Excessive Heat Warning 8, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
216 PM MST TUE AUG 16 2016

.SYNOPSIS...
Very warm and fairly dry conditions will last through Wednesday with
lower desert highs around 110 degrees and isolated storms. A return
of some minimal monsoon moisture looks likely for the latter half of
the week resulting in mostly slight chances for showers and
thunderstorms and slightly cooler temperatures. Some gradual drying
this weekend will mostly confine storms to higher terrain areas of
eastern Arizona with very minimal chances across the deserts.

&&

.DISCUSSION...
Latest WV imagery shows drier air situated across the Mojave Desert
associated with an anticyclone in the eastern Pacific. Dewpoints
have generally been in the 50s across south-central Arizona, but as
low as -1 degree at Las Vegas. Meanwhile, streamline analysis shows
a well-defined short wave trough in the mid-levels near the Four
Corners. Morning Phoenix sounding measured a 500 mb temperature near
-8C, which is close to the 10th percentile.

Isolated showers and ACCAS developed this across portions of the
higher terrain this morning, an indication of the instability present
across the region. Latest CAMs have trended somewhat more active for
this afternoon as well. Models generally show a drying trend both in
total PWAT and low-level mixing ratios, however forcing for ascent
associated with the aforementioned trough should be able to offset
this general decrease in buoyancy.

Afternoon visible satellite imagery clearly shows the mean
anticyclonic flow with activity developing across Yavapai County and the
White Mountains ahead of the Four Corners trough. Thunderstorms have
generally shown movement towards southern Gila and northeastern
Maricopa Counties.

The HRRR has performed well the past few days and the forecast will
continue to be weighted in that direction. Latest mid-level lapse
rates are quite strong across South-Central Arizona and AMDAR
soundings consequently show abundant CAPE above 0C. Scattered
thunderstorms are expected to develop across the higher terrain this
afternoon, which will increase moisture convergence across the lower
deserts. Inverted-V soundings will be favorable for an outflow
boundary to form, which will be needed to initiate additional
isolated storms across the Phoenix area. Isolated storms are
possible and any storms that form will be capable of producing gusty
winds and blowing dust.

&&

.PREVIOUS DISCUSSION...
A somewhat atypical monsoon pattern will continue into the weekend
with a weak upper level trough settling just off the California coast
and possibly transitioning through the Southwest U.S. early next
week. This trough is starting to take shape with the main vorticity
max situated near the California/Oregon coast. Another weaker upper
level trough is currently moving southeastward near the Arizona/Utah
border and is forecast to pass into west-central New Mexico this
evening.

Temperatures today will be very similar to Monday's with highs easily
topping 110 across the southeast California and southwest Arizona
deserts. Excessive heat conditions will again occur across portions
of Imperial County and could even exceed criteria in a few areas
across Riverside County, but feel coverage won't be enough to expand
the Excessive Heat Warning. The south-central Arizona deserts will
again be quite warm, but remain below excessive heat levels. Could
end up adding another day to the 110+ total for Phoenix, but overall
forecast guidance still holds it just shy at 109.

A shift toward a somewhat more moist environment begins Wednesday,
but overall moisture levels will remain low. The upper trough will
shift southeastward through California into tonight, but stall out
just west of Las Vegas. This should allow for a weak moisture return
into southern Arizona for Wednesday, but PWATS will likely remain
below 1.25". This minimal amount of moisture should be enough to work
with as an upper level west to east jet max moves into southern
California opening up diffluent flow over much of Arizona. PoPs for
Wednesday are still mostly focused over high terrain areas of
Arizona, but we can't totally rule out a storm or two making into
into the lower deserts. Quite dry sub-cloud layers will promote gusty
storm outflows and the potential for blowing dust. Further
moistening should occur into Thursday while the upper level jet max
moves over northern Baja into southern Arizona. This should promote
a slightly better chance for storms, first over high terrain areas
and then potentially over the south-central Arizona deserts with the
help of intersecting outflows. As with Wednesday, dry low levels will
likely result in gusty outflow winds and patching blowing dust. High
temperatures during this time should drop at least couple degrees
overall as heights aloft dip and aided by the increased moisture
over Arizona.

Decreasing forecast confidence occurs starting Friday and especially
into early next week as model differences increase. Both the GFS and
European point toward another drying trend into this weekend with the
GFS keeping dry air in place through early next week. The European
leans more toward another increase in monsoon moisture starting
either Sunday or next Monday. Have PoPs decreasing Friday, but then
maintaining near climo PoPs for the weekend. Don't see much change in
temperatures as highs should stay near seasonal normals.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:
Storms will continue to develop and bubble over the far
mountains/higher terrain surrounding the Phx Metro through the
afternoon. Overall storm motion should be to east-southeast, so
activity north of Maricopa County may clip the northern/northeastern
most portions of the Phoenix Metro. Hi-res models indicate outflow
driven winds shifts are highly possible this evening with an initial
estimate of them making it into the Phoenix metro around 02z from an
easterly direction. Some isolated storms/showers may develop in the
wake of outflow or if they intersect, but confidence in their
development as well as their coverage still not high enough to
include mention for KPHX. VCSH was added to the KIWA/KSDL TAFs with
the 18z package. SCT-BKN debris clouds will result overnight, and
CIGs should not get too low even under passing radar
activity...generally 10kft or so.

Southeast California/Southwest Arizona including KIPL and KBLH:

Increasing high pressure building over the region in addition to
some significant sfc drying will keep storm activity mainly east of
the Colorado River Valley tonight. SCT clouds will continue to
develop through the afternoon but gradually clear after sunset.
Winds to maintain a southerly component with speeds generally 10-12
kts at their strongest with occasional gustiness.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday...
Monsoonal moisture makes a return to the area by the late week
period, with enough moisture to support an uptick in storm activity
for the districts. One limiting factor to storm activity possibly
over the lower desert elevations will be westerly surface/low-level
flow. Best coverage for storms and possible wetting rains will be
across the higher terrain of AZ, especially to the
north/east/southeast of Phoenix. With high pressure slightly
displaced out of the region, temperatures will fall back to more
normal values. Min RH's to remain in the 15-30 percent range (lowest
across SE CA), with good overnight recoveries. Outside of
thunderstorms, winds to remain mainly on the light side, following
typical diurnal trends.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected today.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ033.

&&

Visit us on Facebook...Twitter...and at weather.gov/phoenix

DISCUSSION...Hirsch
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Nolte
FIRE WEATHER...Nolte

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE PHOENIX AZ
440 PM MST TUE AUG 16 2016

AZZ024-170030-
SOUTHERN GILA/TONTO NF FOOTHILLS AZ-
440 PM MST TUE AUG 16 2016

...SIGNIFICANT WEATHER ADVISORY...

THE NATIONAL WEATHER SERVICE IN PHOENIX HAS ISSUED A

SIGNIFICANT WEATHER ADVISORY FOR...
  GILA COUNTY ...AROUND GLOBE...MIAMI...AND ROOSEVELT LAKE.

UNTIL 530 PM MST

AT 438 PM MST...DOPPLER RADAR WAS TRACKING SEVERAL STRONG
THUNDERSTORMS IN SOUTHERN GILA COUNTY. THUNDERSTORMS WILL CONTINUE
TO DEVELOP AND EXPAND THE REMAINDER OF THE AFTERNOON. THUNDERSTORMS
ARE CAPABLE OF PRODUCING WIND GUSTS TO 45 MPH AND BRIEF HEAVY RAIN.

WINDS IN EXCESS OF 40 MPH WILL BE POSSIBLE WITH THIS STORM.

LOCATIONS IMPACTED INCLUDE...
GLOBE...HAYDEN...CHRISTMAS...SAN CARLOS...MIAMI...SAWMILL...CUTTER...
HILLTOP...CENTRAL HEIGHTS-MIDLAND CITY...CLAYPOOL AND INSPIRATION.

LAT...LON 3340 11035 3338 11045 3329 11046 3320 11040
      3319 11050 3314 11059 3315 11063 3307 11073
      3304 11072 3299 11076 3299 11079 3309 11086
      3359 11105 3368 11035 3343 11034
TIME...MOT...LOC 2338Z 133DEG 4KT 3333 11070

HV