Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 171136

Area Forecast Discussion
National Weather Service Phoenix AZ
436 AM MST Sat Mar 17 2018

.UPDATE...Updated Aviation and Fire Weather discussions.


An approaching weather system will lead to breezy to windy
conditions over southeast California and Arizona today. The passage
of the system will lead to a chance of showers over south-central
Arizona tonight into Sunday with significantly lower chances further
west. There will be some cooling as well with highs in the 60s on
Sunday. Below normal temperatures through Monday should warm back
into the 80s for the middle of next week. Another weather system
will provide an opportunity for rain and cooler temperatures late
next week.


At 2 am this morning, dry southwest flow aloft was present across
the area, ahead of a deep upper low centered along the northern CA
coast. IR imagery showed clear skies area-wide, and surface
dewpoints over the central deserts were up slightly and in the mid
30s to low 40s for the most part.

Model guidance has been struggling over the past week mainly with
the amount of moisture that would be available to the strong short
wave that is still progged to progressively move east and across
Arizona tonight into Sunday. POPs have vacillated wildly starting
back last Sunday when MRA MOS numbers reached 100 percent for
Phoenix today, then rapidly backed down to 1 percent, then up into
the teens and by yesterday had climbed over 40 percent for the
Phoenix area with the prime time focus being tonight. Now we see the
POPs sharply drop back off and into the teens for tonight. Looking
at IVT forecasts, any "atmospheric river" that might have advected
moisture into the area is progged to stay WELL to our southeast and
over portions of northwest Mexico. As the main short wave rotates
around the back of the upper low, it will spend a bit of time over
the eastern Pacific and should be able to spread some of the Pacific
moisture inland and into Arizona as the wave moves through later
today and tonight.

This still looks to be a rather dynamic system with good QG
forcing/UVV expected. However, a look at moisture/UVV cross sections
show that moisture will not be especially deep as the wave and
associated front move across the deserts. As such, we have pared our
POPs back for tonight with best rain chances expected in the Phoenix
area from midnight through 5 am Sunday. SREF Plumes forecasts have
really backed off again for Phoenix, and the ensemble mean now shows
just 0.02 inches with this system compared to about 0.10 just
yesterday. Greatest outlying member down to just 0.12 inches
compared to over one half inch yesterday. So, SREF clearly not as
bullish for rain chances in the Phoenix area.

Will keep POPs in the low end chance category in the Phoenix area -
won`t completely buy into the sharply lowering POP trend from MOS
guidance. NAEFS POPs remain very high but are likely too "hot" and
overdone but they have not changed much over the past 24 hours. On
Sunday there will be a clearing trend from the west with a few
lingering showers over the eastern portion of the Phoenix area and
best chances remaining over the higher terrain areas to the east of
Phoenix. We will see a nice cool down behind the system as highs in
the central deserts drop into the middle to upper 60s compared to
the lower 70s we will see today. Also, expect breezy to windy
conditions today ahead of the approaching low; for the most part
winds should remain below wind advisory levels except for the far
southwest portion of Imperial County where a Wind Advisory has been
issued and will be in effect through 5 am on Sunday.

For the early to middle portion of next week, high pressure aloft
will build back over the desert southwest allowing for dry
conditions and a sharp warming trend. High temperatures will climb
back into the mid to upper 70s by next Tuesday and then into the
middle 80s over the warmer deserts next Wednesday. We will see
periods of mainly high clouds spreading through the ridge resulting
in partly to mostly cloudy skies Tuesday into Wednesday, but the
high cloud cover will not keep temperatures from reaching into the

For the latter part of next work week - Thursday into Friday -
operational GFS and ECMWF as well as GEFS ensembles continue to
paint a picture that suggests another Pacific low will move inland
and across the area, bringing another chance of showers to the lower
deserts. Confidence in exact timing is low still, but somewhere in
the Thursday/Thursday night time frame seems like the best bet for
rain in the greater Phoenix area. NAEFS POP trend still looks to be
decent with numbers that are a bit overdone but not too much. Expect
quite a bit of cloud cover along with temperatures cooling back into
the 70s with the passage of this next system.


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

A cold front will pass through the region late Saturday night
bringing gusty westerly winds, low CIGs and a chance of light rain
showers. Today skies will be mostly clear with light easterly winds
becoming southwesterly and breezy by early afternoon at 10-12kt
gusting to 15-20kt. By 06Z tonight low CIGS are expected to close-in
at 5-6K with a moderate chance of light rain showers. Gusty westerly
winds, with gusts up to 20-22kt, will also accompany this system.

Southeast California/Southwest Arizona including KIPL and KBLH:

A cold front will move through the region today bringing very breezy
to windy westerly winds at 15-18kt gusting as high as 25-28kt by
midday. Low cloud decks near 8K are also anticipated through late
afternoon to early evening.

Aviation Discussion not updated for amended TAFs.


Monday through Friday...

Very dry conditions together with a warming trend across the
region will predominate through Wednesday. A weather system on
Thursday will bring a chance of light rain from SE CA to AZ.
Seasonably warm daytime temperatures early in the weak will
increase to the mid 80s by Wednesday and then drop back down into
the upper 70s on Thursday. Min RH`s in the 8-15% range through
Wednesday are expected to jump up to the 20-25% with Thursday`s
system. Very breezy to windy southwesterly winds gusting to
20-25kt from SE CA to SW AZ will also accompany the system on
Thursday. Generally fair overnight recoveries should improve to
fair to good by Thursday.


Spotters should follow standard reporting procedures.


CA...Wind Advisory until 5 AM PDT Sunday for CAZ562.



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