Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231212

Area Forecast Discussion
National Weather Service Phoenix AZ
512 AM MST Sun Jul 23 2017

Updated Aviation and Fire Weather sections.


Thunderstorm activity will continue today with activity expanding
into southeast California by Monday morning. Frequent cloud cover
and a moist atmosphere will lead to cooler temperatures through
Monday. This will be followed by drier southwesterly flow aloft
leading to warmer temperatures along with fewer storms across the
region Tuesday through Thursday. More active monsoon conditions
return by next weekend.



Today and tomorrow:
Inverted trough currently situated across southeast Arizona, with
the 300 mb high centered over southwest Arizona. Greatest
divergence associated with the 300 mb high is on the eastern side
of the high. The combination of the inverted trough and divergence
aloft is initiating activity south of Tucson this morning. Most
of the activity should remain in southern Arizona this morning due
to an east to northeast steering flow.

By early afternoon, we will see activity picking up across the
higher terrain north, east, and southeast of our forecast area.
The steering flow will gradually shift from the east/northeast to
the east/southeast by this evening, allowing thunderstorms and
outflows to propagate into our lower desert locations. Stronger
storms will be possible today. Damaging winds will be a
possibility this afternoon due to the 00Z forecast sounding
indicating an inverted V profile & DCAPE values above 900 J/kg.
There is also indication that the steering flow and the flow at
the anvil level will be in opposite directions, which will help
minimize updraft and downdraft interaction and help storms
intensify a bit more.

As we go into the evening hours, the aforementioned inverted
trough and greatest divergence aloft will be in southeastern
Arizona. Thus activity will be ongoing in southeast Arizona late
into the night. The steering flow will continue to shift more to
the southeast, and this will allow more activity to make its way
northward into our lower deserts. As we get into the very early
morning hours, the inverted trough will have shifted into western
Arizona/ southeast California and the greatest divergence will
have shifted into our western CWA as well. This will support the
spreading of activity into southwest Arizona and southeast
California Monday morning and Monday afternoon.

It is important to note that PW values at PHX and YUM will be
near or exceeding record values. The combination of a very moist
atmosphere, dynamic forcing from the inverted trough, and some
decent instability this afternoon through Monday morning will
bring an elevated risk for flash flooding, especially across
southern Gila county and the Phoenix Foothills. A Flash Flood
Watch may be necessary, and if the forecast remains on track, one
may be issued later today.

By Monday afternoon/evening, extensive cloud cover should hold
down instability enough that coverage will be very isolated and
mostly confined to northern Arizona. With the steering flow on
Monday afternoon being out of the south, it will be very hard for
the lower deserts to see much activity.

Tuesday through Sunday:

We start to see a break in monsoon activity Tuesday through
Thursday as dry southwesterly flow starts to situate itself,
pushing moisture north and east. This will keep thunderstorm
chances confined to the higher terrain north and east of Phoenix
each afternoon. 500 mb high begins to situate itself near the four
corners Thursday evening, transporting moisture back into our
region. This will result in a gradual increase in POPs Thursday
night into Sunday.


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

Isolated light showers are possible this morning but confidence
in occurrence at any given TAF site to low to reflect in the TAF.
Otherwise, anticipate thinning of the mid and high clouds late
morning and early afternoon. Storms redevelop over the higher
terrain of north, east, and southeast of metro Phoenix this
afternoon and evening. Mixed signals in the model data as to how
readily storms will develop on the Valley floor. At a minimum,
anticipate outflows from distant storms to affect all TAF sites
during the evening. Thus, the forecasted wind shifts this evening.
Also anticipate that showers and possibly full fledged
thunderstorms could redevelop later tonight in association with a
passing weather disturbance. Given the moisture content in the
airmass, if any thunderstorms manage to form over an airfield,
expect brief but very heavy rain with significantly lowered
ceilings and visibilities. Apart from thunderstorms, expect light

Southeast California/Southwest Arizona including KIPL and KBLH:

Very isolated weak showers this morning will taper off with
clearing later this morning. Thunderstorm development this
afternoon will primarily by limited to La Paz County but could
possibly stray into far eastern portions of Riverside County this
evening. Otherwise, anticipate a lot of mid and high clouds to
overspread the area later tonight and especially Monday morning
with isolated to scattered light showers (mainly near and east of
the Lower Colorado River Valley. Apart from thunderstorms, expect
south and southeast winds to prevail today and tonight with some
gusts up to 20 kts this afternoon (mainly Lower Colorado River

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday:
A drying trend starts Tuesday with a significant drop off in
humidities and storm chances through Wednesday and leveling off
Thursday. Max temperatures trend upward as well to near or
slightly above normal readings. At the end of the week, a return
of Monsoon conditions begins with moisture and storm chances
increasing from east to west.


Spotters should follow standard reporting procedures.




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