Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 271640

Area Forecast Discussion
National Weather Service Phoenix AZ
940 AM MST MON JUN 27 2016

High pressure will move somewhat north and east of Arizona this week
allowing warm temperatures to prevail, but also bring a couple of
weather disturbances through the state. An increase in monsoon
moisture will accompany these weather systems providing a threat of
afternoon and evening showers and thunderstorms the remainder of


A small upper level low is centered just southwest of Yuma this
morning. To the north of that is a larger anticyclonic circulation
centered over the Great Basin. A smaller anticyclone is centered over
north-central Mexico and to the east of that is a cutoff low centered
over Texas. The cyclonic feature over and near our forecast area has
been keeping weak isolated showers/sprinkles going this morning. Also
of note is a zone of strong east-to-west winds in the mid levels due
to gradient between the low and the high to the north. This weakens
as the day wears on. With 1000-700mb average mixing ratios of 7-8
g/kg, there is just enough moisture for CAPE - though there is still
CIN to contend with. But with decent CAPE over southeast AZ and
steering flow from the southeast later on, there is a non-zero chance
of storms surviving over the lower deserts. Not looking at a big day
at this point and hi-res models are not enthusiastic (except for NCAR
ensemble). One caveat is that upper level divergence (associated
with diffluence from the upper low) could extend further south over
our area instead of being focused over FGZ and VEF areas. That could
aid storm development (mainly over La Paz and northern Maricopa
Counties). No changes to the forecast at this time.


Early this morning a well defined inverted trof could be seen moving
westward across far southern/southwestern Arizona, and this feature
was clearly evident in vapor imagery as well as recent plot data.
Circulation around this feature was spreading increasing moisture
westward and into the desert west of Phoenix; latest blended total
precipitable water imagery indicated pwat values over 1.2 inches over
portions of the southwest deserts. The inverted trof created good
upper difluence across the central and western deserts and coupled
with a good uvv field, a field of mostly light showers could be seen
spreading into the western deserts at 2 am. As this inverted trof
slowly pushes west today we can expect a continued slight chance of
showers or thunderstorms across much of the lower deserts to the west
of Phoenix including portions of SE California. Later this afternoon
and evening across south central Arizona, the forcing becomes weaker
behind the exiting inverted trof but there is weak southeast steering
flow and sufficient deeper moisture and instability to allow for a
15-20 percent chance of mainly evening thunderstorms over the central
deserts and a chance of storms over higher terrain east of Phoenix.
This is supported by various mesoscale models such as the NMM6km.
Clouds and moisture working across the lower deserts will keep high
temps today mostly below the 110 degree mark, even over the far
western deserts.

Model guidance, including operational runs of the GFS and ECMWF, as
well as ensemble guidance such as the GEFS and NAEFS, continue to
call for a very monsoonal pattern to develop across the desert
southwest starting Tuesday and continuing through the end of the
work week. Initially the main upper high center forms near the four
corners, setting up a deeper southeast steering flow over most of
the area which will steadily transport increasing and deeper
moisture westward over the lower deserts. PWAT values steadily climb
and are forecast to exceed 1.5 inches across the main monsoon
moisture corridor which sets up just east of the lower Colorado
River Valley and extends into far eastern Arizona. The steering flow
is favorable to bring mountain storms into the lower deserts, and
from time to time there will be disturbances rotating around the
main upper high which will serve to enhance thunderstorm coverage
and intensity. Model guidance typically has a hard time with the
exact timing and track of these features and we generally cannot
place a lot of faith in them more than a day or two down the road.
Overall we are looking at a rather broad brushed forecast with 10-20
percent chances of mainly afternoon and evening storms over the
deserts mainly east of the lower CO river valley, and 30-40 percent
chances across the higher terrain east of Phoenix.

Later in the week the upper high starts to shift towards the
southeast and steering flow becomes more southerly. Still, moisture
remains high and in fact the GFS calls for PWAT value in the central
deserts to approach 2 inches later Thursday into Friday. This would
suggest a much better chance for heavy rains to accompany any storms
that develop. Of course, as moisture increases the temperatures will
fall off and by midweek high temps across south central Arizona will
approach and then fall below seasonal normals. It will stay a bit
hotter further west where the atmosphere is a bit drier, with highs
over 110 expected for much of the week.

During the upcoming weekend, as low pressure sets up along the
Pacific northwest coast, and as the high shifts further to the
southeast, the upper streamline fields and steering flow turn
towards the southwest over most of the area. This starts to bring in
drier air from the southwest and we can expect rain chances to
diminish from west to east with time. By Sunday afternoon,
thunderstorm chances look to be confined mostly to higher terrain
areas east and southeast of central Phoenix as skies become mostly
sunny over the central and western deserts.


South-Central Arizona including KPHX, KIWA, and KSDL:
Much better chances for stronger east/northeast outflow winds are
likely late this afternoon, but confidence in timing is still
somewhat low. Actual storms may not survive to lower elevations,
though mountain obscuration, blowing dust, and frequent lightning for
arrivals/departures to the east may be problematic. Persistent
easterly winds this evening should occur earlier this normal. Sct to
bkn mid and high clouds should persist through Tuesday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
Cigs in a 10K-15K ft range will persist through this afternoon with
some gradual thinning of cloud cover tonight. Virga showers this
morning over the terminals are likely and should dissipate by around
noon. While a south to southeast sfc wind will be preferred through
this afternoon, winds will eventually become more southwesterly this
evening. A few stronger gusts may also be possible for brief
periods, especially this evening.

Aviation discussion not updated for amended TAFs.


Wednesday through Sunday...
Temperatures will continue to "cool" with highs on Wednesday around
105-112F dropping into the 100-110F range by Sunday. As monsoon
moisture levels continue to increase over the region, isolated
thunderstorms are possible each afternoon/evening over much of the
desert southwest, with the highest chances over the higher terrain
of central and eastern Arizona. Accompanying any storm that develops
will be wetting rains, gusty erratic winds, and lightning. This
increase in moisture will support a rise in minimum humidities, with
values ranging in the 15-25 percent range each day with good
overnight recoveries. Outside of gusty storm winds, speeds will
range between 5 to 15 mph with a few afternoon upslope gusts up to
20 mph.


Spotter activation is not expected.




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