Excessive Heat in California, July 9 to 12, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
514 AM MST Fri Jul 9 2021

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
Well above normal temperatures will continue through the weekend,
posing a high risk for health impacts across lower elevation
communities. Moisture levels will remain elevated enough for a
threat of afternoon and evening showers and storms, primarily from
Phoenix eastward. A shift in the weather pattern is expected next
week, with temperatures cooling and increased chances for showers
and thunderstorms.

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.DISCUSSION...
Regional 09.00Z soundings indicate a fairly moist atmosphere in
place (50-75th percentile). Surface moisture also remains
elevated, if not trending slightly upward still in line with the
past 24 hours or so. While that typically would yield high(er)
levels of instability, the RAOBs also measured exceptionally warm
(90th percentile) mid/upper level temperatures, which was reducing
instability. Capping inversions were also noted. Kinematics
remain favorable (theoretically) for terrain- induced convection
to migrate into lower elevations, with mid- level easterly
steering flow of 15 kt (northern AZ) to 30 kt (southern AZ).

Upstream RAOBs (ABQ/ELP) still appear to be quite warm in the
mid-levels, thus mid-level temperatures will likely remain warm
today. Models continue to indicate additional boundary layer
moistening through today in response to the broad GoC/Rim induced
circulation, which should yield some instability increase by
itself. There are indications of moisture/instability pooling
along the Rim leading to an area of fairly uncapped high CAPE by
later today. At lower elevations, CIN becomes more of a factor,
thus there appears to be a limited time/space window for storms to
develop and strengthen later this afternoon/evening, with a push
toward lower elevations given the northeast flow. There remains
moderate (50%) potential for outflow winds of 30+ mph across the
lower elevations of Maricopa and Pinal Counties later today.
Southern Gila County has a low potential for localized heavy rain
(10% of 1"/3hr). Low elevation storms may require mid-level
solenoidal return flow to reduce the capping inversion and
promote instability, meaning convection could come later in the
evening. CAMs indicating more of a threat for 02-06Z across
Southern Gila County and 04-08Z for lower Maricopa/Pinal Counties.

Moisture levels trend down a bit into the weekend, but could
still remain high enough to promote modest instability and with
the established northeast flow, there will still be a chance for
propagating showers/storms Saturday afternoon/evening (albeit
lower than today).

A shortwave trough moving through the central Rockies Sat/Sun will
influence the H5 anticyclone over Arizona and displace it further
west. This yields a warming/drying northerly flow across the
forecast area. Temperatures will already be increasing today in
response to increasing height/thickness values and peaking
tomorrow (Sat). HeatRisk values may still be elevated enough on
Sunday to warrant extension of the Excessive Heat Warning for the
Phoenix area.

Global ensembles pretty consistently forecast a shift in the upper
level pattern early next week, with the H5 ridge migrating more
to the west allowing a weakness in the mid/upper levels to develop
over AZ/NM. Moisture/PWAT values increase and combined with cooler
thermal profiles generate more widespread, uncapped instability.
There may still be enough of a northeast flow to allow storm
migration into lower elevations, but wind fields will be relaxing
some (indicative of an increasing flash flood threat). There will
be various vorticity centers migrating through the area - either
sheared-off energy from the initial development of the trough/
weakness or new development due to convective activity.QPF values
from global ensembles begin increasing Monday and remain elevated
through Wed/Thu. Overall more active thunderstorm period on the
horizon with increased cloud cover and cooler temperatures.

&&

.AVIATION...Updated at 1214Z.

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

Gusty outflow winds, primarily out of the east to northeast, will be
possible late this evening across the Phoenix area. The magnitude
and overall timing of any outflows remains uncertain at this
time. HREF guidance shows around a 30-40% chance for outflow winds
to exceed 30 kts in the Phoenix area with the highest chances
remaining to the east over the higher terrain. Gusty outflow winds
will be capable of producing areas of blowing dust, which could
result in some reduced visibilities at the terminals, primarily at
KIWA. Chances for seeing a few isolated showers/thunderstorms
surviving into the Valley will be low (~15%) with the best chance
of a vicinity shower or thunderstorm mainly remaining across the
eastern part of the metro. For now, have kept VCSH in the TAFs for
this evening at KIWA and KSDL. Otherwise, winds will favor a
westerly component through much of the period with some light and
variable winds this morning. FEW-SCT clouds aoa 15 kft will become
SCT-BKN going through this evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds at KIPL will favor a southeast to southwest component through
the period, while winds at KBLH favor a southerly component.
Afternoon breeziness will be possible with gusts to around 20 kts
possible at KBLH. Otherwise, FEW-SCT clouds aoa 15 kft are expected
through much of the next 24 hours.

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.FIRE WEATHER...
Sunday through Thursday:
The weather will be very hot (5-10F above normal) and modestly
humid (minimum RH 15-30%) Sunday, but transition to cooler (2-5F
below normal) and more humid (minimum RH 15-50%) by Wednesday. The
more cool/humid air will be closer to the AZ/NM border where
overnight RH recoveries will exceed 35% most places with
recoveries potentially exceeding 80% for higher elevations.
Conditions will be somewhat warmer/drier conditions across
southeast California. Chances for showers and storms will increase
across eastern Arizona Monday-Thursday, with storms capable of
producing lightning strikes, locally heavy rain, strong wind
gusts, and long-traveling outflow winds that cause sudden changes
in wind directions. Outside of thunderstorms, winds will follow
typical diurnal/topographic influences with afternoon gusts near
15-20 mph.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Saturday for AZZ530-532>534-537-538-540>544-546-548>551.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Saturday for CAZ569.

     Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Monday for CAZ560>563-566>568-570.

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DISCUSSION...Iniguez
AVIATION...Smith
FIRE WEATHER...Iniguez/AD