037
FXUS65 KPSR 200550
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1050 PM MST Thu Sep 19 2019

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
Dry and mostly clear conditions will affect much of the region
through Saturday with temperatures within a few degrees of
seasonal normals. As early as late Sunday, likely lasting through
the first half of next week, rain chances greatly improve,
especially across Arizona. This next weather system will have the
potential of dropping heavy rainfall across portions of the area
early next week.

&&

.DISCUSSION...
Current upper air analysis shows a dry Pacific low slowly pushing
through the western US bringing breezy conditions this afternoon
with gusts upwards of 20-25 miles per hour. High temperatures will
also drop a few degrees today, and possibly below 100 degrees, by
Friday. Nighttime lows will also cool into the low 70s/upper 60s.

The main concern for the forecast continues to be Sunday through
Wednesday with the potential for significant rainfall as a strong
Pacific low is forecast to impact the Southwest. There is model
agreement depicting the low diving southward along the west coast
while tapping into tropical moisture from tropical storm Lorena.
Models show rich moisture and favorable theta-e advection across
Sonora and southern Arizona by Sunday increasing the potential
for thunderstorms, primarily south of Phoenix. Models converge on
Monday as the time for the best rainfall potential across south-
central Arizona as moisture surges northward. There is growing
evidence that access to the tropical moisture does become severed
late Monday as drier air aloft moves in from the northwest. This
would suggest Monday morning and afternoon as the better timeframe
for rain.

A closer look at both the GEFS and EPS ensembles for total
rainfall reveals two different camps of ideas with one suggesting
a more progressive, fast moving wave (leading to less rain) while
the other favors a slower moving system (leading to more rain). As
such, the model spread for total rainfall is quite varied but
most members in both ensembles show at least some rain across the
area. The GEFS plumes for Phoenix indicates a model mean of
1.20", a slight downtrend from previous runs, while the current
precip forecast from the Weather Prediction Center shows
widespread 0.50-1.00" with a swath of 1.50-2.00" across south-
central Arizona. The possibility that many areas see fairly less
rain does exist but the potential that some places see significantly
more is a concern. The areas that endure excessive rainfall could
easily shift depending on the depth and progression of the trough.
Additionally, this system may produce an environment favorable for
severe storms capable of producing damaging winds and hail,
although the potential for severe weather will depend on the exact
track of the low. Lingering showers are possible for Tuesday and
Wednesday although the aforementioned disconnection from tropical
moisture and push of dry air may limit any activity.

To summarize, model consensus depicts the low diving into the
region and tapping into tropical moisture increasing PoPs and the
potential for excessive rain. Looming uncertainties include the
exact track, depth and timing/progression of the low, along with
rainfall amounts for any particular area. Confidence is growing
that Monday morning and afternoon is the most likely time for the
heaviest rain. Stay tuned for the latest updates!

&&

.AVIATION...Updated at 0550Z

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

Dry southwest flow aloft will keep skies mostly clear through
Friday. Wind directions will favor typical diurnal tendencies.
Wind speeds should remain aob 10-12 kts with gusts to up to 15-20
kts Friday afternoon. Could see a couple hours of southerly
crosswinds again around midday Friday as winds veer towards the
southwest.

Southeast California/Southwest Arizona including KIPL and KBLH:

Skies will remain mostly clear under dry southwest flow. West to
southwest winds should gradually veer northwesterly Friday morning
and then persist out of the north/northwest through the day with
speeds aob 10 kts and limited afternoon gustiness.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Sunday through Thursday:
A low pressure system will lead to cooler temperatures and an
increase in rainfall potential, particularly Monday and Tuesday,
for most locations east of the Colorado River. In addition,
minimum RH values will improve as well as overnight recoveries.
Otherwise, expect some stronger afternoon breezes, especially on
Monday, with gusts as strong as 20-30 mph.

&&

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

&&

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

$$

DISCUSSION...Deems
AVIATION...Benedict
FIRE WEATHER...Deems

NWS PSR Office Area Forecast Discussion