Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 221152

Area Forecast Discussion
National Weather Service Phoenix AZ
452 AM MST Thu Feb 22 2018

.UPDATE...Updated Aviation and Fire Weather Discussions...


Cool conditions will continue to affect the region through the
weekend with several passing weak weather systems. The first will
approach today, causing an increase in winds across southeast
California and some areas of blowing dust. That system will pass
across Arizona Friday, bringing a few showers mainly to northern
Arizona. The weekend looks quite for now, and a slight warming trend
is expected early next week before another weather system moves into
to the area early next week bringing another chance for showers
along with cooler temperatures.


Early this morning, a large upper trof remained entrenched across
the western CONUS; latest plots and raobs depicted dry southwest
flow aloft over AZ with 500mb heights only around 555DM across south
central AZ. IR imagery at 2 am showed partly cloudy skies east of
Phoenix with genly clear skies over the western deserts. Surface
dewpoints were very low ranging from 10 to 20 over the central
deserts to as low as 2 at Blythe.

The weather pattern that is currently in place is not an overly wet
one for our area but from time to time disturbances moving through
the large scale trof have the potential to bring isolated to
scattered showers to the lower deserts. Most of the time we will be
between bouts of precip though. With the very large trof in place we
will be dominated by cyclonic flow aloft and the airmass will stay
on the cool side. Most of the days over the 7 day forecast period
will stay well below seasonal normals with highs in the low to mid
60s. There is only one day out of the next 7 where the forecast high
reaches 70s; currently the normal high for Phoenix is 72 degrees.
Many days we expect highs at least 10 degrees below seasonal normals.

The first shot for some rain will come on Friday as a disturbance
well to our northwest drops southeast and swings across the state;
it will have a rather dry overland trajectory and there is no
evidence of any atmospheric river to provide good moisture advection
into the area. Best rain chances will occur over south central AZ
Friday afternoon and evening, with most of the rain expected over
higher elevations north and east of Phoenix. Guidance over the past
couple of days, and several model runs, continues to back off on the
rain chances for the central deserts; MOS POPs have been steadily
falling and the latest MEX MOS for Phoenix has just a 14 percent
chance on Friday. This looks to be a system that will provide some
wind initially out west (this afternoon/evening) and then a fast
moving cold front lowering desert highs into the mid 50s to low 60s
Friday. As previously mentioned, the front will be accompanied by
isolated to scattered showers most numerous over higher elevations
north and east of Phoenix. The forecast high for Phoenix Friday will
be just 58 degrees. With respect to the wind today, gusts to around
40 mph are possible over favored higher terrain locations in far SE
CA, especially Joshua Tree NP and we have issued a Wind Advisory for
the western portion of the park from noon through 11 pm. Areas of
blowing dust are also in the forecast this afternoon due to the
gusty winds that are expected.

For Saturday through next Monday, dry west/northwest flow aloft
continues and dry short waves continue to pass by to our north. High
temps will stay very cool through the weekend with Phoenix only
reaching to around 64 by Sunday. Monday may be the warmest day of
the next 7, at least for the Phoenix area, and the high will crest
at just 70 degrees which is 3 degrees below normal.

The next significant chance for showers looks to occur during the
early to middle portion of next week, in the Tuesday/Wednesday time
range. At the moment, model guidance is very inconsistent with the
timing and track of the main upper trof that will be moving thru;
the GFS is much more progressive than the ECMWF in terms of their
operational runs and the ensembles from both are very chaotic with
lots of spread between the members. Ensemble means from both lie
somewhere between the operational solutions for these models.
Overall our confidence is low in terms of picking a certain 6-12
hour best window for rainfall, but we will raise our POPs and go
with a rather broad brushed forecast. We expect isolated to
scattered showers Tue into Wed, with best coverage expected over
south central Arizona Tuesday night into the middle of the day on
Wednesday. Of course, this will usher in another cooling trend
dropping high temperatures back into the 60s. Phoenix will fall to
around 64 by next Wednesday under partly to mostly cloudy skies.


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

No aviation impacts through Friday morning. Skies will remain
clear to mostly clear through early Friday morning until a 6-8K ft
cloud deck forms by late Friday morning. Winds will be on the
light side and mostly follow diurnal trends, but west winds will
be a bit more predominant than usual.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light winds this morning will become breezy to windy this
afternoon out of the west. Wind gusts to 25 kts at both KIPL and
KBLH are likely through early evening with some slackening of
gusts during the overnight hours tonight. Skies to remain clear to
mostly clear through the period.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday:

Drier westerly flow aloft will develop Saturday into early next
week, 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 by next Tuesday. Winds will be fairly light Saturday through
next Monday. A weather system is likely to affect the region for
the middle of next week bringing increasing chances for rain and
breezy conditions.


Spotters should follow standard reporting procedures.


CA...Wind Advisory from 11 AM this morning to 10 PM PST this evening
     for CAZ560.



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