315
FXUS65 KTWC 151004
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
304 AM MST Sun Sep 15 2024

.SYNOPSIS...

Increasing moisture will result in chances for thunderstorms
across southeastern Arizona through Monday. Heavy rainfall, strong
wind gusts, and 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...

In the upper levels this morning, a deep trough was located over
the west coast states, while a zonal jet streak was placed from
the southern California coast through southern Arizona. Further
down in the atmosphere, southerly 700mb flow up to 25 knots was
detected by GOES derived motion winds, driving a moisture plume
northward into southeastern Arizona. The GOES Air Mass RGB as
well as the precipitable water products show the richer
atmospheric moisture becoming more widespread our region, which
coinciding with the approaching upper level trough will likely
produce scattered convection today and tonight. High- resolution
guidance signals that the richest moisture axis should be centered
over Santa Cruz and eastern Pima (potentially into Pinal and
western Cochise as well) counties this afternoon, which is where
HREF guidance also highlights the highest chances for
thunderstorms and heavy rain. The previous two HREF runs have
signaled the potentially impactful chance of heavy rainfall with
40 to 60 percent chances of a half inch in an hour and a 10
percent chance of an inch in an hour rainfall accumulations within
a 15 mile radius of any point within the richer moisture axis.
While this may not match the heaviest rainfall rates of prime
monsoon thunderstorms, the potential for multiple rounds of
thunderstorms as well as the location of this activity near
susceptible surfaces such as urban areas or washes still poses an
isolated flash flooding threat. Additionally, moderately dry air
in the mid- levels will allow for a strong wind gust threat while
20-30 knots of 0-6 km shear should produce an isolated hail
potential.

Convection may persist overnight, or feature additional convection
moving in from Mexico, as synoptic ascent intensifies with the
mid-level height falls of the incoming trough. With low to mid
level moisture continuing to surge northward, this will contribute
to the the departure from the normal diurnal convective cycle.
There remains some fairly major discrepancies in the 00Z high-
resolution model suite in timing and coverage overnight, however
given the larger scale pattern a continuation of activity across
portions of southeastern Arizona looks likely.

The extent of overnight activity will play a large part in
convective coverage on Monday, however the incoming trough and
continued southerly moisture feed may be enough to overcome any
previous convective overturning. An additional uncertainty is the
eastward push of dry air associated with the upper level trough,
which is expected to occur Monday and Monday night. A slower push
would keep precipitation chances in Tucson-Nogales through the
day, with a quicker push keeping chances to the east.
Thunderstorm severity Monday will have to be monitored as bulk
shear values begin to exceed 30 knots.

Tuesday onwards, moisture should push east of the region while a
secondary wave keeps the large scale trough in place over the
western CONUS. This will also likely produce cooler than normal
temperatures through this coming week. The tightened pressure
gradient from this trough will also likely create breezy
afternoons each day across the region.

&&

.AVIATION...Valid through 16/12Z

FEW 9k-12k clouds becoming SCT-BKN 6k-9k after 15/19Z. Isolated to
scattered thunderstorms should develop after 15/19Z, with brief
visibility restrictions, mountain obscurations, localized heavy
rain, and gusts to 45 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. Additional thunderstorm activity may
develop during the overnight hours and again on Monday. 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.
&&

$$

Public...Edwards
Aviation...Edwards
Fire Weather....Edwards

Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson