162
FXUS65 KTWC 191615
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
915 AM MST Fri Jul 19 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...It seems like every morning this week there have been
weak showers with a few embedded thunderstorms across western Pima
county during the mid-morning hours. Why is that? Well, last
night we saw a convective complex decay over Southeast Arizona a
bit earlier than I was anticipating with most of the showers
tapering off before midnight. This decaying system has slowly
drifted SW during the early morning hours. At the same time, another
yet stronger complex in Sonora Mexico overnight moved west into
the northern Gulf and pushed out a strong outflow northward just before
daybreak. This outflow can currently be seen on visible imagery
moving north across southwest Arizona. The intersection of these
two features is what has resulted in the shower/thunderstorm
activity this morning. These storms will weaken over the next hour
or two and have minimal impact.

The big forecast question for today is to gage how much activity
will we see across Southeast Arizona today. I was initially
thinking yesterday that the convection late last night would
delay thunderstorm initiation today...but it seems that the storm
activity ended early enough that it shouldn`t have too much
impact. We are currently seeing sunny/mostly sunny skies as of 9
am MST, and we should realize plenty of surface heating to
destabilize for the afternoon/evening.

The upper pattern is defined by the 500 mb centered over central
Arizona this morning. This feature will slowly drift WNW toward
west central Arizona by this afternoon, then up toward southern
Nevada by Saturday afternoon. This will serve to setup a more
favored NE flow off the White Mountains/Mogollon Rim (rimshot
pattern) that generally results in stronger thunderstorms
impacting Southeast Arizona. Today is kind of that transition day
where all the ingredients are not setup yet like it will be for
this weekend. Expect storms to fire up in the higher terrain and
push SW into the lower elevations by late this afternoon into this
weekend. The 19/12 HREF reflects this thinking by showing a 70%
chance of outflow winds in excess of 30 kts begining across
northern Greenlee at 23Z, then expanding SW into the rest of
Graham and SE Pinal counties by 02Z. The latest GOES derived
precipitable water imagery shows a general increase in PWAT values
east and NE of Tucson (increasing from near 1 inch up to 1.25
inches) over this time yesterday morning. Dry enough to aid strong
thunderstorm downdrafts/outflows, but also enough to help
increase the potential for heavy rainfall. The 19/12Z HREF
depicts a 50% probability of 3-hour rainfall exceeding an inch
with the storms that move off the higher terrain in central
Graham and Greenlee counties. Not great, but not bad. It just
means that we will begin to transition from just wind was the
primary thunderstorm threat to a more dual threat with wind and
heavy rainfall as we transition into the weekend.

It may just be my blurry vision after working a full week of
dayshifts, but it looks like I can see a weak mid-level
disturbance in the GOES water vapor imagery just to the SE of
Lordsburg NM (associated with thunderstorm activity last night
across central NM). This feature should help initiate convection
across Cochise and Santa Cruz county during the mid-afternoon
hours today. We have moistened up a bit in these areas as well
over the past 24 hours, so the treat of a strong thunderstorm
outflow is present...but lower than yesterday. The 19/12Z HREF
has this area painted with a 50% chance of outflow winds 30+ kts
starting at 22Z.

So in general, it looks like we will see storms this afternoon
fire up near the White Mountains in northern Graham and northern
Greenlee counties, as well as the higher terrain in Cochise
county early this afternoon. Both areas will push off the terrain
to the SW and W and expand in coverage by the late afternoon and
early evening hours. Gusty outflow winds and localized brief heavy
rain will accompany the stronger storms.

As mentioned yesterday, as we move into this 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 355 AM MST Fri Jul 19 2024/

There is substantially less nocturnal convection currently
ongoing on across southeast Arizona and along the International
Border compared to this time yesterday. Scattered clouds are seen
across most of the area with a complex of storms across north
central Mexico. Meanwhile, an MCV is developing overhead.

The lack of lingering nocturnal convection will lead to another
afternoon with isolated to scattered showers and thunderstorms.
CAMs/HREF are specifically depicting storms forming off the White
Mountains and higher terrain areas around noon to 2 pm MST before
some of the members hint at the storms pushing off the elevated
terrain into the valleys similar to yesterday. While those members
may hold onto some of the convection into the valleys, the
maintenance of convection yesterday afternoon over the valleys may
hinder the storms from remaining quite as robust as the low levels
were worked over a bit. Thus, looking at more isolated chances for
storms in the valleys this afternoon with scattered expected over
the higher terrain lasting into. For the storms that can propagate
off the higher terrain, gusty outflow winds and isolated instances
of flash flooding may occur.

Looking at the upper level pattern, the mid/upper level high
currently centered over north central AZ will migrate west/northwest
resulting in the flow pattern gradually becoming
northerly/northeasterly. Over the weekend into early next week this
pattern will support an uptick in storms forming off the Mogollon
Rim/elevated terrain and propagating southwest.

Guidance has backed off on temperatures for tomorrow, but the
probability of hitting 105 or higher tomorrow at Tucson
International Airport is around 75 percent. Inevitably, it will be
another warm desert day.

&&

.AVIATION...Valid through 20/12Z.
SCT-BKN clouds at 8k-12k ft AGL diminishing between 19/12Z and
19/18Z before developing again associated with SCT TSRA/SHRA thru
20/06Z. 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 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.

&&

$$

Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson