Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 01/01/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 516 PM MST Thu Dec 31 2020

.UPDATE...Updated Aviation discussion.

&&

.SYNOPSIS...
Dry conditions and slightly below normal temperatures are
expected through the middle of this weekend. Low temperatures
will dip into the 30s for many lower desert locations each night
through Sunday. A dry weather system moving through the region
today will clear out during the day Friday with trailing breezy
northerly winds along the Lower Colorado River Valley. Increasing
high pressure this weekend will result in warming temperatures,
with highs returning to near normal. A few weather disturbances
next week may bring chances for precipitation to the high terrain
to the north, but the lower deserts look to remain dry.

&&

.DISCUSSION...
A broad shield of thick mid-level clouds have developed over much
of AZ, downstream of a positively tilted upper level low seen
over southeast CA on satellite early this afternoon. Besides some
light virga echoes on radar, and possibly high elevation light
snow showers in southern Gila county, no precipitation is expected
over the lower deserts. Recent aircraft soundings out of Phoenix
show a very dry sub cloud layer. The clouds, and minor cooling
aloft, should keep temperatures this afternoon closer to 60
degrees. HREF keeps the clouds around through early Friday
morning over south-central AZ. This should help moderate
temperatures and keep Friday morning lows a couple degrees warmer
than this morning, but still quite chilly with most desert areas
in the upper 30s to low 40s.

As the upper low continues to slide southeast across the area, a
30-35 kt northerly 925 mb wind will develop, after sunset this
evening, over southeast CA and along the Lower Colorado River
Valley. Given the timing, stronger surface winds will be limited
mainly to the ridge lines through the overnight hours, but surface
gusts up to 25-30 mph will be possible after sunrise Friday and
through the early afternoon. The gradient will relax and winds
will subside by Friday evening as the upper low continues pushing
east-southeast through northern Mexico.

Friday into Saturday will see the progressive pattern shift a bit
farther to the north allowing a dry and somewhat zonal west
northwesterly flow to set up across the Southwestern U.S.
Generally clear skies are expected for much of Friday and
Saturday, but very little warming will be realized under the zonal
flow, keeping high temperatures a couple degrees below normals.
Overnight lows will continue to be quite chilly each night through
the weekend with the majority of the lower deserts dipping into
the 30s each night and the typical colder spots likely near
freezing.

A more active weather pattern for the Southwestern U.S. still
looks likely for next week with three potential upper level
troughs. However, trends keep these potential troughs as quick-
moving progressive systems and recent ensemble runs are favoring
tracks north of our CWA, more towards northern AZ and Utah with
mainly west-east trajectories. Low pressure systems that follow
this pattern typically lose a good deal of any modest moisture
they may have while traversing over the Sierra mountains. The most
likely scenario for the first system moving through on Tuesday is
for it to be mostly dry, generally only providing a chance of
light showers over higher terrain areas. Latest ensemble runs have
trended drier and weaker with the second system during the
Thursday time-frame. The 12Z GEFS and ECMWF ensemble are now
nearly completely dry for the lower deserts through next week.
NBM temperature guidance is also reflecting the weaker troughs,
keeping forecast highs right around seasonal normals through next
week.

&&

.AVIATION...Updated at 0015Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Mid clouds (with bases near 12k ft) associated with a low pressure
system will gradually move eastward through central Arizona tonight,
followed by a clearing trend Friday morning. Otherwise, winds will
remain light with variable directions at times through Friday
evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

A clearing trend will continue this evening as a low pressure system
moves gradually eastward through Arizona. Otherwise, northerly winds
of 10 to 15 kt will subside at KBLH tonight. Another period of
breezy conditions is likely by late Friday morning. Winds will then
abate by mid-late afternoon.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Dry conditions are expected at least through the weekend with
temperatures remaining at or slightly below normal. Humidity
levels will be fairly stable with minimum RHs across the deserts
typically in the 15 to 25 percent range each day. Many lower
deserts will see humidities climb into the 45 to 60 percent range
each night. Winds will mostly be light through the period with
the exception of some northerly breeziness on Tuesday as a mostly
dry weather system moves through the region.

&&

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

&&

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

$$

DISCUSSION...Kuhlman
AVIATION...Hirsch
FIRE WEATHER...Kuhlman