Wildfire in Arizona, August 21 to 31, 2020

Area Forecast Discussion
National Weather Service Tucson AZ
924 AM MST Fri Aug 21 2020

.SYNOPSIS...A better chance of thunderstorms through the weekend,
with temperatures closer to average for late August. High pressure
building back through Arizona will tend to limit thunderstorm
coverage and push temperatures back up next week.

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.DISCUSSION...Morning visible satellite imagery shows clearly the
MCV located to the SW of Casa Grande and moving towards the NW.
Still some residual cloudiness across parts of Pima, Pinal and Santa
Cruz Counties with generally sunny skies elsewhere. Adequate
moisture is in place with PWAT values around 1" for eastern areas
and 1.5" across the western deserts with the 12Z KTWC sounding
coming in at 1.3". However, after the widespread convection last
night, the atmosphere is worked over with considerable CIN to
overcome. The combination of the CIN and the subsidence behind the
MCV will result in a down day for most locales. Any convection will
be most likely to develop across the White Mtns later this afternoon
and perhaps on some of the higher terrain near the Int'l border but
confidence on that is fairly low. We've gone ahead and nudged PoPs
down slightly for this afternoon given the lackluster setup. Any
storms that do develop across the White Mtns will try to push SW
this evening but may struggle moving into some of the lower
elevations. Given the cooler and more moist boundary layer it will
seem downright cool today with highs only around 100 in Tucson.

With the area of high pressure aloft remaining in a favorable
location across northern Arizona to near the Four Corners this
weekend, that will lend itself to a continued good flow pattern for
monsoonal activity. The best activity may remain east of Tucson
Saturday with perhaps Sunday being the more active day but will take
a closer look at that for the afternoon package. See previous
discussion below for additional information.

&&

.AVIATION...Valid through 22/12Z.
SCT-BKN clouds around 15k ft MSL decreasing through the morning
hours with SCT cu redeveloping around 10k ft MSL this afternoon.
ISOLD-SCT TSRA/SHRA this afternoon into this evening. SFC wind gusts
to 40 kts with strongest TSRA along with MVFR CIGS/VSBYS and
mountain obscurations. Otherwise, SFC wind variable less than 10 kts
this morning becoming WLY-NWLY this afternoon 10-15 kts with gusts
to 25 kts. Strongest winds at KSAD. Wind becoming light and variable
late this evening into Saturday morning. Aviation discussion not
updated for TAF amendments.

&&

.FIRE WEATHER...With better moisture in place, expect broader
coverage of afternoon/evening showers and thunderstorms through
Sunday. 20-foot winds generally remain less than 15 mph outside of
any thunderstorm gusts which could reach 40+ mph. Minimum relative
humidities 25-30 percent across the valleys through Sunday along
with good overnight recoveries. Drier air and hotter temperatures
next week slightly reduces shower/thunderstorm coverage and humidity
values by 5 to 10 percent.

&&

.PREV DISCUSSION...With the deeper moisture we have imported from the
south over the past 48 hours, our thunderstorm coverage was much
more seasonal today, with a decaying thunderstorm complex now west
of Tucson. A weak remnant circulation is noted in the radar imagery
over the Tohono O'Odham Nation with showers expected to continue to
weaken.

Storms will struggle with early cloud cover and the decaying
circulation drifting slowly eastward. Our best chance will be with
initial development over the areas with the best insolation closer
to the New Mexico border this afternoon. Outflows pushing eastward
will try to spread storms further west in the late afternoon, but
the easterly flow is weaker and will not lend quite as much support
to easterly outflows.

The high center is expected drift to the 4 corners area over the
weekend, with storms ramping up and likely peaking Sunday afternoon.
As the flow amplifies upstream and downstream, the ridge will have
room to build back through Arizona early next week, with high
pressure strengthening overhead once again. That will tend to bring
a return of hotter temperatures with storm coverage likely
struggling. We may need another round of Excessive Heat headlines
for portions of the area by Monday afternoon, but a lot will depend
on how our thunderstorm situation evolves.

&&

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

&&

Public...Lader
Aviation...Carpenter
Fire Weather....Carpenter
Prev Discussion...Meyer

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