Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/22/20

Area Forecast Discussion

National Weather Service Phoenix AZ

 454 PM MST Mon Dec 21 2020

.UPDATE...Updated 00Z Aviation Discussion.


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.SYNOPSIS...
With high pressure overhead, temperatures will be well above
normal today and possibly flirt with the record in Phoenix. A cool
down is expected Tuesday through Thursday as a weather system
brushes the region, bringing temperatures back down to near
seasonal normals. A few cold spots may see sub freezing
temperatures Thursday morning. Periods of high clouds are expected
through the week, particularly on Thursday. Heading into the
weekend, the weather will be quiet and typical for late December.

&&

.DISCUSSION...
The large scale weather pattern shows the large UL ridge across the
West CONUS with an elongated, meandering trough dominating the EPAC.
UA plots and the latest ACARS soundings indicate SE flow aloft with
a weak UL trough axis positioned across the AZ-MX border at H5.
There has been a slight uptick in moisture since yesterday with
local PW values hovering near about 0.3 in., although conditions
remain quite dry with extremely clear skies seen on midday satellite
imagery.

Earlier today low temperatures generally nudged up a few
degrees in most places to the upper 30s to low 40s although there
were still some lingering cold spots across the deserts that
observed freezing/subfreezing lows. Above normal highs today will
push into the low to mid 70s with a high of 76 expected at Phoenix
Sky Harbor, very close to the lowest record high for Phoenix of 77
degrees on this Winter Solstice day.

The ridge will migrate east into the Central Plains tonight and
tomorrow with a low pressure system trailing close behind falling
into the Rockies northeast of the region. As a result a cooling
trend beginning tomorrow/Tuesday will be reinforced starting Tuesday
night. Cyclonic flow develops aloft along the southwest flank of
the Central CONUS trough through midweek. Sufficient back-door
filtering of cold air will is likely take place with highs cooling
to the mid 60s and close to seasonable norms by Thursday, and
lows fall back into the 30s. With the lows in the 30s there is a
chance of some scattered cold spots developing with freezing to
subfreezing readings. However, the likelihood of increasing sky
cover and mostly cloudy conditions on Wednesday night and Thursday
may help mitigate some/most of the overnight freezing. Otherwise
periods of breezy northerly winds are also likely during the
midweek period along with a very minor increase in POPs in the
high country well to the east to northeast of Phoenix.

For late week Christmas Eve/Christmas day ensemble cluster tool
analysis provides good confidence on the reemerging prominent high
anomaly positioning north of the region and no regional
precipitation. There is improving confidence on the position of a
small low pressure disturbance encroaching from the west-southwest
towards/into the region below the high. No precipitation is still
strongly favored with very modest rain signals remaining low key with
forecast POPS remaining very low with this late week system.

In any case, the late week disturbance does usher in a pattern
change to a much more zonal/progressive one across the region for
the Holiday weekend period. The region will also have decent
chances of PWs maintaining values closer to 0.5.

For early next week there is slight relaxing of cluster
uncertainty on the longitudinal position of a low pressure anomaly
disturbance ranging from offshore CA to the Desert Southwest.
Modest QPF/precipitation and POPs are improving for the very
late weekend/post weekend period, however the very progressive
pattern still creates significant temporal and spacial system
positioning issues.

&&

.AVIATION...Updated at 2354Z.

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

Very light winds, 6 kts or less, will prevail at all terminals
through the next 24 hours. Wind directions will still favor
typical diurnal trends with extended periods of variability or
calm winds. FEW to SCT high clouds will pass over the area tonight
through Tuesday morning.

&&

.FIRE WEATHER...
Wednesday through Sunday:

Near normal temperatures and dry conditions will persist through
the forecast period under periods of high cloudiness. Humidity
values will bottom out Thursday, morning values in the 20-40
percent range and afternoon values near 10 percent, then gradually
increase through the weekend. Winds will mostly be light, with
exception of increased breezes on Wednesday. No rain is expected.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...Sawtelle
AVIATION...Benedict
FIRE WEATHER...Iniguez