Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 231055

Area Forecast Discussion
National Weather Service Phoenix AZ
355 AM MST SUN OCT 23 2016

Updated Aviation and Fire Weather sections.


Beginning today, a large Pacific low pressure system will move into
the western states. This low pressure system will draw a
considerable amount of subtropical moisture into the region with
threat of showers and thunderstorms starting this afternoon and
continuing through Monday night. The best chance of showers will
occur Monday over south central Arizona, with afternoon
thunderstorms possible. Drier and warmer conditions will return
by the middle of the week.


The weather forecast for the next 2-3 days has not changed
appreciably and we are still looking at somewhat cooler and wetter
conditions developing across the area with scattered showers and
embedded thunderstorms to move into the central deserts by Monday
afternoon. However, the preponderance of model guidance as well as
various MOS numbers suggests that we may not see as much
precipitation with this event, and there is a chance that Phoenix
may not even drop below 90 degrees on Monday due to the
moisture/showers being less prevalent over the central deserts. So
POP trends have actually been backing off somewhat compared to the
past 2-3 forecast packages.

Still, we are seeing a large upper trof developing along the west
coast, and southerly flow ahead of the trof will begin spreading
considerable moisture into the area during the day today. Already,
blended total pwat imagery shows a tongue of over 1 inch creeping
north and approaching the southern AZ border as of 2 am this morning
and IR imagery finally shows some mainly high clouds getting into
far southeast California as well as the southwest AZ deserts. As
moisture increases and UVV fields slowly increase (we still see
modest mid level Q-convergence fields developing out west today) the
atmosphere will wet up from above and a few showers will likely
develop over the deserts to the west of Phoenix - especially over
far southeast California. Rain chances today in the greater Phoenix
should stay below 10 percent however. Despite significant increases
in cloud cover, the southerly low level flow will keep warm air
advection going and allow high temps today to climb back into the
low to middle 90s.

For tonight into Monday, short wave energy continues to move out of
the deeper trof and ride up to the northeast leading to increasing
UVV fields and better chances for precipitation. Despite this, the
models suggest that the bulk of this energy and strongest UVV will
stay well to the northwest of central Phoenix and the final short
wave that passes across the area Monday into Monday night may be a
bit less impressive that we previously thought. So, POPs tonight
stay mostly in the slight chance category, increasing into the
chance category by Monday morning; on Monday the best chances for
rain showers or embedded storms will occur over south-central
Arizona. Given the decent amounts of wind shear expected Monday,
some jet dynamics and CAPE running 400+ j/kg in places, there is
potential for a few stronger and more organized storms mainly during
the afternoon and early evening. Stronger storms will locally
enhance rain totals but overall we are not looking at significant
rains with this event - most places should see rain less than one
quarter of an inch with this event. Again, model POP trends have
been decreasing with time; latest NAEFS and SREF POPs have backed
off with this event this trend will be accepted. POPs for Phoenix on
Monday have been lowered to around 40 percent with higher numbers
across the higher terrain north and east of Phoenix. The
preponderance of guidance suggests that high temps in the Phoenix
area Monday may stay at or above 90, but we will go with 89 for now.
If the MAVPHX guidance had its way Phoenix would see a high of 93
Monday but this seems too high.

A gradual clearing and drying trend from the west is expected Monday
night into Tuesday as the last short wave moves through central
Arizona and then quickly exits east into New Mexico, followed by
building high pressure and increasing subsidence. By Tuesday
morning there should be just a lingering slight chance of showers
over the higher terrain east of Phoenix with mostly sunny skies
developing over the central and western deserts. High temps will
climb slightly Tuesday and into the low 90s from Phoenix westward due
to increasing sunshine and mixing.

For Wednesday into Thursday, strong high pressure aloft is forecast
to build back into the desert southwest, as another large area of
low pressure sets up just off the west coast and pumps up the ridge
downstream and over our area. High temps will climb back into the
middle 90s, with a high of 95 forecast in Phoenix by Thursday; that
would be over 10 degrees above seasonal normals. It should stay dry
for the most part, though a minor short wave lifting out of the deep
trof will brush by far southeast California Thursday leading to a
slight chance of a shower over the higher terrain of Joshua Tree NP.

Latest ECMWF and GFS have come into much better agreement for the
latter portion of the extended forecast, and the GEFS members also
have come into better alignment; we expect that in general the ridge
will remain rather strong over the desert southwest but there will
be continued short waves moving out of the trof along the west
coast, riding across and over the top of the ridge and bringing in
some modest moisture and UVV, enough to warrant a slight chance of
showers over the western deserts - mainly over higher terrain areas
of southeast CA. The central Arizona deserts should remain dry and
warm through the period however. Latest NAEFS POPs trends also
support the relatively dry conditions during the Friday through
Saturday period.


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

A light wind regime will continue today and tonight. Moisture will be
on the increase today and tonight ahead of a Pacific weather system.
Isolated thunderstorms are anticipated over southeast Arizona this
afternoon. There is a slight chance of showers and thunderstorms
developing over metro Phoenix tonight. Couldn`t rule out the
possibility of outflow from distant storms reaching a TAF site.
Probability of storms directly affecting any of the TAF sites is too
low to reflect in the TAFs at this point. Otherwise, increasing
clouds (bases mainly AOA FL100).

Southeast California/Southwest Arizona including KIPL and KBLH:

Light winds will become southerly by midday with occasional gusts to
15 kts this afternoon (mainly near and east of the Lower Colorado
River Valley). Moisture will be on the increase today and tonight
ahead of a Pacific weather system. There is a slight chance of
showers and thunderstorms this afternoon and tonight. Couldn`t rule
out the possibility of outflow from a distant storm reaching a TAF
site. Probability of storms directly affecting any of the TAF sites
is too low to reflect in the TAFs at this point. Otherwise,
increasing clouds (bases mainly AOA FL100).

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday...

There will be a lingering slight chance of showers Tuesday over the
higher terrain of south-central Arizona. Otherwise, temperatures
slowly warm and humidities slowly decline into Thursday. Another
weather system brushes the Desert Southwest late Thursday through
Friday for an increase in humidities and a slight chance of showers -
mainly for southeast California and southwest AZ. No strong winds
are anticipated through the forecast period. Minimum humidities stay
at or above 20% on the lower deserts for the most part and overnight
recovery remains at least good.


Spotters should follow standard reporting procedures.




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