932
FXUS65 KTWC 132142
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
242 PM MST Fri Sep 13 2024

.SYNOPSIS...Dry conditions with above normal high temperatures
today. Cooler temperatures, with highs falling below normal will
occur over the weekend and persist through next week. There will
be a chance of showers and thunderstorms Saturday morning along
the International Border, spreading across all of Southeast
Arizona by Sunday. Dry conditions then return next Tuesday.

&&

.DISCUSSION...Sometimes it is necessary to follow in the
footsteps of the great philosophers like Plato and Socrates, to
dig-deep within oneself to answer the substantive questions that
plague human consciousness. So I ask this question to ponder, if
tropical moisture nudges into Southeast Arizona during the late
periods of the Monsoon season and results in a decent chance for
rainfall, are those storms considered Monsoon thunderstorms? The
answer is yes since the Monsoon season (defined as June 15th thru
September 30th) was created to compare equal time periods from one
season to another...regardless of the nature of the convection.
Although we are expecting showers and thunderstorms this weekend
into early next week, they will feel different than the convection
we typically see during July and August.

The big picture is defined by Tropical Storm Ileana located at 11
AM MST this morning at 22.4N/109.3W (near the tip of the Baja)
moving NNW at 7 kts. Thee latest NHC track for this system is for
it to continue to weaken as it moves north becoming a tropical
depression by 5 AM Sunday before eventually moving into the
central Sonora coastline. We will NOT be impacted by this system,
but a slug of tropical low-level moisture out ahead of this
feature will push up into Southeast Arizona before getting ejected
out of the area again early next week. This moisture is progged
to move into the area tonight, with the 13/13Z NBM increasing
surface dewpoints at KTUS from the mid 40s this afternoon into the
lower 60s by Saturday morning.

The upper pattern is defined by a larger-scale trough across the
West with a more amplified trough expected to develop south along
the West Coast this weekend into early next week. At first glance,
you would expect to see the SE periphery of this trough interact
with the increasing moisture across our neck of the woods
resulting in showers and thunderstorms. However, the mid-upper
flow pattern isn`t really moving and we will only see very weak
diffluence/forcing across Southeast Arizona through this weekend.
Its not until Monday when a shearing jet streak rotating along the
base of the upper trough tracks across Southeast Arizona
increasing the potential for stronger storms. Although we will
see instability as the moisture moves into the area, the lack of
a forcing mechanism will limit the strength and intensity of the
showers and thunderstorms Saturday and Sunday.

So, what can we expect? Scattered showers and thunderstorms will
develop Saturday afternoon with weaker showers potentially
continuing overnight. The best instability will be between 12Z
Sunday into 12Z Monday. I can see scattered to numerous showers
with embedded thunderstorms persisting over a long period of time
extending from Sunday morning into Monday morning. Monday, as we
begin to see some forcing aloft, an increase in mid-level shear
and drier air moving in from the WSW we will see an increase in
the potential for stronger storms to develop, including storms
with heavy downpours.

According to the 13/13Z NBM, the probability of of KTUS seeing a
24 hour rainfall Sunday in excess of a (0.01 inch/0.25 inch/0.50
inch/1.00 inch) is (74 percent/35 percent/21 percent/8
percent)...down from the 22 percent of rainfall in excess of an
inch seen on the 13/07Z run. Looking at the probability of a 24
hour pcpn in excess of 1 inch between 12Z Sunday-12Z Monday is the
greatest (20-35 percent) across the Tohono O`odham Nation and SE
Pinal. In general, expect 0.50-0.75 inches this weekend with
isolated locations across the higher elevations between 1.00 and
1.25 inches. Although this sounds like a lot of rain, it is
expected to fall over a longer period of time (not from a single
storm). This rainfall runoff would result in water flowing in
normally dry washes and potentially significant ponding of water
in low-lying areas...but the potential for flash flooding is
minimal this weekend. However, Monday we will see a more typical
Monsoon storms with the potential for heavy rainfall.

Otherwise, the influence of the West Coast trough will be
significant for Southeast Arizona during the early to middle
portions of next week. It will scour out the rest of the moisture
and we will finally start to experience the cooler conditions we
all look forward to. Yay.

&&

.AVIATION... Valid through 15/00Z.
FEW-SCT clouds at 10k-13k ft AGL across most of southeast Arizona
through midday Saturday, then SCT-BKN clouds at 8k-11k ft AGL with
a chance of -SHRA/-TSRA aft 14/19Z. SFC wind out of the SSE 10-15
kts with gusts to 25kts until 14/03Z then decreasing to 12 kts or
less. Winds Saturday will favor a SLY/SWLY direction during the
afternoon/early evening hours. Aviation discussion not updated
for TAF amendments.

&&

.FIRE WEATHER...Dry conditions are expected across most of southeast
Arizona today, with a slight chance of showers along the
International Border overnight. Min RH values today will be 13-23
percent in valley locations and 18-28 percent in the mountains.
Increasing moisture Saturday through Monday will result in
scattered to numerous showers and thunderstorms, along with min
RH values increasing into the 20-40 percent range in the valleys
and even higher in the mountains. 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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