Scientific Forecaster Discussion
fxus65 ktwc 231558
Area forecast discussion
National Weather Service Tucson Arizona
857 am MST Fri Aug 23 2019
Synopsis...a more active monsoon pattern will continue today and
Saturday with temperatures returning to near or slightly below
normal. A drying trend resumes Sunday with isolated storms limited
to near the international border as high pressure starts to build
back in overhead. Dry conditions generally prevail Monday and
Tuesday with above average temperatures returning. Thereafter,
moisture returns by Wednesday for another chance for showers and
thunderstorms will then exist toward the latter half of next week.
Discussion...the latest satellite imagery shows debris clouds
across the eastern half of the forecast area from last evenings
convection. However, this cloud shield continues to erode quickly
this morning and expect this trend to continue through the remainder
of the morning hours. Surface dewpoints are currently in the upper
50s to upper 60s across southern Arizona, with blended total precip
water imagery between 1.0-1.3" precipitable water across most of the forecast area
(except White Mountains which was around 0.7" pw). The 190823/12z
ktwc sounding had a precipitable water of 1.41", with an Li of -4. The morning
sounding did have a little bit of a worked over appearance from last
night's convection, but with plenty of moisture in place and
sunshine from clearing skies don't see much in the way of convective
inhibition once temperatures begin to heat up.
The latest high res model runs show initial convection firing in the
White Mountains and the sky islands south and east of Tucson, with
areas south of Tucson coming into play by the early to mid afternoon
hours. Steering flow is rather weak with general movement of storms
to the east/northeast. Will have to rely on outflow boundaries to
get storms into the lower deserts/northern portion of the County Warning Area.
Overall, the peak timing for maximum storm coverage today will be
between 2 PM and 8 PM, with the main threats being heavy rainfall
and strong winds/small hail with the stronger storms. If storm
outflows do manage to work into northern Pima/southern Pinal
counties, blowing dust could also be a concern, especially along the
Overall, forecast package looks to be in good shape. No updates this
morning. For more information on the latter periods of the forecast,
please refer to the previous discussion and other sections below.
Aviation...valid through 24/18z.
Isolated -tsra/-shra developing east to south of ktus 18z-20z today.
Scattered to numerous -tsra/-shra across much of Southeast Arizona
later this afternoon into this evening. Expect decreasing coverage
of -tsra/-shra tonight with the bulk of -tsra/-shra ending by
daybreak Saturday or at end of valid period. Brief wind gusts to 45
kts and MVFR cigs/vsbys with the stronger thunderstorms and rain. Otherwise, cloud
decks mainly 12k-18k ft mean sea level. Surface wind this afternoon wly/nwly 8-
15 kts with gusts near 20 kts. Surface wind variable in direction
mostly under 12 kts at other times and aside from tstm gusts.
Aviation discussion not updated for taf amendments.
Fire weather...scattered to numerous thunderstorms across much of
Southeast Arizona this afternoon and evening. The bulk of
thunderstorms should end by daybreak Saturday. Scattered
thunderstorms Saturday afternoon and evening favoring southeast to
southwest of Tucson, and across the White Mountains. Expect only
isolated thunderstorms Sunday followed by dry conditions across much
of the area Monday into Tuesday. Scattered mainly afternoon and
evening thunderstorms should return next Wednesday and Thursday.
Aside from thunderstorm wind gusts, 20-foot winds will be terrain
driven mainly under 15 mph.
Previous discussion...after an active evening last night, things are
much quieter this morning as convection has diminished and we are
left with some debris cloudiness as infrared satellite shows cloud tops
have been warming over the past few hours. Moisture has increased
dramatically with ample moisture transport into southern Arizona due
to a strong Gulf surge. Dewpoints are currently in the 60's from
Tucson westward and in the 50's to the east of Tucson.
Correspondingly, the latest GPS precipitable water values are in the 1.5 inch
range for Tucson.
Today is likely to be another active day across much of the region.
There are still some uncertainties, especially with how much solar
insolation is available and how worked over the lower atmosphere
will be from the convection last night. In the upper levels, an 300 mb
trough axis will be located across the eastern half of the area and
may help aiding lift as anvils also track east to southeastward. In
the lower to mid levels, a general slow steering flow of the storms
towards the west is expected which is favorable since it will be
away from the anvils. The latest cams including the hrrr, uofa WRF-
NAM and uofa WRF-GFS are all fairly active today. Given the deep
moisture available, slower moving storms and antecedent wet
conditions from yesterday in some locales, the primary concern will
be heavy rain and the potential for flash flooding. The most likely
location to see widespread storms will be along the international
border across Santa Cruz and SW Cochise counties with isolated to
scattered storms elsewhere. We did consider a possible Flash Flood
Watch for portions of the area, but confidence is still a bit low so
will defer possible issuance to the day shift.
A favorable pattern is once again expected on Saturday with isolated
to scattered showers and thunderstorms, especially for the southern
half of the area where the deepest moisture is expected. A drying
trend will commence on Sunday as the mid/upper level starts to build
southward and strengthen with increasing heights. There will still
be enough moisture for isolated showers and thunderstorms near the
international border Sunday afternoon with generally dry conditions
Thereafter, dry conditions are generally expected to prevail Monday
and Tuesday as the mid/upper level high peaks in intensity as well
above normal temperatures return with the heat peaking Tuesday.
The hot and dry period early next week will be somewhat short lived
as the combination of the mid/upper level high shifting a bit
northward and increased lower level moisture will result in an
uptick in convective activity starting Wednesday with slightly
cooler temperatures following by late next week as well.