Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/22/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 449 PM MST Tue Dec 21 2021

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
High pressure over the region will lead to slightly above normal
temperatures through the middle part of the week, but with
considerable cloudiness each day. A slow moving Pacific weather
system off the West Coast will continue to feed moisture and
clouds into the region through the holiday weekend, while also
providing possible chances for rain by Thursday. An unsettled
weather pattern is likely to continue through the upcoming weekend
with temperatures cooling off slightly and additional chances for
precipitation.

&&

.DISCUSSION...
Temperatures this morning were several degrees warmer than
previous mornings, with the Phoenix area around 50 degrees. This
was likely largely due to the extensive cloud cover across the
area. A ridge axis is also still positioned over the area, with a
588 dm high centered southwest of Baja California, keeping the
full tropospheric profile warm. The high clouds will likely
moderate highs this afternoon, but above normal highs in the mid
to upper 60s are still expected.

Radar has been showing echoes through the day, but no
precipitation is reaching the surface. Aircraft soundings out of
Phoenix show the moisture is mostly confined between 500 mb and
the EL with very dry air in the sub-cloud layer. Under the current
synoptic setup, the stream of mid and high clouds is expected to
continue for several days as deep southwesterly to westerly flow
persists in between the amplified trough off the West Coast and
high down near Baja.

There has been ongoing uncertainty the past few days in terms of
the evolution and timing of shortwaves within the larger trough
pattern, but models today seem to be honing in on Friday into
Saturday with the timing of the first shortwave. The primary
uncertainty at this point with this wave is the latitudinal
trajectory and how quickly it moves through the area. It is with
this wave that the greatest rain chances are expected. EPS and
GEFS show a spike in IVT to 250-750 kg/ms around Friday afternoon
through Saturday morning, with a gradual rise in IVT leading up to
this point. Latest NBM 24-hour probabilities of measurable rain
peak in the 5 AM Fri - 5 AM Sat timeframe, with chances upwards of
80-90%. This better model consensus in a time frame may also
partially explain a boost in the forecast of most likely QPF, as
the temporal spread of QPF shrinks. Latest WPC forecast has seen a
notable boost from the previous forecast cycle with 0.25-0.50" in
many lower desert locations and upwards of 1-2" in the higher
terrain north and east of Phoenix, as well as in western JTNP.
These values are a little more along the lines of the EPS mean and
NBM 75th percentile. However, there is still potential for less
significant rainfall as the greatest dynamic and forcing may
remain just removed from the local area.

.PREVIOUS DISCUSSION..
Latest NBM PoPs continue to push back the timing of rain chances
later this week, now focusing more between Thursday night and
Friday night. However, given the near continuous feed of moisture
and southwesterly modest low and mid level low, orographic
effects/forcing will likely at least provide minimal shower
chances for the Arizona high terrain starting Thursday, likely
lasting through at least the weekend. Deep sub-tropical moisture
is likely to also be over our region at times starting Thursday,
but with an overall lack of organized upper level dynamics/forced
ascent, significant rainfall amounts remain unlikely.

Current WPC QPF amounts for Thursday-Saturday range from 0.25" or
less across the lower deserts to upwards of 0.75-1" across the
high terrain northeast of Phoenix. Forecast temperatures are also
now warmer for later this week, mostly remaining a few degrees
above normal through Saturday before gradually lowering into early
next week.

The blocking pattern may last through the first half of next
week, possibly even into the following weekend providing unsettled
weather over much of the Western U.S. This should provide
additional chances for precipitation for our region at times next
week, but it is impossible to determine when and if any of these
chances would bring significant precipitation to the Desert
Southwest.

&&

.AVIATION...Updated at 2352z.

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

Extensive mid-level cloudiness will persist throughout much of
the TAF period with CIGS ranging between 12-15 thousand feet.
Otherwise, light winds generally following typical diurnal
patterns, but also with extended periods of variability can be
expected.

&&

.FIRE WEATHER...
Thursday through Monday:
High pressure will eventually give way to a slow moving weather
system likely impacting the Desert Southwest at times Friday
through early next week. Wetting rains are likely to occur at some
point Friday into Saturday, especially over the Arizona high
terrain and potentially again early next week. This unsettled
weather pattern should maintain increased moisture levels and
higher RHs through most of the period. Min RHs should start off
between 20-30% on Thursday before climbing to above 40% starting
Friday. Winds will be breezy at times through the period,
particularly across southeast California and the Arizona high
terrain.

&&

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

$$

DISCUSSION...Benedict
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Rogers
FIRE WEATHER...Kuhlman