Forecast Discussion

Forecast Discussion for TWC NWS Office
FXUS65 KTWC 220418

Area Forecast Discussion
National Weather Service Tucson AZ
918 PM MST Sun Jul 21 2019

.SYNOPSIS...Ample moisture and favorable flow will result in
scattered to numerous thunderstorms Monday afternoon into Monday
night. Blowing dust due to thunderstorm gusts may occur Monday
afternoon and evening. Scattered showers and thunderstorms to
continue Tuesday into next weekend.


.DISCUSSION...Plenty of activity this evening with Pima county,
namely the Tohono O`odham Nation, taking the brunt of it with
several severe warned storms thanks to capes in excess of 3000 j/kg.
That activity has moved east of the area and we even noted a small
MCV within the dying convection. There was plenty of water with some
of the storms as noted by the rapid rise on the Vamori Wash. I`m sure
at least some water was flowing in a few other washes in central and
western Pima county as well. We will need to keep that in mind
tomorrow evening when convection moves into that area.

At this time the activity over southern Arizona should be very
limited for the rest of the night with most of the action remaining
across northern Mexico. I made some adjustments to the grids tonight
into Monday to account for current and expected conditions, mainly
less activity tonight and more tomorrow. Actually tomorrow looks to
be the most active day by far this season and I tweaked on pops,
winds and temps for the afternoon and evening hours. Should be an
interesting afternoon and evening!


.AVIATION...Valid through 23/06Z.
Isolated -SHRA may continue southeast to southwest of KTUS late
tonight into early Monday morning. Then scattered to numerous
TSRA/SHRA expected Monday afternoon into Monday night. Brief wind
gusts to 45-55 kts with the stronger TSRA. SCT-BKN cloud decks
mainly 10k-15k ft MSL, except MFVR cigs and vis with storms and
brief IFR conditions possible Monday afternoon with local mountain
obscuration. Surface wind light and variable overnight becoming
westerly 5-15 kts Monday afternoon. Aviation discussion not updated
for TAF amendments.


.FIRE WEATHER...Expect scattered to numerous thunderstorms Monday
afternoon into Monday night. Adequate moisture will then persist
Tuesday into next weekend for scattered mainly afternoon and evening
thunderstorms. Gusty outflow winds expected near thunderstorms.
20-foot winds generally from the northwest under 15 mph but will
transition to be out of the east to southeast Monday morning. Lowest
minimum humidity is under 15 pct in the Gila River Valley today.
Relative humidity unlikely to be under 20 percent anywhere for the
rest of the week.


.PREV DISCUSSION... /issued 315 PM MST Sun Jul 21 2019/
Our large scale pattern is shaping up nicely with a strong ridge
over the desert southwest and the high center consolidating near the
4-corners region to start the week. The surge we were discussing
yesterday continues to increase in intensity with 12-20kts now up to
6k feet on the KYUX Vad Wind Profile. Surface dew points are up 5 to
15 degrees across the board with upper 50s and 60s pushing through
Santa Cruz and Pima counties. Probably maintaining precipitable water
values in the 1.5 inch range from Tucson westward, with up to 1.75
inches south of the border and 1.3 inches in eastern areas. Our main
area of focus this afternoon is in the deeper moisture areas with a
weak convergent area from northern Sonora through Santa Cruz county
and the Tohono O`Odham Nation. Deeper moisture will continue to push
into the area from the south tonight as we set up an easterly
steering flow under the high.

Tomorrow looks like our first widespread thunderstorm outbreak of
the season. SREF plumes have ML CAPE around 1200 to 1500 tomorrow
afternoon with HREF ensembles surface based CAPE in excess of
1500 J/kg. Effective vertical wind shear between 25 and 30 kts.
UA WRF forecasts respond accordingly tomorrow with strong and
organized thunderstorm development through the afternoon (and are
doing well today). Perfect set-up for organized easterly outflow
and blowing dust issues into the lower deserts.

As mentioned before, Tuesday would likely be a recovery day if
the Monday forecast works out. We`ll see on that. We will still
have plenty of boundary layer moisture to recycle the second half
of next week, but are unlikely to maintain a deep fetch of
moisture with an easterly component to our mid level flow
periodically drying out (with limited Gulf of Mexico influence).
However, with the high center finally in a position where it
wobbles back and forth overhead to northwest to northeast of us,
the overall trend will be better with up days and down days.





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