Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Phoenix Arizona
224 PM MST Thu Sep 19 2019
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.
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
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 1815z
South-central Arizona including kphx, kiwa, ksdl, and kdvt:
Upper low strengthening to the northwest of Arizona will result
in increasing dry southwest flow aloft over the next 24 hours.
Although winds in the greater Phoenix area will maintain typical
diurnal direction trends, wind speeds will likely be a bit
stronger during afternoon hours with some gustiness likely. Look
for occasional gusts over 20 kts after 21z and continuing through
02z. There may be a protracted period of mostly southerly winds
now through 21z with sustained speeds around 10 kts at times and
maybe a gust or two into the teens. Winds should swing more to the
southwest by 21z. Winds will then taper off after 02z and
eventually return to the southeast after midnight tonight. Clouds
should not be too much of an issue, look for few-scattered decks near
10-kft today, with skies becoming generally clear after 02z with
a few upper-level clouds aoa 25 kft.
Southeast California/southwest Arizona including kipl and kblh:
An upper low strengthening to the northwest of Phoenix will
result in increasing southwest flow aloft over the next 24 hours.
This will result in elevated southwest to west winds across the
western deserts. Winds should favor the west at kipl, with
strongest gusts between 22z and 06z tomorrow. Winds will favor the
southwest at kblh with similar gustiness during the afternoon and
early evening. At blh and ipl winds will switch to the northwest
after midnight with lighter wind speeds. Skies to be generally
clear next 24 hours.
Aviation discussion not updated for amended tafs.
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 relative humidity 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.
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