Severe Thunderstorm Warning 240, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
115 PM MST Mon Oct 3 2022

.SYNOPSIS...
Modest shower and thunderstorm chances will persist over the next
couple days, becoming somewhat more concentrated over higher terrain
locations. However, a pesky low pressure system will hover south of
the area providing periodic unsettled weather for the remainder of
the week. While not all locations will see rainfall, gusty late
afternoon and evening outflow winds will be common many days this
week. Otherwise, temperatures will remain near to slightly above
normal through the weekend.

&&

.DISCUSSION...
Early afternoon WV imagery and objective analysis depict a well
defined negative PV anomaly and cold core rotating through southern
Nevada with very favorable left front jet quadrant ascent spread
through northern AZ. Showers and embedded storms have been
percolating since the early morning hours indicative of the strong
dynamics associated with this PV lobe. Closer to the local forecast
area, instability was increasing with daytime heating (MLCape 500-
1000 J/kg) ahead of steepening midlevel lapse rates, albeit in an
environment of marginal moisture. The best boundary layer mixing
ratios (10-12 g/kg) were aligned through the lower Colorado River
valley with markedly lower values (7-8 g/kg) towards the Phoenix
metro and points eastward. While mixing ratios in this range would
typically be insufficient for deep convective sustenance into lower
elevations during the warm season, much colder temperatures aloft
(approaching -15C at H5) may support a few storms even into this
more hostile environment.

Confidence is excellent that convective clusters strung through
northern AZ ahead of the aforementioned PV anomaly will continue to
descend southbound aided by cold, dense outflows. Simultaneously,
isolated storms were already forming well ahead of the main
disturbance as ascent increases and become juxtaposed with the
moist nose through the western CWA. Flow over the most preferred
terrain features (i.e. Kofa range, Chocolate mountains, etc.) were
quickly developing into small storm clusters, and should support
outflow winds (DCape > 1000 J/kg) and additional pop up
showers/storms the remainder of the afternoon. Further east into the
Phoenix metro, forecast confidence is lower as reduced moisture
levels and notable CinH complicate the convective picture. HREF
membership are overwhelmingly in support of a strong outflow
boundary (or perhaps a couple) blasting south through the metro with
30-40 mph gusts blasting northern Maricopa County. Whether this
outflow becomes sufficiently deep to spur additional lower elevation
storms is far more uncertain with HREF members generally showing
decaying convective elements into lower elevations with the strong,
gusty winds. However, inhibition should be breakable and pattern
recognition suggests a few isolated storms surviving into the metro.

The main vorticity lobe will translate into southeast Arizona
Tuesday with upstream anti-cyclonic subsidence spreading into the
majority of the CWA. Moisture and instability should still be
elevated enough for deep convective development clipping higher
terrain area of the southeast parts of the forecast area, though the
greatest impact will likely be a decaying easterly outflow punching
westward during the evening hours. Deterministic and ensemble members
remain in good agreement that this original vorticity center will
disassociate and retrograde from the northern jet during the latter
half of the week becoming a cutoff feature meandering along the
international border as the entire Conus pattern enters another
amplification and blocking phase. This pattern is very similar to
last week with a nearly identical cutoff spinning aimlessly across
southern AZ. While moisture and heating will be more restricted than
monsoon season, enough evidence exists that vorticity lobes will
rotate about the central circulation providing focused ascent and
enhanced rainfall chances at times during the second half of the
week.

&&

.AVIATION...Updated 1745Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
PHOENIX AREA TAF IMPACTS: 1) 30-40% probability of outflow winds of
35+ kts between 23Z-02Z.
 2) 20-30% chance of thunderstorms at the terminal between 23Z-02Z.

Winds generally light out of the ESEly direction with a trend toward
SWly directions after 21Z. Thunderstorms over the higher terrain to
the north and northeast are expected to produce outflow winds from
those directions that overspread the Valley floor. Onset of big wind
shift expected between 23Z to 01Z. Less than 10% chance of peak
gusts reaching 50kts. There is even the potential for a stray
thunderstorm to develop over the Estrella Mountains between 20Z-22Z
but confidence too low in that scenario to reflect associated
impacts in the KPHX TAF. Anticipate weakening winds as the evening
progresses. As for sky cover, a SCT-BKN deck with bases AOA FL120 is
expected to thin out later this morning before clouds increase in
the late afternoon in association with thunderstorm activity (expect
ceilings to remain above FL070). Expect clouds to thin out again
later in the evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
KIPL: Light southeasterly winds are expected to prevail during the
daytime followed by modest southwest winds developing after 02Z.
There is about a 10% chance of outflow winds from the northeast
exceeding 30kts during the late afternoon/early evening. Otherwise,
skies will be mostly clear with thunderstorms well removed from the
airfield.

KBLH: Light and variable winds this morning will become southerly by
late morning. During the afternoon, scattered thunderstorms are
expected to develop once again over La Paz and eastern Riverside
Counties. Outflow winds are expected to affect the airfield by 23Z
and there is a 30-50% chance of gusts of 35+ kts along with
visibilities dipping below 6SM in blowing dust. There is a 30% chance
of thunderstorms at the airfield or within the vicinity between 23Z-
02Z. Expect lighter winds and the start of decreasing cloudiness
beginning 02Z.

&&

.FIRE WEATHER...
Shower and thunderstorm chances this week will be primarily limited
to the higher terrain areas of south-central Arizona. However,
portions of the lower deserts will have the potential for isolated
thunderstorms today. High temperatures will remain two to four
degrees above average through Wednesday followed by a modest cooling
trend brings readings very close to average for Friday and Saturday.
Min RH will range from 20-35% across the forecast area while Max RHs
mostly range from 40-65% (some areas locally higher). Apart from
thunderstorms, winds will be light and favor terrain driven diurnal
patterns through Wednesday. The latter half of the week, winds will
favor easterly directions along with some breeziness at times (most
noticeably over south- central AZ).

&&

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

&&

DISCUSSION...18
AVIATION...Young/Heil
FIRE WEATHER...Smith/AJ

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