Mesoscale Precipitation Discussion 0866
NWS Weather Prediction Center College Park MD
631 PM EDT Wed Sep 19 2018

Areas affected...Central/Southern AZ...Western NM

Concerning...Heavy rainfall...Flash flooding likely

Valid 192230Z - 200430Z

Summary...Areas of heavy showers and thunderstorms will continue
through this evening. Flash flooding will remain likely given very
heavy rainfall rates.

Discussion...The afternoon GOES-16 satellite imagery continues to
show an impressive advance of moisture and cloudiness north up
across the Southwest U.S. out ahead of T.D. Nineteen-E over the
Gulf of CA. This moisture coupled with orographics and a
moderately unstable boundary layer via solar insolation has
allowed numerous showers and thunderstorms to develop over the
last several hours, and this activity generally continues to
expand in coverage across central/southern AZ and over into areas
of western NM.

The thermodynamic environment will remain conducive for heavy
showers and thunderstorms well into the evening hours across many
of these areas as MLCAPE values of 1000 to 2000 j/kg remains in
place, especially over southern AZ, along with anomalously high
PWATS. In fact, AMSU and GPS-derived data sets show PWATS still
near or even above 2.25 inches across northwest Mexico and near
the border of southern AZ and southwest NM. These values are
running an impressive 3 to 4+ standard deviations above normal for
this time of the year. This degree of anomalous moisture and
concentration through the vertical column as seen in the latest
CIRA-LPW data sets will set the stage for highly efficient
rainfall processes that coupled with areas of strong
forcing/instability will likely produce rainfall rates of as much
as 2 to 3 inches/hr.

Regarding the level of forcing in place, there is evidence in the
latest GOES-16 Airmass and Day Convection RGBs of a shortwave
trough entering south-central AZ which is helping to greatly
channel moisture northward from northwest Mexico. However, this
energy is helping to provide a bit more focused ascent over
southern AZ and is aiding with the larger scale ascent already
associated with some modest right-entrance region jet dynamics
over on the region.

The recent runs of the HRRR take this shortwave off to the
northeast this evening and across eastern AZ. This will maintain a
relatively organized heavy rainfall threat across a large area of
southern AZ and over into western NM. Expect as much as 3 to 4+
inches of additional rainfall to occur through 04Z, and especially
with the aforementioned rainfall rates. Flash flooding will remain
likely given these totals.

Orrison

ATTN...WFO...ABQ...EPZ...FGZ...PSR...TWC...VEF...

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

LAT...LON   35780702 34760687 33630740 32740768 31710766
            31200842 31130975 31151065 31281159 31601242
            31841307 31871348 32621412 34211396 35111305
            35051208 34721096 34720988 35010915 35760815