557
FXUS65 KTWC 151921
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
1221 PM MST Sun Sep 15 2024

.SYNOPSIS...Increasing moisture will result in chances for
thunderstorms across Southeast Arizona through Monday. Heavy
rainfall, strong wind gusts, and small hail will be possible with
any thunderstorm. Dry conditions then return Tuesday through the
remainder of this coming week. High temperatures will likely fall
below normal Tuesday through the coming week as well.

&&

.DISCUSSION...Despite having a deep slug of lower level moisture
across Southeast Arizona over the past 24-hours, it was been
relatively quiet with only a few showers/thunderstorms that have
tracked across the region. The weak storms that were moving NE
across Graham and Greenlee county earlier this morning have since
weakened and moved off into west central New Mexico.

I know what you are thinking. What about the poor diamondback
rattlesnakes, large chuckwallas and gila monsters that have been
patiently and eagerly awaiting rainfall to begin. Little fellas
awkwardly standing there in the desert with their shower caps on,
holding their miniature bars of soap. Well, their wait will
finally be over as it is shaping up to be a wet 24-36 hours across
Southeast Arizona.

Lets take a look at the favorable ingredients that we are working
with today. First, the larger scale pattern aloft if defined by a
trough of low pressure digging south along the California
coastline. As this feature moves south today, the flow over SE AZ
becomes more diffluent aloft assisting convection that does develop
with a larger scale lift aloft.

Second, GOES satellite imagery continues to show the deeper
moisture spread across all of Southeast Arizona, with the northern
extent of the plume axis (precipitable water values ranging from
1.25-1.40 inches) extending across the eastern portions of the
Tohono O`odham Nation and eastern Pima county. It is the
instability associated with this moisture that will be the primary
reason we will see shower and thunderstorm development this
afternoon. 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. The SPC
Mesoscale Analysis page indicates a sharp gradient of the maximum
Theta-e vertical difference in the lowest 3km extending across the
same area.

The 15/12Z KTWC upper air sounding indicated plenty of dry air
above 550 mb which can be seen on WV satellite imagery rotating
along the southern and southeast periphery of the upper trough.
This will mean that updrafts will entrain dry air into the upper
portions of the storms assisting evaporation and thus increase the
strong downdraft/outflow potential. Although we are moist in the
lower portions of the sounding, it is not quite the ideal profile
for wet microbursts because is it just a little to moist in the
mid-levels of the storm but we can still expect some strong
outflows to 45 mph. Additionally, the sounding also showed 30
knots of 0-6 km shear which should produce an isolated hail
potential with the storms today.

Given all of these ingredients, it looks like the next 24-36
hours will be a decent rainmaker. The 15/12Z HREF indicated the
valleys east of a Nogales-Three Points-Eloy line have a 10-20
percent chance of total rainfall with this event above 0.50
inches, with a 40-60 percent chance in the higher terrain. In
addition, there are scattered locations across Santa Cruz/western
Cochise/eastern Pima and eastern Pinal counties with a 10-20
percent chance of over an inch of rain.

Given the synoptic assist from the upper trough, expect showers
and thunderstorms to persist overnight into Monday morning.
Strong dynamic support with shortwaves rotating around the base
of the upper trough will likely intensify thunderstorms re-
development Monday afternoon. After that, we will see the moisture
scour out Tuesday as a strong westerly flow ushers in dry air.
The upper 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 17/00Z. SCT 9k-12k clouds becoming
SCT-BKN 6k-9k this afternoon and persisting into Monday. Isolated
to scattered thunderstorms should develop today with brief
visibility restrictions, mountain obscurations, localized heavy
rain, and gusts to 45 kts. Thunderstorm coverage will increase
during the evening, and may persist overnight and again Monday,
especially to the east of Tucson. 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, beginning this afternoon and
continuing overnight tonight. Dry conditions return Tuesday and
persist through the remainder of the week with valley minimum
relative humidities in the 15 to 25 percent range, and higher
elevations 20 to 30 percent. Winds outside of thunderstorms
generally remain under 12 mph today, then south to southwest winds
of 15-20 mph and higher gusts will occur for much of this coming
week, mainly during the afternoon and early evening hours.

&&

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

&&

$$

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