Flood in Arizona, February 14 to 15, 2019

Area Forecast Discussion
National Weather Service Tucson AZ
922 AM MST Thu Feb 14 2019

.SYNOPSIS...Abundant moisture will bring widespread rain to
Southeast Arizona late this afternoon into Friday morning. Showers
ending across much of the area by Friday evening, then some lingering
showers this weekend limited mainly to the White Mountains. A colder
system with much lower snow levels will bring the next chance of
valley rain and mountain snow Sunday night into Monday.

&&

.DISCUSSION...Moisture continues to increase across the region this
morning with the early morning sounding at 0.84 inches of PW which
happens to be only 0.02 inches less the 12Z record for the day, and
we are only just getting started. Still looking at peaking in the
1.20 to potentially 1.30 range overnight which if it happened to be
observed during a sounding would be a record for the months of
December through April!

Anyway that moisture will bring widespread lighter showers to the
valleys later today into Friday morning and when combined with 30-45
knots of orographic lift, heavier rain in the mountains where some
hourly rates will be as high as rainfall totals in the valleys. As a
result the expectation is that washes/creeks draining off the
mountains will get flowing with the potential for some of them to
flood. The areas of greatest concern already have an areal flood
watch in effect, now it will just be wait and see how it plays out.
At this time there is no need for updates.

&&

.AVIATION...Valid through 15/18Z.
Isolated SHRA this morning will gradually expand in coverage through
today, becoming widespread this evening and overnight. Cloud bases
continue to lower, reaching 2k-5k ft AGL tonight. SWLY surface winds
increase in the late afternoon, further increasing during the evening
15-20 kts with gusts to 30 kts. Aviation discussion not updated for
TAF amendments.

&&

.FIRE WEATHER...No fire weather concerns for the next week to 10
days. Widespread rain is expected later today into Friday, with the
heaviest rainfall across southwest to west facing mountain slopes
tonight. Due to this rainfall, some flooding of mountain streams will
be possible into Friday morning. Snow level is near 10,000 feet.
Cooler and damp conditions linger through the weekend with another
storm system expected Sunday night into Monday night. This storm will
be colder allowing snow levels to drop near 3,000 feet and cool
weather continues well into next week. Winds will be gusty at times
with each passing system, especially this evening.

&&

.PREV DISCUSSION... /issued 320 AM MST Thu Feb 14 2019/
The upper pattern is defined this morning by a trough of low
pressure off the coast of California, another in the Gulf of Alaska
attempting to kick the fist system out of its current position, and a
generalized 'dirty' zonal flow over Southeast Arizona. As this first
system ejects east this afternoon into Friday, it will bring
widespread rain to Southeast Arizona, with heavy rainfall amounts
expected in the higher terrain from the Catalina mountains near
Tucson, to the north and northeast.

A significant plume of very moist subtropical moisture can be seen
on the latest GOES satellite imagery along the southern periphery of
the mean trough off the coast of California, extending into northern
Baja this morning. With the 40-50 kts of flow in the 850-700 mb
layer expected along the base of the trough as it moves into SE AZ,
this moisture will be the key ingredient for orographic enhancement
of rainfall tonight into Friday morning. The GFS ensemble mean
magnitude of the integrated water vapor transport (IVT), which is a
measure of the strength of an Atmospheric River (AR) event,
indicates between 800-1000 kg/m-s impacting our CWA over an 12-18
hour period begining late this afternoon. This would put us into a
category 2 or 'moderate' AR event, which is defined as mostly
beneficial with some hazardous components to the rainfall event.

The upper dynamics are not expected to be very impressive over
Southeast Arizona with this event, with the main lift occuring to
our north. Given the current location and orientation of of the
upper feature, the speed of the the moisture tracking into the area,
along with the marginal dynamics, I am confident this category 2 AR
will not be as impactful as it potentially could be. In any event,
expect to see between a 0.50-0.75 inches of rain in the valleys,
with 2 to 3 inches of rain in the higher elevations with isolated
locations (especially those on the west and southwest facing
mountain slopes) seeing up to 4 inches of rain. A Flood Watch is in
effect this evening into Friday for the higher terrain of Eastern
Pinal, northeast Pima, Graham and northern Greenlee counties.

Fortunately during a wintertime system like this, rainfall rates
will be much lower than those seen during the summertime. Despite
the very heavy rainfall amounts, moderate-to-heavy rain over a
longer duration will limit the magnitude of the wave of rainfall
runoff that will drain into mountain streams, creeks and washes as
it flows off of the mountains. Expect widespread high-flows of
runoff in creeks and washes, but only a low-to-moderate threat of
flash flooding. In any event, we will push out the messaging of
"Turn Around, Don't Drown!".

Otherwise, this system will move out of the area Friday, with a
secondary system diving down Sunday into Monday. This system is much
cooler, and will likely result in low snow-levels Monday as it
tracks thru the area. There is still some uncertainty in a followup
system after that, with the ECMWF depicting another winter system
impacting the area the middle of next week, whereas the GFS is not
as impressed. Will hold off for now on introducing this third system
into the forecast.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 6 PM MST this evening through Friday morning
for AZZ504-506-509>511-514.

&&

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

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