Weather Service Report

FXUS65 KPSR 231305 AAB

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
605 AM MST Mon Jan 23 2017

.UPDATE...Updated aviation discussions.


The last in the recent series of wet Pacific storm systems will move
through the area today and Tuesday, bringing another around of desert
rain and mountain snow showers. Snow levels this afternoon and
evening will fall to between 5 and 6 thousand feet. Cold front will
move through today and behind it a drier and much colder airmass
will settle over the region Tuesday through Thursday. This will
promote some of the coldest low temperatures so far this year. Clear
skies, a slow temperature warming trend and breezy conditions will
develop by the coming weekend.


The latest in the series of wet Pacific weather systems is moving
through southern California this morning, with its associated surface
cold front just into the far western portions of our CWA. Last
evening's rainfall specifically over the Peninsular Ranges, generated
by a strong SW-NE oriented jet intrusion, has cleared from radar
scopes this morning but will fill back in as the cold front tracks
into Arizona. Temperatures this morning under persistent mid-level
clouds decks are in the low to mid 50s for many locales.

Modest isentropic ascent and orientation to the higher terrain
north-east-southeast of Phoenix within a very humid airmass with
today's frontal passage will produce rainfall amounts still around or
a little less than 0.5 inches for the south-central AZ deserts and
higher totals for the higher terrain north-east of town. Shower
activity will be more spotty and sparse over the southwestern AZ
deserts and Colorado River valley, with many locales struggling to
tick off more than a few hundredths of QPF. Plenty of high soil
moisture and saturation remains from last week's moderate to heavy
rainfall event over south-central AZ, with several area washes still
containing drainage/run-off flow. With QPF amounts ranging 0.30-1.00
inch storm total for the northern Maricopa and central Gila higher
terrain and drainages that feed into washes across those areas,
storm runoff and wash flow is expected again and as such will keep
the flash flood watch out for the northern portions of the CWA. It
could conceivably been transitioned to more of a longer fused flood
watch as rainfall will be occurring over several hours vs. the
rainfall bursts we see with convective events, but the escalation of
flooding concerns is being messaged either way.

Latest hi-res precip plots and FROPA/MSLP plots point to increasing
showers over Yavapai and Coconino counties before expanding southward
towards Phoenix during the day. Prime time for rainfall falls midday
through evening for the south-central deserts with snow levels
falling later into the day and overnight. Portions of Pinal and lower
elevations of Gila county will see healthy rainfall activity
overnight while snow levels head down around 5000ft overnight and
further down towards 4000ft by Tuesday AM. The heaviest amounts of
accumulating snowfall will range in the 5500-6000ft elevations, with
some snowfall possible below those elevations but in amounts at this
time not overly heavy. As such, the Winter Weather Advisory remains
in effect for elevations above 6000ft. The cold front will clear
east of the area early Tuesday with the main vorticity band and
trough axis swinging through the area late Tuesday into Wednesday.
Boundary layer conditions begin to dry during the day Tuesday over
the desert elevations, but remain elevated over the mountains and
light mountain showers may continue into Tuesday.

Late Tuesday into Wednesday, flow aloft becomes more subsident with
dry advection transitioning into the area. Greater temp-dewpoint
spreads coupled with a still cold airmass and clearing skies will
generate some of the coldest temperatures to settle over the region
beginning Tuesday, with the coldest mornings midweek. Coldest
outlying desert locales dip into the low 30s, but are not at freeze
warning thresholds just yet. Be sure to check our social media
channels and weather story graphics for preparation and safety info
for the upcoming cold temperatures.

By the late week and coming weekend, temperatures will slowly recover
to forecast values closer to seasonal averages if not just a little
below average. Another low pressure trough will make a trip through
the Four Corners and Colorado River Valley regions from the north
Friday. Given the overland trajectory of the system and little by way
of moisture advection into the area ahead of it, PoP chances remain
at or close to zero for the weekend. Sensible weather impacts from
this dry trough passage will likely be a prolonged period of
northerly to northeasterly breeziness, with gap/pass winds filtering
through the mountains northeast of Phoenix as well persistent gusty
conditions along and west of the Colorado River Valley. 


South-Central Arizona including KPHX, KIWA,and KSDL:

Surface winds in the greater Phoenix area are expected to stay
mostly southeast this morning around 10kt or so, backing ahead of an
approaching cold front. Winds should swing to the south-southwest by
18-19z or so as the front moves thru. This morning we may have
borderline LLWS (wind shear) conditions as winds 3-5k feet are
already 25kt or stronger from the southwest. For now will leave wind
shear out of the TAFs but again, conditions will be borderline.

Another wet Pacific storm will gradually push across the central
deserts today into tonight bringing showers to the terminals in
waves. For the most part expect CIGS between 5 and 8k feet through
the evening; the first enhanced wave of precip will likely occur mid
morning thru early aftn with some CIGs dropping to around 3-4k feet
along with enhanced showers. Second stronger wave from late aftn
thru mid evening dropping cigs to around 2-3k feet at times and vis
values to 3-4SM in moderate rain/rain showers. Thunder today not out
of the question but confidence too low to mention in the TAFs. Winds
southeast 8-12kt thru late morning then swinging to the southwest
after noon as the front moves thru the greater Phoenix area. May see
some gusts over 20kt at times this afternoon. Clearing from the west
expected after 05z or so, with CIGS mostly climbing to 8k feet or
above and some scattering out of the low/mid cloud decks. 
Southeast California/Southwest Arizona including KIPL and KBLH:

Unsettled period this morning into the afternoon as a cold upper low
moves across the area, should see SCT-BKN decks around 5k feet and
also 8-12k feet into early evening with a few light showers at
times. Clearing from the west by early evening. Gusty west winds
today at KIPL with peak gusts around 30kt mainly 18z-00z, winds more
from the south to southwest at KBLH. Winds to taper off during the
early evening at mostly below 15kt.

Aviation discussion not updated for amended TAFs.


Wednesday through Sunday: Weakened but broad and dry troughing will
settle over the west Wednesday keeping flow aloft westerly with low
level winds north to northwesterly but rather light. Quasi-zonal
flow will develop into the region as high pressure attempts to build
into the west and various shortwave systems ride over the ridge
flow. Temperatures initially will be well below normal but will
gradually warm during the five day period, becoming near seasonal
seasonal normal by Sunday. Humidity values will start highly
elevated with minimum RH values 30-45 percent, gradually drying
during the week and lowering into the mid to upper teens by the
weekend. One stronger storm system passing through the Four Corners
during the late week/early weekend will produce enhanced easterly
surface flow over the Mogollon Rim and northerly flow down the
Colorado River Valley, introducing breeziness and periodic gustiness
from the north and northeast for the districts.


Spotters should follow standard reporting procedures. 


AZ...Winter Weather Advisory from 1 PM this afternoon to 5 AM MST 
     Tuesday for AZZ024.

     Flash Flood Watch from 11 AM MST this morning through this 
     evening for AZZ021>024.



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