Debris Flows in Arizona, July 22, 2020

Area Forecast Discussion
National Weather Service Tucson AZ
855 AM MST Wed Jul 22 2020

.SYNOPSIS...Strong southerly flow will bring increasing moisture
this week. Thunderstorm chances ramp up over the next several days,
with the strongest thunderstorm days likely Thursday and/or Friday.
Some storms will generate heavy rain and strong winds.

&&

.DISCUSSION...The early morning sounding had plenty of moisture
although it may be a slightly overdone as it looks a bit
convectively contaminated. Even with that it is higher and trending
upward over time. To help, clouds have already dissipated west of
Benson with the heating getting off to a good start. Trends for areas
east of there also point toward thinning clouds.

Taking a cruise through the various CAM solutions available shows a
significant lack of consistency in convective development today
which is not surprising given there isn't really a "kicker" to focus
the storm development. They are consistent in the idea that once
storms form, new storms fire off the convective outflows and that is
also expected given the thermal and moisture environment. So the
thought is the most likely initial development would be over higher
terrain with the earliest sun exposure and over central/western Pima
county where the greatest instability exists. This also happens to
be the area with the highest PW and lightest mean wind so we will
need to watch for heavy rain producing storms across the Tohono
O'odham Nation today for any persistent storms. Further east, storms
will move slowly to the north or follow outflows. Overall it should
be a busier day than yesterday

For Thursday and beyond, will take a look at that later today but
don't foresee significant changes and there is plenty of detail in
the previous discussion below.

&&

.AVIATION...Valid through 23/12Z.
Partial clearing through the morning hours and then scattered
TSRA/-SHRA redeveloping this afternoon. FEW-SCT clouds 10-15k ft MSL
and BKN clouds above 15k ft MSL. Normal wind trends with some
gustiness in the afternoon, and possible strong erratic outflow from
any thunderstorms. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Thunderstorm activity will increase across a wider
area today, with a further trend upward Thursday into the weekend.
The main threats will be thunderstorm gusts of 45 mph or higher and
locally heavy rainfall which bring an increased risk of flash
flooding/mud flows on area burn scars. Otherwise normal wind trends,
with 20-ft winds generally less than 15 mph through the period. With
the moisture increase comes high humidity levels resulting in good to
excellent overnight recoveries and day time mins of 25 percent or
higher Thursday onward.

&&

.PREV DISCUSSION... /issued 320 AM MST Wed Jul 22 2020/
Our increasing southerly flow has arrived as expected with a deep
moisture surge underway. 20 mph through 5k feet at Yuma, where the
dew point is almost 70. Widespread dew point increase between 15 and
20 degrees across the board with most locations in the upper 50s and
60s. Precipitable water values up to at least 1.45 inches, with the
HREF probabilities pushing the 1.5 inch line deep into the state over
the next 24 hours. We'll probably top out as high as 1.65 to 1.7
inches Thursday and Friday with NAEFS mean integrated water vapor
transport hitting the 97th climatological percentile Thursday. GEFS
and ECMWF ensembles are showing a significant number of members that
even hit as high as 1.8 inches PW. Starts to make me a little nervous
that we're approaching levels that are actually too much for
widespread strong thunderstorm activity.

Nevertheless, short range mesoscale models as well as NBM and U
of A WRF trends are right on track with our long anticipated
forecast of a round of strong to severe thunderstorms developing
Thursday (maybe even in the morning hours). These should be very
wet storms with high rainfall rates and strong outflows.

Friday still looks busy as well. After being worked over Thursday
and/or Thursday night, there is not much time to recover for
Friday. However, we're still tracking a strong mid level impulse
from the south that should help restart and focus additional
shower and thunderstorm activity by later Friday or Friday night.

Plenty of moisture to keep a daily/nightly chance of thunderstorms
in our corner of the state through at least Monday. By then we'll
be watching a less favorable reconsolidation of the high to our
northwest, but also potentially enough room for a strong easterly
wave to sneak toward us from the east. We'll have to wait and see
on this one.

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

Public...Cerniglia/Meyer
Aviation...Rasmussen
Fire Weather....Rasmussen

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