Area Forecast Discussion
FXUS65 KPSR 161233
Area Forecast Discussion
National Weather Service Phoenix AZ
533 AM MST Sat Dec 16 2017
.UPDATE...To Aviation and Fire Weather Discussions...
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
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:
Clouds will increase and thicken from the west today, varying
15-25kft in height and coverage SCT thru to OVC. Typical easterly
AM winds will become light and variable into the afternoon, speeds
less than 6kts. Easterly winds will return into the evening. Upper
trough moving through the region later tonight into Sunday will be
capable of producing lower cloud layers down between 8-10kft
(possibly lower in the more aggressive model forecasts) for early
Sunday, along with slight rain chances. Introduced VCSH for KPHX
midday Sunday and other area terminals could see at least VCSH
mention including in TAF packages later today.
Southeast California/Southwest Arizona including KIPL and KBLH:
Light wind regime to continue for KIPL and KBLH with headings
varying from southeasterly to northwesterly. High clouds will
persist between SCT thru OVC coverage today with ceilings 18-20kft
possible. Another storm system will drop into the region from the
north overnight and into Sunday, switching surface winds to
northerly headings and supporting breezy conditions. Strongest
winds will be over area ridgetops to the west of the terminals,
but elevated gusts are possible for KBLH (more so than KIPL) near
20-30kts through the day Sunday.
Aviation Discussion not updated for amended TAFs.
Monday through Friday...
Upper low pressure feature that moved into the region on Sunday
will be slow to exit, however end most precip chances across the
eastern Districts. In the wake of the exiting system, winds will
be light and variable with subtle trends toward drainage slopes
and diurnal patterns. Dry northwest flow will linger for Tuesday
and Wednesday before another colder and more potent storm system
sweeps in from the northwest. Breezy west-northwest winds will
develop overnight Wednesday and into Thursday, impacting area
ridgetops first then mixing into the lower desert elevations
during the daytime. There could be some precipitation that
develops over Arizona with this next system, however forecast
model differences are really noticeable after Thursday with how
deep the system is and how long it sticks around the area. Daytime
humidities will be range higher than the last few weeks, 15 to 20
percent for the lower elevations and 30 percent or greater for the
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
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