Freeze Warning 4, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
1108 AM MST Wed Dec 26 2018

.UPDATE...Updated Aviation discussion.

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.SYNOPSIS...
A low pressure system will move through the area today with
isolated showers mainly across the higher terrain east of
Phoenix. Cooler air will be ushered in behind this system
today resulting in a prolonged period of below normal
temperatures. Another storm system will produce light snowfall
across the higher elevations Thursday and Thursday night. Even
colder air behind this system will overspread the Desert Southwest
and persist through early next week. The coldest temperatures of
the season are likely this weekend with freezing conditions
possible across portions of the area.

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.DISCUSSION...
Early morning satellite shows the cold core low centered over
northern Sonora as it begins to lift to the northeast. The MRMS
shows a some scattered light showers around the state with the
next vort max still south of Tucson. The main axis of
precipitation extends from northeastern Sonora north through
eastern Arizona. Northwesterly flow in the midlevels will continue
to push behind the system along with a more subsident regime
across the western half of the state leading to decreasing cloud
cover and very little chance for additional rainfall. Any
lingering shower activity later today will most likely be
confined to the higher terrain well north and east of Phoenix.

Northwesterly flow aloft will continue for the next few days as
another system drops into Arizona. As with the Christmas Day system,
moisture will be limited while the best atmospheric dynamics
favorable for showers, as of now, look more likely to graze the
eastern side of the state resulting in limited PoPs for the
Phoenix area and places further west. However, the big weather
impact will be the much colder temperatures beginning Thursday
morning and lasting through the weekend. The colder, more
continental dry air mass will cool the atmospheric profile several
degrees from current conditions. By Saturday, the 700 mb and 850
mb temperatures will drop below the 10th percentile of climatology
for this time of year translating to the coldest surface
temperatures so far this winter. Morning lows will near the
freezing mark for many Valley locations (suburbs and exurbs) and
significantly colder elsewhere, such as Casa Grande and Globe.
While Globe and other higher elevations (3500 ft) may see freezing
temperatures by Thursday, there is a concern the area could reach
a hard freeze status with temperatures in the mid 20's. Daytime
highs will remain below seasonal normals.

Temperatures will warm gradually through the early part of the
work week, eventually approaching normal. However, model guidance
into next week remains inconsistent and variable offering very
little certainty come the new year.

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.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
A dry and subsident airmass will continue to build across Arizona
in the wake of a low pressure system across New Mexico. However,
some low-level moisture has lingered, which is resulting in areas
of lower clouds near the Mazatzal Mountains northeast of Phoenix.
At the TAF sites, scattered stratocu with bases as low as 6-8k ft
will persist into the afternoon, followed by a clearing trend
after sunset. Otherwise, winds will generally remain light and
follow typical, diurnal patterns.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds will generally be out of the northwest this afternoon in the
wake of a low pressure system that has moved out of the area.
Occasional gusts to 20 kt are possible at KBLH, while weaker winds
are anticipated at KIPL. Winds will dissipate after sunset and
will generally remain light and diurnal through Thursday.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday: Another large and cold area of low pressure
will develop across the desert southwest Thursday night into Friday
allowing for isolated to scattered showers of rain and higher
elevation mountain snow to develop across south-central Arizona. The
most significant showers will affect higher terrain areas northeast
of Phoenix. The showers will diminish Friday evening before ending
with dry conditions expected Saturday through Tuesday.

We can expect very cool high temperatures throughout the period with
most of the lower deserts falling into the 50s each day starting
Thursday. Humidity levels will also stay highly elevated each day
with minimum RH values mostly in excess of 30 percent. From Thursday
into the weekend, winds will stay mostly on the light side with
local breeziness during afternoon hours, favoring west to north and
strongest over deserts west of Maricopa County. Due to persistent
cool temperatures, high humidity, and no overly strong winds, fire
weather concerns will be minimal during the 5 day period.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Freeze Watch from late Friday night through Saturday morning for
     AZZ530-533-534-536-547-552>554-557.

     Hard Freeze Watch from late Friday night through Saturday
     morning for AZZ556-560>562.

     Freeze Watch from late tonight through Thursday morning for
     AZZ562.

CA...Freeze Watch from Friday evening through Saturday morning for
     CAZ560.

     Wind Advisory until 10 AM PST this morning for CAZ560.

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DISCUSSION...Deemer
AVIATION...Hirsch
FIRE WEATHER...CB