Freeze Warning 1, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
845 PM MST Wed Dec 6 2017

.UPDATE...updated aviation discussion

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.SYNOPSIS...
Expect sunny skies this afternoon with lingering breeziness. Tonight
and Thursday, a dry cold front from the north pushes through the
region bringing significantly stronger winds. It will also lead to
cooler temperatures. In fact, desert highs Thursday and Friday will
be in the mid to upper 60s. The coldest low temperatures of the
season so far can be expected Friday morning and Saturday morning.
Thereafter, a warming trend can be expected with above normal
temperatures returning over the weekend and into next week.

&&

.DISCUSSION...
At 2 am this afternoon, the elongated east-to-west oriented short
wave had pushed off to the southeast allowing a drier and more
subsident northerly flow to overspread the area. IR imagery showed
sunny skies across the lower deserts and temperatures were running
around 5 to 15 degrees ahead of this time 24 hours ago. Subsidence
aloft, sunshine and northeast downslope effects all played a role in
the warmer temperatures today. Dewpoints were also dropping fast
with many locations in the central deserts down into the teens;
Blythe had dropped to just 10 at 1 am. The drier air that has been
ushered in will help to cause low temperatures over the deserts to
fall off nicely over the next few days given increased radiational
cooling potential: the greater Phoenix area should see most
locations in the 40s tonight with even colder readings on tap for
Friday morning. Low temperatures over portions of the central
deserts will probably remain a bit elevated tonight due to gusty
northeast winds that will affect the area during the nighttime hours.

The high amplitude upper ridge developing along the west coast will
help to drive another short wave down the front side of the ridge
and into the interior west tonight into Thursday and as this occurs
a dry cold front will push across the area resulting in considerably
cooler high temperatures tomorrow. Central desert highs will fall
into the low to mid 60s and surface dewpoints will fall into the
single digits over most of the area under sunny skies. During the
day on Thursday, we will continue to see gusty north to northeast
winds pretty much area wide with some of the stronger gusts located
across higher terrain/ridgetop areas of far southeast CA as well as
the higher terrain of southern Gila County. Winds may reach advisory
levels for a few hours but overall conditions look borderline for a
wind advisory and we will opt to message the winds instead. Gusty
winds and very low RH (due to single digit dewpoints) will also
result in pockets of dangerous fire weather conditions but again
coverage of red flag conditions will be too borderline to warrant a
watch or red flag warning.

Friday through Tuesday...
Expect the coldest lows of the season so far to be Friday morning.
Anticipate most of the lower deserts will dip into 30s. As such, we
have issued a Freeze Watch for portions of southern Gila County
Friday morning, mainly the area around Globe as well as areas
east/southeast of Globe. This will match up nicely with the Freeze
Watch that Tucson has issued for the same time.

Thereafter, the coastal ridge expands inland and a warming trend
begins with highs on the deserts climbing well into the 70s
again. Winds will be much lighter. Early next workweek, we may see
an increase in high clouds as an upper low (within the southern
branch flow undercutting the ridge) advances toward the southern
CA and Baja Mexico coast. Wouldn't be impossible to see some
precip from that feature but even the generous NAEFS only briefly
reaches low double digit PoPs Tuesday. Otherwise it stays single
digits. Calibrated GEFS guidance was not available. Not enough to
go on at this point in order to introduce mention of precip.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:

Breezy northeast winds are forecast to continue for the next 24
hours as a dry weather system slides south over our area. However,
a few forecast models indicate that the NE surface flow may once
again switch to a light W/SW return flow tonight at KPHX making
for a difficult forecast. If and when this may happen is highly
uncertain but will continue to update the TAF as need be.
Regardless, breezy northeast winds should return to all locations
tomorrow morning and persist through the late afternoon. Skies
will remain clear.

Southeast California/Southwest Arizona including KIPL and KBLH:

Breezy northerly flow will continue for the next 24 hours with
winds slightly weaker on Friday. Once again, the strongest winds
will be focused at KBLH with gusts 25-30kts in the morning
decreasing to around 20-25 kts late in the afternoon. Skies will
remain clear at TAF sites for at least the next 24 hours.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday: The drying trend will continue with
minimum relative humidities dropping to near 15 percent for the
driest locations but contrasted with fair overnight recoveries.
The strongest winds will be felt along the ridgetops and other
higher terrain but will generally be on the lighter side outside
of a few afternoon gusts. A few locations may temporarily flirt
with critical fire weather conditions but these moments should be
fairly brief. Otherwise, temperatures will slowly increase a few
degrees and remain above the seasonal average.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Freeze Watch from Thursday evening through Friday morning for
     AZZ560-562-563.

CA...None.
&&

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DISCUSSION...AJ/CB
AVIATION...Wilson
FIRE WEATHER...Deemer