FXUS65 KPSR 232037

Area Forecast Discussion
National Weather Service Phoenix AZ
137 PM MST Wed Aug 23 2017


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.


Today continues to progress as our best day for thunderstorms
with 20-30 percent chance for storms Maricopa County and eastward.
The synoptic setup brings the California trough closer to the
area from the west while the ridge to the east continues to
migrate north. Together, these features favor a more southerly
flow through the area which has allowed more moisture to permeate.
Dew points across the lower desert have risen into the upper 50s
to lower 60s, and most models indicate 500 to 1000 j/kg of CAPE by
this afternoon with fairly weak capping. As the trough
approaches, positive vorticity anomalies look to spread over
Arizona later today along with some areas of upper level
divergence. Currently, the best atmospheric dynamics are perched
over southeast Arizona, including a healthy pocket of upper level
divergence and some minor perturbations rotating around the
ridge. Combined with the remnants from the overnight convection in
Sonora, southeast and eastern Arizona have already become active
with storms early this afternoon.

The greatest change to the forecast is the addition of blowing dust
into the grids for southern Maricopa County and northern Pinal
County. Convective Allowing Models (CAMs) continue to highlight
stronger convection south over Pima County, while the Storm Scale
Ensemble Opportunity and NCAR ensembles each show a bullseye for
convection over southeast Arizona with diminishing chances closer
to Phoenix. Storm initiation over the Phoenix area may require
some extra lift, such as strong or colliding outflow boundaries,
and that set up is not a given at this time. Folks have a better
shot of being inconvenience by blowing dust as convection south
of Phoenix will be capable of producing northward traveling
outflows that might be strong enough to loft dust between Phoenix
and Tucson. A haboob event is possible, even into the Phoenix
area, but will depend on how storms and outflows progress. The
models have enough varied interpretations of what may happen
tonight to leave some doubt on outflows for the Phoenix area later
this evening. The Tucson office issued a blowing dust advisory
for the northern areas of their CWA as to serve as a watch but I
think there is enough doubt in the event to hold off for now.

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

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.


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

At least scattered thunderstorms are forecast for the higher terrain
of southeast AZ this afternoon and early evening. These storms will
send outflows to the north/northwest and the TAF sites could see
gusty southeasterly winds to around 25kt by 02-03Z. This may result
in some blowing dust making its way into the Phoenix area along with
the gusty winds. For now will hold off mentioning any visibility
restrictions for KPHX or KSDL.

Could see an isolated thunderstorm approach Phoenix area from the
south/southeast early this evening which may impact KIWA. Scattered
showers are more likely for KPHX and KSDL, but we can't completely
rule out some thunder approaching the terminals. Expect BKN decks
around 8-10K feet to develop and persist through midnight.

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 this afternoon. Winds to favor the southeast
for the most part at KIPL, less than 10kt.



Saturday through Wednesday:

Monsoon activity over the forecast area through next Wednesday will
be somewhat sparse. Storm chances will stay confined to the higher
terrain areas east of Phoenix through Sunday 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 12 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.


Spotters should follow standard reporting procedures.




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