Special Weather Statement, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
330 PM MST Tue Aug 31 2021

.SYNOPSIS...The risk for flash flooding remains high today into
Wednesday as the combination of deep tropical moisture and an upper-
level disturbance spreads across Southeast Arizona. Expect
widespread showers and thunderstorms with the potential for heavy
rain. Although the heaviest rainfall will occur this afternoon
through the early morning hours Wednesday, storms will persist into
Wednesday night. The rest of the week will remain active but become
more average in terms of monsoon activity with afternoon showers and
storms possible each day.

&&

.DISCUSSION...The overall pattern is defined by a slug of deep
tropical moisture from the decaying former tropical system (Nora)
moving northward into south-central and southwest Arizona, and an
upper-low spinning off the northern Baja. Things are shaping up to
be a very busy and wet 24 hours across all of Southeast Arizona. As
of 2 PM MST, we have seen a broad area of moderate rainfall develop
across central and western Pima county. This area coincides with the
confluence of the deep moisture/weak residual mid-level disturbance
from Nora, along with a wedge of upper-level diffluence on the
eastern periphery of the upper-low begining to eject NE across
western/central AZ. This area will eventually spread NE as the
trough ejects/shears out. The most favorable time period for heavy
rain will be late this afternoon into this evening, with showers and
embedded thunderstorms likely persisting overnight into Wednesday.
Rainfall amounts across Southeast Arizona for this event will
generally be 0.50" to 1.50" across the valleys, with 2"-3" inches in
the mountains. A Flash Flood Watch is in effect for all of Southeast
Arizona through 5 AM Thursday morning.

This moisture remains across the area tomorrow, but will get shunted
to the eastern half of the forecast area as drier air is advected in
from the southwest around the upper-low. Normally when this type of
situation occurs, we can see strong convection develop along the
upper-level moisture boundary, which in this case will extend from
south-central Pima county into central Graham county. However, given
the residual showers and embedded thunderstorms continuing into
Wednesday morning, there may not be enough instability to support
strong updrafts along this boundary. However, it should be enough to
enhance showers and thunderstorms tomorrow afternoon along this
boundary...with the feature itself shifting east Wednesday evening.

The moist airmass will remain in place late this week, resulting in
mid-grade (average) thunderstorm activity Thursday and Friday. A
broad upper-ridge of high pressure in place over the Southern Plains
Thursday expands west this weekend with the high consolidating near
CA/NV. This open us up to easterly impulses and a general uptick in
thunderstorm activity this weekend into early next week. The upper
high then begins to make its move back down to east-central AZ by
the middle of next week which should result low-grade thunderstorm
pattern for our neck of the woods.

&&

.AVIATION...Valid through 02/00Z.
Scattered to numerous SHRA/TSRA through Wednesday midday with cigs
BKN 7k-10k ft MSL through Wednesday. Storm coverage will decrease
from the west-to-east Wednesday afternoon. The heaviest rainfall
appears to be late this afternoon into the evening. Periods of MVFR
ceilings/vsbys, mountain obscurations, and wind gusts to 45 kts
possible with the stronger storms. SFC wind 12 knots or less, except
when influenced by thunderstorm outflows. Aviation not updated for
TAF amendments.

&&

.FIRE WEATHER...Expect widespread showers and thunderstorms this
afternoon into Wednesday with a threat of heavy rain and flash
flooding. Thursday storm chances taper off some with isolated to
scattered activity mainly from central Pima County eastward. A more
active monsoon pattern then returns this weekend. 20-foot winds will
generally be less than 15 mph when not influenced by thunderstorm
gusts. Minimum RH values will remain well above critical thresholds
this week.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 11 AM MST this morning through late
Wednesday night for AZZ501>515.

&&

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