Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231142 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
440 AM MST Wed Aug 23 2017

.UPDATE...Updated aviation and fire weather discussions.

&&

.SYNOPSIS...
Moisture levels will increase across much of Arizona today,
heralding an increase in chances for showers and thunderstorms.
Further west, dry air will remain entrenched across Southeast
California. The moisture will linger into Thursday before
decreasing as we head into the weekend. Temperatures will remain
above normal through the next week.

&&

.DISCUSSION...

Low level water vapor G16 imagery depicts a decaying MCS over
Sonora Mex (along with an excellent example of gravity waves)
under a broad and weak anti-cyclonic circulation. Higher up, a
trough can be seen approaching the SoCal coast. A small area of
convection did flair up near Globe, possibly due to errant wave
interactions. PWAT values were generally just above an inch.

Looking ahead toward today, pattern seems to be the most conducive
for convection that we`ve seen in the past several days. The
incoming trough will help provide some mid-level lift supported by
the entrance region of an (albeit weak) upper level jet streak and
an increasing southerly flow at 700 mb. Generally speaking,
instability parameters indicate a moderately unstable airmass will
be in place this afternoon. CAMs seem to prefer developing
convection across Pima County this afternoon, with outflow winds
moving northward and generating additional convection into Pinal
and Maricopa Counties. Outflow could easily be strong enough to
create a dust storm, but not enough confidence at this time to
enhance our messaging on that specific threat. HRRR-X appears to
be the most aggressive with that solution while the operational
HRRR is less inclined. SPC SSEO and NCAR Ens are somewhat
inbetween with their solutions. All in all, possible active
afternoon/evening with better forcing mechanisms in play and
moderate instability to work with.

Already by Thursday, the trough lifting to the northeast will
begin to beat back the lower level moisture. Mixing ratios in the
1000-700 mb level will thin to around 9 g/kg by the afternoon,
which is reflected in decreasing instability parameters.
Precipitation chances will remain but will be on the downward
trend.

Heading into the weekend, a broad anti-cyclone gradually develops
across the western states, with the maximum height anomalies
situations over the PacNW. Locally, our temperatures will increase
as any chances for rain greatly diminish. HeatRisk values will
broadly climb into the moderate range, meaning the heat will be an
issue for at-risk folks. We`re unlikely to reach the high category
unless forecast temperatures increase by several degrees.

Early next week, appears a backdoor front will push through New
Mexico and into Arizona. This pattern can bring us some
precipitation (primarily through a northeast steering flow to help
propagate convection off the Rim) but not really the most ideal,
especially as we`d be recycling moisture from the Four Corners
area. It should be enough to bring our temperatures down just a
few degrees.

&&

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

Outside of some variable mid level cloud decks this morning (bases
genly aoa 12k feet) the main weather impact will be a thunderstorms
developing late this afternoon and through the evening hours as an
upper low approaches from the west. High based cumulus to develop
this afternoon, with stronger storms forming over higher terrain of
southeast AZ. These storms will send outflows to the north/northwest
and the TAF sites could see gusty southeasterly winds to 25kt by
02z. After 02Z showers and storms are expected to develop with rain
chances around 30 percent. Expect BKN decks around 8k feet to
develop and persist through midnight. Confidence way too low to
mention thunder in the TAFs at this time outside of VCTS wording -
and we will opt to keep that out of the KPHX TAF for now. For
Phoenix we will just mention VCSH between 03z and 08z but isolated
showers or storms could persist into the morning hours on Thursday.
Otherwise after midnight expect SCT to BKN debris decks 12-15K feet
with some lingering cumulus. This may prove to be an interesting
night, convectively speaking, in the Phoenix area.

Southeast California/Southwest Arizona including KIPL and KBLH:

We expect convection to stay east of the lower Colorado River valley
next 24 hours with mostly clear skies at KIPL and just variable mid
decks at times at KBLH. Winds to favor the south next 24 hours at
KBLH with a few gusts to near 18kt likely during the afternoon.
Winds to favor the southeast for the most part at KIPL, less than
10kt.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
Friday through Tuesday:
A break in the Monsoon continues over the forecast area through
Tuesday with negligible storm chances. Overnight recoveries will
remain fair to good. Storm chances will stay confined to the higher
terrain areas east of Phoenix through Friday before moisture starts
to work back towards the west, bringing a slight chance of
thunderstorms to the south-central deserts by Monday. Humidities
will stay somewhat low each day with minimum RH values mostly
between 11 and 20 percent each day. Apart from thunderstorms, winds
will tend to follow familiar warm season patterns. In addition,
temperatures will stay above normal each day, especially over the
weekend when hotter western deserts should reach to 110 degrees.

&&

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

&&

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

$$

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DISCUSSION...Iniguez
AVIATION...CB
FIRE WEATHER...Hernandez/CB


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