Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/23/21

Area Forecast Discussion

National Weather Service Phoenix AZ

 450 PM MST Wed Dec 22 2021

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS... High pressure over the region will lead to above normal
temperatures through tomorrow despite considerable cloudiness. 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 as early as
Thursday. As this system ejects inland Friday and Saturday, the
probability of widespread and possibly heavy rain will increase. An
unsettled weather pattern is likely to continue into next week with
additional chances of precipitation while temperatures remain at or
below seasonal normals.

&&

.DISCUSSION...
Synoptic analysis reveals an expansive upper level high over the
north Pacific with an upper level low centered a few hundred miles
off of the California coast. Deep subtropical moisture continues to
be advected north around the southern and eastern periphery and
into the desert southwest. ACARS soundings highlight a moist layer
near 700 hPa and another above 500 hPa. Although only scattered
clouds are observed at the moment, IR satellite shows plentiful
cloud cover approaching, which should lead to overcast conditions
by later this afternoon. With warm southerly flow in place and the
current mostly clear skies, high temperatures will approach 70
degrees today across the lower deserts. Temperatures on Thursday
will be similar. Overnight lows will also be elevated. The current
forecast low of 55 in Phoenix tonight would tie the record daily
warm minimum. The only forecast concern in the near term is a weak
wave which will pass through the forecast area on Thursday
morning. High resolution guidance shows just scattered showers,
and with a still dry boundary layer it will be tough for much if
any precipitation to reach the ground in many areas. Accordingly,
PoPs are near 10-20% for most locations.

As we move into Friday the more interesting part of the forecast
takes shape. The slow moving upper level low will begin to
accelerate as the deep upper level longwave trough digs south. As
the upper level low moves into California it will be filling in,
which will tend to weaken its dynamics with time. Although there has
been excellent support for this pattern for several days, ensembles
continue to disagree on the exact trajectory of the decaying low, as
well as how quickly it fills in. These seemingly minor differences
will ultimately dictate whether this system can capitalize on an
environment which is otherwise very conducive for heavy rainfall.
In terms of atmospheric water vapor, PWAT values will climb to
1.2-1.3" across the lower deserts, which is near the all-time
maximum value for December according to the SPC sounding
climatology. Upper level winds will be quite strong with speeds of
35-50 kts at 850 hPa ahead of the trough axis on Friday, with 700
hPa speeds of 45-60 kts. These values are in the 97th-99th
percentile of the ECMWF model climatology. The combination of high
moisture content and strong winds will lead to impressive
moisture advection. Integrated Vapor Transport values will also be
quite elevated, with ECMWF values of 500-700 kg/m/s. The GFS
indicates IVT could reach as high as 800. These values are also
near the all-time max for this time of year. Unlike at present,
this moisture will not just be confined to 700 hPa and above.
Model guidance shows atmospheric river conditions throughout most
of the troposphere.

Due to the aforementioned uncertainty with respect to the incoming
trough position and strength, considerable uncertainty exists in
terms of rainfall totals and to a lesser degree, timing of the
rainfall. The deterministic NBM 4.0 continues to show values of near
0.25" for Yuma increasing to about 0.60" for the Phoenix area, with
higher totals in the high terrain. The 10th percentile NBM has
essentially no rain for the lower deserts except a few hundredths
near Phoenix. On the other hand the 90th percentile has around
0.75" in Yuma increasing to about 1.25" for Phoenix, and values
exceeding 2" in the higher terrain. As it stands now, the NBM has
a 20% chance of exceeding 1" at Phoenix. WPC guidance is tilted
towards about the 75th percentile of the NBM. In summary, the
potential does exist for heavy rainfall, especially over the
higher terrain, but confidence in these higher totals verifying is
low due to continued model spread. Presumably, this uncertainty
should diminish quite a bit in the next 24 hours.

In terms of impacts, the main concern is the potential for flooding
rainfall. The highest risk seems to be the terrain northeast of
Phoenix, including Tonto Creek. However, should the incoming trough
remain more intact and further south as it landfalls, the risk for
potentially flooding rainfall would expand into the lower deserts as
well. Wind will be breezy but is not expected to be a major factor
this time around. Fog formation may be possible but would be tied to
heavy rainfall. For now, NBM guidance only shows a minimal
possibility of this occurring. High elevation snow is not expected
during this first storm system as it is quite warm.

As we head into next week the forecast remains cool and active at
times. WPC cluster analysis shows extremely high confidence that an
anomalously strong trough will remain over the west coast for the
next week, with shortwave troughs occasionally digging a bit
deeper and clipping the area. The main uncertainty lies with
respect to the position of the trough, with the secondary source
of uncertainty relating to its amplitude. About half of the
ensemble members allow a series of shortwave troughs to dig far
enough to the southwest that the forecast area would see
additional storminess. If this scenario were to unfold, some of
the higher terrain areas could even see the risk of snow with snow
levels falling below 5000 feet. On the other hand, a more inland
track would result in cool but mostly dry northwest flow.

&&

.AVIATION...Updated at 1742Z.

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

Winds will be light and variable for the majority of the period
at all terminals. Ceilings will lower overnight with a mid cloud
deck as low as 9-10 kft in Phoenix and 8 kft over KIPL and KBLH
with some occasional virga noted. Clouds will likely become
scattered again by mid to late morning Thursday.

&&

.FIRE WEATHER...
Friday through Tuesday:
Low pressure will lead to cooler temperatures and an increase of
moisture. Min RH values will generally stay at or above 40% while
overnight recovery surges above 60-70%. Winds may be concerning at
times with occasional gusts of 20-30 mph some days, but which days
will be the breeziest remains ambiguous. The entire area could see
wetting rain Friday-Saturday with lingering showers over Gila
County Sunday. Rainfall totals of 0.25-0.50" will be common but
higher totals in the higher terrain are very possible.

&&

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

$$

DISCUSSION...Hodges
AVIATION...Frieders
FIRE WEATHER...12