041 
FXUS65 KPSR 080500 AAB
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1000 PM MST Thu Oct 7 2021

.UPDATE...
Updated Aviation

.SYNOPSIS...
High clouds will continue today and early tomorrow as another 
weather system approaches the region. By Friday, a few light 
showers will be possible across southeast California and portions 
of southwest Arizona. Breezy conditions will also develop, 
particularly for southeast California. The weekend should be quiet
with slightly below normal temperatures. Winds and clouds will 
increase again Monday and Tuesday ahead of the next system which 
should be larger and stronger than recent disturbances. Breezy and
significantly cooler conditions will arrive Tuesday but with 
little in the way of precipitation.

&&

.DISCUSSION... 
Satellite imagery continues to show widespread high clouds 
stretching from SoCal to Arizona as upper level moisture streams in 
from the Pacific. Periods of dense high clouds will begin to clear 
out on Friday with afternoon highs peaking near 90 degrees. An 
offshore trough will begin to lift northeastward on Friday into 
southern California. However, the base of the trough will slow 
considerably and partially detach from the mean flow as a second 
disturbance sliding down the western edge of the trough causes it 
deepen somewhat. This feature will become positively tilted as it 
finally pushes through on Saturday. Although the trough orientation 
will not provide favorable dynamics for lift and precipitation, it 
will be associated with a robust moisture tap. Model IVT may peak as 
high as 500-700 kg/m/s for portions of southern California. This 
moisture rich environment will likely allow for a few showers to 
form. PoPs approach 40% for JTNP, and gradually diminish to the 
single digits towards La Paz and Yuma counties. 

Besides the rain chances, wind will also be a concern. The 
orientation of the trough, steep height falls, and pressure rises 
behind the surface front will all be favorable for breezy conditions 
west of the Colorado River. ECMWF EFI values are near 0.5 to 0.7, 
representing a slightly unusual event. Wind gusts may approach wind 
advisory criteria by Friday evening for typically windy portions 
of Imperial County. 

Temperatures will cool somewhat Saturday through Monday in the wake 
of the weak storm system. Highs will generally rise to the mid 80s. 
Our attention then turns to the anomalously strong trough that will 
crash into the desert southwest starting Tuesday. Ensemble guidance 
is in very good agreement that this trough will develop with 
relatively limited variation in its depicted timing. There is still 
uncertainty with respect to its amplitude. About 3/4 of the WPC 
clusters show a relatively more amplified trough, where the low is 
able to completely close off, with about 1/4 showing a less 
amplified open trough solution. Overall, confidence continues to 
improve that the more amplified solution will pan out. 

How anomalous will this trough be? The EPS and NAEFS both show that 
for 500 mb heights and below it will be in the bottom 1th 
percentile of values for this time of year. ECMWF EFI values for 
surface temperatures show values of -.90 to -.95 over the area. By
any metric this storm will be unusually cold for the middle of 
October. In terms of surface temperature, the NBM shows highs will
likely only rise to the low 70s with lows in the low 50s on 
Tuesday for the lower deserts. For Phoenix, NBM temperature 
probabilities have a 45% chance of highs remaining below 70F and 
a 35% chance of lows falling into the 40s. 

Wind will also be a major factor for this storm system. This system 
will induce a very strong height and pressure gradient as it moves 
in. Even at 5 days out, the ECMWF EFI is showing values of 0.7-0.8 
with SOT values approaching 1. If the forecast remains consistent, 
these values could rise further in the coming days as the ensemble 
solutions with a weaker trough are gradually removed. It is much too 
early to get too specific, but ensemble guidance suggests that some 
of the typically windier spots could see gusts approaching wind 
advisory criteria. 

The chance for rain with this system will remain 
low as a continental trajectory down the interior west will limit 
moisture. Isolated showers will likely be limited to the higher 
terrain in Northern Arizona, such as Flagstaff, the Rim and White 
Mountains. NBM precipitation probabilities have crept up just a 
bit compared to yesterday along the foothills north and east of 
Phoenix due to a slightly more favorable trough orientation and 
moisture values, but rain remains unlikely in the major population
centers. A progressive upper level pattern will allow the trough 
to quickly eject towards the east within a day or two. Rebounding 
heights and dry conditions will favor a return to pleasant 
conditions the second half of next week. 

&&

.AVIATION...Updated at 0006Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Periods of thick cirrus cigs will be common through Friday afternoon 
with only patchy virga otherwise. Current light westerly winds to 
switch back to easterly shortly before midnight. The daytime switch 
back to west on Friday should occur earlier and more distinctly than 
recent days, with a bit of gustiness likely as well, with lighter 
westerly winds likely to continue through most, or all of Friday 
night. 

Southeast California/Southwest Arizona including KIPL and KBLH:
Thick high cirrus cigs will gradually lower through Friday morning 
with decks falling as low as 8K ft AGL. While virga will be 
expansive, a few light showers reaching the surface will be possible 
Friday morning at KIPL. These virga/light showers Friday could lead 
to some variability in wind directions/speeds at times. Some 
enhanced breeziness is possible Friday morning in response to an 
approaching weather system, but likely will not become pronounced 
until later in the day Friday as winds aloft increase and mix down 
to the sfc.

&&

.FIRE WEATHER... 
Saturday through Wednesday: 
The next weather system will brush through the Southwest on Friday, 
bringing low end rain chances to southeast California and southwest 
Arizona, along with breezy conditions and a slight uptick in 
afternoon humidity to all areas. Wind gusts may reach 20-30 mph 
Friday afternoon. Temperatures and humidity will fluctuate a bit 
over the weekend before the next system approaches on Monday. Much 
stronger wind gusts are possible Monday across southeast California 
before spreading to all areas on Tuesday. Temps will cool by 10-15 
degrees for Tue/Wed giving a minor boost to RH levels. 

&&

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

&&

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

$$

DISCUSSION...Hodges/12
AVIATION...Percha/18
FIRE WEATHER...12/Iniguez