642
FXUS65 KTWC 022210
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
310 PM MST Sun Jun 2 2024

.SYNOPSIS...Temperatures will remain above normal this week, with
dangerously hot temperatures likely Thursday. Sunny skies and
afternoon breeziness can be expected over the next few days. A
weather system moving into the area will bring a chance of
thunderstorms Friday and Saturday. These storms will produce
little if any rainfall, but will be capable of producing gusty and
erratic outflow winds.

&&

.DISCUSSION...Current pattern defined by a broad ridge of high
pressure over Mexico with a weak dry trough of low pressure moving
through the Great Basin. The southern end of the trough axis
moving through Arizona today has kept afternoon temperatures 1-3
degrees cooler than this time yesterday, but still hot with
valleys across Southeast Arizona still expected to top out in the
upper 90s to near 100 degrees. This trough is also responsible for
breezy conditions this afternoon southeast of Tucson, with a few
spots gusting up to 30 mph (below critical fire weather thresholds).
Expect Monday to be more of the same as the overall upper pattern
remains similar.

A weak upper-low seen on satellite imagery developing near off
the coast of southern California (28.7N/127.0W) this afternoon
will be impactful to our neck of the woods later this week. Before
this happens through, we will see the impacts of a longwave
amplified ridge developing across the western states, including
Arizona, this week. Ridge axis slowly translates east, over
Arizona Wednesday and into New Mexico Thursday. Highest
thicknesses values expected Thursday afternoon, with both 02/12Z
operational runs of the GFS and ECMWF suggesting 850-700mb
thicknesses over Tucson of 1690-1700m. These values would
correspond to a high temperature at KTUS between 107-111 degrees.
The NBM 25th/75th percentile for Tucson on Thursday afternoon is
106/110 with the operational NBM 109 degrees. These values are
slightly warmer than previous runs, but I am confident it wont get
much warmer as additional amplification of the ridge will be
difficult given its orientation. In any event, an Excessive Heat
Watch is in effect for the valleys of Southeast Arizona Thursday.
It is possible that we will need to extend the excessive heat
watch into Friday as it too has been trending warmer with every
run. After coordination with surrounding offices, we have decided
to wait because it is possible that more significant mainly mid
and high cloud cover Friday afternoon will help moderate temps.

Focus then shifts back to that developing upper low. This feature
slowly digs south to off the Baja Peninsula and then northeast to
the Baja Spur Friday. This will result in decent diffluence/
synoptic lift over Southeast Arizona Friday and Saturday. Limited
moisture will initially move into the area within the mid-to-upper
levels. This will result in significant virga across the majority
of forecast area Friday afternoon and evening, moistening the
atmosphere from the top-down Friday night. Expect mainly dry
thunderstorms Friday with light rain possible more on Saturday.
The problem is that the sub-cloud layer will remain very dry, so
the potential will exist both days for strong and erratic outflow
winds.

The ensemble members are all over the place with how the upper low
ejects next weekend. The GEFS members typically track it north
along the Colorado River with a more progressive ECMWF bringing it
through southern New Mexico. The positive impact of this system
will be to cool temperatures back down from the excessive heat on
Thursday to near normal levels Saturday and Sunday.

&&

.AVIATION...Valid through 04/00Z.
SKC with a few high clouds through the period. SFC winds SWLY
12-17 kts with gusts to 30 kts into the early evening, before
gradually diminishing to less than 10 kts overnight. Wind Saturday
will be again be SWLY 12-17 with gusts to 30 kts. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Dry conditions and high temperatures 2 to 5
degrees above normal the next few days, becoming 5 to 8 degrees
above normal the second half of the week. Dangerous heat is
anticipated across the valleys of Southeast Arizona Thursday and
possibly again Friday. Minimum relative humidity values 5-10% in
the lower elevations 8-15 percent in the mountains through the
week, along with poor overnight recoveries between 20-30%.

Afternoon winds should be breezy this afternoon and again Monday
with southwest winds 15-20 mph with gusts to around 30 mph (more
likely south to southeast of Tucson). Otherwise, an approaching
weather system will result in the potential for dry thunderstorms
with gusty outflow winds Friday and Saturday.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...

Excessive Heat Watch from Thursday morning through Thursday
evening for AZZ501>509.

&&

$$

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