741
FXUS65 KTWC 181618
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
918 AM 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.

&&

.UPDATE...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. In the
short term, we are currently seeing scattered light showers with
embedded thunderstorms across far western Pima county this morning
as a decaying complex from convection along the International
Border during the early morning hours continues to push off to the
west. Thicker cloud cover extends as far east as Santa Cruz
county. Its possible that this feature may delay convective
initiation this afternoon across south-central Arizona while it
remains in the subsident wake of this departing mid-level feature.

In any event, 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+kt 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 stroms 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.

&&

.PREV DISCUSSION... /issued 344 AM MST Thu Jul 18 2024/

Residual thunderstorm activity across southern Cochise and Santa
Cruz counties continue to linger at the time of this writing
forming off outflows from evening convection in Sonora. CAMs have
somewhat struggled on resolving this and similar activity over the
past few nights, but anticipate it gradually diminishing as it
pushes west into south central Pima county this morning.

PWATS are currently running around normal for this time of year at
around an inch near the NM state line to around 1.3 inches in the
Tucson Metro to around 1.5 inches across western Pima county. The
overall upper air pattern over the past couple of days has been
characterized by a mid/upper level high drifting west over AZ/NM.
The flow pattern resulting from the high is currently
easterly/southeasterly, but as the high continues to meander
northwest over the next few days we will see the flow pattern
gradually becoming more easterly/northeasterly heading into the
weekend. The northeasterly flow regime favors storms forming off the
higher terrain/Mogollon Rim and then propagating into the lower
deserts of Southeast Arizona through the late afternoon hours. Right
now, Saturday and Sunday appear to be the best chances for
convection of that nature.

For today, HREF/CAMs are depicting scattered showers and
thunderstorms primarily forming over Santa Cruz and southeastern
Pima counties this afternoon as well as in the White Mountains and
elevated terrain. The main concerns with these storms continues to
be gusty outflow winds and locally heavy rainfall leading to the
possibility for isolated occurrences of flash flooding. Temperatures
will be a degree or two warmer across southeast Arizona today and
tomorrow resulting in isolated areas of major HeatRisk. Decided to
hold off on heat related products at this time as there is not
enough widespread Major HeatRisk to warrant a headline.

&&

.AVIATION...Valid through 19/12Z.
SCT-BKN clouds at 8k-12k ft AGL gradually diminishing from east to
west after 18/12Z and forming again after 18/19Z associated with SCT
TSRA/SHRA development. MVFR conditions in TSRA, with mountain
obscurations, vsby reductions, and wind gusts to 40+ kts. Outside of
TSRA, SFC winds at most locations will remain under 12 kts favoring
SWLY thru NWLY directions 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 mid July moisture levels expected through at least the
upcoming weekend. 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 will primarily be southwest through northwest into the
weekend.

&&

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

&&

$$

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