000
FXUS65 KPSR 212255
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
355 PM MST Thu Sep 21 2017

.UPDATE...
Updated Aviation and Fire Weather sections.

&&

.SYNOPSIS...
An area of low pressure will overspread the Desert Southwest
tonight and Friday and remain in place over the weekend. The end
result will be substantially cooler temperatures along with
occasionally breezy conditions...especially tonight. Precipitation
chances will be neglible and limited to only some sprinkles over
the higher terrain locations. Temperatures will only slowly warm
back to near average next week.

&&

.DISCUSSION...
Rest of Today through Saturday...
Wind Advisory conditions are underway early this afternoon west
of the Lower Colorado River Valley. METARs in the Imperial Valley
have had gusts of 40-45 mph along with visibilities dipping down
to 3 miles. These conditions (including the rest of the Advisory
area) will continue the rest of the afternoon and most of the
evening before winds become considerably lighter. At times, expect
even lower visibilities in more dust/sand prone areas within the
Advisory. However, that will not be the only area where blowing
dust may be a problem. It will still be quite breezy in areas east
of the Lower Colorado River Valley. The more dust prone areas of
Pinal County may see patchy blowing dust as well (possibly getting
into portions of the southeast Valley). The cold front, which as
roughly along a line from near Salt Lake City to Los Angeles at 2
pm, will continue advancing eastward tonight and reach the Phoenix
area well after midnight (closer to sunrise).

The front is associated with a very large region of low pressure
affecting much of western North America. The southern end of the
system has very little moisture with it and thus we have no precip
chances worth mentioning through Friday. But, temperatures drop
nicely with highs in the upper 80s to around 90 on the lower
deserts on Friday. With the upper trough over the interior West
and a reinforcing short wave tracking through the southern end of
it, the front stalls out over eastern AZ. In addition, a little
bit more moisture gets pulled into the system and thus there will
be a slight chance of some very weak convective showers over the
higher terrain of south-central AZ late Friday and into Saturday.
Plus high temperatures will be slightly cooler as well (mainly
over our eastern areas).

Sunday through Thursday...
The overall troughing structure will be maintained over the SW Conus
into the middle of next week, however heights will slowly rise as
weakening and filling of the trough begins. Downstream eastern Conus
ridging will also weaken allowing for a pattern shift/adjustment by
the end of next week. At this point, operational and ensemble output
diverge with respect to how much vorticity/circulation remains
centered over Arizona or gets displaced by high pressure building
from the east Pacific. Regardless, temperatures will undoubtedly
moderate much closer to the seasonal normal beyond the middle of the
week.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Southwest and west winds this afternoon and evening will be
stronger than usual due to the approach of a dry cold front.
Anticipate the front will reach metro well after 07Z but there
will not be a distinct wind shift with it. Also anticipate surface
winds to remain strong enough after sunset that LLWS criteria will
not be met for TAF purposes. However, winds in the lower levels
above the surface will be notable. Anticipate there will be a
period of time where light southeast winds will be common after
09Z before southwesterly winds resume by mid morning Friday.
Otherwise, little or no cloudiness.

Southeast California/Southwest Arizona including KIPL and KBLH:
A cold front extending from roughly Salt Lake City to near Los
Angeles as of 22Z will progress reach the Lower Colorado River
Valley (LCRV) by 03Z-04Z. Strong southwest winds gusting 30-40 kts
will be common near and west of the LCRV through 04Z before
weakening. Expect areas of blowing dust reducing visibility well
below 10SM with brief localized reductions of visibility down to 1
to 3SM. Winds will not be as strong east of the LCRV. Expect less
wind on Friday. Otherwise, little or no cloudiness.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday:
A low pressure system will linger over the interior West through
Tuesday before beginning to shift eastward. Temperatures will
climb to near or slightly above normal by Thursday. Humidities
will be on the low side with minimum readings remaining well
below 20% on the lower deserts and overnight recovery only fair to
good. Anticipate north and northwesterly breeziness Sunday into
Tuesday over southeast CA and southwest AZ. Otherwise, light
winds.


&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Wind Advisory until 1 AM PDT Friday for CAZ560>568.

&&

$$

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DISCUSSION...AJ/MO
AVIATION...AJ
FIRE WEATHER...AJ

NWS PSR Office Area Forecast Discussion