000
FXUS65 KPSR 030500 CCA
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
959 PM MST Mon Aug 2 2021

.UPDATE...Updated 06Z Aviation Discussion.

.SYNOPSIS...
Storm chances will continue to trend down through midweek as drier
air moves in. Meanwhile, high pressure will strengthen over the
region. The combination will lead to a warming trend with highs
climbing well above normal. It will be hottest over southeast
California and southwest Arizona with a number of locations
reaching, or even exceeding, 115 degrees Tuesday and Wednesday. The
Phoenix area will not be quite as hot. But, highs will be around 110
degrees. During the latter half of the week and over the weekend,
storm chances are expected to slowly increase from east to west.
Also, high temperatures are expected to trend down
considerably...especially over south-central Arizona.

&&

.DISCUSSION...
Upper air analysis shows that a high amplitude ridge remains parked
over the western US while a weak trough, easily seen on objective
analysis at the H5 and H3 levels, passes by to our north. Meanwhile,
air mass RGB imagery shows relatively drier air pushing into Arizona
from the northwest, setting up a dry/moist boundary across the
southern half of the state. Late morning and early afternoon
thunderstorms have already developed along the Mogollon Rim and
White Mountains, close to the moisture boundary and where there is
some divergence aloft due to that weak trough.

Keeping an eye on the higher terrain of Gila and eastern Maricopa
County where daytime heating will destabilize the atmosphere, and
satellite derived instability imagery shows a pocket of 250-450 j/kg
CAPE. Already one shower has rained over northern Maricopa County,
and the latest HREF shows more storms developing over the
Superstitions, Four Peaks and Gila County within the next few hours.
Storm chances east of Phoenix are around 20-30% but isolated
coverage should limit impacts. Any storms that do develop will
capable of producing strong winds and locally heavy rain. The flash
flood threat is less than previous days but remains a concern over
wildfire burn scars.

NNW steering flow will try to push activity south but storms
surviving over the lower desert is not likely. Hi-res guidance does
show a small signal for outflows, with chances around 10-20%, and
wind gusts reaching 20-30 mph. The dust threat at those speeds will
be low and localized.
Chances for storms developing along outflows is even lower, below
10%, and supported only by the most aggressive guidance.
Thunderstorm related impacts are most likely to be limited to the
higher terrain areas well east and south of downtown Phoenix.

After today, ensemble guidance remains consistent showing some
strengthening of the aforementioned ridge with H5 heights rising
into the 594-598 dm range across southern California and much of
Arizona by Wednesday, placing near 97.5th percentile of climatology.
Meanwhile, drier air will continue to settle over the region, with
PWATs dropping from 1.53" as measured on the KPSR sounding this
morning to around 1.00" by Wed/Thu according to global ensembles.
Increasing thickness and clear skies will result in much warmer
temperatures with numerous places reaching 110 degrees by tomorrow.
The highest heat will occur along the Colorado River and southeast
California where NBM probabilities indicate a 30-50% for highs
reaching 115 degrees. As such, the previous Excessive Heat Watch was
upgraded to a warning.

With the impending heat, now is a good time to consider how
relatively seasonably cool Phoenix has been as of late. Since July
11th, the last time Phoenix reached 110 degrees, Sky Harbor has been
above normal just three times reaching 109 degrees on July 13th,
20th and 21st.
While the high temps in and around Phoenix are not forecast to warm
quite to the levels as places farther west, they will push into the
High HeatRisk and possibly excessive heat levels. NBM probabilities
give KPHX a 60-65% chance of reaching 110 degrees on Tue and Wed,
and a 10-20% chance for reaching 115 degrees. Trends may not favor
record heat, but when considering the recent coolness of the last
two weeks, potential heat impacts warrant enough concern for an
Excessive Heat Warning for the Phoenix metro and some surrounding
areas Tuesday and Wednesday.

For the latter half of the week and the weekend, there is good model
agreement on a pattern change wherein the ridge gets flattened by a
trough passing to the north. Also there will be a large inverted
trough moving westward across Mexico with the northern end at least
brushing Arizona. Another feature of interest is a smaller short
wave (possibly the one currently over the Great Basin) riding over
the ridge and then southward through the southern Rockies. That
feature (or a portion of it) may phase up with the larger inverted
trough. The end result appears to be an uptrend in moisture/humidity
that begins as early as Wednesday night. PWAT looks to peak on
Saturday. The combination of weaker high pressure and substantially
higher atmospheric moisture content is expected to cause high
temperatures to trend down significantly. Also, storm chances
increase from east to west. But, it appears storm chances will
struggle to get above single digits over southeast California. As an
aside, the temperature trend will be heavily affected by how the
convection plays out (more cloud cover and rain cooled air from
outflows - depending upon where that takes place). It does not
appear at this time that we will be looking at a similar situation
as what we had in recent days with the unusually strong inverted
trough passage from last month.

&&

.AVIATION...Updated at 0459Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Typical
diurnal trends are favored through Tuesday afternoon, with wind
speeds generally 10 kts or less and periods of variability during
diurnal wind shifts. Isolated thunderstorms on Tuesday are expected
to remain well to the east of the Phx area (most likely only over
the White mtns/western NM), with westerly winds likely lingering
well into Tuesday night. Clouds bases should remain mainly aoa 12
kft with FEW to SCT coverage.

Southeast California/Southwest Arizona including KIPL and KBLH: No
significant aviation weather concerns anticipated through Monday the
TAF period. Winds will generally remain light and diurnal under
mostly clear skies.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Temperatures are expected to peak on Wednesday with highs at or
above 110 on the lower deserts (at or above 115 at a lot of places
in southeast CA and southwest AZ). Humidities will be at their
lowest with Min RH at or below 15% on the lower elevations and up
to 20% on the higher terrain. Max RH will be lowest over eastern
Riverside County (20-25%) and not as bad elsewhere on the lower
deserts (30-50%) and better over the south-central AZ higher
terrain (50-70%). Atmospheric moisture content trends upward
during the latter half of the week (starting as early as Wednesday
night) and high temperatures trend downward significantly. As a
result, Min RH increases 10-20% by Sunday and Max RH increases
20-30%. Storm chances trend upward from east to west but at this
time are expected to be limited to AZ. Apart from thunderstorms,
winds will tend to favor typical diurnal patterns Wednesday. A
change is anticipated beginning Thursday over portions of
southeast CA and southwest AZ with southerly breeziness and that
continues through Sunday (most noticeable during midday through
evening).

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM Tuesday to 8 PM MST Wednesday
     for AZZ530>533-535>537-539>544-546-548>551-553>555-559.

CA...Excessive Heat Warning from 10 AM Tuesday to 8 PM PDT Wednesday
     for CAZ561>570.

&&

$$

DISCUSSION...AD/AJ
AVIATION...Percha/Benedict
FIRE WEATHER...AJ

NWS PSR Office Area Forecast Discussion