Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 201148

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

.UPDATE...Updated Aviation & Fire Weather Discussions.


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.


Today and Friday...
Stream gauge along Agua Fria River at Rock Springs jumped 3 feet
by around 1230am which reflects the runoff associated with the
flooding that occurred last evening in/near Mayer. Though this
flow is within the banks, it is certainly deep enough to be too
dangerous to try to cross through it. Fortunately it is trending
down and expect this will continue. Areal Flood Warning originally
in effect through 445 am. Will probably need to extend it a
little while longer or perhaps transition to another product
depending upon the trends. Convection looks to be winding down
across Arizona. There had been some decent storms that moved
across southern Gila County earlier. The RAP has been depicting a
small vort max tracking westward across central AZ that may have
been contributing though hard to see a distinct circulation in
water vapor imagery. Recent runs of the HRRR depicted showers
trending down before 15Z but not before reaching eastern portions
of metro Phoenix. There may still be some redevelopment as other
high res models also hint at some isolated showers early this
morning over the central AZ deserts. 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 dropoff. This will also mean storms being better wind
producers and thus outflow boundaries would tend to be more

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 outflows.


Spotters should follow standard reporting procedures.




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