Hard Freeze Warning 3, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
905 PM MST Mon Nov 28 2016

.SYNOPSIS...A few light valley rain and mountain snow showers will
occur mainly from Tucson eastward into Tuesday morning. Much cooler
temperatures and the coldest lows of the season so far Tuesday night
and Wednesday night, with widespread freeze or hard freeze
conditions for Southeast Arizona. Temperatures will remain below
average through the week, and another storm system may bring a
chance of valley rain and mountain snow Friday into the weekend.

&&

.DISCUSSION...IR satellite imagery depicts mostly cloudy to cloudy
skies across southeast Arizona this evening, with broken to overcast
mostly lower level clouds as per several surface observations valid
8 pm MST. A broad upper trough extended from the intermountain west
eastward into central CONUS plains. A speed max/vort lobe was
embedded within the southwestern periphery of this upper trough as
per the 29/00Z upper air plots. IR satellite imagery also depicted a
distinct back edge to the lower level clouds, with clearing skies or
mostly clear skies from KPHX northwestward into southern Nevada.

A linear extrapolation of this back edge of lower clouds feature
would suggest that clearing skies would encroach upon the Tucson
metro area within a few hours, or perhaps around 11 pm - midnight or
such. However, some hi-res models such as the HRRR and the 29/00Z
Univ of AZ WRF-NAM suggests a potential for at least isolated valley
rain/mountain snow showers from near Tucson eastward to the New
Mexico into Tuesday morning.

Thus, am inclined to make no changes to the inherited official
forecast at this time. Expect isolated to scattered valley rain/
mountain snow showers into Tuesday morning. The exception will be
across western Pima County where precip-free conditions are
expected. Any rain amounts at lower elevations will be quite light,
or mostly less than 0.10". Snow accumulations (as noted in the
Previous Discussion) should only be from 1-2 inches or so above
5000 feet.

A few lingering showers may occur Tuesday afternoon across southern
Cochise County. Clear skies or mostly clear skies will then prevail
Tuesday night into Wednesday morning, setting the stage for the
coldest night of the season thus far. A Freeze Watch and a Hard
Freeze Watch will continue from late Tuesday night through Wednesday
morning. Please refer to AWIPS product PHXNPWTWC - WMO header WWUS75
KTWC for more information.

&&

.AVIATION...Valid through 30/06Z.
Expect isolated to scattered -SHRA and mountain -SHSN mainly KTUS
eastward to the New Mexico border into Tuesday morning. MVFR
conditions will occur especially across higher terrain. A few
lingering -SHRA mostly from KFHU-KDUG Tuesday afternoon, then
clearing skies Tuesday evening. Surface wind east/south of KTUS
wly/nwly at 10-18 kts with gusts to 25 kts. Surface wind KTUS
vicinity westward generally less than 10 kts thru the period.
Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...A few valley rain and mountain snow showers will
occur from Tucson eastward into Tuesday morning, then a few showers
across southern Cochise County Tuesday afternoon. Dry conditions
will then occur Wednesday into Thursday. A low pressure system may
then bring more wintry weather and gusty east winds Friday into
this weekend.

&&

.PREV DISCUSSION /145 PM MST/...Latest water vapor imagery shows
northwesterly flow aloft across the forecast area with one final
disturbance near the CA/NV border moving southeast. There is still
considerable low to mid level clouds north of Tucson and partly
sunny conditions with mainly cirroform clouds for the remainder of
the forecast area. Latest model trends have generally backed off a
bit on this final moisture starved system tonight into Tuesday
morning. Still looking at a few showers and mountain snow showers
starting as early as this afternoon across the White Mountains, then
continuing tonight into Tuesday morning, mainly from Tucson north
and eastward. Another inch or two of snow is possible in the
mountains from Tucson north and east with snow levels will be around
5000 feet. This system will also keep temperatures from crashing
overnight.

The aforementioned disturbance will move east of the area Tuesday
morning with any residual chance of showers coming to an end. With
skies clearing through the day from west to east, this will set the
stage for the coldest night of the season for Wednesday morning. A
freeze watch remains in effect for most desert locations from Tucson
westward except the far western deserts near Ajo and Organ Pipe
National Monument. A hard freeze watch is also in effect for much of
Santa Cruz County and the higher elevations of South Central Pima
County including the Arivaca and Sasabe areas which haven't had a
widespread freeze yet. The eastern valleys of Graham/Greenlee/
Cochise counties will also see a hard freeze but these locations
have had a widespread freeze already this season so will not issue
any headlines there.

Fairly benign and continued cool on Wednesday and Thursday with dry
westerly flow aloft. Thursday morning will be rather cold once again
but likely a couple of degrees warmer than Wednesday morning. By
Friday into the weekend our attention focuses toward a closed low
that will move south from the Great Basin into Arizona and then
possibly northern Mexico. Still lots of uncertainty with this system
as the ECMWF has been depicting a much more progressive and open
wave solution compared to the GFS/CMC for the past couple of runs.
Will lean towards the GFS/CMC and GEFS ensemble mean which is
somewhat slower and more closed off than the ECMWF. Regarding
sensible weather, will maintain slight chance to low end chance PoPs
starting Friday, then continuing into Saturday before winding down
on Sunday. Early indications are that snow levels will be about 5000
feet with this system.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Freeze Watch from late Tuesday night through Wednesday morning for
AZZ502-504>506.

Hard Freeze Watch from late Tuesday night through Wednesday
morning for AZZ503.

&&

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