Freeze Warning 2, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
405 AM MST Mon Feb 3 2020

.SYNOPSIS...
As a cold winter weather system moves into the area, an abrupt
change will take place today as the associated cold front sweeps
through the region, lowering high temperatures into the 50s. There
will be very little precipitation with the front, with just isolated
to scattered rain and high elevation snow showers over southern Gila
County this afternoon into the evening, with just partly cloudy
skies expected over the deserts. Well below normal temperatures will
last through Wednesday with overnight lows around freezing tonight
and Tuesday night. Increasing high pressure during the latter half
of next week should push daytime high temperatures back to normal by
Thursday or Friday.

&&

.DISCUSSION...

A deep, very cold and positive tilt upper level trof continued to
develop across the western CONUS early this morning, with the main
upper low over Utah shifting towards the south/southeast. the
associated cold front was pushing into far southeast California and
is expected to race rather quickly and progressively east across the
Arizona deserts today with the front moving into the Phoenix area
during late morning/early afternoon. Operational and ensemble
guidance, including GEFS/NAEFS/CMC output continue to be in very
good agreement with respect to the large scale pattern including
track and timing of the large upper trof. One thing that guidance
has been strongly suggesting recently is that this will end up being
a VERY dry system in terms of any rain or snow, and the only
significant impacts will be very cold temperatures (including
freezing temps over the deserts Tuesday and Wednesday) as well as
strong gusty winds over the western deserts today into Tuesday.

Latest GEFS plumes has now flat-lined with all members running at
0.0 inches of rain in Phoenix for the entire week. Latest WPC QPF
forecasts have basically pulled all measurable rain out of the CWA
except for a small amount over southern Gila county this afternoon
mostly east of Globe. As such POPs continue to be trimmed back with
near zero values in the Phoenix area. Given the very meager amounts
of QPF expected in southern Gila county this afternoon into the
evening, any snow that falls will be light and generally less than
one inch. Part of the reason behind the limited precipitation is the
very dry air that spills into the deserts behind the cold front;
dewpoints are forecast to plummet into the single digits today and
remain in the single digits or low teens through at least Tuesday
afternoon.

Behind the cold front today, strong gusty northwest to north winds
will continue over the western deserts, including favored higher
terrain areas of far southeast California including Joshua Tree NP.
A Wind Advisory will remain in place for much of eastern Riverside
County through 4 pm this afternoon. Gusty north flow behind the
exiting trof persists through Tuesday and we have issued another
Wind Advisory for similar areas for much of the day on Tuesday.
Patchy blowing dust and sand are in the forecast for both days
associated with the stronger wind gusts across the lower elevations.

As previously mentioned, the biggest impact with this system will be
the very cold temperatures. We expect afternoon highs to fall into
the 50s today through Wednesday with the coldest temps likely to be
on Wednesday as Phoenix only reaches to 54 degrees. Very cold low
temperatures are a good bet for both Tuesday and Wednesday mornings
with the coldest lows likely to be on Wednesday. Based on guidance
from the WR situational awareness table, NAEFS percentiles for 850mb
temps on Tuesday put most of our lower deserts below the 1st
percentile relative to climatology, with similarly low values on
Wednesday morning. 850mb temps generally are forecast to be in the
minus 2-3C range, and thickness values plunge well below 540dm both
mornings across the Arizona lower deserts. There are a couple of
mitigating factors that could limit the freeze potential, one of
which is the possibility of lingering breeziness (or at least winds
up to around 10 mph). There may also be some high clouds at times
during the morning hours. Still, the potential looks to significant
enough to warrant replacing our Freeze Watch products with Freeze
and Hard Freeze Warnings. On Tuesday we will leave most of the
greater Phoenix area out of the warning, but simply message the
borderline freeze potential. Most of the zones on Tuesday will be
those bordering Tucson and Las Vegas. We will have a Hard Freeze
Warning both Tuesday and Wednesday for the Globe/San Carlos areas in
part to blend with the Tucson forecasts. Wednesday may still be the
coldest morning and most of the Arizona lower deserts will be in a
Freeze or Hard Freeze Warning status, including the greater Phoenix
area.

As the cold upper trof continues to slide off to the east later in
the week we can expect very dry conditions as high pressure aloft
starts to build into the area from the west. A steady warming trend
will be in the picture Wednesday into the weekend under generally
sunny days and clear nights. By Thursday afternoon highs in the
Phoenix area will climb into the low 60s and by the weekend we can
expect highs in the low to mid 70s. As we move through the day on
Sunday and into early next week there is the potential for another
precipitation producing upper trof/closed low to settle into the
desert southwest but any rain chances of consequence will likely not
occur until next Monday. Sunday will see an increase in mid/high
clouds ahead of the developing trof.

&&

.AVIATION...Updated at 0525Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Veering winds with a frontal passage and the potential short 2-3
hour period of southerly cross runway winds during the high traffic
period of Monday morning will be the primary aviation impacts. E/SE
sfc winds prevail through the night, however above the boundary
layer, SW winds will strengthen resulting in marginal LLWS
conditions late tonight. In reality, the actual abrupt
speed/direction change may stay above the 2000kt AGL threshold.

The influence of a quick moving cold front may cause a winds to veer
to a due southerly direction for a couple hours between sunrise and
mid morning Monday, with speeds possibly reaching around 10kt
causing problems with runway configurations. By late Monday morning,
winds should fully veer to a W/SW direction; and increased
afternoon mixing should allow 20-25kt gusts to be common throughout
all terminal locations. Only brief clouds above 7K ft are possible
with any cigs remaining above 20K ft. Wind speeds will decrease in
speed with loss of any gusts Monday evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
Increasing strong wind gusts and very localized reduced visibilities
due to blowing dust will be the greatest aviation impacts through
Monday afternoon. A front will push through terminal late tonight
shifting winds to a W/NW direction. A period of gusts above 30kt is
possible creating the potential for BLDU visibility restrictions.
Slantwise visibilities may be more impacted.

KIPL may see a period of cigs below 6K ft Monday morning with
confidence in a moderate category. Otherwise, gusty winds will
persist into the afternoon for both terminal sites before gradually
weakening late Monday afternoon and evening.

&&

.FIRE WEATHER...
Wednesday through Sunday: The cold weather system will slowly exit
the region on Wednesday keeping temperatures below normal through at
least Thursday. A warming trend will allow high temperatures to
approach seasonal normal Friday and then climb into the 70s over the
weekend. Winds will still be a bit breezy on Wednesday, but mostly
in higher terrain areas. Light winds are then seen late in the week
into the first part of next weekend. Minimum RH values start low on
Wednesday, ranging from single digits in southeast California to the
mid teens in the central Arizona deserts. The minimum RHs will climb
steadily through the week and into the weekend, running from mostly
15-20% across the lower deserts to 25-30% over Gila County through
Saturday and climbing into the 20 to 30 percent ballpark over the
deserts on Sunday under partly to mostly cloudy skies.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...Freeze Warning from midnight Tuesday night to 9 AM MST Wednesday
     for AZZ533>544-546>552-554>556-559.

     Freeze Warning from midnight tonight to 9 AM MST Tuesday for
     AZZ533-534-547-551>556-559-561.

     Hard Freeze Warning from midnight tonight to 9 AM MST Tuesday
     for AZZ560-562.

     Hard Freeze Warning from midnight Tuesday night to 9 AM MST
     Wednesday for AZZ553-560>562.

CA...Wind Advisory until 4 PM PST this afternoon for CAZ560-561-564-
     568-570.

     Wind Advisory from 7 AM to 7 PM PST Tuesday for CAZ560-561-564-
     568-570.

     High Wind Warning until 7 AM PST this morning for CAZ562.

     Wind Advisory until 5 AM PST early this morning for CAZ566-567.

&&

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