Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 191123

Area Forecast Discussion
National Weather Service Phoenix AZ
423 AM MST Sat Aug 19 2017

.UPDATE...Updated aviation and fire weather discussions


Atmospheric moisture content begins to trend upward over the
weekend for the Desert Southwest. Accordingly, rain chances begin
over southeast and east-central Arizona today and expand across the
region to the deserts of California by early next week. Temperatures
will remain near normal through next week.


The past few days of unseasonably dry weather look to be coming to
an end as monsoon moisture will be returning to the area this
weekend and ushering in generally low-grade monsoon conditions -
mainly to southwest and south-central Arizona and not so much far
southeast California. At 3 am, IR imagery showed a dissipating MCS
over northwest Mexico shedding debris clouds which were spreading
into southeast Arizona, but most of the lower deserts of AZ and far
SE California were clear. Latest Phoenix sounding depicted the dry
airmass currently in place with at PWAT of just 0.62 inches. Surface
dewpoints over the central deserts mostly ranged from the mid 30s to
mid 40s, with values running around 50 further west along the lower
Colorado River valley and into the Imperial area.

Latest plot data showed a weak upper low setting up along the far
southeast California coast, and the upper high was east of the area,
roughly along the AZ/NM border over far east-central AZ. The gist of
the synoptic pattern, as depicted by operational models as well as
GEFS ensemble output, has the upper low continuing to strengthen as
it sits in place to our west, keeping the upper high off to our east
and providing a mostly southerly flow into the area. This will
gradually import monsoon moisture into the area, first into south-
central Arizona and by late this weekend and into early next week,
into the deserts of far southeast California as well. High res
models do not bring much moisture into south-central Arizona today
and convection will be minimal, mainly a few storms this evening
over higher terrain areas mostly east of Globe. Better chances for
storms will develop Sunday into early next week but overall we
expect just slight chances over lower deserts and low end chances
across the higher terrain east of Phoenix. Disturbances rotating
around the low could add dynamics to the mix and locally enhance
storm chances, but the timing of any of these features is
problematical at best. There will also be difluence aloft ahead of
the low, but most of this appears to stay focused north of eastern
Riverside and La Paz Counties and again will not be much of a factor
for storm development over our western deserts.

High temps this weekend should stay near seasonal normals under the
influence of the low to our east.

Starting Tuesday evening and continuing into midweek, the low will
start to spread drier air into southeast California and far southwest
Arizona as flow aloft turns slightly southwesterly and this will
serve to confine storm development to areas east of the lower
Colorado River valley. On Thursday the low is forecast to open up
and move inland, bringing more dry air further east across the lower
deserts and as this occurs, storm chances from Phoenix west fall
with POPs to be in the single digits. Slight chances for storms will
continue over the higher terrain of southern Gila County into the
end of the work week. High temps Tuesday into Thursday will stay a
bit below normal, especially over south-central Arizona, under the
influence of the low and its slightly cooler airmass. Temperatures
should climb slightly Friday as low continues to weaken and high
pressure aloft becomes slightly stronger.


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

Winds this morning will remain light and sometimes variable before
switching back to westerly earlier than usual (18-19Z). A few
afternoon gusts up to 15 kts out of the west will also be
possible. Increasing moisture will bring FEW to SCT decks around
12kft late this morning and afternoon but no storms are expected.

Southeast California/Southwest Arizona including KIPL and KBLH:

Mostly clear skies will prevail as winds remain southerly at KBLH
today and from the southeast at KIPL. Models indicate westerly
winds will likely form again at KIPL this evening (01-03Z)
although they will remain light. No other aviation concerns.

Aviation discussion not updated for amended TAFs.


Monday through Friday: A moderate amount of monsoon moisture will
persist through at least mid week bringing a chance of storms (20-
30%) to the higher elevations each day and a slight chance (~10%) for
the lower elevations of south-central Arizona. Slightly drier air
moving in late week will likely confine storms to the high
elevations on Thursday and Friday while all areas will see near
seasonal temps. Minimum humidities will hover around 20% for the
lower elevations while the higher elevations of Gila County will be
much more favorable, around 30-40%.


Spotters should follow standard reporting procedures.




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