Flash Flood Watch 4, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
831 AM MST Fri Aug 28 2020

.SYNOPSIS...Still hot today with isolated to scattered showers and
thunderstorms. An increase in tropical moisture will bring a
better chance of showers and thunderstorms this weekend into
early next week. High temperature will be much cooler due to the
added cloud cover and shower activity. Heating back up by the
middle of next week.

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.DISCUSSION...It's a pretty straight forward forecast in the near
term today. The 12Z KTWC sounding came in with a 1.23" PWAT value
which is quite similar to 24 hours ago. Still a bit drier across
Cochise County with PWAT values around 1 inch. The mid/upper level
water vapor imagery shows the high pressure aloft has shifted to the
east over NM with southerly flow across the area. Meanwhile,
Tropical Depression Hernan near the mouth of the Gulf of California
and Tropical Storm Iselle to the southwest of the lower Baja
peninsuala will both help to transport moisture our way over the
upcoming days. A Gulf surge will begin to move into AZ later this
afternoon and increase tonight setting the stage for an active
weekend. Meanwhile, for today, isolated to scattered showers and
thunderstorms are expected mainly from Sells eastward this afternoon
and evening. Not a lot of CAPE to work with /generally 500 J/KG or
less/ and the storms will be lower topped as EL levels are lower due
to warming around H3 but given DCAPE values around 1500 J/KG we'll
see some gusty outflow winds and a few pockets of heavier rain with
the stronger storms. For more info beyond today, see previous
forecast discussion below.

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.AVIATION...Valid through 29/18Z.
SCT-BKN clouds 10-15k ft MSL mainly thru the forecast period.
Isolated to scattered -TSRA/-SHRA aft 28/17z. Expect the smoke haze
to linger through the period. Aside from thunderstorm outflow gusts
to 40 kts, surface winds generally remain less than 12 knots with
afternoon/evening winds out of the northwest and variable at other
times. Aviation discussion not updated for TAF amendments.

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.FIRE WEATHER...Isolated to scattered showers and thunderstorms
today. This weekend into early next week moisture increases
leading to a chance of showers and thunderstorms area wide.
Temperatures will remain hotter than normal with minimum
humidities 10-20 percent across the valleys and 20-30 percent in
the mountains, with good overnight recoveries through Friday.
Saturday into next week minimum humidities will generally be above
25 percent with normal to slightly below normal temperatures.
20-foot winds generally remain less than 15 mph outside of any
thunder.

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.CLIMATE...Yesterday the Tucson airport recorded the 53rd high
temperature of 105 degrees or hotter this year, tying 1994 for the
most in a calendar year. Today would be a record setting 54th day
with a forecast high of 106 degrees.

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.PREV DISCUSSION...The center of the upper high that was over west
central NM last evening will drift east today into eastern NM
bringing about southerly flow to the area. PW values across the area
this morning ranged from around 1" east to 1.50" west. Isolated to
scattered showers this afternoon into this evening across most of
the area with the exception being across western Pima county where
it will be dry most of the day outside of a few light showers this
morning.
Gusty winds, blowing dust and brief heavy rain main concerns with
storms. Another hot day but just below Excessive Heat criteria.
Tucson can set a record all- time yearly cumulative record today.
See climate section below. A few showers lingering into the
overnight hours.

Tropical storm Hernan could enter the Gulf of California per the
latest advisory from the NHC. This system will initiate a surge of
moisture up the Gulf of California today, likely hitting Yuma late
today. The increased moisture on Saturday, with PW values ranging
from 1.25" east to 1.75" west, will bring a better chance of showers
and thunderstorms with the same hazards as today. With added cloud
cover highs on Saturday, highs will be 5 to 8 degrees cooler than
today.

Still some uncertainty in the Saturday night through Monday forecast
with respects on the remnant moisture from of Tropical Storms Hernan
and Iselle moving across the area interacting with upper level trof
to our west. GEFS produces a good amount of rain to the area,
considering what has occurred this monsoon, while the ENS is not as
rambunctious. Will be leaning with the wetter GEFS for now. WPC
Excessive Rainfall outlook (ERO) has most of the area under marginal
risk for some heavy rain. This could cause some problems with a few
of our burn scars, especially the Bighorn burn scar. We may also end
up just clouded over with lots of showers and the occasional
embedded heavy shower. NAM/GFS MOS highs on Sunday for Tucson are
only in the mid to upper 80s. If there is more sun, then the threat
of thunderstorms producing very heavy rain is higher with PW values
ranging from 1.50" east to slightly over 2" west. This would
increase the flash flood threat.
Lots to consider over the next 24 hours. Took the middle of the road
with high temperatures Sunday and Monday. Sure would be nice to see
a good rain event since most of the area was downgraded to Extreme
drought (D3) status on this weeks Drought Monitor map.

Moisture starts to be pushed east on Tuesday, the first day of
meteorological Fall, as an upper level trof moves thru the four
corners. Still enough moisture around though for a chance of
afternoon and evening storms. Highs will warm back up to around
normal to start September.

Thereafter, high pressure aloft over California will expand over the
state on Wednesday and Thursday. NW flow will shunt moisture to our
south and heat the area back up again. Both ECMWF and CMC thickness
numbers suggest near all-time September record highs will be
approached in a few locals at the end of next week.

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

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