Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 170327
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
825 PM MST Sat Dec 16 2017

.SYNOPSIS...
A compact weather disturbance will bring chances for light rain and
mountain snow to the eastern half of Arizona Sunday into early
Monday. Temperatures will cool substantially with this system
bringing the coolest temperatures of this winter so far. Beyond
Tuesday, dry conditions with some warming will settle over the area
during the middle of the week before another potentially colder
weather system swings into the region by the end of the week.

&&

.DISCUSSION...
A very tight PV anomaly/cold core has descended into the northern
Baja this evening driven by an objectively analyzed 100+kt jet
streak punching down the west side of a narrow trough axis. With the
jet nose rounding the basal portion of the trough, the PV anomaly
was quickly obtaining a negative tilt; and upward ascent through
vorticity advection and deformation were beginning to materialize
through northern Mexico and far southern AZ. In fact under northern
Baja -23C H5 cold core, several convective elements and lightning
strikes have been noted since late afternoon.

The maturing cyclone will propagate along the international border
over the next 24-36 hours providing increasing mid/upper level
moisture and lift to the southeastern quarter of AZ. However,
regional 00Z sounding data revealed little quality moisture (KTWC
sampled PWAT only 0.3 inches) and satellite derived PWAT suggests
little additional input upstream (possibly increasing into a 0.50-
0.75 inch range). Regardless, several operational and high
resolution models indicate robust ascent squeezing out every bit of
available moisture very late tonight into Sunday morning through the
eastern parts of the CWA (including much of the Phoenix metro). The
first round of vorticity forced lift will likely be followed by
another round of isold showers tied somewhat closer to daytime
heating and weakly capped instability of around 100 J/kg. Made some
targeted increases to POPs/QPF tomorrow morning while also adding a
slight thunder chance based on instability forecasts. In the near
term, have adjusted some short term parameters based on current
trends.

&&

.PREVIOUS DISCUSSION...
/208 PM MST Sat Dec 16 2017/
After high temperatures in the upper 60s to around 70 today,
temperatures will dip even further on Sunday with highs mostly in
the middle 60s, likely a bit lower across eastern Arizona under
mostly cloudy skies. Gusty winds will also be an issue across
southeast California after midnight tonight through Sunday
afternoon. The strongest northerly winds will be focused across
central Riverside County, including JTNP, where Advisory level winds
are expected.

On Monday as the low moves off to our east, some wrap around
moisture will linger over the higher terrain of southern Gila County
with slight chances of showers through the morning. By Monday
afternoon, we expect mostly sunny or sunny skies along with desert
highs in the mid to upper 60s. Upper ridging will build into the
area Tuesday into Wednesday for mostly sunny days, clear nights and
a modest warming trend allowing the warmer deserts to reach into the
lower 70s by Wednesday afternoon.

For the latter portion of the extended, Thursday into Friday, latest
guidance has become more insistent that another large and rather
cold upper trof will drop into the area from the north and gradually
develop with a closed low eventually forming somewhere over southern
Arizona or northwest Mexico. GFS and ECMWF differ on details and
moisture, but we have sufficient confidence to add some slight
chances to the forecast mainly over southern Gila County on Friday.
This deepening trof will usher in another bout of much cooler air so
a marked cooling trend has been introduced with desert highs falling
into the low to middle 60s over the deserts by Friday. Phoenix is
now expected to see a below normal high of just 63 next Friday under
mostly sunny skies.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Isold showers, shifting winds, and lowering cigs will all be
aviation concerns Sunday morning though afternoon around Phoenix.
East winds will be preferred this evening and overnight, potentially
becoming a light south wind as the first light showers move north
after sunrise.

Cigs should generally lower below 10K ft, then possibly flirting
with 6-7K ft later Sunday afternoon with the next wave of showers.
Cannot totally discount a very isold TS Sunday afternoon, but
chances are far too remote to mention. Winds may also become highly
variable Sunday afternoon around any shower activity.

Southeast California/Southwest Arizona including KIPL and KBLH:
Gusty north winds will be the primary aviation concern Sunday behind
a cold front and departing weather system. The initial north winds
will settle over the region this evening, however tighter pressure
packing late tonight and Sunday morning will yield stronger gusts
(particularly at KBLH). By late Sunday afternoon, these wind gusts
should abate. Cigs around a 10K ft range tonight will fall to near
7K ft for a time Sunday afternoon, though confidence in coverage and
duration is limited.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday...
Upper low pressure feature that moved into the region on Sunday
will be slow to exit, however end most precip chances across the
eastern Districts. In the wake of the exiting system, winds will
be light and variable with subtle trends toward drainage slopes
and diurnal patterns. Dry northwest flow will linger for Tuesday
and Wednesday before another colder and more potent storm system
sweeps in from the northwest. Breezy west-northwest winds will
develop overnight Wednesday and into Thursday, impacting area
ridgetops first then mixing into the lower desert elevations
during the daytime. There could be some precipitation that
develops over Arizona with this next system, however forecast
model differences are really noticeable after Thursday with how
deep the system is and how long it sticks around the area. Daytime
humidities will be range higher than the last few weeks, 15 to 20
percent for the lower elevations and 30 percent or greater for the
higher terrain.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Wind Advisory from 11 PM this evening to noon PST Sunday for
     CAZ560-568.

&&

$$

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DISCUSSION...MO/Kuhlman/CB
AVIATION...MO
FIRE WEATHER...Nolte



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