Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 160922

Area Forecast Discussion
National Weather Service Phoenix AZ
220 AM MST Sat Dec 16 2017

Quiet weather is expected today before the next weather system moves
in. This one still looks to bring chances for rain and mountain snow
to the eastern half of Arizona Sunday and into early Monday.
Temperatures will also be cool with this system, potentially
bringing the coolest temperatures of the winter so far. Tuesday and
beyond will see a return of dry and gradually warming weather before
another cooling trend sets in for the latter part of next week.


Early this morning, high pressure aloft continued to dominate
Arizona`s weather pattern, keeping a very dry airmass in place
across the area; latest Tucson sounding indicated just 0.2 inches of
PWAT and the 2 am central desert surface dewpoints remained very low
and mostly in the teens to low 20s. High pressure aloft will be
weakening and shifting off to the east today as a rather cold but
not overly wet Pacific upper trof approaches from the northwest.

IR imagery early this morning showed a shield of considerable
mid/high clouds dropping into the area and spreading into northwest
AZ and far SE CA; this moisture is ahead of a developing trof that
will eventually bring scattered showers mainly to the eastern
portion of Arizona on Sunday. We still do not expect this system to
be overly wet and QPF values remain low; latest SREF plumes data
only indicated a median of less than 0.05 inches of rain in Phoenix
on Sunday and WPC rainfall totals for the event are well below one
quarter of an inch over any of the lower deserts. A look at latest
blended total PWAT satellite imagery showed very low moisture
content off of northern Baja /southern CA and no atmospheric rivers
are expected to be available to import moisture into the area ahead
of this system.

For days now, operational models and ensemble guidance have
struggled to resolve this approaching trof; as it dives
southeastward out of the Pacific northwest and into the desert
southwest, progs have tried to close off a portion of the low but
the track of the closed low remains very uncertain - even at this
late date. Models have waffled quite a bit on how much moisture will
be pulled into southern AZ ahead of the low and POP forecasts have
varied wildly from run to run. Latest 00z forecasts have taken a
huge step back with POP forecasts for the deserts; one example is
the latest MAV guidance from the GFS which now calls for just a 3
percent chance of rain in Phoenix Sunday afternoon compared to 50
percent or more from earlier runs. ECE MOS from the European now
calls for less than 50 percent rain chances in Phoenix for Sunday,
down from about 90 percent just one run ago. Most of the latest
operational runs, and most of the GEFS ensemble members now want to
close off an upper low by Sunday morning, but keep the center over
northern Baja/gulf of CA and they then track the low to our south,
mostly through northwest Mexico giving best forcing/moisture and
rain chances to southeast Arizona and not the central/southcentral

Despite being only a day or so away from this event, guidance is
still struggling and due to the poor consistency we opted to keep
forecasts pretty much as is - and so did our neighboring offices.
So, we still call for a chance of showers developing late tonight
and continuing into Sunday afternoon before tapering off Sunday
night as the low shifts slowly off to the east. Higher POPs, into
the likely category, remain over higher terrain areas to the east of
Phoenix Sunday. This looks to be a relatively high POP but low QPF
event for our area. Latest model trends have also correspondingly
raised our high temps for Sunday and we have made a nod to these
trends by pushing our highs on Sunday up several degrees; Phoenix is
now forecast to rise to near normal with high of 65 on Sunday.
Little change in these temps is expected Monday.

On Monday, as the low moves off to our east, some wrap around
moisture will linger over the higher terrain of southern Gila County
and we will keep a slight chance of morning showers in the forecast.
Should mention that latest trends keep snow levels a bit higher than
earlier runs and as such - given the low expected QPF - there will
be little if any accumulating snow in the forecast and any that
falls should stay generally above 5000ft. By Monday afternoon we
expect mostly sunny or sunny skies along with desert highs in the
mid to upper 60s.

Upper ridging will build into the area Tuesday into Wednesday for
mostly sunny days, clear nights and a modest warming trend allowing
the warmer deserts to reach into the lower or even middle 70s by
Wednesday afternoon.

For the latter portion of the extended, Thursday into Friday, latest
guidance has become more insistent that another large and rather
cold upper trof will drop into the area from the north and gradually
develop with a closed low eventually forming somewhere over southern
Arizona or northwest Mexico. GFS and ECMWF differ on details and
moisture, but we have sufficient confidence to add some slight
chances to the forecast mainly over southern Gila County on Friday.
This deepening trof will usher in another bout of much cooler air so
a marked cooling trend has been introduced, with desert highs
falling into the low to middle 60s over the deserts by Friday.
Phoenix is now expected to see a below normal high of just 63 next
Friday under mostly sunny skies.



South-Central Arizona including KPHX, KIWA, and KSDL:
Light winds will prevail through the TAF period as directions
favor the east. However, there is a chance a period of variable
flow may occur late Saturday afternoon/early evening before
easterlies return later in the evening. Thick high clouds will
move in early Saturday although the terminals will remain dry
throughout the day.

Southeast California/Southwest Arizona including KIPL and KBLH:
Light winds will continue tomorrow at KBLH and KIPL with
directions favoring the south and east. However, winds may be so
light variable conditions are possible for several hours. Thick
high clouds will also persist through much of the day although
terminals should remain dry. Surface winds will switch to the
north late Saturday/early Sunday as a front approaches from the

Aviation Discussion not updated for amended TAFs.


Sunday through Thursday...
Fast moving upper trough will continue to cut through Arizona on
Sunday, interacting with moisture from northern Mexico and now
introducing at least slight chances for precipitation from
Maricopa County eastward. Much cooler temperatures will result
with the change in airmass, with the potential to fall below
seasonal normals. Current fire weather forecast elements support
very sparse and spotty red flag conditions west of the Colorado
River for an hour or two, but nothing at this time warranting more
than mention here. Sunday`s trough quickly exits the region,
leaving northerly over the Districts with forecast winds to be
light and possibly subtly following drainage tendencies. Another
trough will begin to approach the region by mid-week, possibly
increasing winds and bringing even cooler air into the region for
next weekend.


Spotters should follow standard reporting procedures.




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