660
FXUS65 KTWC 142152
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
252 PM MST Sat Sep 14 2024

.SYNOPSIS...Increasing moisture will result in a chance of showers
and thunderstorms across southeast Arizona through Monday. The
highest chances and most widespread coverage will be Sunday into
Monday. Dry conditions then return Tuesday through the remainder
of next week. High temperatures will be right around normal today
and then several degrees below normal Sunday through the rest of
next week.

&&

.DISCUSSION...The latest 2 PM MST NHC information on Tropical
Storm Ileana has it positioned near 25.5N/108.9W moving north at
5 kts. Looking on satellite imagery, it still continues to look
like it is rapidly losing steam as it moves onshore near Los
Mochis, Mexico. Again, this tropical system will NOT impact
Southeast Arizona, but tropical moisture north of this feature
over the northern portions of the Gulf of California will continue
to track northward into our neck of the woods. This deeper
moisture started to make it way into Southeast Arizona overnight
this morning, with surface dewpoints in lower 60s this morning,
which have since mixed back down into the mid 50s early this
afternoon. The latest GOES Total Precipitable Water satellite
imagery indicated PWAT values between 1.10 and 1.25 inches across
the SW third of AZ. It is the instability of this moisture that
will assist shower and thunderstorm development this afternoon.

Given there is a lack of dynamic support aloft for convection, I
would expect to see storms develop today along the gradient of the
best moisture convergence in the lower levels which would be
across Santa Cruz and eastern Pima county including the eastern
portions of the Tohono O`odham Nation. This is also supported by
the SPC mesoanalysis indicating the sharpest gradient of the
maximum Theta-e vertical difference in the lowest 3km extending
across the same area. The latest 14/12Z HREF members aren`t too
excited with convection today, but there is a 10 percent bullseye
for a max 4-hr updraft of 20 m/s across central and western Pima
county. What does that mean exactly? The HREF thinks the stronger
more established storms will develop there this afternoon. I agree
with that, but think the HREF may be a tad underdone with the
precipitation today. I doubt that we will have heavy downpours
today, but I cant rule out some locally heavy rainfall amounts
that might result in some ponding of water in low-lying areas.

The heavier rainfall should start to impact Southeast Arizona
Sunday into Monday as the slug of deeper moisture tracks across
east the rest of the forecast area. Upper dynamics are still weak
Sunday but a little better as upper trough nears and the flow
becomes slightly diffluent. The 14/12Z GFS has the area of
maximum vertical Theta-e in the lowest 6 km spreading across the
CWA Sunday, with the maximum values occuring Sunday night into
mid-day Monday. This is supported with an area of 50-70 percent
likelihood for 20+ m/s updrafts on the 14/12Z SPC HREF graphics
with an axis extending across Pinal, eastern Pima and SW Cochise
county between 5-9 pm MST tomorrow evening. These deeper updrafts
will make it more likely for taller storms, making it more likely
they will be able to entrain the drier air aloft into the upper
portions of the storms. This will in turn increase the potential
for strong downdrafts/gusty outflow winds and heavier downpours.

The threat of heavy rain should be confined to Sunday afternoon
into Monday morning. The 24-hr LPMM at 12Z Mon on the 14/12Z HREF
is quite robust with QPF amounts between 0.75 and 1.50 inches in
SE Pinal, eastern Pima and Santa Cruz counties. There is a 20-40
percent chance of rainfall in excess of 1.50 inches within 5 miles
of the Catalina, Tucson, Baboquivari and Santa Rita mountains
Sunday into Monday morning. The valleys will still generally see
lighter rainfall amounts, with generally 0.33-0.66 inches expected
and less than a 10 percent chance the Tucson Metro Area will get
over an inch of rain.

After Monday, the drier air along the southern periphery of upper
trough will scour moisture and we will remain dry for the second
half of the forecast period. The trough will position itself over
the Desert Southwest into next weekend. This will keep afternoon
temperatures in check across the entire forecast area, with
Tucson experiencing those awesome temperatures in the upper 80s to
near 90.

&&

.AVIATION... Valid through 16/00Z.
SCT-BKN clouds at 6k-9k ft AGL. A chance of -SHRA/-TSRA this
afternoon and evening through 15/05Z and again tomorrow. MVFR
conditions with the stronger -TSRA Sunday afternoon with brief
ceiling and visibility restrictions, mountain obscurations,
localized heavy rain and gusts to 40 kts. Outside of thunderstorm
wind gusts, SFC wind will be 12 kts or less and SWLY/WLY in
direction during the afternoon/early evening hours and variable in
direction at other times. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...There will be a chance of showers and thunderstorms
through Monday, with the highest chances and most widespread
coverage Sunday into Monday. Dry conditions return next Tuesday
and persist through the remainder of next week. Min RH values
today will be 20-30 percent in valley locations and 30-45 percent
in the mountains, rising another 10 percent on Sunday and Monday.
20-foot winds will generally be 15 mph or less through the
weekend, with the exception of gusty and erratic winds in/near
thunderstorms. South to southwest winds of 15-20 mph and higher
gusts will occur for much of next week, mainly during the
afternoon and early evening hours.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

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