Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 201600

Area Forecast Discussion
National Weather Service Phoenix AZ
900 AM MST Thu Jul 20 2017

Thunderstorm chances will be limited to Arizona through the
weekend before expanding into southeast California Monday. As is
typical, chances will be best over the higher terrain. A gradual
decrease in storm chances begins next week as drier flow develops.


Numerous MCV`s are floating around the SW Conus this morning
encapsulated in a weakening flow pattern characterized by a
pronounced easterly wave lifting north through New Mexico and a more
subtle anti-cyclone over southern AZ. Of nearly equal importance is
increasing westerly flow aloft impinging into California and Nevada
which will act to reconfigure the flow pattern over the next 24
hours. As the H2 easterly wave lifts further north into Colorado
later today, the confluent westerly flow at jet level will begin to
spread into much of Arizona limiting the development of more
organized thunderstorm systems beginning as early as this afternoon.

Otherwise, one consequence of all the small vorticity centers
collected with ample moisture in the H7-H5 layer is the amount of
accas lingering over the region. The flat nature to the clouds is an
indication of the higher static stability; and the resultant modest
reduction in insolation may also have additional negative influence
on storm development later today (not to mention impacts on
temperatures). Convection will be favored through far SE and NE
Arizona this afternoon with more of a subsidence background covering
the bulk of the forecast area (sans Gila County where numerous slow
moving storms are likely this afternoon). Thus, have strongly
limited POPs to the eastern foothills and mountains of the CWA and
would only expect weaker outflow boundaries elsewhere around the
Phoenix metro.


Today and Friday...
A larger cyclonic feature lies over New Mexico. While the axis of
the associated inverted trough will track through eastern AZ, it
looks like the vorticity advection will mainly affect the Colorado
Plateau. On the larger scale, flow aloft weakens for this afternoon
and evening in the wake of the inverted trough and the main high
pressure region over the CONUS shifts emphasis a bit further east.
With weak steering flow, storms will be less likely to advect off of
the higher terrain. Blended TPW imagery indicates that precipitable
water is starting to trend down. GFS and NAM show a downward trend
for today and again Friday. The ECMWF less so. The declining
moisture will also tend to lead to less storm coverage. However, it
is not a sharp drop off.

Saturday and Sunday...
Steering flow becomes more distinct with an anticylonic center
taking position near Las Vegas. This would be favorable scenario
for storms to reach metro Phoenix. Northeast or easterly
component to the steering flow looks to continue through Sunday.
Storm chances spread further west as well.

Monday through Wednesday...
A transition begins during the next work week with a turn to a
more southwesterly component to the flow and a gradual drying
trend. Thus tried to depict a gradual downtrend to the PoPs.


South-Central Arizona including KPHX, KIWA, and KSDL:
East winds will continue at terminals until the usual switch to
westerly occurs around noon.  Thunderstorm chances decrease for
today as relatively light steering winds will have difficulty moving
storms from the mountains down into the valley. A weak outflow
generated from thunderstorms in the higher terrain may possibly
move through the TAF sites this evening but chances look low. For
now, will continue with a persistence forecast.

Southeast California/Southwest Arizona including KIPL and KBLH:
No convection is expected today as any activity should remain well
off to the east in Arizona.  Typical southeasterly winds will
prevail at KIPL with southerly winds at KBLH.

Aviation discussion not updated for amended TAFs.


Saturday through Wednesday:
Afternoon and evening thunderstorms will continue into early next
week, although the most concentrated storms will be confined to the
higher elevations north and east of Phoenix. Occasionally heavy rain
will be possible at any location impacted by storms. Otherwise,
minimum humidity will remain around 20-30% in the lower deserts and
between 40-50% for the higher elevations of Gila County. Typical
summertime breezes can be expected except near thunderstorm


Spotters should follow standard reporting procedures.





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