Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 230432

Area Forecast Discussion
National Weather Service Phoenix AZ
932 PM MST Sun Jan 22 2017

Another Pacific storm will approach the area tonight. Increasing
clouds are expected, with a chance of showers over portions of
southeast California, spreading east across the remainder of
southern Arizona later tonight and Monday. Snow levels Monday
afternoon and night will fall to between 5 and 6 thousand feet.
After the cold front Monday, a drier and much colder airmass will
settle over the region Tuesday through Thursday, resulting in some
of the coldest low temperatures of the year. Clear skies with a
warming trend will return next Friday through Sunday.


A swath of moderate rainfall has spread across southern California,
with radar-estimated totals approaching one inch across parts of
Joshua Tree National Park. This rainfall is occurring within a plume
of relatively high precipitable water (0.9 inch or greater per SPC
mesoanalysis), oriented from the northern Baja coast into the lower
Colorado River Valley. Although this band has shown some signs of
weakening in the past hour, observational trends suggest light rainfall
will affect parts of Imperial and eastern Riverside counties, and
eventually far southwestern Arizona, through the late evening.
Short-term PoP and sky grids have been adjusted to account for this.

Otherwise, the forecast for increasing rainfall chances into south-
central Arizona by tomorrow afternoon appears on track. Although
rainfall amounts are not expected to be particularly high across the
lower deserts (generally less than 0.5 inch), higher totals north and
east of Phoenix amidst saturated soil conditions will lead to a
heightened risk of flash flooding over area washes. Timing of a cold
front moving through central Arizona during the afternoon also
appears on track, and will coincide with falling snow levels into
Monday night.


Tuesday through Thursday...

After the cold front, some of the coldest temperatures of the season
are forecast to settle over the region Tuesday through Thursday. Low
temperatures Tuesday and Wednesday nights will likely fall into the
low to mid 30s on the deserts, but not cold enough yet for a Freeze
Warning.  Clearing skies are also expected.

Friday through Sunday...

Mostly clear skies with a slow warming trend is forecast this
period. Afternoon temperatures will finally approach season norms by
next Sunday.


South-Central Arizona including KPHX, KIWA,and KSDL:
East winds will likely continue through 12-15Z, before beginning to
veer towards the south ahead of a cold front. Timing of a cold front
is currently estimated at 18Z, which will arrive with an abrupt
change to westerly surface winds. A few gusts to 20 kts may also
occur in the afternoon. The exact timing of precipitation affecting
terminals still remains somewhat uncertain, but ceilings falling to
5000-7000 feet by 13Z appears probable, with periods of lower
ceilings possible associated with any showers.

Southeast California/Southwest Arizona including KIPL and KBLH:
Observations suggest southerly winds are strengthening across the
lower Colorado River Valley, and stronger winds are expected to be
realized at both KIPL/KBLH by 06-08Z. A cold front will shift
eastward across the area, and result in a wind shift towards the west
10-12Z. Strong gusts up to 30 kts are possible, especially during the
afternoon. Periods of light to moderate rainfall may also affect KBLH
through about 06Z, with a temporary lowering of ceilings possible to
around 5000 ft. Any shower activity beyond this time should remain
spotty through Monday afternoon.

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday: Weakened but broad and dry troughing will
settle over the west Tuesday and Wednesday keeping flow aloft
westerly. Additional weak shortwaves may move on-shore over southern
CA but fall apart before reaching the districts in southeast CA.
Quasi-zonal flow will develop into the region as high pressure
attempts to build into the west and various shortwave systems ride
over the ridge flow. Temperatures will stay at or below seasonal
normals as minimum humidities gradually dry into the 20 to 40 percent
range through the week. One stronger storm system passing through the
Four Corners during the late week/early weekend will produce enhanced
easterly surface flow over the Mogollon Rim and northerly flow down
the Colorado River Valley, introducing breeziness and periodic
gustiness from the north and northeast for the districts.


Spotters should follow standard reporting procedures.


AZ...Winter Weather Advisory from 1 PM Monday to 5 AM MST Tuesday for

     Flash Flood Watch from Monday morning through Monday evening for



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