Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 172133
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
230 PM MST Tue Jan 17 2017

.SYNOPSIS...
Weak high pressure will persist over the region through Wednesday
with dry conditions and near average temperatures. Beginning
Thursday, a series of strong Pacific storm system will traverse the
region. Extensive rain and high elevation snow are likely Thursday
and again Friday through Saturday afternoon. A brief break is likely
Saturday night and Sunday before another storm system affects the
region early next week. Temperatures will turn much cooler
starting Friday with highs up to 10 degrees below normal.

&&

.DISCUSSION...
A positively tilted trough axis continues to linger across New
Mexico and Arizona this morning with WV and visible satellite
imagery clearly showing a well defined vorticity center rotating
through southern Gila County. Although regional radar imagery
currently has little in the way of returns, congestive CU in
satellite suggests a few light showers possible into the early
evening before loss of daytime heating.

Expansive sfc high pressure will continue subjugate the region
Wednesday and much like this morning, some very isolated river
valley fog will be possible. This will particularly be the case
though the Gila River valley of northern Pinal County where
crossover temperatures should be reached. Otherwise with shortwave
ridging aloft spreading into the forecast area, temperatures should
realize readings near or somewhat warmer than today under mostly
sunny skies (the last time we`ll be uttering those words for many
days to come).

As has been the case the past several weeks, a pronounced strong
zonal jet extends from the central to eastern Pacific basin with
numerous embedded shortwaves ready to break into the western Conus.
The first wave will eject in a negative tilt fashion over the
Southwest Thursday with an attendant narrow frontal boundary pushing
across the San Diego mountains. Model output indicates the trailing
residual tail of an atmospheric river becoming entrained along this
front. Given the impressive dynamical ascent/frontogenetical
forcing, numerous showers should blossom Thursday afternoon and
progress into eastern AZ Thursday night. A favorable, deep and moist
SW flow looks excellent for orographic ascent and have increased
rain/snow chances near and east of Phoenix. Snow levels near 6000 ft
will crash to around 5000 ft by Friday morning with most snowfall
AOA the 5500 ft level. This will be just the first surge of moisture
into a cold environment and have started with a Winter Storm Watch
Friday evening for a prolonged snow event at higher elevations.

A deeper, yet still negatively tilted secondary wave will blast into
the region Friday into Saturday on the nose of a 180kt polar jet.
Strong pre-frontal warm air advection late Friday morning and
afternoon in combination with 25-45kt of low level southwesterly
flow will promote rapid shower development through isentropic
processes in a deep 290K-300K layer. The main midlevel forcing will
arrive later Friday evening along a steep low level cold front.
Concurrently, an event better defined atmospheric river will punch
into southern California and the Baja peninsula advecting northeast
towards southern and central AZ. This is when steadier and more
expansive rainfall will occur; and given all the premium ingredients
in place, have little doubt in precipitation potential hedging POPs
towards the categorical range.

Snow levels will also drop further Friday night towards a 4000-4500
ft level Saturday morning. Due to the aforementioned stronger
dynamics and higher moisture levels in the atmospheric column, QPF
amounts will be significantly higher than the Thursday system. Have
hedged precipitation amounts towards ensemble means yielding 0.50-
0.75" around the Phoenix area, lesser amounts over SE California and
SW Arizona, and localized amounts up to two inches over the high
terrain east of Phoenix. Significant snow amounts may be realized
across higher elevations, especially above 5000-5500 ft where
prolonged 36 hour amounts may breach the one foot mark. Some
lingering rain and snow showers are likely to the east of Phoenix
during the day Saturday, coming to an end by evening. Winds Friday
afternoon into Friday night will also be quite breezy, and even
windy at times over the ridges and peaks which would only exacerbate
the winter weather problems of southern Gila County.

Weak ridging builds in behind the exiting system on Sunday giving a
brief reprieve from the wet weather. High temperatures Sunday will
barely budge from the previous mostly below 60 degree highs on Friday
and Saturday, but at least some partial sunshine should occur.

The third storm system to affect the Desert Southwest should arrive
sometime Monday, but model spread has increased some since
yesterday. The 00Z operational GFS was running 6-12 hours faster
than most of its ensemble members and the ECMWF, however the 12Z
iteration was closer to the preferred ensemble mean. Regardless, the
best precipitation chances should be Monday afternoon into Monday
night. Confidence is still not very high in the exact timing and how
much of an impact the system may bring, but much of the area should
again see rain with higher elevation snows again being an issue.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Expect the winds to remain light to occasionally calm and favor a
westerly direction through late tonight, then gradually favoring an
easterly heading around 05-08Z. Otherwise, skies will be mostly
cloudy with mid level CIGS from 10-15K ft before gradually
scattering out this evening to late tonight. There is a chance of
some isolated patchy early morning fog with vsbys temporarily
lowering to 1-2 SM, however the confidence in its formation at the
terminals was not high enough to include in the TAFS at this time.

Southeast California/Southwest Arizona including KIPL and KBLH:
Expect light winds at KIPL and light to occasionally breezy winds at
KBLH favoring northerly directions this afternoon, while gradually
favoring a more westerly component late tonight by around 05-06Z.
Otherwise skies will be clear to mostly clear with no aviation
impacts expected.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday:
A series of low pressure systems will bring periods of precipitation
along with persistent below normal temperatures through early next
week. The strongest storm system will likely move through the area
Friday night, bringing wetting rains and mountain snow. Widespread
breezy conditions will also accompany the system Friday and
Saturday. However, fire danger will remain low due to the relatively
moist conditions. After a brief respite Sunday, conditions will
again deteriorate Monday with increasing winds and precipitation
likely.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter reports of rain and snow will likely be needed later this
week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...Winter Storm Watch from Thursday evening through Saturday
     morning for AZZ024.

CA...None.

&&

$$

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DISCUSSION...MO/Kuhlman
AVIATION...Sawtelle
FIRE WEATHER...Hirsch



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