Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 161534
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
835 AM MST Mon Jan 16 2017

&&

.SYNOPSIS...
A large weather system over the region will linger through tonight
continuing to circulate clouds over much of Arizona along with
isolated showers across eastern Arizona. Weak high pressure will then
take over for Tuesday and Wednesday leading to a slight warm up and
drier conditions. Starting Thursday, another series of Pacific
storms will begin to traverse the region. Good chances of rain and
high elevation snows are forecast for Thursday through Friday
night. A brief break is likely Saturday night and Sunday before
another storm system affects the region early next week. Temperatures
will turn much colder starting Friday with highs up to 10 degrees
below normal.

&&

.DISCUSSION...

The beginning of a break between systems this morning, as the upper
low that brought the rainfall over the past few days has now moved
off well to our east, now tracking into central KS this morning.
However, a broad upper trof continues to lag behind this low center
over the Desert Southwest. The presence of the upper trof is keeping
considerable cloudiness, and even a few light showers going over
much of South-Central AZ. Areas to the north and west of the main
trof axis, including Western Maricopa, La Paz, and Yuma counties
have seen enough dry air bleed in to allow at least some clearing
over those regions. The combination of these clear skies and a still-
moist boundary layer has allowed some patchy areas of dense fog to
form along parts of I-10 west of Phoenix and I-8 in the Yuma area,
requiring the issuance of a Dense Fog Advisory until 11 am LT. At
temperatures warm, this fog is expected to dissipate by then. Latest
High-Res HRRR model guidance is keeping a few light showers going
over southern Gila County into this afternoon before dissipating
this evening as the upper trof finally begins to shift off to the
east. In the short term, outside of adding fog to the grids and some
minor adjustments to the hourly temperature grids, inherited
forecast look good.

&&

.PREVIOUS DISCUSSION...
A somewhat pleasant start to the week will turn wet and dreary
starting Thursday as three separate storm systems are likely to
affect the Desert Southwest through early next week. Currently, a
broad area of troughing remains situated from the central Plains
through southern California. Abundant low and mid level moisture
lingers over the eastern 2/3 of Arizona which is resulting in areas
of stratus and stratocu. A weak mid level circulation within the
broad trough is seen on water vapor imagery over Baja California.
This will slide to the east northeast today ending up just to our
southeast late this afternoon. This should provide enough lift for a
continued threat of isolated showers mainly to the east of Phoenix.
Not anticipating much in the way of rainfall, but some locations
could pick up an additional few hundredths of an inch. Temperatures
likely won`t recover much today over yesterday with highs again
several degrees below normal.

Subsidence will gradually increase over the area by Tuesday as the
secondary low center lifts into New Mexico. We should see more
sunshine on Tuesday, but clouds will likely linger over eastern
Arizona throughout the day. Upper level ridging finally moves in
from the west on Wednesday, but only briefly as a fast moving Pacific
low will be approaching the California coast. Temperatures will
recover somewhat on Wednesday, but we`ve lowered our forecast highs a
degree or two to nearly match normal readings.

The system on Thursday will be the first in a series of waves riding
on a very strong jet branch circulating around the southern edge of
a strong polar vortex situated over northern Alaska. A good consensus
among the models shows this Thursday system not deepening all the
much, but still coming into the Desert Southwest with a good amount
of moisture and energy. For now the main PV anomaly is forecast to
quickly move through central Arizona Thursday afternoon and evening.
Strong low level southwesterly flow will transport modest amounts of
moisture into our region while also providing favorable upslope
conditions. Rainfall amounts over the deserts will generally be a
tenth of an inch or less with up a quarter to a half an inch over
the high terrain north of Phoenix.

Quickly following behind the Thursday system will be a stronger and
more significant system for Friday afternoon through Saturday
morning. Stronger dynamics on the nose of the 180kt polar jet over
the eastern Pacific will provide for better lift over our region
while a strong surface cold front traverses the area on Friday. 850mb
winds up to 40 kts will bring windy conditions later Friday into
Friday night and promote abundant upslope shower activity. A
weakening atmospheric river will likely bring enough moisture into
the region to lead to a much more significant rain event. Up to an
inch over the south-central Arizona deserts and up to two inches
over the higher terrain will be possible. Much colder air will
accompany this system dropping snow levels down to around 5K feet
potentially leading to several inches of snow across the higher
terrain. Rain and snow will likely linger from Phoenix eastward into
Saturday until the trough slides east Saturday night.

Models remain in good agreement through early next week with a weak
ridge moving over for Sunday before an even colder and just as strong
upper level trough moves into the region for next Monday and Tuesday.
This system will likely bring another decent amount of rain and
higher elevation snows with snow levels possibly dropping to as low
as 3K feet late Monday night into Tuesday.

Forecast temperatures through the active weather period of Thursday
through next Monday are not of high confidence, but it seems likely
many desert areas will struggle to reach 60 degrees starting Friday.

&&

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

Conditions remain relatively moist in the lower levels of the
atmosphere and consequently low clouds will be the main aviation
weather concern through Tuesday. A cloud deck with bases around 8k
feet prevails across much of the Phoenix area this morning. However,
some scattered lower clouds with bases near 5k ft are likely near
KPHX this morning. The lowest cigs are expected near KSDL, where
bases will drop as low as 2500 ft.

Latest probabilistic guidance suggests conditions will improve area-
wide around 18z as drier air works its way into Central Arizona. Some
areas of stratocu will persist during the afternoon with a clearing
trend likely thereafter through the evening and into Tuesday morning.
Meanwhile, winds will generally remain light and diurnal through the
period.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather impacts anticipated. Winds will generally remain
light and diurnal at KIPL/KBLH through at least Tuesday morning.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday:
High pressure returning to the region Wednesday will result in near
normal temperatures. Thereafter, a series of low pressure systems
will bring periods of increased chances of precipitation along with
persistent below normal temperatures through early next week. Minimum
humidity levels near 30 percent are expected through Thursday before
increasing closer to 50 percent for Friday and Saturday. Winds
through Wednesday will be fairly light with breezy to windy
conditions likely from Thursday through Saturday. However, fire
danger remains low due to the relatively moist conditions.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures. Skywarn radio
Net operations are not expected to be needed this week.

&&

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

$$

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DISCUSSION...Percha/Kuhlman
AVIATION...Hirsch
FIRE WEATHER...Hirsch


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