Area Forecast Discussion

FXUS65 KPSR 151227
Area Forecast Discussion
National Weather Service Phoenix AZ
527 AM MST Fri Dec 15 2017
.UPDATE...To Aviation and Fire Weather Discussions...
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 more so 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:
Northeast-east sfc and above sfc winds to persist through the
morning hours across the Phoenix airspace. Early morning aircraft
soundings show speeds of 20kts or so 1500ft above the sfc, so
threat for LLWS is diminishing and once the sun is up, end
altogether. Daytime winds will maintain easterly headings through
midday, before sfc pressure gradients relax generating light and
variable afternoon winds. Skies to remain mostly clear, with FEW
to SCT cirrus coverage AOA 25kft through the daytime and early
Southeast California/Southwest Arizona including KIPL and KBLH:
Much calmer winds in store today, maintaining north-northeast
headings for KBLH, with speeds generally 8kts or less. Winds for
KIPL will stay light and mostly variable in nature. Skies to
remain mostly clear through the early evening, before mid-level
clouds begin to thicken and increase from west by Saturday
Aviation Discussion not updated for amended TAFs.
Sunday through Thursday...
Fast moving upper trough will continue to cut through Arizona on
Sunday, interacting with moisture from northern Mexico and now
introducing at least slight chances for precipitation from
Maricopa County eastward. Much cooler temperatures will result
with the change in airmass, with the potential to fall below
seasonal normals. Current fire weather forecast elements support 
very sparse and spotty red flag conditions west of the Colorado 
River for an hour or two, but nothing at this time warranting more
than mention here. Sunday's trough quickly exits the region,
leaving northerly over the Districts with forecast winds to be
light and possibly subtly following drainage tendencies. Another 
trough will begin to approach the region by mid-week, possibly 
increasing winds and bringing even cooler air into the region for 
next weekend. 
Spotters should follow standard reporting procedures.
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