Excessive Heat in Arizona, August 5, 2019

Area Forecast Discussion
National Weather Service Tucson AZ
933 AM MST Mon Aug 5 2019

.SYNOPSIS...More hot daytime temperatures can be anticipated again
today and Tuesday along with isolated afternoon and evening
thunderstorms. A more favorable weather pattern for showers and
thunderstorms will bring increasing rain chances the second half
of the week into the coming weekend. This will also moderate
temperatures closer to normal.

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.DISCUSSION...A mid-level ridge is currently over most of Arizona,
and will likely push temperatures to near record levels today.
This morning's 12Z sounding is similar to yesterday's 12Z sounding
which yielded a 107 degree high for Tucson and even hotter further
west. Because of this similar temperatures are expected today. As
a result an Excessive Heat Warning is in effect until 8 PM MST
this evening from Tucson westward.

Thunderstorm activity today will likely be a little more active
than yesterday. MLCAPE values will range from 500-1000 J/kg with
the best instability in Cochise County. Surface dewpoints are in
the upper 50's to lower 60's with PWAT between 1.3-1.5 inches, so
there is sufficient moisture in place. Hi-res models show storms
hugging the higher terrain mainly to the south and east of
Tucson. Storms may struggle to move away from the mountains thanks
to the weak steering flow with the high over Arizona. The main
impacts of any storms will be locally heavy rainfall and gusty
winds. Current forecast looks on track so no changes right now,
see the previous discussion for longer term details.

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.AVIATION...Valid through 06/12Z.
Expect a more activity today, especially south and east of TUS.
Isolated SHRA/TSRA is expected for KTUS with potential for gusty
outflow winds, especially later in the afternoon and early
evening. Scattered SHRA/TSRA is expected for the other terminals
with gusty and erratic winds near any TSRA and localized MVFR CIGS
and VSBYS possible. Cloud decks will start as FEW/SCT and become
SCT/BKN around 8-12K FT AGL by the afternoon and evening with
another SCT-BKN deck at 25k ft AGL. Clouds will thin after sunset.
Sfc winds generally out of the northwest 5-10 knots this
afternoon becoming variable and less than 8 knots overnight.
Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...The daily cycle of afternoon and evening showers and
thunderstorms will continue through the forecast period with
coverage expanding to the entire CWA Wednesday into the weekend.
Minimum RH values remain above 20-25 percent, and overnight
recoveries will be very good. Min RH values will rise in the 25-35
percent range by the middle of the week with continued very good to
excellent RH recoveries. Even with the abundant moisture,
temperatures will be several degrees above normal today and Tuesday.
Temperatures will likely cool closer to normal by the middle and
end of the week. Aside from gusty erratic winds from thunderstorm
outflows, wind trends stay light and terrain driven.

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.PREV DISCUSSION... /issued 307 AM MST Mon Aug 5 2019/
Another day with broad high pressure over the Desert Southwest
pushing temperatures to several degrees above average. With a weak
flow we haven't been able to import much deep moisture lately,
and have primarily been recycling the moisture in place. This has
seen surface dew points drop from the lower and middle 60s to the
mid to upper 50s, and precipitable water values drop from 1.6
inches to 1.3 inches. Still enough to work with, but with limited
convective potential for valleys, mountains will still tend to
favor mountain locations today.

With the desert heat, pressure differential and outflows pushing
into the gulf, we should see some flavor of gulf surge over the
next 24 hours. It's not looking terribly strong, say 10 kts up to
4k feet or so. NAEFS mean 700mb specific humidity is up over the
next 48 hours, but the most significant moisture increase will be
below this. The best thing we have going for us is an inverted
trough wrapping under the high and pushing across Chihuahua and
Sonora over the next 72 hours. This will help organize storms a
little better.

As high pressure elongates into Texas by Wednesday, this will tend
to engage more activity from the Gulf of Mexico in the resulting
easterly and southeasterly flow with potential easterly waves and
additional moisture. Our forecast will ramp up thunderstorm
coverage and QPF values as the week progresses. At some point we
will have a some up days that wipe out the next day, but there's
still some timing and trajectory issues with the busier
thunderstorm period into the weekend. Building Tuesday and
Wednesday into a big day Thursday? GEFS plumes are showing surface
CAPE in the 1400 to 2200 J/kg range Thursday with elevated shear
as well.

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.TWC WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM MST this evening for AZZ501-
502-504-505.

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