Mesoscale Precipitation Discussion 0994
NWS Weather Prediction Center College Park MD
844 AM EDT Mon Oct 22 2018

Areas affected...Nevada...Arizona...Utah Border Region

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

Valid 221243Z - 221530Z

Summary...Another round of showers and some thunderstorms were
forming near the Arizona/Nevada border and moving northward. 
Rainfall rates were not excessively...generally under half an inch
per hour...but these cells will be moving over areas which have
already had one or more rounds of convection recently.  This could
lead to localized problems due to run-off or ponding, especially
in normally dry arroyos and washes.

Discussion...Flow aloft was becoming difluent/divergent as a wave
makes its way across California into the Great Basin.  SPC
Mesoscale Analysis showed that ML CAPES were running at or above
500 J per kg and Precipitable Water values just under an inch. 
While the storms have a history of producing hail, which may skew
the rainfall estimate a bit, rainfall rates have generally been in
the range of half an inch per hour to three-quarters of an inch
per hour. Given the forward speed of the cells, these rainfall
rates should not be a problem. However, radar imagery early this
morning showed additional convection developing upstream which may
move over areas which have already experienced one or more rounds
of rainfall in the past 24 hours.  This is consistent with the
hi-resolution guidance showing additional precipitation through
1530Z or so. 

As a result, there could be isolated problems with run-off or
ponding of water...especially in normally dry washes or
arroyos...during the morning. 

Bann

ATTN...WFO...SLC...VEF...

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

LAT...LON   37441352 37301339 36931336 36581328 36321332
            36081349 35781394 35281490 35421505 35991482
            36651449 37411423