Mesoscale Precipitation Discussion 0916
NWS Weather Prediction Center College Park MD
1113 AM EDT Tue Oct 02 2018

Areas affected...Arizona

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

Valid 021512Z - 022100Z

Summary...Rain will continue today across Arizona, associated with
a plume of tropical moisture from the remnants of Rosa. The rain
will be heavy at times, with some thunderstorms possible. Rain
rates should peak in the 0.5 to 1.0 in/hr range in areas of
steadier rain, and could exceed 1.0 in/hr if thunderstorms can
develop in the afternoon in areas that receive some clearing of
cloud cover. This should lead to a continuation of flooding across
portions of Arizona through today.

Discussion...As of the 15Z advisory cycle, NHC says that Rosa has
degenerated into a trough and is no longer a tropical cyclone.
Nevertheless, a plume of tropical moisture will continue to extend
into Arizona and provide a threat of flash flooding through today.
A GPS-PW observation near the US-Mexico border showed a value of
2.05 inches, close to recent RAP analysis. The PW values varied
with terrain and latitude, but they were generally exceeding the
99th percentile in the CFSR climatology for late-September to
mid-October over the entire state of Arizona. RAP forecasts show a
relatively strong southerly LLJ persisting over south-central and
central Arizona, moving very little over the next six hours. This
was sampled well by the radar VWPs from KIWA and TPHX, with 850mb
winds around 35 knots, also at the upper end of climatology for
the region. PHX did not have a 12Z sounding, but the highest 850mb
wind speed measured at that site in the SPC upper air database is
listed as 29 knots. The RAP places the nose of the strongest 850mb
moisture transport very close to (perhaps just north of) the
Phoenix Metro Area for much of the day. This is where the highest
radar reflectivity values are currently situated, and the colder
cloud tops on IR satellite channels. Therefore, the expectation is
that a broad shield of rain is likely to persist in the general
vicinity of the strong northward moisture flux in the core of the
LLJ through the day, with rain rates perhaps highest near the nose
of the LLJ, and where orographic ascent would be focused to the
north toward the Mogollon Rim. Therefore, the highest confidence
in flooding issues during the day today corresponds to the same
area: from Phoenix Metro north to near Flagstaff and the Rim.
Although rain rates may only peak in the 0.5 to 1.0 in/hr range,
their persistence over 3-6 hours should lead to flash flooding.

There is also the question of whether instability will be able to
build further south. Multiple NWP model forecasts indicate that
this will happen in SW/SC Arizona, although thus far the cloud
cover has not shown many signs of scouring out. CIRA Layer PW
product does show some drier air beginning to arrive in far
southern Arizona above 700mb (behind the decoupled mid-level
remnant circulation of Rosa), and this could theoretically help
increase convective instability. However, some clearing would also
likely be necessary. If this occurred, MLCAPE values in excess of
1000 j/kg would be able to support some stronger convection, and
this could more than offset the possibility of reduced
precipitation efficiency to lead to some risk of 1+ in/hr rain
rates on areas that have already received some heavy rainfall in
recent days. Confidence is a little lower in the eventual
development and placement of this flash flood threat, but it does
exist, particularly after 19-20Z.

Lamers

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

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

LAT...LON   36241182 35420961 32511095 31431253 32001401
            33611379 35661431