Mesoscale Precipitation Discussion 0909
NWS Weather Prediction Center College Park MD
200 PM EDT Sun Sep 30 2018

Areas affected...Sonoran Desert of Southern AZ...Ext Southeast
CA...

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

Valid 301800Z - 302300Z

SUMMARY...Slow moving, increasingly efficient heavy rainfall
production across deserts of SW AZ, pose localized FF risk through
afternoon.

DISCUSSION...GOES-16 1-minute Visible imagery depict developing
convection along the AZ/Mexican boarder with TCU extending NNW to
SSE.  This orientation aligns well with convergence band set up
along NE Sea of Cortez air and mean low level flow parallel to the
coast.  The Cortez air mass is sampled by TDs in the upper 70s to
80F at San Felipe.  Strong 15-20kt sfc to 850mb flow streams
moisture/instability northward into the lower Colorado River and
Gila River Vallyes and adjacent deserts.   Due to extremely
hot/moist low levels and modest dry air aloft (per CIRA LPW
suite), SBCAPEs to 5000 J/Kg and MLCAPEs to 2500 J/kg are
estimated to fuel updrafts.  GPS TPW values are steadily
increasing from the south as narrow band of 700mb moisture (per
RAP and CIRA LPW) lifts north into SW AZ, raising TPWs from near
1.0" this morning to 1.4" currently (1.6" upstream in W Sonora).

As such current cells are capable of 1.5"/hr rates.  Mean cell
motions are very slow to support hourly totals at or around 1.5"
which would exceed the low FFG values in the hard-pan desert
setting.  Still, given weaker flow, cells are likely to be a bit
more scattered in nature and overall coverage would be limited for
FF to occur.

Cells are expected to lift NNW along the 700 RH maxima into NW
Yuma county/S La Paz, where greater mid-level convergence will
occur with the intersection of confluent mid-level flow from SW to
NE outflow channel from TC Rosa, this slowing may further compound
rainfall totals here, even though moisture/instability will be
lessened further from the source of high latent heat air near the
Gulf.  Upper level right entrance to 65kt 3H Jet will also support
outflow to maintain updraft strength through the afternoon hours. 


Gallina

ATTN...WFO...PSR...TWC...

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

LAT...LON   34121388 33881326 32931310 32461307 31941295
            31971324 32281423 32741500 33851458