-- Discussion containing changed information from previous version are highlighted. --
734
FXUS65 KPSR 151211
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
511 AM MST Sun Sep 15 2024

.UPDATE...
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Updated 12z Aviation Discussion.
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&&

.SYNOPSIS...
A weather system will approach the region from the northwest
today while moisture increases across eastern Arizona. This will
result in modest chances for showers and thunderstorms focused
more over the higher terrain east of Phoenix. Chances for rain
will continue into Monday, but once again the best chances will be
over higher terrain areas. Dry and mild conditions are then
expected starting Tuesday with temperatures running five to ten
degrees below normal through Friday.

&&

.DISCUSSION...
Talk about a dramatic shift in rain chances. Model trends over the
past 12 to 24 hours show considerably less moisture getting
advected northward into southern and central Arizona with much of
this moisture now slated to be mainly east and southeast of the
Phoenix area. This is likely because TC Ileana has dissipated
earlier than expected with little to no convection now associated
with the disturbance. As a result, the moisture advection into
Arizona will be much less than what was expected with PWATs now
likely only maxing out around 1.3" later today. Given the less
moisture, instability will also be much more limited with HREF
MUCAPEs falling well short of 1000 J/kg over much of the area.

Looking at the latest guidance, there will still be a roughly
20-40% chance of showers and thunderstorms starting this afternoon
as far west as Phoenix but more so to the southeast and east of
Phoenix. The expected lower instability will also reduce the
potential for severe weather impacts. The incoming Pacific trough
from the northwest will help to increase winds aloft and thus we
will still have modest shear, but the limited instability will
play a bigger role in the reduced strong to severe storm
potential. Guidance still shows the potential for scattered
showers and isolated thunderstorms through Monday morning, but now
the bulk of the activity is expected to be east of the Phoenix
area. The drier air that was supposed to start moving eastward
through the Phoenix area by Monday afternoon now looks more likely
to move through by sunrise Monday morning. This could end rain
chances altogether for Phoenix on Monday, but the CAMs still show
a few isolated showers possibly around the eastern part of
Phoenix through Monday afternoon. The best chances for convection
on Monday should again fall over the higher terrain east of
Phoenix with still some potential for strong winds aloft to get
mixed down to the surface with any stronger thunderstorms.

The expected lower moisture and less unstable environment means
QPF amounts have also gone down dramatically. The latest guidance
shows barely any QPF from central Phoenix and areas west to maybe
0.1" over northern and eastern portions of Phoenix. Higher QPF
amounts upwards of 0.5-0.75" are shown across higher terrain areas
of Gila County. Localized higher amounts of over an inch will
still be possible over the higher terrain, but the potential for
heavy rainfall and any flooding is also noticeably lower than it
was last night. The drier air pushing eastward across Arizona
should then mostly end rain chances over eastern Arizona by
around midnight Monday night.

The down trend in temperatures has stalled out a bit now that
the rain chances have lowered. Forecast highs for today are now
mostly just over 100 degrees across the lower deserts, including
the Phoenix area. As the trough enters northwestern portions of
the region on Monday, there will be some cooling over the western
deserts as highs drop into the lower 90s, but Phoenix may still be
able to reach 100 degrees Monday. The trough moving into the
region will also bring increased winds Monday with gusts ranging
from 30-40 mph across southeast California to 20-30 mph across
south-central Arizona. As the trough lifts to the northeast on
Tuesday, the cooler air should finally push through the rest of
the area with highs topping out in the upper 80s across the
western deserts to the lower 90s in the Phoenix area. Through
latter half of the work week, we will still stay under a
troughing pattern with another Pacific system tracking
southeastward through California into the Desert Southwest later
in the week. We may see a little bump in moisture ahead of the
weather system Wednesday into Thursday, but for now we are not
anticipating any chances for rain. The system will at least
reinforce the cooler air across the region as forecast highs
should stay in the upper 80s to lower 90s through Friday, while
overnight lows dip into the upper 50s to upper 60s. Higher model
uncertainty is seen thereafter with a stronger ridge building off
the West Coast, but yet another potential trough diving southward
toward our region. If the ridge does end up moving into our
region, we may see temperatures creep toward the 100 degree mark
again, but if the trough wins out temperatures may stay below
normal for a few more days.

&&

.AVIATION...
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Updated at 1210Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Primary aviation impact over the next 24 hours will be the
potential for VCTS/TS impacts this afternoon into the evening.
There is low to moderate confidence in a few rain showers
developing and moving northward for a few hours starting around
13-14Z this morning. A period of S crosswinds is expected by
midday and may continue through 22-23Z before shifting more SW.
Convection is expected to begin around 20-21Z in the surrounding
areas and there is low confidence activity will spread into the
Phoenix area between 22-03Z and potentially impact some of the
terminals. Any storm will be capable of locally strong wind gusts,
with a 30% chance for >30 kt gusts. Assuming thunderstorms move
into the vicinity late this afternoon and into the evening, one of
the key uncertainties during that time would be the exact wind
directions, as winds may exhibit periods of erratic behavior.
However, directions are expected to begin shifting SE-E as the
evening progresses. Another round of VCSH/VCTS is anticipated overnight
beginning around 08-09Z and lasting for several hours. Lowest
cloud bases will be around 7-8K ft AGL, with SCT to BKN coverage
through most of the period.

Southeast California/Southwest Arizona including KIPL and KBLH:
No weather concerns will exist through Sunday under mostly clear
skies. Winds will generally favor a W`rly component at KIPL
through the period, with the potential for gusts to around 20-25
kts for a period this evening. South winds will be favored at
KBLH through the TAF period with breezy afternoon conditions, as
gusts reach up to 20-25 kts.
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&&

.FIRE WEATHER...
A weather system will brush by the region through Monday, while
moisture is expected to increase enough across the eastern
districts to bring chances for showers and isolated thunderstorms
from this afternoon through Monday afternoon. The best chances for
wetting rains will be over the higher terrain east of Phoenix
where chances peak around 50% Sunday night. Winds will
predominately be out of the south starting this afternoon through
Monday morning before turning southwesterly and becoming gusty
Monday afternoon. MinRHs through Monday will range from the mid
teens across the western districts to 20-30% over the eastern
districts. Dry air will fully push through the area on Tuesday as
below normal temperatures take hold across the region. Dry
conditions with temperatures running 5-10 degrees below normal are
likely to persist through the rest of the work week before
temperatures push back into the normal range next weekend.

&&

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

$$

DISCUSSION...Kuhlman
AVIATION...Whittock/Benedict
FIRE WEATHER...Kuhlman