Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 201122

Area Forecast Discussion
National Weather Service Phoenix AZ
422 AM MST Sun Aug 20 2017

.UPDATE...Updated aviation and fire weather discussions


An increase in monsoon moisture over the next couple days will
lead to rain chances moving back into the area, especially across
southeast and south-central Arizona. Rain chances will last into
the middle of the week before another drying trend lowers chances
by Thursday or Friday. Temperatures over the next week will
remain generally near normal.


At 3 am this morning, plot data depicted a developing upper low
along the far southern California coast; this feature will become
more prominent in the days to come as southerly flow ahead of it
steadily imports monsoon moisture northward and into the lower
deserts. Western deserts were basically clear this morning, but a
disturbance moving north out of Mexico could be seen moving into far
eastern Arizona and bringing considerable cloud cover into southeast
and east-central Arizona as well as a few light showers over
portions of southern Gila County. As the day progresses, this weak
low will slide north along the AZ/NM border, bringing a few showers
and afternoon thunderstorms to the higher terrain of southern Gila
County before lifting off to the northeast and moving out of the
area. Otherwise, monsoon moisture will continue to spread north and
northwest leading to a slight chance for late afternoon and evening
storms mainly east of the lower Colorado River valley. PWAT is
already increasing; the latest Phoenix and Tuscon soundings
indicated values just over one inch. The 3 am surface dewpoints over
the central deserts had climbed 5 to 15 degrees over the past 24
hours and were running in the upper 40s to mid 50s.

Over the next few days, through about Tuesday, the low to our west
will continue to strengthen; southerly flow ahead of it will import
increasing monsoon moisture north and west leading to mainly slight
chances for afternoon and evening desert showers and thunderstorms.
For the most part we are looking at low grade monsoon conditions as
moisture is not forecast to become excessive; 850mb dewpoints
generally stay below 10C and PWAT values do not rise much above one
inch. CAPE values also remain modest each day, mostly below 300
j/kg. Vort spokes rotating around the upper low will add occasional
dynamics/lift to the equation but they are problematical to time,
and most of the difluence aloft is forecast to stay north of our
area. With high temperatures expected to stay generally near
seasonal normals each day, we won`t have the benefit of excessive
solar heating during the afternoon to spur significant convection.

Operational runs from the GFS and ECMWF as well as GEFS ensemble
output continue to call for the low to our west to open up and move
inland Wednesday into Thursday. As this occurs, wind fields become
elevated over the central deserts and although steering flow becomes
southwesterly, divergence aloft as well as QG forcing will combine
with sufficient moisture and instability to force scattered
thunderstorms over south-central Arizona starting Wednesday
afternoon and running through the evening hours. POPs have been
raised into the 20-30 percent ballpark for the greater Phoenix area
Wednesday night, which looks to be the most active portion of the
entire 7 day forecast period. By Thursday evening, drier air will
have overspread much of our western deserts, pretty much eliminating
any threat for storms for areas west of Maricopa County, and largely
confining storm chances to the higher terrain east of Phoenix. This
will be the case into the weekend as POPs lower into the single
digits across most of the lower deserts.

High temperatures Thursday will fall a few degrees and below
seasonal normals under the influence of the low moving into the
area. However Friday into the weekend we expect a warming trend as
high pressure aloft again takes hold over the desert southwest; as
the ridge builds aloft 500mb heights climb above 592dm and we can
expect high temps to climb several degrees above normal. By
Saturday, the Phoenix area should see highs near 107 with readings
approaching 110 over the hotter western deserts.


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

Easterly flow will persist through the morning before a switch to
westerly happens in the afternoon (20-21Z). Monsoon moisture will
be increasing across our area today and will continue to bring
FEW to SCT decks around 12kft. Some models suggest a few showers
could develop near the TAF sites between 17-20Z today but
confidence is low on that happening. Additionally, thunderstorms
will be possible later in the afternoon, however, not confident
enough to put them in the TAFs right now.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will remain southerly at KBLH through the TAF period.  For
KIPL, expect winds to remain light and sometimes variable through
the early afternoon favoring the east to southeast direction. By
late afternoon winds will be switching back to the west at KIPL.
Models indicate a few showers/storms may form in the vicinity of
the airports late this morning and through the afternoon, however,
it is very unlikely. Only mentioning FEW to SCT decks in the TAFs
for now.

Aviation discussion not updated for amended TAFs.



Tuesday through Saturday: 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 higher
elevations on Friday while all areas will see near seasonal
temps. Minimum humidities will hover around 15-20% in the lower
deserts while the higher terrain of Gila County will be much more
favorable, around 30-40%. Models diverge towards the weekend
although continued mountain storms looks to be a safe bet.


Spotters should follow standard reporting procedures.




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