Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 150840

Area Forecast Discussion
National Weather Service Phoenix AZ
140 AM MST Fri Dec 15 2017

Quiet weather is expected today and most of Saturday before the next
weather system moves in. This one is looking a bit stronger now,
with chances for rain and mountain snow now in the forecast for the
eastern half of Arizona Sunday and into early Monday. Temperatures
will also be quite cool with this system, likely bringing the
coolest temperatures of the winter so far. Tuesday and beyond will
see a return of dry and gradually warming weather.


Early this morning, the upper ridge axis remained near the west
coast, keeping a dry north/northeast flow aloft going into Arizona.
PWAT values from the latest soundings showed just 0.10 at Flagstaff
and 0.30 at Tucson. Surface dewpoints over the central deserts were
mostly in the teens or low 20s. IR imagery at 1 am indicated clear
skies area-wide. For today we can expect continued sunny skies as
high pressure aloft continues to dominate our weather pattern. High
temperatures over the warmer deserts will again be well above
normal, climbing into the low to mid 70s.

Operational as well as ensemble guidance has started to congeal upon
a solution for this weekend that involves better chances of rainfall
for south-central Arizona (tho still not overly heavy amounts of
rain). There are some minor differences in the details but the
overall picture involves the upper ridge retrograding a bit off of
the west coast and allowing a rather vigorous short wave to dive
quickly southeast out of the Pacific Northwest and into Arizona. As
it approaches on Saturday, considerable mid/high clouds spread into
the lower deserts for partly or mostly cloudy skies. High temps
start to fall off as heights fall and clouds increase. By late
Saturday both operational GFS and ECMWF as well as most GEFS
ensemble members propose a large upper trof forming over the desert
southwest, situated squarely over Arizona. GFS is more of an open
wave with the latest run (but that could easily change with the
next run or two) and the ECMWF has a large closed low centered over
the far northern Gulf of CA. In both cases they rapidly develop high
mean RH in the column over southern Arizona and areas of light rain
or showers will start to break out across south central AZ by early
Saturday morning. Best rain chances for south-central Arizona will
be mid morning through late afternoon; POPs have been raised again
into the likely category for the higher terrain east of Phoenix, and
into the good chance category for the Phoenix metropolitan area.

Dynamics look to be rather impressive with healthy amounts of PVA,
mid level Q-convergence and low/mid level UVV. There are no
atmospheric rivers forecast to bring large amounts of moisture to
far southern CA or Arizona and mean RH really increases quickly
despite no real moisture tap potential (based on current satellite
imagery). It is likely that much of the RH increase is due to rapid
cooling of the airmass moreso than large amounts of moisture
ingested into the system. As such, QPF is forecast to be rather low
with this system. Best forcing and precip potential is during the
day Sunday before all of the cold air arrives, so we are not looking
at any significant snowfall either. Given the sharp lowering of
heights and thicknesses and the increase in clouds and showers, high
temps will fall well below seasonal normals for a change with
central deserts falling into the upper 50s to low 60s, and only
slight warming is expected Monday.

Models agree that the trof/closed low will be quick to push off to
our east overnight Sunday night into early Monday as strong DVV and
mid level divergence of Q move into the deserts; by 12z Monday rain
chances will be down into the single digits east of Phoenix and
skies from the Phoenix metro westward will be generally clear.

High pressure aloft will rebuild over the desert southwest during
the early to middle portion of next week allowing high temperatures
to gradually warm back above seasonal normals with warmer deserts
climbing back into the low to mid 70s by next Wednesday. Another
short wave is forecast to drop through the four corners Thursday but
as of now it likely will be dry for our area and POPs will stay in
the single digits. It will usher in some cooler air again, dropping
central desert highs back into the upper 60s.


South-Central Arizona including KPHX, KIWA, and KSDL:
A north-northeasterly flow has developed across the Phoenix area
in the wake of the departing trough. Winds will veer overnight to
the east-northeast while subsiding somewhat. However, latest model
guidance suggests stronger winds around 30 kt just above the
surface will persist overnight, which will create isolated pockets
of low-level wind shear. For Friday, winds will predominantly
remain out of the east less than 10 kt, except for the late
afternoon when a period of light and variable winds is possible.
There is also a slight chance that local topographic influences
will yield a period of light southwesterly winds at KPHX early
Friday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather impacts expected. The windy conditions from
earlier today will continue to subside overnight across
southeastern California. Winds will remain out of the north at
Blythe around 10 kt Friday, while light and variable winds will
prevail at KIPL.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday...
A fast moving upper trough will cut southward through SoCal and
dive off the western Baja coast Saturday into Sunday, developing
another round of elevated northerly winds over southwest AZ, the
Lower Colorado River Valley and portions of southeast CA. With
cooler high temperatures in the forecast daytime humidities range
10-20 percent over the weekend with mostly fair overnight
recoveries. Current fire weather forecast elements support very
sparse and spotty red flag conditions for Sunday west of the
Colorado River for an hour or two, but nothing at this time
warranting more than mention here. Some recent changes to model
forecasts now have some moisture in play to the south across
northern Mexico. Some increased in humidities regionally and even
remote chance for light precip (mainly eastern-southeast AZ) could
develop. Quasi-zonal flow will settle over the region by early
next week, keeping temperatures on the more mild side but still
slightly above seasonal normals.


Spotters should follow standard reporting procedures.




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