955 
FXUS65 KPSR 221112
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
410 AM MST Fri Dec 22 2017

...Updated Aviation...

.SYNOPSIS...
A warming trend will begin today, though below normal 
temperatures will persist across the region. High pressure
building across the Western states will result in a return to
above normal temperatures Saturday and will continue through next
week.

&&

.DISCUSSION...
Latest water vapor imagery shows the vort max that produced the 
breezy and chilly conditions Thursday continues to weaken while 
becoming elongated east of the Four Corners. With the mid-level 
trough axis already located east of the area and moving steadily 
eastward, thickness rises will prevail today resulting in a 
warming trend. However, in the wake of the trough, some of the 
coldest air of the season is currently affecting the Desert 
Southwest. Several hours of sub-freezing conditions are likely 
across portions of La Paz County, Pinal County and Gila County 
where a Freeze Warning remains in effect for this morning. 

With conditions starting out relatively cool this morning, 
temperatures will struggle to reach the 60 degree mark across the 
lower deserts this afternoon, given the lack of late-December 
insolation. Below-normal temperatures are expected again tonight 
and the latest blended guidance indicates that spots along the 
Colorado River Valley including Parker and Blythe may see a few 
hours of sub-freezing temperatures, due to the lighter winds and more
favorable conditions for radiational cooling.

A more discernible warming trend is expected by Saturday as the
eastern Pacific ridge migrates eastward. Positive height 
anomalies will return to the region, heralding a period of 
westerly flow and benign weather along with above normal 
temperatures through at least Wednesday. Thereafter, operational 
models diverge with the latest GFS suggesting a short wave trough 
diving southward through the southern Rockies will bring a slight 
cooling trend later in the week. Considerable uncertainty is 
evident in the ECMWF ensembles and the official forecast 
represents a consensus, with temperatures generally dropping back 
a few degrees but remaining above normal. However, this pattern is
not favorable for significant moisture transport and 
precipitation is unlikely the remainder of the year.

&&

.AVIATION... 
South-Central Arizona including KPHX, KIWA, and KSDL; and
Southeast California/Southwest Arizona including KIPL and KBLH: 

No major aviation concerns through Saturday morning under mostly 
clear skies. Winds will be quite variable through the period, but 
will generally favor typical directional patterns around the Phoenix 
metro. Southeast California terminals will tend towards a NW 
component. All areas will experience periods of nearly calm 
conditions. 

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Temperatures will steadily rise through early next week as high 
pressure builds over our area with above normal temperatures 
returning by Sunday. Afternoon humidity minimums will range 
between 15-25% for most days while overnight recoveries will 
generally be poor to fair as dry air remains in place. Winds now 
look to remain generally light which will help keep fire danger to
a minimum despite the relatively dry conditions. The one 
exception may be that locally breezy conditions could develop 
Saturday night into Sunday mainly over elevated locations of 
south- central AZ. No precipitation is expected through the 
forecast period.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Freeze Warning until 9 AM MST this morning for AZZ533-552-553-
     556>558-560>562.

CA...None.

&&

$$

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DISCUSSION...Hirsch
AVIATION...MO
FIRE WEATHER...Wilson