Mesoscale Precipitation Discussion 0977
NWS Weather Prediction Center College Park MD
312 PM EDT Sat Oct 13 2018

Areas affected...Arizona

Concerning...Heavy rainfall...Flash flooding possible

Valid 131912Z - 140100Z

Summary...Rain will continue across portions of Arizona into the
afternoon, and the persistent rain over several hours may lead to
flooding. Additional showers and thunderstorms producing heavy
rain may redevelop in southern Arizona, and these thunderstorms
could produce rain rates in excess of 1 in/hr, which could lead to
flash flooding.

Discussion...A well-defined mid-latitude cyclone was situated in
the general vicinity of the Lower Colorado River Valley. On GOES
water vapor channels, and with GOES derived motion winds, the
mid-upper level low was quite evident over southern California
with highly diffluent flow to the east. This diffluent flow was
sustaining an area of rain over central Arizona, and this seems
likely to continue into the early-mid afternoon hours. This was
also occurring on the nose of a 850-700mb low-level jet, with KIWA
and TPHX radar VWPs showing around 15 knots of SSE flow in those
layers. Rain rates have not been particularly high in the area of
rain, generally less than 0.5 in/hr, but embedded convective bands
have reached 0.5 to 1.0 in/hr per WSR-88D dual pol and MRMS
estimates, as well as Arizona Flood Warning System rain gauges.
Even though these rates are a little under FFG values, the
cumulative effect of persistent rain with occasionally heavy rain
bands has been sufficient to generate some flooding in the area
near and immediately southwest of Phoenix. Additional flooding
could develop in nearby areas into central Arizona provided that
large-scale forcing sustains this area of rain as expected.

Renewed convective development will be possible in the afternoon,
particularly in the Sonoran Desert region of southern and
southwest Arizona. Visible satellite imagery does show some
clearing ongoing in this region with MLCAPE just beginning to
build to around 500 j/kg per RAP analysis. This is forecast to
build further to approach 1000 j/kg in some areas during the
afternoon given the combination of diurnal heating, cooler
mid-level temperatures associated with the closed low aloft (below
the 20th percentile for October at 500mb), and anomalously high
low-level moisture content (850mb specific humidity above the 97th
percentile). Instability reaching those levels would be sufficient
to support thunderstorms, and rain rates potentially exceeding 1
in/hr. Recent runs of the HRRR attempt to focus this in two areas:
(1) From the remote parts of the desert in western Pima County
northeast toward Phoenix Metro. There's some evidence of this
happening already with a trail of convective clouds on visible
satellite and some developing showers on KIWA radar. (2) Along an
axis of stronger surface moisture convergence near an analyzed
trough near the Colorado River in far western Arizona. Afternoon
convection should not be expected to be confined to these areas,
but they may be favored for additional heavy rain and thus flash
flooding.

Lamers

ATTN...WFO...FGZ...PSR...TWC...VEF...

ATTN...RFC...CBRFC...

LAT...LON   35371136 33941016 32640988 32081040 31521200
            32271447 33951421 35321344