Damaging Winds in Arizona, July 21, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
220 AM MST Sat Jul 21 2018

.SYNOPSIS...
Thunderstorm chances and areal coverage will progressively wane
through the weekend as strong high pressure gradually builds over
the region. Into the first half of next week, storms will likely
become relegated only to the highest terrain areas as hot
temperatures begin to bake the lower elevations. In fact, the
hottest weather of the year will cover the area with afternoon highs
nearing record levels by next Tuesday. Cooler and more humid
conditions are expected starting next Thursday as high pressure
weakens and easterly flow aloft starts to spread monsoon moisture
back into the area.

&&

.DISCUSSION...
Early this morning, the large upper high that will be bringing
excessive heat to the deserts next week was continuing to slowly
build westward, although the high was still centered well east of
Arizona. H5 heights were up to 595dm over the lower deserts per the
latest 00z plot data, and Phoenix reached a high of 109 yesterday.
On the smaller scale, and in the very short term, a weak short
wave/vort max/mcv could be seen embedded in the flow around the
high, moving northwestward out of northwest Mexico and into far
southeast Arizona at 2 am. This feature was causing scattered
showers and some thunderstorms southeast of our area and we expect
some of this convection to move into south central Arizona after 3
am and continue into the mid morning hours. Latest SPC mesoanalysis
graphic showed a tongue of MLCAPE over 1000j/kg nosing up into
eastern AZ with the axis of CAPE east of Phoenix. Some weak
difluence also shows up in upper streamline fields, helping support
convection. So we will likely be dealing with this through the
morning today.

After the wave moves north of our area later today, guidance
continues to suggest that we will be steadily moving into low-grade
monsoon conditions for the remainder of the weekend, becoming pretty
much "no-grade" monsoon conditions for the early part of next week
as excessive heat develops over the desert southwest and envelops
our area. By Sunday we expect just a few afternoon/evening storms
over higher terrain areas east of Phoenix with partly to mostly
sunny skies over the deserts from Phoenix westward. For next Monday
through Wednesday only very isolated convection is likely during the
late afternoon and evening and that should pretty much stay east of
Globe each day with mostly sunny days and mostly clear nights
expected over the deserts.

Model guidance has been very consistent over the past several days,
or more, that a VERY large upper ridge would shift westward, build
and eventually center itself over south-central Arizona with 5000mb
high centers approaching 600dm near Phoenix by Tuesday afternoon.
GEFS ensemble members have been remarkably consistent as have
operational GFS and ECMWF runs for days. Confidence in this heat
event is very high as this is reflected in the Excessive Heat
Warning that remains in effect for the entire CWA Tuesday and
Wednesday, and over the majority over the lower deserts Monday.
Guidance from ESRL Situational Awareness Table - EFI temperatures -
continues to call for high temps between the 95th and 100th
percentile during the warning period and many best performing
temperature guidances such as ECMWFBC suggest highs near or above
records at times. For example, this guidance forecasts a high of 119
in Phoenix on Tuesday which would break the old record of 116 for
the day. At this time it still appears that Tuesday will be the
hottest day of the warning, with the hottest deserts approaching 120
degrees. We did add a few zones across south-central Arizona into
the warning for Monday, including a few that bordered our neighbor
to the southeast.

For the latter part of the extended, the upper high is forecast to
shift a bit further to the north and west, allowing a deeper
easterly flow aloft to develop across southern Arizona. This will
allow increasing amounts of monsoon moisture to spread back into the
lower deserts, ramping up our convective chances and lowering our
high temperatures. Expect highs to fall below heat warning levels on
Thursday with hottest readings west of Phoenix mostly staying below
114. Continued falloff in temps expected into Friday and next
weekend.

&&

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

After a quiet late evening, with westerly winds continuing, a
weather disturbance moving into the region from the SE is expected
to produce at least some light shower activity afternoon after
midnight into the early morning hours on Saturday. A few
thunderstorms are also possible, but just going with VCSH in the
TAFS for now. Cigs could briefly fall to as low as 8k feet as well.
Expecting a shift to southeasterly winds when these showers/TS move
into the Greater Phoenix area. Winds are expected to shift back to
westerly and skies to begin to gradually clear by mid-morning
Saturday, with these westerly winds continuing well into Saturday
night.

Southeast California/Southwest Arizona including KIPL and KBLH:

Anticipate quiet conditions through the TAF period. Winds to remain
mainly out of a southerly direction at KBLH and a southerly
direction at KIPL through the TAF period.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday: Anticipate little to no thunderstorm
potential Monday through Wednesday - except perhaps very slight
chances over the higher terrain of south-central AZ. Instead, much
above normal temperatures can be expected with highs around 115
common on the lower deserts by Tuesday. Minimum humidities
decrease to near 15% lower elevations and 20-30% higher terrain.
Overnight recovery decreases to fair to good. Winds will favor
westerly directions. During the latter half of the week,
humidities trend upward and temperatures trend downward. Slight
chances of storms slowly expands over south-central AZ.

&&

.CLIMATE...

Record high temperatures next week:

Date         Phoenix         Yuma
----         -------         ----
July 23    114 in 2014    117 in 2014
July 24    116 in 2014    117 in 2014
July 25    115 in 1943    120 in 1943

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM Monday to 8 PM MST Wednesday
     for AZZ532-534>544-546-548>551-553>556-559>561.

     Excessive Heat Warning from 10 AM Tuesday to 8 PM MST Wednesday
     for AZZ530-531-533-545-547-552-562.

     Heat Advisory from 10 AM Tuesday to 8 PM MST Wednesday for
     AZZ557-558-563.

CA...Excessive Heat Warning from 10 AM Monday to 8 PM PDT Wednesday
     for CAZ567-570.

     Excessive Heat Warning from 10 AM Tuesday to 8 PM PDT Wednesday
     for CAZ560>566-568-569.

&&

DISCUSSION...CB
AVIATION...Percha
FIRE WEATHER...AJ