Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 161155 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
455 AM MST Tue Jan 16 2018

.UPDATE...Updated aviation and fire weather discussions.

&&

.SYNOPSIS...
High pressure will prevail across the region most of the week with
above normal temperatures and dry conditions. A weak disturbance
will move across the area this evening resulting in only limited
precipitation chances at higher elevation locations. A more
significant storm system will move through the area over the weekend
bringing much cooler temperatures, stronger winds, and a chance of
light precipitation.

&&

.DISCUSSION...
Very little change in the overall synoptic flow will occur through
the next 72 hours as a high amplitude blocking pattern remains stuck
in place. Several low amplitude shortwaves will break through the
western Conus ridge axis, eventually contributing to a poleward
retrogression of blocking high pressure and opening more progressive
flow across the country. Given this pattern transition, consistency
among model suites and among different model runs is surprisingly
good yielding better than expected forecast confidence through the
weekend.

In the short term, an impressive 1050mb sfc high (and deep cold
airmass) stationed over the central plains has built enough of a
pressure gradient into higher terrain to push a westward moving cold
front over the Continental Divide into western New Mexico. While
this boundary will largely lose definition heading into Arizona,
enough cool air will bleed into central Arizona and combine with
thicker higher clouds to retard high temperatures some 5F-10F versus
Monday. In addition, the enhanced pressure gradient will create a 25-
30kt easterly LLJ later this morning - a portion of which should mix
to the surface. The thicker cloud cover may inhibit better
mechanical mixing, however sufficient gusts may occur in an area of
freshly plowed fields yielding some localized blowing dust though
widespread coverage should not be anticipated.

Otherwise, a very weak shortwave will break through the ridge axis
this evening becoming absorbed in the downstream NW flow. Better
moisture will be almost exclusively limited above the H7 layer such
that only higher elevations might experience some sprinkles and
others only seeing virga. More so, this shortwave will only act to
reinforce the cooler airmass situated over the eastern half of the
forecast area into Wednesday. A sharp shortwave ridge will fold back
into the region Thursday and Friday providing a return to
H5 heights approaching 580dm and abnormally warm sfc conditions.

The first signs of a pattern change will arrive Friday as a
progressive shortwave and well defined cold front charge into the
region. The 00Z operational GFS remains about 6 hours faster than
the ECMWF and ensemble mean (not unusual), but overall model
agreement is very good. Given the WNW trajectory, initial positive
tilt, and evolution towards deepening only after moving onshore,
it`s unlikely better quality moisture will be swept into the steep
frontogentical band during FROPA Friday evening/night. In fact, only
a slim minority of GEFS members depict any QPF at lower elevations
with this system. It`s actually far more likely the influence of
residual moisture trapped in the cold core aloft and combined with
daytime heating and orographic influences Saturday afternoon will
yield the best shower chances.

In all likelihood, this system will yield higher impacts unrelated
to precipitation, but rather wind and temperatures. The most notable
impact will probably be strong winds and blowing dust across
southeast California and western Arizona around and after FROPA
Friday evening. Given this timing, some mountain rotors would be
favored in western Imperial County and stronger ridge top gusts
elsewhere in SE California. With the wave more positively tilted,
the pre-frontal gradient in Arizona will be weaker and strongest
winds tied to the post-frontal gradient and height falls aloft. Thus
by the time the front reaches central Arizona, much of the nocturnal
decoupling will have already occurred and winds much less an issue.

Otherwise, an abrupt cooling trend will be the other notable feature
with this weather system as temperatures may hover below average all
weekend. Although the overall pattern does not conceptually match a
widespread freeze event, the outlying, sheltered valley locations
will be almost certainly be touching the freezing mark Sunday and
Monday mornings. Given the progressive nature to the pattern, its
unlikely anything colder would occur, and a moderating trend towards
more typical late January temperatures will quickly follow next
week.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL and
Southeast California/Southwest Arizona including KIPL and KBLH:

A weak weather system moving across the state will lead to
considerable mid and high level cloudiness today, with cigs much of
the time. Expect bases from 15k feet on up with most of the BKN to
OVC cigs aoa 18k feet. Winds in the greater Phoenix area will favor
the east for most, or all of, the day today; Phoenix will likely not
return to the west this afternoon or evening. A few mid day gusts to
around 20kt are possible as well. Expect light winds out west,
favoring the north to northeast at KBLH and the west at KIPL.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday:
Above normal temperatures will continue on Thursday as the forecast
area remains dry.  Winds will generally remain light as well.
Afternoon minimum humidity will range between the upper teens and
mid 20s Thursday, even for the higher elevations, while overnight
recoveries will range between 40-60%. Breezy/windy conditions look
to materialize Friday/Friday night as a relatively dry weather
system moves through our area. Clouds and humidity levels will
increase Friday. The weather system may bring a isolated to
scattered showers to south central Arizona later Friday night
through Saturday evening with some higher elevation snow possible
east of Phoenix. It will also turn much cooler Saturday into Sunday
behind a passing cold front with high temperatures into the low to
mid 60s. It will turn drier over the weekend as minimum RH values
fall into the teens to around 20 percent. It will be less windy
Saturday, but local breezes from the west to northwest will still
occur mainly during the afternoon. Even less wind is expected
Sunday. On Monday, high pressure aloft will bring dry and warmer
conditions with near seasonal normal high temperatures and minimum
RH values in the mid teens over the deserts.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.

&&

$$

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DISCUSSION...MO
AVIATION...CB
FIRE WEATHER...Wilson/CB



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