MESOSCALE PRECIPITATION DISCUSSION 0810
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1120 AM EDT THU SEP 14 2017

AREAS AFFECTED...FAR ERN NV AND FAR NRN AZ INTO SRN UT

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 141519Z - 142115Z

SUMMARY...AN INCREASING THREAT FOR FLASH FLOODING WILL EXIST FROM
FAR ERN NV INTO SRN UT AND FAR NRN AZ THROUGH 21Z. TRAINING CELLS
WITH RAINFALL RATES PEAKING NEAR 1 IN/HR ARE EXPECTED.

DISCUSSION...WATER VAPOR IMAGERY AT 15Z SHOWED A POTENT CLOSED LOW
OVER SRN CA NEAR THE NV BORDER MOVING ENE. 12Z RAOBS AND RAP
ANALYSES DEPICTED A SHARP 70 KT JET STREAK AT 250 MB SOUTH OF THIS
LOW OVER SRN CA WITH STRONGLY DIFFLUENT AND DIVERGENT FLOW OUT
AHEAD OVER FAR ERN NV INTO SWRN UT AND NWRN AZ. CONVECTION WAS
ALREADY UNDERWAY  OVER THE WRN UT/AZ BORDER AHEAD OF THE MID-LEVEL
VORTICITY MAX AND UPPER LEVEL JET STREAK. THE HIGHEST CAPE WAS
LIMITED AT 14Z TO THE TRI-STATE REGION OF NV/UT/AZ WITH 500-1000
J/KG MUCAPE VIA THE LATEST SPC MESOANALYSIS. 12Z SOUNDINGS FROM
VEF AND FGZ SHOWED ELEVATED INSTABILITY OF ABOUT 600 J/KG.

THE CLOSED LOW IS FORECAST TO OPEN UP/WEAKEN THROUGH 00Z AHEAD OF
A LARGER UPPER TROUGH AMPLIFYING ACROSS THE PACIFIC
NORTHWEST...BUT FORCING AHEAD OF THE UPPER WAVE WILL BE ENOUGH TO
CONTINUE TO DRIVE CONVECTION INTO MUCH OF THE AFTERNOON. COOLING
MID-LEVEL TEMPERATURES WITH THE APPROACHING MID-LEVEL TROUGH WILL
AID IN EXPANDING INSTABILITY...500-1000 J/KG AS FAR NORTH AS I-70
WITH LOCALLY HIGHER VALUES INTO NRN AZ WHERE SKIES WERE MOSTLY
CLEAR THIS MORNING.

PRECIPITABLE WATER VALUES WILL VARY FROM 0.7 TO 1.2 INCHES ACROSS
THE MPD THREAT REGION ACCORDING TO CURRENT GPS VALUES AND RAP
FORECASTS...WHICH WHEN COMBINED WITH WEAK INSTABILITY SHOULD BE
ENOUGH TO PRODUCE RAINFALL RATES NEAR 1 IN/HR. TRAINING APPEARS
LIKELY WITH UNIDIRECTIONAL SSWLY FLOW FROM THE LFC-EL...HOWEVER
THERE ARE SOME MIXED SIGNALS. AS ONE TRAVELS FARTHER NORTH...700
MB FLOW SHOULD BE WEAKER WHICH MAY KEEP STORMS MORE PROGRESSIVE IN
NATURE...BUT UPPER FORCING WILL BE STRONGER HERE. FARTHER SOUTH
INTO NRN AZ...700 MB FLOW WILL BE STRONGER...NEAR 70 KT...BUT
STRONGER UPPER FORCING WILL BE DISPLACED TO THE NORTH. FOR THESE
REASONS...THE FLASH FLOOD THREAT WILL STRADDLE THE UT/AZ BORDER
WITH THE THREAT INCREASING INTO THE AFTERNOON AS BETTER CAPE
COMBINES WITH APPROACHING FORCING FOR ASCENT.

OTTO

ATTN...WFO...FGZ...GJT...SLC...VEF...

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

LAT...LON   39521026 39490948 39120916 38370915 37770923
            36550992 36101089 35991156 36071212 36601290
            37111415 38141420 38701351 39031222 39321125