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fxus65 kpsr 201207 
afdpsr

Area forecast discussion
National Weather Service Phoenix Arizona
507 am MST Sat Jan 20 2018

Update...
updated aviation and fire weather sections.

&&

Synopsis...
a notable storm system will move into the southwest today and into
Saturday. While it still does not look like a very wet system,it
will still bring scattered showers to the central deserts this
afternoon and evening and will result in increasing winds and a
significant drop in temperatures. High temperatures over the lower
deserts will be very cool today and Sunday with many areas in the
50s to around 60; starting next Monday temperatures will slowly
rebound as we move into the middle of next week. Morning low
temperatures will be rather cold early next week, especially Monday
morning, as the deserts drop into the 30s.

&&

Discussion...
at 2 am this morning, the deep and cold open wave Pacific trof was
bearing in on the Southern California coast; the surface cold front
had already moved into the far southeast California deserts and had just
passed through the Imperial Valley. Wind gusts to around 50 mph
occurred briefly in parts of el centro associated with the frontal
passage; due to the unexpected strong winds we issued a Wind
Advisory for southwest Imperial County thru 12z this morning. So far
it has been a very dry frontal passage and the surface radar mosaic
showed little if any echo over the western deserts. Infrared imagery
showed considerable Lee cirrus spreading across much of western Arizona
and far southeast California, with skies virtually clear over the Phoenix
area.

Operational guidance, including GFS and European model (ecmwf) runs, maintain good
consistency with the timing and track of the trof and associated
cold front and gefs ensemble members are pretty much all on the same
Page. Surface front should move into the central deserts late this
morning and pass through the Phoenix area by late morning to early
afternoon. The most significant precipitation will develop later
this morning and affect areas to the north and east of central
Phoenix this afternoon. One minor wrinkle; the system again appears
to be a bit more dynamic and moist than we previously thought; GFS
has raised its quantitative precipitation forecast and gefs plumes suggest many locations in the
Phoenix area may pick up one tenth of an inch or more with this
system. Mex/mra pops for Phoenix area are also higher and now in the
50 percent ballpark for today. Naefs pops are a bit higher as well,
especially east of Phoenix where they call for categorical rain/snow
this afternoon. We have raised our pops a bit with values rising
over 70 percent across the higher terrain east of Phoenix, and into
the 30-40 percent ballpark over the Phoenix Metro. Snow level lowers
to around 4500 feet this afternoon, then below 4000 feet tonight
across south-central Arizona but given the low quantitative precipitation forecast we do not expect
significant new snow accumulations across the higher terrain east of
Phoenix.

Although the main cold front passes quickly east and into New Mexico
this afternoon, a secondary short wave/vort lobe will rotate around
the back side of the departing trof and lead to a continued threat
for scattered showers and higher elevation snow from Phoenix
eastward this evening. Pops have been raised overnight, with Phoenix
staying in the 20-30 percent range this evening. Given that we will
probably see more cloud cover overnight, and thus less radiational
cooling, low temps Sunday morning have been raised a few degrees and
the low in Phoenix is now expected to be about 41 degrees. It now
appears that the coldest morning with this system will actually be
Monday morning, when many of the lower deserts fall into the 30s,
and the outlying deserts drop to around freezing. For now we still
do not expect to issue a freeze warning due to temps not quite cold
enough, and readings near freezing not quite widespread enough.

Otherwise, for Max temps, today still looks to be very cool with
most of the lower deserts falling into the mid 50s to low 60s. Still
going with a high in Phoenix today of just 59 degrees, nearly 10
degrees below seasonal normals. Very little change is expected on
Sunday, as Phoenix should still struggle to reach 60 degrees under
mostly sunny skies.

For Monday into the middle of the week, the weather pattern still
looks to be dry with periods of high clouds passing through the
area. A gradual warming trend will take place as the Desert
Southwest comes under a dry northwest flow aloft with weak high
pressure building in overhead. High temps in Phoenix should finally
reach to around 70 on Tuesday and then climb into the low to
possibly the middle 70s Wednesday into Thursday. Guidance still
suggests that a deep and cold upper trof may develop over the
western Continental U.S. By late next week. This still looks to be dry for our
area and we only have some single digit pops in place Thursday into
Friday, but the biggest impact may again be much cooler temperatures
developing by next Friday. For now we have lowered our Friday Max
temps into the mid 60s over the central deserts; time will tell if
we need some additional cooling.

&&

Aviation...
south-central Arizona including kphx, kiwa, and ksdl:

A cold front over southwest Arizona and extending northeastward across
north-central Arizona (and beyond) as of 12z will continue moving
eastward as the day progresses. Anticipate the front to track
across Metro Phoenix in the 14z through 18z time frame bringing a
wind shift from southeast to southwest (reaching phx between 15z-
16z). Gusts look to be modest with the front (10-20 kts). It will
be accompanied by stratocumulus with bases of fl060-080 along with
scattered showers for a few hours. Can't rule out phx ceiling
touching bkn050 in a shower but confidence too low to show that in
the taf. Anticipate some temporary thinning of clouds in the
afternoon with more frequent gusts of 20-25 kts. Cloud bases will
lower again after 00z this evening with isolated showers but
anticipate they will remain above bkn050. Expect more numerous
showers and lower cigs over higher terrain north and east of Metro
Phoenix (higher mountain tops obscured). Anticipate clearing from
west to east beginning after 06z.



Southeast California/southwest Arizona including kipl and kblh:

Very strong west winds over Imperial County will decrease markedly
by 14z this morning. Elsewhere, winds haven't been nearly as
strong (gusting 20-25 kts). Anticipate strengthening of westerly
winds after 18z with gusts of 25-30 kts common. Local blowing dust
is possible but too isolated to depict in tafs. Most areas will
have cloud bases at or above fl100 except for La Paz County which will
have some local ceilings of fl070-090 with isolated showers until
about 19z.



Aviation discussion not updated for amended tafs.

&&

Fire weather...
Monday through friday:
a warming trend can be expected Monday through Thursday with lower
deserts highs climbing well into the 70s again Wed-Thu. Humidities
trend down early in the workweek but overnight recovery remains
good. Winds will generally be light and favor north and
northeasterly directions with some local breeziness Tuesday. A dry
weather system moves through late in the workweek for breezy
conditions (starting thursday) and cooler temperatures by Friday.

&&

Spotter information statement...
spotters should follow Standard reporting procedures.

&&

Psr watches/warnings/advisories...
Arizona...none.
California...Wind Advisory until 4 am PST early this morning for caz562-566-
567.

&&

$$

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