Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 061225 AAA

Area Forecast Discussion
National Weather Service Phoenix AZ
525 AM MST Tue Dec 6 2016

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


A weak upper storm system will pass by to our north today and
Wednesday, increasing clouds and cooling temperatures slightly. Dry
weather with seasonable temperatures and occasional passing high
clouds will return Thursday and last through the weekend.


Troughing remains entrenched over much of the North American
continent with various shortwave circulations dotting traversing the
flow. Areas of broken to overcast cloudiness continue this morning
downrange from the Sierra Nevada, indicative of increasing ML winds
and RHs. Water vapor overnight imagery paints some areas of ML/UL
moisture, with the driest air well removed to our south.

Of the various shortwaves upstream over the west, one moving through
the Pac NW currently will drive as far south as central UT with any
PVA/Div Q forcing fields of interest skirting the north state line
b/w AZ and UT. Surface high pressure, albeit weak, over central
Mexico along with the low pressure shortwave moving southward towards
the area will orient LL wind fields southerly in nature. This will
draw up some moisture currently sampled on 00z Mexican RAOBs of 850
Tds near 0C into the region. Per SPC Sounding Climo, 0C dewpoint at
850 this time of year for Tucson is near 75th percentile, with the
daily averages usually running around -5C. NAM and GFS Bufr soundings
also indicate for a brief window this morning and Wednesday morning
that T/Td traces approaching saturation near 850mb with drier layers
aloft and through the ML. Since forcing will to far removed from the
north to enact on this thin moisture layer, increasing LL cloud
fields/decks are more likely. Last few hi-res model runs as well as
the NCEP and UA WRFs indicate some light radar returns are possible
over the SW deserts as early as this AM -- which may more than likely
just be the thickening clouds along with some limited virga. At their
peak, 850mb Tds could crest 5C, approaching regional daily maxes for
this early in December. Again, deep moisture isn`t prevalent and
PWATs struggle to reach forecast values higher than 0.60", so chances
of measurable precip are non-zero but not broadly high enough for
widespread light shower mention today and tomorrow. Any areas that
stand the slightest chances for any precip activity are across the
higher terrain of northern AZ/Mogollon Rim. Under increasing clouds
and subtle CAA, temperatures today and Wednesday will drop a few
degrees but stay within normal values.

Late Wednesday into Thursday, LL reverses back to northerly headings
ushering in some dry advection as the upper shortwave tracks out into
the Plains. Longwave troughing remains over the CONUS for the late
week and weekend with 500mb heights ranging 576-582dam over the
forecast area. Not much variance will result in the diurnal curve
with somewhat steady state and mild temperatures settling in for the
late week and weekend.


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

Weather disturbance passing by to the north will usher in
significant amounts of mid/high clouds today and tonight with bases
mostly aoa 15k feet. Expect BKN-OVC cigs much of today and into the
evening. May see a few decks down around 10k feet at times. One
potential issue...several models suggest that there will be a very
low level moisture feed into the central deserts ahead of the
approaching system with the potential for a SCT-BKN cloud deck
around 5-6k feet to develop later this morning then persisting into
the evening hours or even through much of tonight. For now will call
for just SCT decks in the TAFs including at KPHX but potential is
there for BKN decks later today. Otherwise winds to favor normal
diurnal tendencies next 24 hours for the Phoenix area TAFs with
typical speeds less than 10kt.

Southeast California/Southwest Arizona including KIPL and KBLH:

No real aviation concerns for the next 24 hours as we will mainly
see variable amounts of high clouds spreading across the deserts and
affecting the TAF sites. Most cloud bases aoa 15k feet with BKN
decks at times aoa 20k feet. Winds will be on the light side
today...becoming slightly stronger from the west at KIPL later today
and into the evening with speeds up to 15kt. Winds at KBLH to favor
the south/southwest from late morning thru this evening...then
turning to the north/northwest.

Aviation discussion not updated for amended TAFs.



Thursday through Monday: Near normal temperatures and mostly dry
conditions are likely this week into early next week with periods of
high clouds moving across the area at times. Winds each day will
mostly stay on the light side with typical afternoon breeziness
favoring the southwest or west. Through Friday, minimum RH values
will be elevated across Arizona with values mostly above 20 percent;
from the lower Colorado River Valley west we can expect values
between 10 and 17 percent. Readings become elevated area-wide over
the weekend, with minimum RH values mostly between 20 and 30 percent.


Spotters activation is not expected this week.




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