Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 242054

Area Forecast Discussion
National Weather Service Phoenix AZ
155 PM MST Fri Feb 24 2017

.Update...Aviation weather discussion.


Cooler than average temperatures will persist through early next
week with two fast moving weather disturbances moving into the
region Sunday and again Monday night. The weather system Monday
night will have better potential for spreading rain across the area.
Dry conditions and much warmer temperatures will return by the
middle of next week.



Tonight through Saturday...

After one more cool night, with a very dry airmass, light winds, and
mostly clear skies allowing for near-ideal radiational cooling
conditions, temperatures on Saturday are expected to warm 3-5
degrees over today`s highs, as light southerly flow below 800mb
develops ahead of the next shortwave that will be affecting our area
on Sunday. This should allow some of the warmest lower desert
locations in SW AZ and SE CA to reach/break 70 degrees. One thing
that could hold back the warming a bit is increasing mid and high
cloudiness ahead of the next system on Saturday afternoon. However,
at this point, it does not appear that the clouds will become dense
enough to significantly reduce the amount of solar radiation
reaching the sfc.


The first of two shortwaves is expected to pass over the region
during the day on Sunday, in a rather brisk westerly flow aloft.
Although moisture will be quite limited with this system, due to the
rather short time it will be spending over water, as it tracks
southward from Vancouver Island to central CA (PWATS struggling to
reach 0.5 inch), there appears that there will be enough
cooling/dynamics aloft to trigger at least some light shower
activity as it passes over our cwa, with most of it occurring over
the higher terrain of south-central AZ. Given the slightly greater
amounts of available moisture that are now showing up on the latest
GFS/GFS ensemble outputs, have raised POPS a bit for this period.
Still, the vast majority of any measurable rainfall is still
expected to be confined to the higher terrain north and east of
Phoenix, with the lower elevations still only seeing (mostly) virga
and sprinkles. Although the clouds and precip will hold daytime
highs down a bit, overnight lows are expected to be boosted  up to
near (or slightly above normal) values by these clouds.

Monday and Tuesday...

Latest global model suites continue to show the 2nd, and final
shortwave dropping southward into the region during this period.
Although the GFS, and to, a lesser extent, its ensemble members
continue the drying trend of the past few model runs a bit more, the
EURO is still showing a wetter, and a bit slower solution, as it
seems to pull more subtropical moisture, that is associated with
another shortwave that is forecast to move off to our south (across
northern Mexico), into the Desert Southwest during this period.  The
Canadian is also showing a similar soln to the EURO. Given all of
these model differences, have only made some very slight changes to
the forecast during this period (best chances for rain during the
Monday night/Tuesday timeframe), keeping POPS in the chance category
across the lower deserts of south-central AZ, and in the likely
category for the higher terrain north and east of Phoenix.

Wednesday and Thursday...

The flow pattern will become more predominated by longwave features
during the second half of next week with mean troughing over the
eastern Conus and ridging through the NW. There is excellent model
agreement that a nearly cutoff wave will spin and meander over
northern Mexico albeit completely devoid of appreciable moisture.
While H5 heights in a 570-576dm range are fairly typical for early
March, increased sun angle and insolation should push temperatures
back into a near to above normal range. Kept forecast highs fairly
conservative in the mid/upper 70s for the most part, however would
not be shocked to see reading eclipse the lower 80s by the end of
next week.


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

Dry westerly flow aloft producing mostly clear skies with light and
variable wind under 6 knots at least through 20z Sat.

Southeast California/Southwest Arizona including KIPL and KBLH:

Dry westerly flow aloft producing mostly clear skies and light north
wind under 6 knots.

Aviation Discussion not updated for amended TAFs.



Sunday through Thursday...
Dry conditions should prevail for most areas on Sunday ahead of an
approaching Pacific low pressure system. This weather system will
bring chances of wetting rains mainly later Monday through Tuesday
morning focused especially across the high terrain of Arizona.
Below normal temperatures are expected through next Wednesday with
a significant warming trend likely late next week. Breezy
conditions on Monday are expected in most areas with breezy
conditions persisting down the Lower Colorado River each afternoon
through Thursday. Increased humidities with minimum readings
between 25 to 35% through Tuesday will dry out starting Wednesday
with afternoon readings closer to 15% over the deserts for
Wednesday and Thursday.


Spotter reports will not be needed through the weekend.




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