Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 182040

Area Forecast Discussion
National Weather Service Phoenix AZ
140 PM MST Thu Jan 18 2018

High pressure will prevail across the region through Friday with
above normal temperatures and dry conditions. A notable storm system
will move into the Southwest late Friday and into Saturday.
While it still does not look like a very wet system,it will result
in increasing winds and a significant drop in temperatures. After a
cool Saturday and Sunday, temperatures will slowly rebound heading
into the first part of next week.


A high amplitude ridge is pushing through the region with the
ridge axis now over western Arizona with 500mb heights as high as
582dm. Some high cirrus is beginning to spill through the ridge,
but for the most part skies will remain mostly clear through
tonight. Under the increasing heights aloft, some slight warming
will boost afternoon highs into the middle 70s across the majority
of the lower deserts. A positively tilted Pacific trough will
move into the Western States through Friday, reaching southern
California as early as Friday evening. Increasing southerly flow
ahead of the trough will allow for winds to become gusty across
southeast California and southwest Arizona by late Friday
afternoon and into the evening hours. Even warmer temperatures are
anticipated for Friday before the trough moves into the region.
Highs should easily reach into the middle 70s across the lower
deserts with a few spots likely nearing 80 degrees before clouds
increase by sunset Friday. The previous discussion for this
weekend into early next week still applies.


Model guidance has been very consistent over the past few days
regarding this upcoming weather system; both the operational GFS
and ECMWF are in lock-step and most of the GEFS ensemble members
agree nicely with respect to the track and timing of this upper
trof. The open wave nature of this large trof has aided in the
models ability to accurately move this system through our area;
large closed upper lows are always harder to time. A look at IVT
(Integrated Vapor Transport) continues to show a weak atmospheric
river moving into Arizona ahead of the trof Friday into Friday
night, but we will not be seeing a lot of moisture spreading into
the area and QPF values will stay very low over the deserts. SREF
plumes output for Phoenix show a few members picking up less than
0.05 inches on Saturday and that`s about it.

We still expect the cold front to race through the western deserts
Friday night, then move through south-central Arizona during the day
on Saturday; POPs will stay low and in the slight chance category on
Friday night, and will increase as the day wears on Saturday; by
late morning to early afternoon the best chances for rain and
mountain snow will focus over the higher terrain areas to the north
and east of Phoenix. Although the snow level will drop to around
4000 feet by Saturday afternoon, given the very low QPF expected
there will not be any significant snow across southern Gila County
and no real impacts are expected. Following the front on Saturday,
much cooler air filters across the deserts resulting in high
temperature readings mostly in the upper 50s to low 60s. The greater
Phoenix area will struggle to reach 60 on Saturday, the forecast
high of 59 is nearly 10 degrees below the seasonal normal of 68.

After skies clear from the west overnight Saturday night, the much
cooler and drier air combined with good radiational cooling
potential will result in a cold morning with many lower deserts
dropping into the 30s. Phoenix should see a low Sunday morning of
about 39 degrees; although some of the outlying deserts especially
over portions of northwest Pinal County will drop to near freezing
we do not expect conditions to be quite cold enough to warrant the
issuance of a Freeze Warning. The following morning, Monday morning,
will have similarly cold temperatures with Phoenix again expected
to have a low of 39 degrees.

Otherwise, for Sunday into the middle of next week, a drier west to
northwest flow aloft will overspread the desert southwest; gradually
rising heights will result in a modest warming trend. Highs Sunday
will stay quite cool with Phoenix only reaching to around 61, and
then each day will see a slow bump-up with highs pretty much staying
in the 60s through Tuesday. The low 70s should return by next
Wednesday. For the most part mostly sunny days and clear nights are
expected but there will be occasional high clouds moving through the
area during the extended forecast period.



South-Central Arizona including KPHX, KIWA, and KSDL:  No major
aviation concerns for the next 24 hours as winds remain light and
mainly out of the east.  Light and variable flow is likely for KPHX
this afternoon before returning back east late this evening and
continuing through much of the afternoon on Friday. Otherwise, only a
FEW to SCT high clouds can be expected.

Southeast California/Southwest Arizona including KIPL and KBLH:

Very light winds will continue through tomorrow morning with
extended periods of variable flow less than 5 kts. Southerly
breezes will begin at KBLH late Friday morning ahead of our next
weather system although light winds will continue at KIPL through
much of the day before picking up Friday evening. Ceilings will
not be of concern through our TAF period with only a FEW to SCT
high clouds.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday:
A cold front will clear the districts by Saturday afternoon
resulting in much cooler conditions over the weekend and into early
next week. Numerous light showers will be possible over higher
terrain locations east of Phoenix Saturday (both rain and snow),
however widespread wetting amounts will be highly unlikely. In
addition, a few stronger westerly wind gusts near 30 mph will be
possible behind the cold front, though winds will weaken
substantially next week. Despite the cooler weather, minimum
humidity levels will still fall into a 15-25% range with good
overnight recovery. Temperature will moderate back to near average
into the middle of next week though dry weather will continue.


Spotters should follow standard reporting procedures.




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