Spring Valley, Arizona Weather Forecast Discussion

918
FXUS65 KFGZ 180742
AFDFGZ
Area Forecast Discussion
National Weather Service Flagstaff AZ
1242 AM MST Thu Sep 18 2025
.SYNOPSIS...Chances for storms increase over the next few days as
moisture from the remnants of Tropical Cyclone Mario move into
the area. Cooler temperatures and generally light winds are
expected to close out the week with a return to dry weather and a
warming trend to start out next week.
&&
.DISCUSSION...Today and Friday...The remnant low of what was
Tropical Storm Mario has ejected off the Pacific into southern
California, resulting in a large pull of sub-tropical moisture
into the Lower Deserts. Across northern Arizona, this moisture is
still yet to be seen across the entirety of the area. Surface
dewpoints still remain near 20-30F north of I-40, to near 40-50F
to the south. How this moisture gradient evolves through the day
will be important as it will be the division between a wetter day
and a dry day. Current hi-res guidance keeps this boundary largely
in place, with only a subtle northward shift as southerly flow
increases, and in turn increases moisture advection. As such, the
best chances for showers and thunderstorms this afternoon look to
mainly be south and west of a line from the Grand Canyon to
Flagstaff to St. Johns.
Much of the activity from today looks to diminish with the loss of
daytime heating later today. However, increasing divergence aloft
looks to aid in keeping storms going into the overnight for a
portion of the area. Latest hi-res guidance has a complex of storms
moving into Yavapai County around midnight, and gradually
progressing northeast through the overnight.
The third, and more widespread, round of storms looks to hit on
Friday. With more widespread moisture in place, combined with large
scale-acent and daytime heating looks to lead to widespread showers
and thunderstorms developing the by the afternoon. Activity across
far northern Arizona into the Four Corners region may initially be
delayed given lingering debris clouds from overnight convection.
Thus, storms look most likely to from the differential heating
boundary and south (likely I-40 and south). Coverage should then
continue increase through the remainder of the day.
With deeper moisture in place and a fairly efficient precipitation
environment, the threat for flash flooding will be the greatest
concern. This will be especially true for any areas of training
thunderstorms or rain sensitive locations (i.e. burn scars and slot
canyons). While widespread severe weather is not anticipated, a
couple strong to severe thunderstorms can not be totally out ruled.
Instability looks to be the greatest over Yavapai County (HREF
guidance shows SBCAPE near 800-1000 J/Kg) each afternoon, this will
be the most probable area for stronger storms.
Saturday through Wednesday...Heights look to rise over the weekend
and into early next week as high pressure tries to build over the
southwest US once again. At the same time, an upper-level low looks
to sit off the California coast. Of the operational 00Z guidance,
the CMC still favors the faster solution, along with the GFS now.
While the ECMWF continues to favor a slower ejection of the low and
a more amplified ridge over Arizona. Solutions become even less
clear looking at the ensemble, with some members between the EPS,
GEFS, and CMCE showing a significant downward trend in PWAT, while
another handful show the moisture remaining over northern Arizona.
With this in mind, the current forecast of warming temperatures and
minimal precipitation chances seems reasonable for the extended
period.
&&
.AVIATION...Thursday 18/06Z through Friday 19/06Z...Mainly VFR
expected. ISO-SCT SHRA/TSRA mainly along and south of a KGCN-
KFLG-KJTC line after 18Z. Gusty and erratic winds and brief MVFR
conditions possible in and around TS. Outside of storms, expect
daytime SW-NW winds 5-15 kts becoming light and variable overnight.
OUTLOOK...Friday 19/06Z through Sunday 21/06Z...Mainly VFR
expected. SCT-NMRS -SHRA/-TSRA spreading NE Thursday night into
Friday, becoming ISO-SCT on Saturday. Gusty and erratic winds and
brief MVFR conditions possible in and around TS. Outside of storms,
expect daytime S-SW winds 5-15 kts.
&&
.FIRE WEATHER...Today and Friday...Scattered showers and
thunderstorms moving from south to north today, becoming more
widespread tonight and Friday. Wetting rain is likely. Generally
light winds, trending southwesterly 5 to 15 mph.
Saturday through Monday...Scattered showers and thunderstorms
continue on Saturday, turning warmer and drying for Sunday and
Monday. Winds mainly light.
&&
.FGZ WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
PUBLIC...Humphreys
AVIATION...Meola
FIRE WEATHER...Humphreys
For Northern Arizona weather information visit
weather.gov/flagstaff

NWS FGZ Office Area Forecast Discussion