Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 270449
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
949 PM MST MON SEP 26 2016

.SYNOPSIS...
A developing low pressure system over northwest Mexico and the Baja
Peninsula will slowly move north toward southwest Arizona tonight.
Variable cloudiness, cooler temperatures, and a threat of showers
and afternoon thunderstorms are forecast. Partial clearing is
expected Wednesday, with a renewed chance of showers Thursday.
Dry and stable weather will return Friday through next Monday.

&&

.DISCUSSION...
Cut-off low continues to circulate over the western coast of the
Baja Peninsula this evening, producing uniform east-southeast winds
from the sfc on up thru 300mb. Earlier wind, cloud and haze/lofted
dust intrusion into the south-central Arizona deserts this afternoon
was due to first PV band circulating and lifting through the
cyclonic flow. Additional PV band is visible on water vapor imagery
this evening, with storm activity arching from across Baja Norte and
portions of central Sonora this hour. While most over the south-
central Arizona deserts saw only a few sprinkles to trace amounts of
rainfall during the afternoon, dewpoints have trended upward
returning to the 50s and even a few low 60F readings. Light radar
returns continue to track in the easterly mean flow towards the
Colorado River. While there are faint radar returns on area scopes,
sub-cloud condition remain dry so outside of some sprinkles and
virga, heavier showers are not in the cards for the rest of this
evening even across the western CWA. Grid updates focused on getting
dewpoints a bit higher than earlier forecasts to account for current
obs. Elevated easterly winds will persist into the evening, but with
reduced speeds compared to those from earlier today so the threat for
blowing dust should be reduced during the overnight hours.

&&

.PREVIOUS DISCUSSION...

Tonight through Tuesday Night...

The developing cutoff low pressure system over northwest Mexico/Baja
CA circulated a significant amount of moisture into southern AZ. Mid
and upper level moisture remains deeply saturated, resulting a
threat of sprinkles and or light showers over portions of southwest
and south central AZ tonight.

The Baja cutoff low pressure system is still forecast to slowly
migrate north toward Yuma by Tuesday evening, then into the Mohave
Desert just southwest of Las Vegas by Wednesday afternoon. As a
result, an unsettled weather pattern will continue though Tuesday
night with periods of clouds and a slight chance of showers and or
afternoon thunderstorms.

Wednesday...

We may get a little break in the weather in the subsidence southern
end of the passing low pressure system as it moves into the Mohave
Desert. However surface dewpoints will higher over the region for
humid day.

Thursday...

Another shortwave trof is forecast to move into southwest and south
central AZ from the southwest direction. The threat of showers and
thunderstorms are forecast over portions of southwest and south
central AZ Thursday afternoon and evening.

Friday through Monday...

A drier and more stable pattern is forecast this period under
increasing westerly flow aloft.

&&

.AVIATION...

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

Easterly winds to persist for the Phoenix area terminals through the
evening. Another round of elevated and periodically gusty winds are
possible tomorrow. Initial band of clouds/virga/wind continue to
clear to the west-northwest with a narrow pocket of clearing that
should transition through the area over the next few hours resulting
in FEW to SCT skies briefly. Additional mid-level cloud decks will
then transition into the area towards the early morning hours with
additional potential for virga/very light rainfall. Earlier lofting
of dust from regionally strong easterly winds may impact slantwise
vsbys during the morning hours Tuesday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Easterly winds will persist into the evening and make a gradual turn
to more northerly headings during the early Tuesday morning hours.
BKN to OVC mid and high level decks will also persist, along with
some lofted dust moving in from the east which may impact
slantwise/long-range vsbys in the morning. Some virga/sprinkle may
present overnight but be brief enough in nature that even VC mention
in the TAFs would be a bit much.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday...A low pressure system across the Desert
Southwest will bring an increase in moisture that will persist
through Thursday. The chance for showers and thunderstorms will be
highest across northeastern Arizona and across the higher terrain
north and east of Phoenix. However, some rain is possible in the
lower deserts, particularly Thursday. Drier conditions are
anticipated Friday and through early next week as southwesterly flow
develops ahead of an area of low pressure in the Pacific Northwest.
Winds will follow typical diurnal headings and remain on the lighter
side, with the exception of some afternoon breezes up to 15 to 20
mph in southwest Arizona and southeast California Saturday and
Sunday.

&&

.SPOTTER INFORMATION STATEMENT...
Skywarn radio Net activation will not be needed.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

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DISCUSSION...Nolte
PREVIOUS DISCUSSION...Vasquez
AVIATION...Nolte
FIRE WEATHER...Hernandez



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