Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 230454
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
954 PM MST Thu Oct 22 2020

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
A gradual cooling trend is expected over the next several days as
high pressure south of the region weakens. Considerably cooler
conditions will move into the region early next week as a strong
storm system settles across the Desert Southwest. Decent chances
for showers and possibly a few thunderstorms are also expected as
early as Sunday afternoon, likely lasting through Monday night.
The best chances for significant rainfall will fall over the high
terrain north and east of Phoenix where total rainfall amounts
could exceed one inch.

&&

.DISCUSSION...
The cooling trend commences with a broad longwave trough now in
place across much of the U.S.. The next few days the cooling will
be gradual, with afternoon temperatures likely remaining several
degrees above normal through Saturday. Forecast highs this
afternoon and Friday are expected to be in the lower 90s to right
around 90 across the lower deserts before dipping into the upper
80s by Saturday. The environment is still fairly dry across the
region. However, there was a push of low level moisture up from
the Gulf of California last night that led to some dense fog and
shallow stratus this morning in the Imperial Valley. Low level
moisture levels will continue to climb gradually with southerly
flow into the region. HREF and hi res model soundings, suggest
fog/low stratus is a possibility again Friday morning in the Yuma
to Imperial Valley area.

The main forecast concern over the next week continues to be the
potential impacts from an anticipated deep positively tilted
trough digging southward through the Western U.S. over the weekend
before reaching our region late Sunday into Monday. There is
still very good model agreement that the vorticity maximum with
the trough will pass directly through AZ and temperatures will
fall significantly, but there is still higher model spread and
increased forecast uncertainty in terms of the strength of the
trough, how far west it digs, and precipitation onset, placement,
and amounts.

The first part of Sunday should be relatively quiet just ahead of
the main system, but winds are likely to become breezy to locally
windy out of the southwest by early afternoon. ECMWF and GFS show
a weak to moderate IVT plume, downstream of the low pressure
center currently over the EPAC, advecting into the Desert
Southwest as early as Saturday, leading to a significant increase
in mid and high clouds and helping moisten the atmosphere through
Sunday. There is still a good deal of uncertainty on when exactly
the first showers and possibly thunderstorms will develop. For
now, the most likely scenario calls for a mostly dry first part of
Sunday, but with increasing chances of showers and thunderstorms
by late Sunday afternoon and evening as the EPAC low progresses
east inland, just ahead of the more poignant trough digging down
from the north. Precip chances Sunday are especially favored in
upslope areas just east and northeast of Phoenix. Conceptually,
this makes sense as there should be some additional moisture
advection in the mid levels on Sunday while colder air begins to
filter southward through most of the region starting Sunday
afternoon. There are also some hints of frontogenetical forcing
through south-central Arizona later on Sunday. If we were to have
any chance of stronger thunderstorms with this system, it would
likely occur later Sunday afternoon into the early evening hours
when models generally show the highest available CAPE as well as
fairly strong bulk shear.

For late Sunday night through most of Monday, the main PV anomaly
and the strongest upper level energy associated with the system
dropping down from the north are likely to affect at least the
eastern 2/3 of Arizona. Keep in mind, there is also increasing
model spread by this point, so there can still be some shift in
the timing and the placement of the strongest upper level energy.
There are still a wide range of possibilities as far as rain
chances and possible rainfall amounts through Tuesday, but it
does seem likely much of the eastern half of Arizona will see at
least some rainfall with considerably lower chances closer to the
California state line. The latest forecast QPF amounts are barely
higher than the previous forecast with a range of 0.2-0.5" in the
Phoenix area and locally higher over the higher terrain east and
northeast of Phoenix. Any significant changes to the track of the
low pressure system will ultimately affect potential rainfall
amounts. Latest WPC cluster guidance, still shows a large variance
among the EPS, GEFS, and CMOS members with how far west the
trough digs. Beyond Monday, there are still some ensemble members
(~20%) keeping a cutoff low over AZ through Wednesday. However at
this time, the other 80% of members have the vort max pushing east
of the area by Tuesday night. So for now, it seems most likely
consequential rain chances will come to an end by sometime late
Tuesday.

Another factor that may play a roll in precipitation amounts with
this event will be precipitation placement. A lot of dry air is
expected to advect south on the back side of the trough with a
strong northerly wind. GEFS shows PWATs crashing by Monday
evening, especially along the Lower Colorado River. As a side
note, 925-850 mb winds in southeast CA and southwest AZ may reach
40-50 kts late Monday afternoon through Tuesday morning. Which may
meet wind advisory criteria and lead to elevated fire weather
concerns if these winds are able to still mix to the surface. The
best available moisture will likely remain just downstream of the
trough axis and could become focused over certain areas.
Frontogenetical banding and storms may lead to localized to areas
of higher rainfall amounts. While the current forecast QPF for the
Phoenix area is only around a quarter of an inch, the range of
solutions is still anywhere from no rain to 1.5" as suggested by
the 12Z EPS spread.

The temperature forecast during the upcoming weather system is
definitely of higher confidence compared to the rain forecast.
Highs Sunday are likely to only top out in a 80-85 degree range
just before the cold front moves through, while highs Monday and
Tuesday are likely to struggle to get into the lower 70s.
Overnight lows will become quite chilly starting Monday night with
readings dipping into the 40s in most areas. We should eventually
see temperatures climbing back to around normal readings at some
point during the latter half of next week.

&&

.AVIATION...Updated at 0500Z.

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

There are no aviation weather concerns through the TAF period. Winds
will remain light, mostly AOB 7 kts, with directions following
typical diurnal trends. Cloud cover will increase through the day
Friday but cigs will remain above 20k feet.

Southeast California/Southwest Arizona including KIPL and KBLH:

At KIPL, winds will remain westerly with speeds AOB 7 kts through
the TAF period. Low clouds and fog could redevelop Friday morning
but confidence for any aviation impacts is too low to mention in
the TAFs at this time. At KBLH, winds will generally favor
southerly to south-southwesterly direction with speeds 5 to 10
kts.


&&

.FIRE WEATHER...
Sunday through Thursday:
A major cooling trend is anticipated during this period. Modest
cooling continues Sunday with lower desert highs mostly in the mid
to upper 80s and by Tuesday readings are anticipated to be in the
upper 60s for most lower desert locations. This is due to the
passage of a low pressure system from the north. It will also lead
to breezy to windy conditions Sunday and Monday with continued
northerly breeziness on Tuesday over southeast CA and southwest
AZ. In addition, there will be an opportunity for rainfall (mainly
over south-central AZ) in the Sunday through Tuesday time frame.
In fact, current projections are that the higher terrain of south-
central AZ could get storm total rainfall amounts ranging from
0.5 to 1.5 inches (less than 0.5 for the lower elevations -
especially away from south-central AZ). Humidities will increase
(most noticeably over south-central AZ) with Min RH values on the
lower elevations of 20-30% and Max RH values of 50-70%. Humidities
will begin to trend down from west to east on Monday.

&&

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

&&

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

$$

DISCUSSION...Benedict/Kuhlman
AVIATION...AD
FIRE WEATHER...Percha/Kuhlman


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