Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 240935

Area Forecast Discussion
National Weather Service Phoenix AZ
235 AM MST Sat Feb 24 2018

Cool conditions will continue to affect the region through the
weekend with several passing weak weather systems. A slight warming
trend is expected next Monday before another weather system moves
into to the area early next week bringing another chance for showers
along with cooler temperatures.


At 2 am this morning, a fast moving upper shortwave was pushing east
of the area, bringing dry subsident northwest flow aloft along with
a cooler and drier airmass to the deserts. Surface dewpoints over
the central deserts were in the 20s, and Blythe was down to 10. IR
imagery showed clear skies area wide. Although winds have tapered
off behind the exiting wave/cold front, there remains enough breeze
to keep temperatures from dropping to their full potential and most
deserts should stay at least slightly above the freezing mark this

For the next few days, a dry but continued cool pattern will persist
over the area, fast moving disturbances will pass by to our north,
such as one moving through the four corners Sunday, keeping
conditions dry and unseasonably cool. High temps this weekend will
stay well below normal with deserts readings in the low to mid 60s.
Temps Monday will climb at least several degrees with Phoenix
finally rising to around 70 as mid/upper level flow becomes a bit
more southerly.

Model guidance has started to lock in on the timing/moisture of the
next system that will affect our area for the early to middle
portion of next week. At this time, another wet Pacific low will
move across the area Tuesday into Wednesday, with the best window
for significant rains to be Tuesday night for south central Arizona.
Both the ECE and MEX POPs have by far the highest values Tuesday
night - both near or above 50 percent - and the MRA MOS for Phoenix
is now up to 58 percent. Both the GFS and ECMWF initially form a
closed low just off the central CA coast, and as the low swings in
from the west, it will be able to tap into some moisture from the
Pacific. No atmospheric river is being advertised, but we will
likely see a well organized frontal band move inland and into the
western deserts Tuesday, then across central AZ Tuesday night into
Wednesday, bringing scattered to numerous showers to the area. POPs
have been raised accordingly and will likely be raised further as we
close in on this next event. It will also bring much cooler
temperatures; desert highs will fall back into the 60s Tuesday and
the cooler central deserts will lower to around 60 on Wednesday.

For the latter part of the week, GFS and ECMWF are in somewhat
better agreement, and although ensemble member spread remains high,
the members are not quite as chaotic as they were 24 hours ago.
Basically, we should see a very large upper low develop over the
western CONUS, but the center will remain well to our
north/northwest and as it moves east, Arizona will remain on the far
southern flank under somewhat dry westerly flow aloft. POPs will
stay on the low side for Thursday and Friday, generally single
digits or slight chance numbers with skies to be partly cloudy. High
temperatures will rise gradually with warmer deserts climbing into
the low 70s by Friday.


South-Central Arizona including KPHX, KIWA, and KSDL:
Weak virga showers/sprinkles have shifted into far eastern
portions of the Greater Phoenix area. Gradual clearing can be
expected through 06Z, with only periodic scattered clouds between
8-10 thousand feet. Wind gusts have also diminished across most
area terminals. An atypical westerly winds should persist
overnight, before a weakening pressure gradient results in more
variable directions after sunrise Saturday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

Gusty winds have diminished at KIPL/KBLH this evening, with a
north to northwest direction generally favored. Mostly clear skies
can be expected overnight, with weaker wind speeds Saturday.

Aviation Discussion not updated for amended TAFs.


Sunday through Thursday:

Drier westerly flow aloft will dominate Sunday and Monday,
allowing temperatures to gradually climb and approach seasonal
normals by Monday. Minimum RH values will drop into the 12 to 18
percent range through Monday with improvement into the 20s
starting next Tuesday. A weather system will likely bring good
chances of wetting rains on Tuesday with chances lingering into
Wednesday. Light winds through Monday will increase for Tuesday
and Wednesday as the weather system moves through the area.


Spotters should follow standard reporting procedures.




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