Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 210917

Area Forecast Discussion
National Weather Service Phoenix AZ
215 AM MST FRI OCT 21 2016

Dry weather with mostly clear skies will continue across the Desert
Southwest through Saturday. Temperatures will also continue to be
unseasonably warm. By Sunday, a large Pacific low pressure system
will move into the western states. This will draw a considerable
amount of subtropical moisture into the region with slight chances of
showers starting Sunday afternoon persisting through Tuesday, mainly
over south central Arizona. The best chance of showers will occur
Monday over south central Arizona, and isolated afternoon
thunderstorms are possible as well.


Strong high pressure aloft continues over the desert southwest with
500mb heights running around 590dm across much of Arizona as seen in
the recent 00z plot data. Atmosphere continues very dry with just
0.21 inches of PWAT at Flagstaff and slightly less than one half
inch at Tucson. Surface dewpoints over the central AZ deserts at 1
am were running in the mid 30s to around 40. IR imagery early this
morning showed clear skies area-wide. Locally breezy east winds
yesterday helped add some downslope warming to the mix and allowed
the high temperature in Phoenix to climb to 97 degrees. Similar
conditions are expected today as the strong high pressure aloft
will persist and breezy low level east winds are again expected to
produce some downslope warming effects. High is forecast to again
reach 97 in Phoenix today. Little change in the forecast is
anticipated Saturday, as the upper ridge starts to shift off to the
east with a dry south/southwest flow aloft moving into the area.
Temps will fall but only slightly under mostly sunny skies.

The dry and quite warm weather will be coming to an abrupt end
starting Sunday and continuing into the early part of next week as a
deep upper trof developing along the west coast starts to tap
significant amounts of subtropical moisture and then spreads it
north into the lower deserts of southern Arizona and southeast
California. By late Sunday afternoon, a plume of deep moisture with
PWAT values at or over one inch spreads into the lower deserts
mostly to the west of Phoenix. IVT imagery from the GEFS/GFS depicts
a plume of 250-300+ units of moisture transport moving into western
Arizona and the ensemble situational awareness table shows PWAT of 2
to 3 standard deviations above climo associated with this plume.
Initially we will see thick mid/high clouds spreading in from the
south and with time the atmosphere will wet up from above. In
addition there will be some QG forcing at play as short waves eject
from the deep upper trof and move northeast across the area. GFS
forecasts a prominent area of mid level Q-convergence moving into
the deserts west of Phoenix by Sunday afternoon and this field
remains in the area into the day on Monday, eventually translating
east and into south central Arizona. So by Sunday afternoon we
expect 15-20 percent chances for showers over the deserts west of
Phoenix with POPs just below 10 in the greater Phoenix area. There
is minimal CAPE but this is mostly east of Phoenix so there may be
an isolated higher based storm developing across the higher terrain
of southern Gila County later on Sunday.

Overnight Sunday night the rain chances increase to near 30 percent
over south-central AZ with around 20 percent chances persisting out
west. Best threat for rain and potentially heavy rain will occur on
Monday as the best QG forcing moves in from the west and acts on a
very moist and slightly unstable airmass. POPs have been raised
Monday into the 30-50 percent range over south-central AZ with best
chances across higher terrain areas north and east of Phoenix. POP
trends Sunday through Monday have been based to a large extent on
the current NAEFS POPs trends which seem very reasonable although
slightly overdone.

High temperatures Sunday come down a few degrees due to the cloud
cover and showers, but stay in the 90s due to expected warm air
advection under the influence of brisk southerly flow into the
state. Cooler temps will occur Monday as the airmass wets up and
considerable cloud cover continues; expect the greater Phoenix area
to fall into the mid to upper 80s Monday.

On Monday night the last in series of shortwaves exits to the east
of Phoenix and we will see a marked drying and clearing trend from
the west. There will be a lingering chance of showers with an
isolated evening storm mainly to the east of central Phoenix and by
Tuesday as higher pressure starts building in from the west, there
will be just a slight chance of a shower over southern Gila County
as mostly sunny skies develop from the Phoenix area westward.

Dry and warmer weather is expected Wednesday into Thursday as high
pressure aloft builds across the area from the west. High
temperatures will again climb into the low to mid 90s over the
warmer lower deserts under mostly sunny skies.


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

East to northeasterly winds will continue through Friday as the
pressure gradient across central Arizona remains in place. Winds
will remain AOB 10 kts through mid Friday morning, but then become
gusty at times during the late morning and afternoon. No aviation
impacts through at least Friday night.

Southeast California/Southwest Arizona Including KIPL and KBLH:

Winds will generally remain AOB 10 kts through the period and favor a
northerly direction through around noon Friday. Winds may become more
light and somewhat variable Friday afternoon. Skies will remain


Sunday through Thursday...

As a ridge of high pressure begins to shift east, disturbances from
the south-southwest will move into the area with an increase in
moisture and chances of scattered rain showers and isolated
thunderstorms through Tuesday. Expect the best chances for wetting
rains on Monday and Monday night over the lower deserts with even
better chances over the higher elevations to the east. Dryer
conditions spread across the region by Wednesday as a ridge builds
over the area. Minimum RH of 22-30 percent on Monday will fall to 17-
22 percent by Thursday. Slightly breezy southwesterly winds with
gusts to 15 mph will develop Monday afternoon. Overnight recoveries
will be good to excellent.



Spotter activation will not be needed this week.




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