Excessive Heat in California, June 27 to 28, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
420 AM MST Sun Jun 27 2021

.UPDATE...
Updated Aviation

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.SYNOPSIS...
The hottest day of the week will be today a lot of lower desert
locations at, or above, 110 degrees. Storm activity is expected to
remain over eastern portions of Arizona today but some portions
of south-central Arizona could get some gusty winds from distant
storms. Storm chances begin an upward trend on Monday and increase
from east to west through midweek. This will be due to a major
increase in humidity along with one or more weather disturbances.
While rainfall occurrence at any given location is not certain, this
will be the first prolonged window of opportunity this Summer. Storm
chances are anticipated to trend down late in the week. High
temperatures decrease significantly toward midweek then slowly
increase late in the week.

&&

.DISCUSSION...
The synoptic pattern over North America features a relatively
narrow, but high amplitude, ridge over the west (axis along the West
Coast), a large positively tilted trough downstream, and then a
ridge over the East Coast. A smaller, yet noteworthy feature is
Hurricane Enrique. It is forecast to peak as a low end Cat 2 today
and then slowly weaken as it tracks northwestward eventually
dissipating over southern Baja on Friday. Today, we will certainly
be feeling the influence of the ridge with additional warming for
highs about 3-5 degrees higher than yesterday. Thus, the Excessive
Heat Warning remains in effect for our southeast CA and Lower
Colorado River Valley zones where highs of 115 will be common today.
Lofted smoke from wildfires over northern AZ is anticipated to put a
subtle dent in the high temperatures over portions of south-central
AZ but highs are still anticipated to be close to high Heat Risk.
Temperatures lower Monday but remain just high enough in populated
areas to hold on to the Excessive Heat Warning. Some of that cooling
may be offset by higher dew points.

The southwestern-most extent of the upper trough lies over New
Mexico and eastern AZ. One short wave within that trough has already
moved through northern AZ and into NM. Models continue to depict
another short wave from the north moving through far eastern AZ and
western NM late today and Monday. Latest CAMs generally agree that
thunderstorms today will be limited to the White Mountains and SE AZ
with only 10-15% storm chances over our portion of Gila County near
and east of Globe. But, outflows from distant storms will be
possible over a larger area with a 10-30% probability of significant
wind gusts over eastern portions of our forecast area (~10% for east
Valley). For Monday, there is also agreement amongst the CAMs on the
idea of convection occurring further west than today such that metro
Phoenix and northwest Pinal County will have noticeable storm
chances (20-30% per NBM).

However, wind would be the main issue with these storms with strong
wind probabilities increasing another 20% over south-central AZ. A
key difference for Monday isn't so much the position of the
aforementioned short wave since it will primarily be over New Mexico
at the mid-levels. But, a backdoor front spilling over New Mexico
will help advect some lower level moisture into AZ thanks to the
easterly pressure gradient it generates. Steering flow will continue
to be northeasterly. the further into Yavapai County that storms can
develop, the more likely northern Maricopa County (incl. the Phoenix
area) will see storms. But, sustaining the storms off the higher
terrain will still be a challenge.

In the Tuesday through Thursday time frame, the short wave that had
been over New Mexico (having broken off from the main trough) will
be slowly moving westward across southern AZ and far northwest
Mexico as an inverted trough. It may dissipate over southeast
California. There may also be other perturbations that phase up with
it and/or follow it. Also during this time, Enrique makes its
presence felt from afar with tangential moist advection into the
Desert Southwest. NAEFS and EPS means show PWAT values peaking at
1.50-1.75 inches over southern AZ and far southeast CA on Wednesday
with little change Thursday. The PoPs generally follow that trend.
However, the timing and track of any waves will be tricky. If
Tuesday wound up being an active day, it's less likely that
Wednesday would have better storm chances. Trying to pin down the
day to day trends for any given location will be very difficult.
Thus, relying on a multimodel blend approach (NBM). That being said,
the NBM PoPs seem a bit high compared to a variety of other
guidance. Made some minor adjustments to try to reflect
climatological/diurnal factors a bit better. With the passage of the
main inverted trough, steering flow will shift from a northerly
component to a weaker southerly one which will have implications on
storm behavior. Overall, the substantial increase in moisture during
this time will lead to more heavy rain potential - not just wind.
The wildfire scars will be the most vulnerable to any heavy rain
situations. With the increase in clouds, outflows, etc., high
temperatures will trend down nicely toward midweek.

Late this week, it appears there will be a slow downtrend in
moisture availability as the focus of high pressure aloft weakens
and sags southward and becomes centered roughly over AZ. But with
lingering moisture available, storm chances linger as well. Highs
are anticipated to slowly trend upward as well.

&&

.AVIATION...Updated at 1120Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Somewhat variable winds this morning should settle on an easterly
component before becoming more erratic again early this afternoon.
The switch to the typical afternoon westerly fetch may be
complicated by a competing pressure gradient yielding a tendency
towards a N/NW component by late afternoon. Distant storms over
eastern Arizona will likely contribute to a hastened switch to E/NE
winds during the mid evening with ~10% chance of periodic gusts near
20kt into Monday morning. Otherwise, a periodic thicker elevated
smoke layer should be common today though restrictions to slantwise
visibility may be more localized.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation concerns will exist through Monday morning under clear
skies. Light and variable winds will be preferred throughout the
daylight hours, then a trend towards westerly winds at KIPL and
northerly winds at KBLH should materialize tonight.

&&

.FIRE WEATHER...
Tuesday through Saturday:
A major increase in atmospheric moisture content and storm chances
takes place across the forecast area Tuesday and Wednesday with
little change Thursday. Min RH increases from 20-30% most places on
Tuesday to 30-40% on Wednesday and Thursday. High temperatures trend
downward as well. Max RH increases from 30-50% most places on
Tuesday morning to 45-65% by Thursday morning. RH trends down slowly
thereafter. High temperatures will be at, or slightly below normal
on Tuesday and decrease several degrees (mainly over south-central
AZ) on Wednesday with little change Thursday. Slow warming
thereafter. Apart from thunderstorms, no strong wind events are
anticipated but there will be east and southeast breeziness on
Tuesday.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Monday for AZZ530-532.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Monday for CAZ560>570.

&&

DISCUSSION...AJ
AVIATION...MO
FIRE WEATHER...AJ