Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/21/20

Area Forecast Discussion

National Weather Service Phoenix AZ

 436 PM MST Sun Dec 20 2020

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
High pressure will build across the Desert Southwest today into
Monday, resulting in a decent warming trend. Temperatures will
warm into the low to mid 70s on Monday for the lower deserts with
a near record high possible for Phoenix. Cooler, drier and breezy
conditions are likely on Wednesday with temperatures returning to
seasonal normals and lasting into next weekend.

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A large West Coast ridge dominates the large scale weather
pattern with an upper level trough positioned well off to the
east. Upper level plots and local ACARS soundings indicate
northerly component flow aloft with a very dry airmass in place
associated with the prominent ridge. Recent local soundings also
confirm the dryness with PWs only near 0.20 in. Earlier today it
was another rather chilly morning and the third straight morning
that Phoenix Sky Harbor reported a low of exactly 40 degrees, with
most city and lower desert locales again reporting lows in the
low to upper 30s.

The ridge will build and shift inland over the Western US through
early this week. The warming trend that began Friday will
continue today through tomorrow/Monday when a high of 75 degrees
is expected for Phoenix. This would be only 2 degrees shy of the
record high for tomorrow of 77 degrees--which also happens to be
the lowest record high temperature of the entire year for the
Phoenix Area. Fortunately, getting near this Winter time record
high in Phoenix does not produce any heat risk to speak of.

A cooling trend beginning Tuesday will receive a boost Tuesday
night to Wednesday morning as a low pressure system drops through
the Rockies northeast of the region. As a result cyclonic flow
develops aloft along the southwest flank of the Central CONUS
trough. Enough back-door filtering of cold air will probably occur
midweek to see highs cool to the mid 60s and close to seasonable
norms by Wednesday, as lows fall back into the 30s, including some
scattered freezing to subfreezing readings for the lower deserts
and the city. 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 the second half of the holiday week ensemble cluster tool
analysis provides good confidence on the prominent high anomaly
positioning north of the region and no regional precipitation.
Although there is less confidence on the exact position of a
relatively modest low pressure disturbance trying to slide in from
the west-southwest towards/into the region below the high. For
the weekend there is plenty of cluster uncertainty on the position
of a low pressure anomaly disturbance ranging from offshore CA to
the Desert Southwest. No precipitation is still strongly favored
with very modest rain being much less favored. Increases in cloud
cover and PW beginning the second half of the week have
moderate/decent confidence, along with highs in the upper 60s late
this week and into the weekend.

&&

.AVIATION...Updated at 2345Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
There are no aviation weather concerns for the rest of tonight as
wind speeds remain light, AOB 7 kts, while directions follow
typical diurnal trends. For tomorrow morning, there could be
periods of low level wind shear at a more northeasterly direction
and speeds near 20-30 kts up to 2k feet above the surface.
However, confidence for this occurring and meeting TAF criteria
are too low to mention outside of this discussion at this time. By
tomorrow afternoon, there is also uncertainty as to whether or
not the westerly winds will develop with some thought that
easterly winds could prevail all day, albeit, at speeds at or
below 7 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:
There are no aviation weather concerns as wind speeds will remain
light AOB 6 kts through the TAF period. At KIPL, the westerly
winds this evening will prevail until the early morning when
directions are most likely to become easterly. At KBLH, the
westerly winds this evening will become northerly again a few
hours before sunrise.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Near normal temperatures and dry conditions will persist through
the forecast period under periods of high cloudiness. Minimum RHs
will generally fall into a 10-20% range while overnight max RHs
mostly top out between 25-45%. Winds will be light the bulk of the
period, but north to northeasterly breezy to locally windy
conditions are expected across the Lower CO River Valley and
portions of central Arizona Wednesday into early Thursday. Wind
gusts to 30 mph will be possible throughout the Lower CO River
Valley and up to 25 mph across south-central and eastern Arizona.
The combination of dry and locally windy conditions on Wednesday
will likely promote an enhanced fire danger across the Lower CO
River Valley.

&&

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

&&

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

$$

DISCUSSION...Sawtelle
AVIATION...AD
FIRE WEATHER...Kuhlman