Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231808 AAB
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1108 AM MST Tue Jul 23 2019

.UPDATE...
Updated 18Z aviation discussion.

&&

.SYNOPSIS...
After clouds thin out today, expect storm development over higher
terrain locations in the afternoon. Anticipate at least isolated
shower and thunderstorm development over the lower deserts in the
evening...most likely over south-central Arizona. Locally heavy
rain and areas of blowing dust are possible. High pressure will
slowly become centered over Arizona for a downtrend in storm
chances starting Wednesday. High temperatures will also
increase...especially during the latter half of the week.

&&

.DISCUSSION...
Today...
Deep convection over our forecast area has dissipated early this
morning. However, there is an MCV (left over from the earlier
convection) that is tracking right over our CWA. Within that,
there are some bands of very light precip/sprinkles. This feature
will slowly track west-northwestward moving into the Mohave desert
by late morning/midday. With the high centered to the north and
northeast, there will be deep southeasterly steering flow. This
should help prevent us from being clouded over the whole day
(which would hinder destabilization). While surface dew points are
high and there is a lot of moisture at cloud level, the
1000-700mb mean mixing ratios are not overwhelming - but not
insufficient either. Just how much destabilization occurs remains
in question. If the clouds are too slow to depart, that will hurt
instability. Also, the evening storms have had a cooling effect on
the boundary layer. That being said, all indications are that the
Rim and Yavapai County (and Coconino) will be more active today
than they were yesterday. Plus, Gila County looks to be active
once again. That presents the opportunity for outflows. With the
axis of deep moisture roughly over the Lower Colorado River
Valley, propagation to the southwest should be favored. But, the
outflows will likely not be quite as strong with less DCAPE.
Meanwhile, southeast AZ looks to be less active with more
stabilization having occurred. CAMs indicate that at least some
evening storm activity will develop over metro Phoenix. In fact,
there will be at least a slight chance of showers/storms over much
of the forecast area. They also indicate overnight activity -
suggestive of a disturbance from the southeast (but hard to see in
wind, vort/pv model fields). Anticipate a situation tonight with
less wind production but perhaps better precip coverage.

Wednesday through Monday...
There is general model agreement that the storm activity over the
lower deserts trends down beginning Wednesday. This is due in large
part to an increase in stability caused by the the high becoming
centered over Arizona. That also decreases steering flow which
makes it harder for storms to advect from higher terrain. There
also looks to be a west to east decline of moisture with time. The
moisture doesn`t drop to nothing. In fact, 1000-700mb mean mixing
ratios don`t drop dramatically over most of Arizona. Thus, there
will still be higher terrain storms (mainly outside of our CWA).
With less storms and clouds, and the location of the high, max
temps will climb. At this time, it looks like we will remain below
Excessive Heat thresholds. Over the weekend, there are some
indications that the high center may shift westward a little bit
which would make for somewhat more favorable steering flow. It
remains to be seen. For now, have temps trending closer to normal
with "low grade" monsoon PoPs.

&&

.AVIATION...Updated at 1808 UTC

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

A high confidence that easterly winds will be maintained at the Phx
area terminals through the remainder of the day today as 15-20 kt
easterly winds in the low/mid-levels of the atmosphere continue to
be mixed down to the sfc. Thunderstorm outflows are once again
expected early this evening, but the potential for any thunderstorms
to develop between 02-06Z will depend on how much cloud cover we
have and how early the outflows are able to tap into any instability
with daytime heating. Confidence in thunderstorms developing along
these outflows is highest at KIWA where VCTS has been included, but
is more conditional at the other sites where PROB30s are in place.
Easterly outflows are most likely to occur first at KIWA and KPHX,
whereas KSDL and KDVT are more likely to have north or northeasterly
outflows first from convection that develops along the Mogollon Rim
despite the general southeasterly steering flow. Dust potential is a
bit more limited today because weaker and less organized outflows
are expected, with moist downbursts from any convection that
develops the more likely cause of wind gusts in excess of 35 knots.
Easterly winds are expected to persist into the morning hours on
Wednesday, along with some lingering light showers and mid-level BKN-
OVC cigs. Westerly winds will likely return Wednesday afternoon as
winds aloft weaken.

Southeast California/Southwest Arizona including KIPL and KBLH:

Although there is a very slight chance of an isolated shower and
thunderstorm at both KIPL and KBLH, lack of a clear triggering
mechanism like outflows or terrain will most likely prevent any from
developing. Assuming no thunderstorms develop, winds will generally
remain southeasterly at KIPL and south-southeasterly at KBLH with
some cloud cover and relatively weak flow aloft keeping surface
winds below 15 knots. Latest high-res model output continue to
indicate that showers and potentially an isolated thunderstorm may
develop late in the overnight hours with the assistance of a low-
level jet and deep moisture across the Imperial valley, which is
reflected by VCSH in the KIPL TAF.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday:
Expect thunderstorms to be limited to the higher terrain of south-
central AZ. Max temperatures will climb to several degrees above
normal Friday and Saturday before beginning to trend down. Any
thunderstorms that manage to form will be very effective at
producing strong winds but with little wetting rain. With the
rising temperatures, minimum humidities decline as well but are
expected to remain well into the teens. Overnight recovery
declines some but remains at least fair (better over higher
terrain).

&&

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

&&

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

$$

DISCUSSION...AJ
AVIATION...Percha/Hopper
FIRE WEATHER...AJ


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