Excessive Heat and Heat in Arizona, August 2 to 7, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
921 AM MST Thu Aug 2 2018

.SYNOPSIS...
High pressure will meander across the southwest United States
through this weekend resulting in slightly above average
temperatures and periodic thunderstorm chances. Storms will be most
favored across higher terrain locations north and east of Phoenix,
though occasionally will spread into lower elevations. Slightly
drier conditions by the end of the week may confine thunderstorms
away from major population centers before moisture spreads back into
the region next week.

&&

.DISCUSSION...

An interesting set up is taking place this morning across south-
central Arizona. While the morning soundings from Tucson and
Phoenix show near-median precipitable water values for this time
of year (per the SPC sounding climatology), and some modest low-
level drying has occurred since yesterday, satellite imagery
reveals a few notable features that may support an increase in
thunderstorm potential this afternoon. Mid-level
cloudiness/altocumulus has increased this morning from the
northeast, and shallow convection has occasionally been noted
across southwest Maricopa County. A fast-moving gravity wave is
also approaching from northern Sonora and will spread across
south-central Arizona later this afternoon, with associated
enhancements to the cloud field occurring.

12Z suite of hi-res convective allowing guidance has increased
its signal showing a cluster of storms developing across Pima
County into southwest Maricopa and Pinal Counties later this
afternoon. Although steering flow is quite weak, thunderstorms are
expected to spread from south to north towards the Phoenix area
later this afternoon and evening. With weak large-scale forcing
for ascent and new thunderstorm convective development largely
dependent on outflow boundaries, coverage is still uncertain. But
weak inhibition should be supportive for thunderstorm development
aided by terrain features through the early evening. Can't rule
out a few strong wind gusts, locally heavy rain, and patchy
blowing dust with any thunderstorms/outflows that develop. The
primary update to the forecast this morning was to increase
precipitation chances across south-central Arizona.

Farther west, also cannot rule out a few thunderstorms developing
into western Imperial County this afternoon. A remnant MCV
continues to slowly move across the higher terrain of northern
Baja California, and may spark thunderstorm development this
afternoon. It's not out of the question that thunderstorms spread
into the western Imperial Valley this afternoon.

&&

.PREVIOUS DISCUSSION...

The setup for today does not seem to differ all that much from
yesterday, so early thinking is we will see more of the
same...which is just like any early August day. The current
midlevel water vapor channel still shows signs of yesterday's
decaying convection over Southeast Arizona and the Rim Country,
with some development of light showers/clouds further west along
the axis of a gravity wave. The 0Z upper air analysis still
indicates the upper level high parked to our west with the center
of the midlevel circulation situated closer to the center of the
state providing an east-southeasterly storm motion by Thursday
afternoon. Most of the convective allowing models continue to
paint the typical suspect locations for early convection,
including the Rim and Southeast Arizona, but with questionable
coverage for lower desert locations. The HREF's nearest
probability parameter also highlights these areas for storms this
afternoon so perhaps the possibility of outflow induced
lift/convection is in play, meaning it is conceivable for some
storms to penetrate into the Greater Phoenix area. Nevertheless,
any outflows from the south may again create blowing dust
problems for Pinal County. Further west, a deep plume of moisture
will remain over Southeast California, especially through
Imperial County. Some high based storms may dot the area early
Thursday morning but additional convection late in the afternoon
is also not out of the question.

The forecast for Friday and beyond has not changed all that much
as an approaching shortwave trough over the Pacific Northwest will
weaken the high over us and push the midlevel center further
east. At this time, a drier westerly flow will prevail over our
area so the best storm chances will stay in the higher terrain
well north and east of Phoenix into eastern Arizona. A stray
storm or two for lower desert locations is possible Friday, but
any storms will likely not be cause for anyone to change their
Friday night plans for.

Saturday and Sunday look even drier with little chance for storms
in our forecast area. By early next week, the high is forecast to
reposition closer to the Four Corners bringing a moist easterly
to southeasterly flow into the CWA and justifying progressively
increasing storm chances through the week. Otherwise, daytime high
temperatures will remain hot in the 105-110 degree range for the
warmest desert locations.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

Monsoon moisture remains in place, and we should see pretty good
heating this afternoon helping to fuel another round of monsoon
storms. However, weak south/southwest steering flow aloft will limit
chances for storms to reach into the greater Phoenix area. Can't
rule out a storm developing in, or moving into, the area after 02z
this evening but confidence too low to mention on the TAFs right
now. Some high-res guidance does suggest storms might work into and
affect the terminals, so a modest wind shift has been added
reflecting southerly outflow winds after 02-03z along with some high
based CU and BKN mid decks. Expect west winds by early afternoon
with some afternoon gusts into the teens and if outflow winds don't
materialize, the west winds will likely persist well beyond midnight.

Again, confidence is low to moderate that storms will develop and
move into the Phoenix area this evening but we can't rule out the
possibility until either moisture significantly decreases or a
stronger more westerly flow aloft develops.

Southeast California/Southwest Arizona including KIPL and KBLH:

Storm chances in the western deserts not zero this morning, but too
low to mention in the TAFs. Will simply keep mid/high level SCT to
BKN decks going at the terminals with high based CU possible at
times. Winds to favor the south at KBLH with some afternoon
gustiness. Winds to favor southeast at KIPL til early this evening
when winds have a chance to swing more towards the southwest.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Thunderstorm chances will become more relegated to higher terrain
areas of Gila County over the weekend as the overall atmospheric
moisture content declines. In fact, most locations will probably not
see any storms or wetting rains through the weekend time frame.
Steadily through next week, storm and wetting rain chances will
migrate from higher terrain areas back westward into lower
elevations. Otherwise, minimum humidities will remain in a 15-30%
range following fair to good overnight recovery. Apart from
thunderstorms, winds will be light except for typical afternoon and
evening upslope breeziness. Little temperature trend expected during
the five day period, with high temperatures remaining a few degrees
or so above seasonal normals each day.

&&

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

&&

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

DISCUSSION...Rogers/Deemer
AVIATION...CB
FIRE WEATHER...MO/CB