Area Forecast Discussion
Issued by NWS Phoenix, AZ

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498
FXUS65 KPSR 152205
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
305 PM MST Fri Dec 15 2017

.SYNOPSIS...
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.

&&

.DISCUSSION...
Dry north to northeast flow currently exists across the region
around an area of high pressure. This extremely dry air mass is
very evident on both the 12Z FGZ and TWC soundings, where PWATs
were .08 inches and .27 inches respectively. This dry air mass is
evident on satellite imagery as well, where not even a single
cloud exists in the sky. Subsidence, increased heights, and clear
skies are providing us with another day of above average
temperatures with highs expected to reach the low to mid 70s in
most lower desert locations.

Models continue to be in good agreement about our next rain
maker. Confidence is growing that a short wave will dig into our
region Saturday, with a large trough developing over AZ by Sunday
morning (GFS is still an open wave and the ECMWF continues to
show a large closed low developing near the AZ/Mexico border). As
the short wave digs into our region tonight into tomorrow, there
will be an increase in mid to upper level cloud cover starting in
southeast California and then spreading eastward into central AZ
by Saturday morning.

So the big questions remain, how much moisture will accompany
this system and will we see any rain? Well, taking a look at the
Integrated Water Vapor Transport, there isn`t any sort of
atmospheric river providing us with moisture. This system looks to
develop its own deep layer of moisture, mainly over south-central
Arizona. However, with at least two out of the three ingredients
needed for precipitation (moisture and lift) there is high
confidence we will see some areas of light rain/showers during the
morning hours mainly from Maricopa county eastward. The best time
frame continues to be the 12Z to 18Z time frame on Sunday
morning. With little to no instability and forecast PWAT values
near average, not expecting any significant rainfall, but some
rain is better than no rain! Since the best chances for
precipitation are before the cold airmass arrives, significant
snowfall is not a concern either, but some light dusting will
occur across the higher terrain in southern Gila County including
Hilltop.

In addition to some rain chances, this system will be bringing us
cooler temperatures with highs expected to drop into the upper
50s/lower 60s across the lower deserts in Arizona on Sunday. Since
heights will be building behind the front in southeast California
on Sunday, temperatures will be a few degrees warmer. Low
temperatures on Sunday night will be in the 30s and 40s, with some
of the higher elevation zones seeing near freezing temperatures.

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.

&&

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

High pressure building into the region from the northwest to keep
winds quite light (mainly 6 kts or less) through the taf period.
Wind direction to mainly remain easterly through the taf period at
the Phx area terminals, except possibly a short period of
light/variable conditions during the late afternoon/early evening
hours on Saturday. Light/variable conditions at both KBLH and KIPL
this evening to become light southerly late tonight and Saturday.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
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.

&&

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

&&

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

$$

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DISCUSSION...Hernandez/CB
AVIATION...Percha
FIRE WEATHER...Nolte



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