Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 120954

Area Forecast Discussion
National Weather Service Phoenix AZ
254 AM MST Tue Dec 12 2017

High pressure parked across the western United States will result in
dry and quiet weather through the weekend. Temperatures will be
well above average for this time of year with highs in the middle
and upper 70s across the lower deserts through Friday and
generally in the lower 70s this weekend. Mostly clear skies will
be punctuated at times with varying amounts of high clouds.


A bit drier conditions over the region have allowed for skies to
become clear in all areas except for the southeast quarter of
Arizona. The broad cyclonic circulation to our south remains in
place as part of the persistent Rex Block, but the feed of mid
and upper level moisture has diminished quite a bit leaving only
patchy areas of clouds. Height packing aloft will increase today
across southern Arizona allowing an increase in easterly flow with
850mb winds increasing to around 20-25kts by 18Z. This easterly
flow will keep at least some moisture in the mid levels through
the rest of today allowing for at least partly cloudy skies across
southern Arizona. The stronger winds above the boundary layer
will partly mix down to the surface creating a bit of breezy
afternoon and giving a bit of a boost to our temperatures. Highs
today should easily top out in the middle and upper 70s, possibly
reaching the current record high of 79 degrees in Phoenix.

Starting Wednesday, the broad low to the south will begin to sink
farther south as a fast moving shortwave trough slides down
through the Four Corners area. The passage of this shortwave will
have little affect on our weather, but we will see drier air
temporarily filter in from the north and a couple degree drop in

The long lasting high amplitude ridge over the Western U.S. will
weaken somewhat late this week into the early part of the weekend
as a fast moving Northern Pacific trough passes through the
Pacific Northwest Friday and then through the Great Basin and
Desert Southwest on Saturday. So far models indicate nearly all of
the upper level dynamics will slide by to our northeast and then
east. Very little system moisture is advertised and any
precipitation will miss us well to the north and east. It does
seem more likely that a bit of a cool down is in store with the
trough passage, but temperatures are still forecast to remain a
good five degrees above normal for Saturday and Sunday.

Above normal temperatures is a good bet for at least the first
part of next week as the Pacific ridge to our west rebuilds a bit
and heights aloft across our region remain above climo normals
for the period. Though it is pretty far in the extended, both the
GFS and European are depicting a significant Pacific low pressure
trough moving into the Western U.S. sometime late next week. If
this comes to fruition, precipitation chances are likely to return
and much cooler temperatures are a good bet.


South-Central Arizona including KPHX, KIWA, and KSDL:
Easterly winds will take hold this evening and will persist
through at least the next 24 hours. Models indicate stronger
easterlies will form at the terminals late tomorrow morning as
higher momentum air mixes down. Gusts around 15-20 kts look likely
through much of the afternoon but could reach as high as 25 kts.
Lighter easterlies will return early Tuesday evening as the
boundary layer decouples. Outside of SCT to BKN high clouds, no
other aviation concerns.

Southeast California/Southwest Arizona including KIPL and KBLH:
Typical diurnal winds are forecast through the next 24 hours with
speeds at or below 10 kts. FEW high clouds above 20 kft will
persist with no other aviation concerns.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday...
With a dry fair weather pattern dominating the region expect
continued above normal maxTs in the mid to upper 70s. Min RH
values in the 7-15 percent range are expected to increase slightly
to the 11-17 percent range by Saturday. Winds will be mainly
light and favor normal diurnal patterns. Otherwise generally fair
to poor overnight recoveries are expected for most of the period.



Record highs for December 12th:

Phoenix: 79 in 2010
Yuma: 85 in 1958 and 1950


Spotters should follow standard reporting procedures.




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