Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 181552

Area Forecast Discussion
National Weather Service Phoenix AZ
850 AM MST Sun Mar 18 2018

A weather system will produce lingering light showers over higher
terrain areas east of Phoenix today, and also bring cooler high
temperatures. Below normal temperatures will continue through
Monday,but should warm back into the 80s by the middle of the week.
Another weather system will provide an opportunity for rain and
cooler temperatures later in the week.


Within a larger western Conus trough, a notable PV anomaly was
lifting from northern Arizona into SE Utah allowing a frontal
boundary to surge into eastern Arizona. The associated midlevel
moisture gradient was also pushing eastward quite rapidly this
morning with pronounced subsidence and a dryslot punching into
central Arizona. Light showers will remain likely over the next
several hours across Gila County in conjunction with vorticity
advection and orographic ascent, however a "crisp" early spring day
will result for the remainder of the area. Based on satellite
presentation and short term observational trends, have refined some
of the forecast for the remainder of the day clearing conditions
quicker and resulting in a delightful afternoon. In fact, highs this
afternoon only in the 60s could be the coolest afternoon the area
experiences for the next 7-8 months (last time Phoenix had a high in
the 60s in April was 2012).


/354 AM MST Sun Mar 18 2018/
For Monday through Wednesday, guidance remains consistent in calling
for high pressure aloft to build over the deserts southwest bringing
dry conditions along with a sharp warming trend. The warmer lower
deserts will climb to near 80 degrees by Tuesday and into the mid
80s Wednesday. It will be generally sunny on Monday, but there will
be periods of mainly high clouds spreading through the ridge Tuesday
through Wednesday resulting in partly to mostly cloudy skies. Clouds
will not be thick enough however to keep the high temperatures from
rising into the 80s.

For the latter portion of the work week - Thursday into Friday -
model guidance is still consistent in calling for another Pacific
trof to move through the area, bringing a chance of showers to the
lower deserts. However, there remain some serious timing issues
especially between the operational GFS and the ECMWF. GFS is about
12 hours faster than the ECMWF, calling for the main precip event in
the Phoenix area to be during the day Thursday while the ECMWF calls
for the main event Thursday night into Friday morning. As an example
of the discrepancy, the GFSBC max temp guidance for Thursday gives a
high of just 71 degrees in Phoenix with the showers/frontal band
moving thru. The ECMWFBC, from the slower European, calls for a
high of 88 degrees.

GEFS ensembles are all over the map at this point and very chaotic;
NBM POP guidance is broad brushed and NAEFS POPs are high but mainly
in the chance category, favoring timing of the slower European. Due
to these issues, we have a broad brushed forecast, with 20-30
percent numbers for much of Thursday/Thursday night, then tapering
off on Friday. If we had good confidence in exactly when the front
would move through and rain would fall, the POPs for that 6 hour
window would be in excess of 50 percent. Due to low confidence, POPs
are much less than that for any given 6 hour window. Hopefully these
issues will be resolved soon.

Drier westerly flow aloft returns over the weekend for partly to
mostly sunny skies and no chance of rain, but heights do not rise
all that much, so high temperatures stay closer to seasonal normals.


South-Central Arizona including KPHX, KIWA, and KSDL:
Today skies will be partly cloudy with moderate to breezy westerly
winds at 8-12kt with gusts of 15-20kt, especially during the
afternoon. BKN low cloud decks near 6-8K should scatter-out early to
mid morning. Light winds are expected by this evening with a deck of
high cirrus clouds passing through the area this evening and

Southeast California/Southwest Arizona including KIPL and KBLH:
Moderate westerly winds at 8-12kt will will turn to the north by mid
to late morning. Meanwhile expect periods of SCT-BKN high clouds
this afternoon and tonight.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday...
Expect warm and dry conditions ahead of a weather system that will
pass across the region on Thursday with drier air returning by the
end of the week. The weather system on Thursday will bring a chance
of light rain from SE CA to AZ. Seasonably warm daytime highs on
Tuesday will increase to the mid 80s by Wednesday and then drop back
down into the upper 70s to low 80s on Thursday. Min RH`s near 10%
range on Tuesday are expected to spike up to the 22-28% with
Thursday`s system. Breezy southwesterly winds gusting to 17-22kt
from SE CA to SW AZ are also expected with Thursday`s system.
Generally fair overnight recoveries should improve to fair to good
by Wednesday night and Thursday night.


Spotters should follow standard reporting procedures.





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