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FXUS65 KPSR 242346
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
446 PM MST Sat Jul 24 2021

.UPDATE...Updated Aviation Weather Discussion.

&&

.SYNOPSIS...
A low pressure system will continue to drift slowly towards Arizona.
With abundant Monsoon moisture in place, the threat of showers and
thunderstorms will continue through Sunday. The strongest storms
will be capable of producing heavy rainfall and areas of flash
flooding. Temperatures will also be well below normal going into
the new week. Warmer and drier conditions are anticipated next
week as the low pressure system lifts northward into the Great
Basin.

&&

.DISCUSSION...
Unsettled weather continues across the state as many areas continue
to receive rainfall today. Many areas across eastern parts of the
Valley have received measurable rainfall over the past six hours
with some locations seeing upwards of 0.50" to 1.00". Officially,
Phoenix has received 0.17" of rainfall since midnight, bringing
the storm total over the past couple of days up to 1.18". The
total for the monsoon season is now up to 1.72", which is the 12th
wettest up to this point in the season. Temperatures across
south-central Arizona continue to remain well below normal with
many lower elevation locations currently in the upper 70s to low
80s. Warmer temperatures remain focused to the west across
southwest Arizona and southeast California where temperatures are
in the 90s.

Radar this afternoon shows a lull in shower activity across the
Valley with visible satellite imagery showing some breaks in the
clouds across parts of south-central Arizona. Areas that see
extended breaks in the clouds this afternoon will have the potential
to see higher temperatures in the mid to upper 80s. Meanwhile, water
vapor imagery and mesoanalysis data this afternoon showed the well-
defined circulation of an upper low currently centered near the
Arizona/New Mexico border with an additional weak circulation
showing up a little further southwest over eastern Arizona. The
atmosphere remains very moist across the region with the 24.12Z PHX
sounding recording a PWAT value of 2.08", which is near record
territory for this date. Instability has remain low so far today in
the cool environment with little lightning activity detected.
Additional shower activity and some isolated thunderstorms will be
possible through the remainder of the afternoon with CAMs showing
most activity remaining over the higher terrain north and east of
Phoenix. The main threats with any of these would be locally heavy
rainfall/flash flooding. A Flash Flood Watch remains in effect
across south-central Arizona through Sunday evening. The strong wind
threat looks to remain low with HREF guidance showing less than a
15% chance of outflow winds exceeding 35 mph.

The closed low near the Arizona/New Mexico border will continue to
drift further westward into Arizona. As this occurs, chances for
precipitation will once again increase overnight into the early
morning hours with PoPs climbing to around 60-70%. Chances for
rainfall spread west into southeast California going into tomorrow
afternoon as the low continues its westward progression and provides
more upper level forcing over the region. Heavy rainfall and some
localized flash flooding will remain possible through tomorrow
before chances decrease going into the new workweek.

Ensembles remain in good agreement that warmer and drier conditions
develop during the first half of the week as the upper low lifts out
in the Great Basin. Driest day will likely be Monday. Thereafter,
models depict a persistent easterly component to the steering flow
with the Monsoon High situated well to our north. This will at least
keep rain chances at or above climatology through the week. Latest
NBM also indicates return to near normal temperatures is likely by
the middle of the week.

&&

.AVIATION...Updated at 2350Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Weather will continue to have multiple aviation impacts. For now,
cloud decks are slowly clearing which may offer some improvement
of cigs for this afternoon. So far, trends do not favor widespread
clearing. Southerly to southeasterly winds will predominate this
afternoon and evening, especially if the aforementioned clearing
remains underwhelming. There is a 20% chance a few terminals could
see a westerly switch if enough clearing/surface heating occurs.
Meanwhile, a very damp and energetic atmosphere will have the
potential for producing waves of multiple showers and embedded
thunderstorms. Right now, the best timing for these waves to
impact the terminals is late tonight-Sunday morning between
08-12z, and then again Sunday late morning/afternoon. Lightning,
heavy rain (reduced vis) and gusty winds are the main threats with
any showers/storms. Unfortunately, confidence in any particular
timing of showers is low and could develop outside of the times
discussed above. Cloudy skies with unseasonably low cigs will
continue for much of the next 24 hours.


Southeast California/Southwest Arizona including KIPL and KBLH:
There are no major aviation weather concerns at the moment.
However, the damp and energetic atmosphere could bring showers,
storms, and low cigs to the terminal area. There is a chance for
late night/early morning showers tonight and then again Sunday
afternoon. Lightning,heavy rain (reduced vis) and gusty winds are
the main threats with any showers/storms.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Storm activity takes a substantial trend downward during the first
half of the week as an upper low exits the desert southwest and
drier air moves into the region. The least active day looks to be
Tuesday with storm chances confined to south-central Arizona.
Accordingly, humidities trend downward with Min RH values
decreasing to the 15-30% range on the lower deserts by Wednesday
(25-35% higher terrain) with little change thereafter. Max RH
values on the lower deserts decline to 30-50% (lowest over SE CA)
by Thursday morning (50-80% higher terrain). High temperatures
start off several degrees below normal Tuesday and climb to near
normal readings over south-central AZ by Thursday (2-4 degrees
above normal SW AZ and SE CA). Little change thereafter. Apart
from thunderstorms, winds will favor familiar diurnal patterns.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through Sunday evening for AZZ534-537>563.

CA...None.
&&

$$

DISCUSSION...Smith/Hirsch
AVIATION...AD
FIRE WEATHER...Smith/AJ

NWS PSR Office Area Forecast Discussion