Flood Watch 2, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
940 AM MST Thu Aug 31 2023

.SYNOPSIS...Expect active thunderstorm days across Southeast
Arizona today and Friday before decreasing into the weekend. The
main hazard from storms today will be strong thunderstorm outflow
winds capable of producing dense blowing dust, especially to the
northwest and west of Tucson. Tomorrow, the threat for strong
winds diminishes, but the threat of heavy rainfall increases. High
temperatures near normal today before decreasing 5 to 8 degrees
below normal Friday and Saturday.

&&

.DISCUSSION...Today is shaping up to be an active Monsoon day across
Southeast Arizona as there are many factors coming together to
increase the potential for strong organized thunderstorm outflows/dense
blowing dust, especially from Tucson west and northwest.

Like a good chef, its important to take stock of the ingredients
we have. First, we are in a state of transition with the mid and
upper level high pressure areas. At 500 mb, the upper high is
starting off along the northern Arizona/New Mexico border and will
move east toward northeast NM by this evening. As this happens,
this will shift the mid-level flow from easterly to southeasterly
across our neck of the woods, not only increasing the directional
difluence over south-central Arizona but additional stretching
deformation from the upper high shifting away will result in
a stronger lifting mechanism. A bit higher up at 300 mb, the
ridge-axis that extends across the southern half of Arizona
shifts east, with the axis becoming a N-S orientation over New
Mexico this evening. The combination of this enhanced lift in the
mid-to-upper levels will enhance thunderstorm activity from
Tucson west and northwest.

The second ingredient is the moisture. The 31/12Z KTWC sounding
depicted a PWAT of 1.20 inches, which matches up with the GOES TPW
satellite imagery which depicts 1.25 inches over Tucson this
morning, with deeper values near 1.50 inches to the west and dry
values near 0.80 inches to the east along the Arizona/New Mexico
border. This drier air will limit thunderstorm coverage across the
eastern portions of the forecast area today. However, given the
southeast flow at 500 mb, this will enhance the downdraft
potential for storms across south-central arizona as it will aid
in the entrainment of dry air into the updrafts in the middle-
levels of the storms.

The third ingredient is the increasing steering flow between
700-500 mb. Initially the flow will start off at 10 kts, but
increases to 25 kts by this afternoon. This will help storms
transition from isolated pulse-type storms into more organized
multi-cellular storm clusters.

All of these ingredients increase confidence that we will see a
more significant Monsoon day across the area today. This is
reflected in the HREF or ensemble of high-resolution models. The
31/12Z HREF suggests 70-90 pct chance of wind gusts from storms
in excess of 30 kts (10-30 percent in excess of 50 kts) across
Southeast Pinal/central and eastern Pima and Santa Cruz counties
between 3-10 PM, with the potential generally moving west and
northwest into the evening hours. The mid-shift issued a Blowing
Dust Advisory for the potential for a Dust Storm/Haboob formation
between Tucson and Phoenix and this looks like it is still on
track to develop. In addition to the threat for lower visibility
in blowing dust, what sometimes gets forgotten is the health
impact of blowing dust to those individuals with heart disease
and respiratory sensitivities. Its a good idea to make sure these
individuals avoid being outside if a dust storm develops.

In any event, today will not be the only active day. Friday will
likely be busy as well for different reasons. As the mid and upper
highs get settled into NM, we see a deeper southerly flow across
the area. This will increase the PWAT values across the area in
general, with 1.50-1.75 inches west of Tucson and 1.00-1.50 inches
to the east. This deeper moisture profile will increase the
potential for heavy rainfall, with the 31/12Z HREF localized
probability matched mean 24-hour precipitation showing pockets of
1-2 inch rainfall amounts west of a line from Sierra Vista to
Oracle. The NBM has also latched onto the potential for heavier
precipitation, with a 36 percent probability of exceeding 1.00
inch at the Tucson International Airport tomorrow, and 46 percent
in Nogales. The WPC has elevated us into the 'Slight' category in
their Excessive Rainfall Outlook (ERO) for Day 2 (tomorrow), and
this looks good to me given all the ingredients.

&&

.AVIATION...Valid through 01/18Z
SCT-BKN clouds with scattered to numerous SHRA/TSRA primarily from
KTUS west and northwest aft 31/21Z thru 01/08Z with ISOLD SHRA/TSRA
overnight. Brief MVFR CIGS/VSBYS, mountain obscurations and gusts
to 35-45 kts and possible BLDU associated with the TSRA reducing
VSBY below a mile. Otherwise typical wind trends generally less
than 12 kts. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Increased shower and thunderstorm activity
today and Friday for the forecast area. Drier air moving in
across the area late this upcoming weekend will bring a downturn
in thunderstorms which is expected to continue into early next
week. 20-foot winds will generally 15 mph or less outside of any
thunderstorm outflow winds. High temperatures will be 7-10 degrees
this afternoon, before lowering back to near normal by the end of
the week.

&&

.PREV DISCUSSION... /issued 235 AM MST Thu Aug 31 2023/

Some small showers are out and about early this morning that will
dissipate, leaving behind a few mid and high level clouds, but
will leave plenty of sunshine to have ample heating and create
some instability. By the afternoon hours, that upper level high
that has been causing our heat over the last few days will be
drifting to the Arizona/New Mexico border. That movement coupled
with the upper level diffluence will lead to some more storm
coverage than what we saw yesterday afternoon. Current convective
allowing models are focusing mainly Tucson westward for the
greatest chances for storm activity. The main threats from any
storms will be locally heavy rainfall, localized flooding, and
gusty to strong outflow boundaries picking up blowing dust. The
HREF is showing large areas of 70% probabilities of surface winds
being 30 kts or greater and a fairly small area of 10% probability
of surface winds being 50 kts or greater for this afternoon and
evening, therefore a blowing dust advisory is in effect for this
afternoon from 3 PM to 7 PM.

Looking into Friday, the activity from this afternoon and evening will
certainly play a part in what will unfold. As always, left over
cloud cover limiting heating/instability is possible. Outside of
that unknown, the type IV monsoon pattern where the upper level
high is to the east of us located over the Great Plains while a
digging trough is to the west of us located over Northern
California looks to hold. This pattern has the southerly flow,
allowing for a moisture surge with possible PWAT to increase
mainly areas Tucson westward to 1.5 inches to up to 2 inches in
Western Pima county.

Looking ahead, the trough begins to work its way back into the
mean flow on Saturday and Sunday, bringing a drier westerly flow,
thus decreasing coverage of storms once again. By Wednesday the
high begins to reconsolidate along the Arizona/New
Mexico/International border, bringing back our warmer
temperatures.

High temperatures are on the decline today, with it dipping to
near normal temperatures. After today, there is a bit of a cool
down with Friday and Saturday being near 5 to 8 degrees below
normal, before slowly warming up to near normal the begining of
the work week.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Blowing Dust Advisory from 3 PM this afternoon to 7 PM MST this
evening for AZZ501-502-504-505.

&&

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Preliminary Local Storm Report
National Weather Service Tucson AZ
823 AM MST Thu Aug 31 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0530 PM     Rain             6 ENE Tucson            32.24N 110.81W
08/30/2023  E1.03 inch       Pima               AZ   Trained Spotter

            Camino Seco just south of Wrightstown. 1.03
            in 30 minutes.

&&

GRG