Flash Flood in California, March 10, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
230 PM MST Tue Mar 10 2020

.SYNOPSIS...
A slow moving and wet storm system will bring waves of widespread
rain to the region today through Friday, with high temperatures
remaining in the low 70s across the lower deserts. The heaviest
rain, along with possible thunderstorms, is expected Thursday
evening into Friday morning. The highest rainfall amounts are
expected north and east of Phoenix with flooding impacts to creeks
and typically dry washes likely. Another much colder storm system is
becoming more likely for early next week.

&&

.DISCUSSION...
The first wave of our several-day storm system has pushed
through, which brought some light showers to the area earlier this
morning. Near Phoenix totals were less than a tenth of an inch,
with somewhat higher totals towards the west, where scattered
showers are continuing at this hour. Associated with this initial
wave a surge of subtropical moisture increased PWAT to 1" near
Yuma. With continuing moisture transport and scattered showers,
PWAT values should remain at or above 1" for much of the next few
days.

After the initial showers this morning, much of the early
afternoon has been quiescent under mostly cloudy skies. This will
change this evening as a shortwave rotating around the upper level
low off of the California coast moves in this evening. In fact,
an increase in shower activity is already occurring over southeast
California with a few lightning strikes also observed. While
instability is not that high, MUCAPE of 100-200 J does support the
continued risk of thunder in our western zones over the next
several hours. As the evening progresses strong upper level
divergence and increasing cyclonic vorticity advection will lead
to ascent of the atmospheric column. With the very warm and moist
airmass in place precipitation production will be efficient. Total
precipitation near Phoenix through Wednesday morning should be
less than half an inch in most areas.

Considering the forecast from Wednesday onwards, our attention
turns to the low pressure system off of the California coast. This
low pressure area will attempt to move inland over the next 24
hours, however, a filament of vorticity wrapping around the back
of the low will cause it to stall and even move south temporarily.
Model consensus is suggesting a noticeable break in shower
activity/coverage across the warning area Wednesday night through
Thursday morning. This coincides with the deep plume of moisture
shifting more to east and southeast of the area. However, as the
aforementioned low pressure center moves south it will reorient
from a positive tilt to a more neutral tilt. This reorientation
will help draw deep moisture back to the region later Thursday.
The low will also begin to track northeast and eventually right
through central AZ Friday. With this track, some of the most
significant rain is expected to fall during the Thursday evening
through Friday morning timeframe. Strong dynamics, deep moisture,
and MUCAPE of 300-500 J will support higher rain rates and
convection, with a higher potential for flash flooding, especially
with grounds already saturated. WPC has issued a slight risk for
excessive rainfall during this period for a good portion of the
CWA, with an expansion of coverage compared to their previous
iteration. Lingering showers and storms will be possible Friday
afternoon before the low finally exits the region Friday night.

QPF totals remain high, with model agreement finally honing in on a
narrower range. The GFS ensemble is the driest with a mean storm
total of about 1.3" of additional rain in Phoenix. The ECMWF
ensemble shows about 1.8" and the Canadian ensemble has about 2".
While significant spread remains in each of the ensembles, this is
to be expected due to the convective element of this portion of
the system. Overall, most lower desert locations are expecting at
least 1.00". Highest amounts are expected in central AZ, with
around 1.5 to 2" in the Phoenix area and 3-4" in the high terrain
north and east of Phoenix. Rain rates today and Wednesday
continue to not warrant a flash flood watch. One may be needed
Thursday into Friday, but confidence is not high enough on exact
intensity, location, and coverage of the heavier rain at this
moment. Main concerns with this event continue to be the expected
runoff into creeks/rivers and dry washes leading to elevated water
rises with impacts to low water crossings. An aerial flood watch
remains in effect for Tonto Basin with Tonto creek expected to
exceed Alarm stage.

High temperatures through most of the week will likely hover in the
low-70s across the lower deserts, with mild overnights lows. The
coolest day will likely be Friday, with highs struggling to get out
of the 60s as the cold core low passes over the region. Heading into
early next week, global ensembles continue to favor an anomalously
cold system dropping down the west coast and clipping through the
area. This would bring much colder weather to the Desert SW and
potentially another chance for rain, as well as a chance at lower
elevation snow. At a week out, it is still too early to get into
specifics, however a general continuation of this unsettled weather
pattern seems likely.

&&

.AVIATION...Updated at 1844Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Radar this morning showed most of the shower activity outside of the
Phoenix area. The latest HiRes guidance continues to show a break in
activity across the Phoenix area into the afternoon before showers
begin to push back in later this afternoon into the early evening.
CIGs this morning have been in the 6-7 kft range but should drop
more later today. Winds are expected to favor an easterly component
through much of the period, though erratic winds will be possible
with any showers that move through.

Another round of showers later this afternoon and early evening are
expected to lower CIGs as they push through. MVFR CIGs will be
possible with any heavier bands of rain showers that move through,
potentially dropping CIGs to as low as 2-3 kft. Showers may result
in a brief reduction in slantwise visibilities. Showers begin to
exit later this evening with slight improvements in CIGs. Not
confident that we'll much improvement in CIGs overnight. As of now,
CIGs look to fall in the 6-7 kft range for the Phoenix area. Could
see a few showers and an isolated thunderstorm pop up tomorrow
afternoon, but not confident enough to include in the TAFs at
this time.

Southeast California/Southwest Arizona including KIPL and KBLH:

Showers continue to push through southeast California this morning.
Could see shower activity continue throughout the afternoon and
early evening hours with MVFR CIGs possible with heavier bands.
Showers will also have the potential to result in a brief
reduction in visibilities. Winds may become erratic at times
though speeds look to mostly remain aob 10 kts. CIGs do not look
to improve a whole lot as SCT-BKN bases around 2-4 kft and BKN-
OVC around 5-7 kft will be possible through the period. Can't rule
out a few showers and possibly a thunderstorm popping up tomorrow
but not confident enough to mention in the TAFs.

&&

.FIRE WEATHER...
Thursday through Monday: Wet and humid conditions are expected
through Friday lingering showers over south central Arizona into the
day on Saturday. A slow moving storm system will bring widespread
rain chances across most of the districts through Friday with rain
moderate or even heavy at times. Isolated storms will be possible on
Thursday mainly during afternoon and early evening hours. Humidity
levels will stay highly elevated through the five day period. High
temperatures will stay near or a bit below seasonal normals each day
with the coolest day on Friday as central deserts fall into the mid
to upper 60s. Winds each day will generally be light around 15 mph
or less, but higher gusts will be possible with any storms. Drier
conditions return by sunday following the exit to the east of the
large Pacific storm system.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch from 9 PM MST this evening through Wednesday
     afternoon for AZZ556.

CA...None.
&&

DISCUSSION...Hodges/Benedict
AVIATION...Smith
FIRE WEATHER...Benedict/CB