus bases will be around 7-8K ft AGL this
afternoon, while SCT to BKN mid and high clouds this evening and
tonight will mostly be aoa 15K ft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds at KIPL will primarily be southeasterly with speeds
generally aob 10 kt through the TAF period, with the exception of
some gustiness the next couple hours. Some isolated storm
activity is possible this afternoon and evening in the mountains
to the west of KIPL, which could push outflow winds into the
Imperial Valley and cause a brief westerly shift. At KBLH,
southerly winds will prevail through the TAF period. Winds speeds
will generally be in the teens through this afternoon with gusts
reaching into the middle 20 kts. FEW-SCT mid and high level
clouds will continue through the period.

&&

.FIRE WEATHER...
Shower and thunderstorm chances will persist through the next
couple of days, with the highest chances expected across south-
central AZ. With any thunderstorm activity, strong winds with
potential for outflows to travel long distances and cause sudden
wind shifts as well locally heavy rainfall and frequent lightning
can be expected. Storm activity is still expected to trend
downward significantly by the middle of the week as drier air
filters into the region. Aside from the thunderstorm winds, the
overall general winds will continue to follow the typical diurnal
tendencies with some afternoon upslope breeziness. MinRH values
through Monday will remain elevated at between 20-35%, but then
trend slightly downward heading towards midweek with values
ranging between 15-25%. Temperatures during the next several days
will average near to slightly above normal.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Kuhlman/Lojero
AVIATION...Benedict/Berislavich
FIRE WEATHER...Lojero/Kuhlman