Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 170950

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

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:
There are few weather concerns tonight through tomorrow morning as
winds will follow typical diurnal trends. However, gusty winds and
lower ceilings will pose concerns beginning late in the morning
Saturday and lasting through the afternoon. Ceilings may drop to
the 5-6k feet range with coverage of FEW to SCT. The winds will be
generally stronger, 10-15 kts, with some gusts as well. There is
a chance for showers late tomorrow but there is too much
uncertainty on coverage and timing to include in the TAFs at this

Southeast California/Southwest Arizona including KIPL and KBLH:
The winds will continue to gust for a few more hours tonight
across the area but will subside somewhat by the early morning.
However, even stronger gusts up to 25-30 kts will resume late in
the morning through much of the afternoon. In addition, ceilings
may also lower below 10k feet near noon and lasting through the

Aviation Discussion not updated for amended TAFs.


Sunday through Thursday...

A cool and drier airmass on Sunday will gradually be replaced by
high pressure and warmer temperatures heading into the week.
Significantly below normal highs of 65-70 on Sunday will be replaced
by above normal 80s by Wednesday. Very breezy westerly winds from SE
CA to SW AZ on Sunday morning, with gusts of 18-24kt, will spread
east by midday into the central and eastern areas before tapering
off late in the day. Min RH values are expected to fall mostly to
the 10-15% range across the deserts by Monday. Overnight
recoveries will generally be fair.


Spotters should follow standard reporting procedures.


CA...Wind Advisory CAZ562 through 5 AM PDT Sunday.


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