Severe Thunderstorm Warning 142, NWS Tucson, AZ

Area Forecast Discussion
National Weather Service Tucson AZ
210 PM MST Thu Jul 18 2024

.SYNOPSIS...Temperatures will remain 4 to 7 degrees above normal
through the rest of this week, with typical mid-July moisture levels
across the region. This will result in the daily chances for showers
and thunderstorms. The main threats will continue to be strong gusty
outflow winds and heavy rainfall leading to localized flash
flooding.

&&

.DISCUSSION...Welp. That low-grade Monsoon day we saw yesterday
across Southeast Arizona was short-lived. We can expect to see a
ramp up in a activity over the next several days, begining today.

Today is shaping up to be a more typical Monsoon day across the
entire forecast area. To set the big picture, the 500 mb upper
high is centered across eastern Arizona and the 300 mb high center
is positioned near the bootheel of New Mexico. Both of these
features will drift west to northwest today, becoming stacked over
central Arizona by this afternoon. As this happens, we not only
cool slightly in the mid-to-upper levels, but we also find
ourselves with weak lift due to a stretching deformation aloft.
During the Monsoon season, it is sometimes the most subtle
features or mechanisms that differentiate a quiet or active day.
The NNE flow pattern today isn't quite the favored NE "rimshot"
for us today, that will come overnight and more specifically
tomorrow and into the weekend, but it will help force storms off
the higher terrain into the lower elevations across Graham and
Greenlee counties this afternoon.

The latest GOES derived Precipitable Water satellite imagery
indicates dry air (between 1.0 and 1.2 inches) situated east of
Tucson with the 18/12Z KTWC sounding indicating a value of 1.49
inches over Tucson, and deeper PWAT values to 1.75 inches across
the SW corner of Arizona. There is a deeper pocket of dry values
near 1.0 inch along the International Border in Cochise county.
This is mainly drier air residing between 700-500 mb which should
increase the downdraft potential (DCAPE) this afternoon across
Santa Cruz as the easterly flow tried to entrain this into storms
that develop this afternoon. The 18/12Z SPC HREF indicated an area
of 70% probability of 30+ kts winds within 25 miles of a given
point across Santa Cruz, Eastern Pima and the southern portions of
the TON at 00Z this afternoon, with even a 10% chance of wind
gusts in excess of 50+ kts withing 25 miles of a given point in
the Tohono O'odham Nation. The drier air to the east of Tucson will
also increase the potential for strong thunderstorm outflows there
this afternoon for the storms that develop over the higher
terrain and push off into the lower elevations.

Strong thunderstorm outflow winds will be the primary threat from
storm this afternoon, with an increasing threat of heavy rainfall
as storms push into the deeper moisture across central and western
Pima county this afternoon/evening.

There is still hints from the high resolution CAMS runs of
nocturnal convection, especially with the 18/14Z HRRR, across
Southeast Arizona tonight as the lift associated with the
shifting upper high center will interact with the elevated CAPE.
Nothing strong, but if it does happen it may hinder thunderstorm
activity Friday despite the improving NE flow/rimshot potential.

As we move into the weekend and next week, the upper high positions
itself near southern NV which would suggest the stronger NE flow
pattern will continue into next week for our neck of the woods.
The strongest days are still looking like Saturday and Sunday as
the mid-level flow is the strongest for Southeast Arizona.

&&

.AVIATION...Valid through 20/00Z.
SCT-BKN clouds at 8k-12k ft AGL thru 19/05Z and again aft 19/18Z,
with diminishing cloud cover between 19/05Z and 19/18Z. SCT
TSRA/SHRA thru 19/05Z. SCT TSRA/SHRA developing again aft 19/19Z.
MVFR conditions in TSRA, with mountain obscurations, vsby reductions
and wind gusts to 40+ knots. Outside of TSRA, SFC wind at most
locations 12 knots or less favoring a WLY/NWLY direction during the
afternoon/early evening hours and variable in direction at other
times. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...Deeper moisture remains across southeast Arizona,
with typical July moisture levels expected into at least early next
week. This will result in the daily cycle of mainly afternoon and
evening showers and thunderstorms. The main threats will be strong,
gusty and erratic outflow winds and heavy rainfall leading to
localized flash flooding. Minimum relative humidity values will be
20-30 percent in most lower elevation locations and 25-40 percent in
mountain areas. Away from thunderstorms, 20-foot winds will
generally be 15 mph or less, with afternoon/early evening gusts to
20-25 mph and light winds overnight. Wind direction during the
afternoon hours will primarily be west to northwest through the
weekend, especially from Tucson westward and in the upper Gila
River valley.

&&

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

&&

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