Heat in Arizona, August 27, 2024

Area Forecast Discussion
National Weather Service Tucson AZ
847 AM MST Tue Aug 27 2024

.SYNOPSIS...There will be increasing shower and thunderstorm
coverage today and tonight, persisting through Wednesday. A slow
decrease in coverage will take place late in the week and over the
weekend. High temperatures will be within a couple of degrees of
normal through the weekend.

&&

.UPDATE...Latest satellite imagery shows partly to mostly cloudy
skies from Tucson westward with a weak circulation just north of
Tucson resulting in a few light showers. Skies are clear to partly
cloudy to the south and east of Tucson. PWAT values this morning
range from 0.9 inches for the far western deserts to around 1.1 to
1.2 inches for most locales from Tucson eastward. Latest HREF/CAMs
show scattered to numerous showers and thunderstorms developing
early this afternoon, mainly from the Tucson area eastward. HREF
mean SBCAPE values are around 500-1000 J/KG this afternoon from
Tucson eastward with DCAPE locally in excess of 1000 J/KG. Expect a
few strong thunderstorms with the potential for gusty winds and
areas of heavy rain including localized flash flooding due to slow
storm movement from the south to southeast at around 5 mph. Showers
and thunderstorms have the potential to continue well into the
evening hours tonight with the possibility of an MCS this
evening/tonight east of Tucson. Current forecast in good shape this
morning. No updates.

&&

.DISCUSSION...In the large scale high pressure was centered over
the mid Mississippi valley and in south central California, with a
storm system moving into the Pacific northwest and an easterly wave
pushing westward across west central Texas. Current PWAT values
range from 0.75" in far western Pima County to 1.25" from Tucson
southeastward. Over the next 48 hours, the Pacific storm system will
track east along the Canadian border and the easterly wave will
stall out in the Big Bend of Texas. Mid level flow across the desert
southwest will turn more southerly allowing some deeper moisture to
move back into the eastern half of the forecast area. By late in the
week and this weekend, the upper high currently over California
shifts northeast into the northern Great Basin. Drier air will begin
to mix in from the west. However, not enough to scour us out, with
isolated convection mainly from Tucson eastward. High temperatures
will be within a couple of degrees of normal through early next
week.

For today and tonight, the hi-res models all point to an active
monsoon day. Both the University of Arizona WRF model and HRRR
indicate convection initiation between 17Z-18Z, with a substantial
increase during the afternoon hours, persisting well into the
evening hours. Interestingly, both of these models hint that an MCS
will develop over northeastern portions of the forecast area (Graham
and Greenlee counties into western New Mexico) during the evening
hours tonight, lasting well into the overnight hours. MLCAPE values
will be 500-1000 J/Kg during the afternoon and evening hours today.
On the other hand, DCAPE values are not that impressive and the
strongest modeled thunderstorm winds are 40-45 mph. The strongest
cells may still have a wet microburst potential, but this will be
very isolated. The main threat today/tonight appears to be a heavy
rainfall potential, especially if training cells occur. WPC has the
eastern two-thirds of the forecast area in a marginal risk for
excessive rainfall today/tonight.

For Wednesday, the forecast really depends upon what happens
tonight. If convection persists well into the overnight and/or early
morning hours Wednesday it may more of a down day than currently
forecast. The models still indicate scattered to numerous showers
and thunderstorms, mainly from Sells eastward. The WPC marginal risk
for excessive rainfall is also shifted slightly eastward, mainly
east of a Nogales to Tucson line.

For Thursday through Saturday, moisture begins to decrease for areas
out west which will decrease storm chances in those areas. Isolated
to scattered showers and thunderstorms are expected from Tucson south
and eastward.

For Sunday and Monday of next week, moisture continues to decrease
across the forecast area, with isolated storm chances confined
mainly to the International Border south/southeast of Tucson and in
the White Mountains.

&&

.AVIATION...Valid through 28/12Z.
FEW-SCT clouds at 8k-12k ft AGL and SCT-BKN clouds AOA 15k ft AGL
along with ISOLD -SHRA/-TSRA thru 27/19Z. Between 27/19Z and 28/07Z,
SCT-BKN clouds at 6k-9k ft AGL and SCT-NMRS SHRA/TSRA from Sells
eastward. Slowly decreasing clouds and precipitation aft 28/07Z.
Visibility restrictions, wind gusts up to 45 kts, heavy rain, small
hail and mountain obscurations possible with the stronger TSRA.
Outside of thunderstorm wind gusts SFC wind 12 kts or less, favoring
a SLY/SWLY direction during the afternoon hours and variable in
direction at other times. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...There will be an increase in the coverage of showers
and thunderstorms through Wednesday, then a gradual decrease in
storms late in the week and weekend. Min RH values will be 20-30
percent in most valley locations, with minimum's of 15-20 percent
west of Tucson across central/western Pima County. Outside of any
thunderstorm outflow winds, 20-foot winds will generally be 15 mph
or less, with afternoon/early evening gusts to 20-25 mph and light
winds overnight.

&&

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

Zell

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