Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 221120 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
420 AM MST Tue Aug 22 2017

.UPDATE...Updated aviation and fire weather discussions.


A decrease in monsoon moisture will confine any chance of thunderstorms
to far eastern Gila County today. It is not until Wednesday that
an increase in moisture will allow storm chances to return to the
valley. Another drying trend starts to lower chances Thursday
evening while temperatures gradually warm into the weekend.



Latest satellite imagery shows mostly clear skies over our CWA
this morning while northern Arizona and neighboring states are
seeing some showers and thunderstorms. This is all associated
with an upper level low that is situated just off the California
coast where various vort maxes are gradually pinwheeling around
the pressure center.

For the Phoenix CWA, we continue to sit under a drying
southwesterly flow which has brought PWs down to around an inch
here in Phoenix. Because of this benign flow pattern, we are not
expecting any storms across the deserts this afternoon. However,
terrain induced upslope flow may trigger weak isolated storms in
far eastern Gila County this afternoon but even that looks to be a
stretch given the relatively dry conditions.

It is not until Wednesday does the flow pattern change, albeit
slightly. An upper level high, currently situated over northern
Mexico, will gradually move up into eastern Arizona while the
previously mentioned low moves inland. Consequently, vort maxes
rotating around the high and low may broadbrush our area late
Wednesday into Thursday morning providing the upper level dynamics
to possibly trigger convection. Additionally, with the high
positioned to our east, a bit of moisture will likely be pumped
into our area. The GEFS mean currently shows only 1.3 inches of
PW Wednesday afternoon through Thursday morning which will likely
be a limiting factor as to how much convection will form during
this time frame. Nonetheless, we will continue to advertise a
slight chance of storms (~10%) across the south-central AZ
deserts Wednesday evening to Thursday morning.

As the upper level trough moves through, another reinforcing shot
of dry air comes marching in Thursday afternoon and will
effectively shut off all convection in our CWA by late Thursday
evening or early Friday morning. Models advertise that these dry
conditions will largely persist through the weekend as an
expansive ridge of high pressure builds over the west coast. While
large, some of our best verifying guidance suggests highs may
only top out in the upper 100s Saturday and Sunday which is just a
few degrees above normal for this time of year.

Beyond this, the flow becomes more convoluted and various
solutions being to present themselves. The GFS and ECMWF show that
by early next week the ridge may position itself far enough north
to bring a moistening easterly flow to our area. If this comes to
fruition, an uptick in moisture and monsoon activity could be
realized. Given how far out this is, have gone with PoPs
closer to climatology.


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

No aviation concerns expected for at least the next 24 hours as
drier southwest flow aloft keeps any thunderstorms confined well to
the east of Phoenix and over higher terrain areas of southern Gila
County. Look for FEW to SCT mid level decks this morning with bases
aoa 12k feet, clearing by afternoon with just a few higher based
cumulus floating around. Winds to follow typical diurnal tendencies
at the TAF sites with speeds mostly below 10kt.

Southeast California/Southwest Arizona Including KIPL and KBLH:

No aviation concerns for at least the next 24 hours as drier
southwest flow aloft will dominate the weather producing mostly
clear skies. Expect a few mid decks mainly this morning with bases
genly aoa 12k feet becoming clear by this evening. Winds to be on
the light side, favoring the south at KBLH and the southeast at KIPL
during the afternoon. We may see lots of light variable wind reports
from both sites next 24 hours.

Aviation discussion not updated for amended TAFs.


Thursday through Monday:
Thunderstorm activity will wane through the end of the week with
any lingering storms focused only over the higher terrain of Gila
County. More expansive thunderstorms may be possible after Sunday
next week. As the atmosphere dries and warms, afternoon minimum
humidity levels will hover in a 15-25% range with fair to good
overnight recovery. Wind will maintain a typically weak summer speed
with nothing unusual expected through the weekend. For the most part
high temperatures each day will stay above seasonal normals, with
the warmest days likely to be this weekend as hotter western deserts
approach 110 degrees.


Spotters should follow standard reporting procedures.




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