Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 181128

Area Forecast Discussion
National Weather Service Phoenix AZ
425 AM MST Thu Jan 18 2018

...Updated Aviation and Fire Weather...

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.


At 2 am this morning, skies were basically clear across the area
although there were a few high clouds starting to approach far
western Arizona; they should thin as they move through the ridge and
into Arizona. For today, expect the high pressure ridge just to our
west to slide east and squarely overhead allowing slight warming as
most of the lower deserts reach the low to mid 70s under mostly to
partly sunny skies. The ridge will shift off to the southeast Friday
as a deep, cold but somewhat dry Pacific upper trof approaches the
southern California coast. Gradients will tighten out west Friday
afternoon, leading to breezy or locally windy conditions mainly over
far southeast California, with gusts over 30 mph expected in the
favored higher terrain locations west of the lower Colorado River
valley. Friday should be the warmest of the next 7 days, as a
southerly component to the low level flow ahead of the approaching
cold front will lead to some warm air advection, pushing deserts
highs into the mid to upper 70s in many locations.

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; and
Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation impacts through Friday morning with only passing
high cirrus across the region. Easterly winds around the Phoenix
metro will be persistent today, and the typical westerly switch will
likely be delayed and/or muted versus a typical afternoon. Instead,
there may be an extended late afternoon/early evening period of
nearly calm winds (light and variable at the very least). Similar
variability in direction will also occur at the SE California

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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