MESOSCALE PRECIPITATION DISCUSSION 0398
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
554 AM EDT SAT SEP 27 2014
 
AREAS AFFECTED...EXTREME SOUTHEAST NV...SOUTHWEST UT...NORTHWEST
AZ 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 270950Z - 271500Z
 
SUMMARY...ADDITIONAL FLASH FLOODING WILL BE LIKELY THROUGH THE
MORNING HOURS AS CONVECTION CONTINUES TO REDEVELOP AND TRAIN OVER
THE SAME AREA.

DISCUSSION...THE LATEST WV IMAGERY CONTINUES TO SHOW A STRONG UPR
TROUGH AND ASSOCD CLOSED LOW MOVING SLOWLY THROUGH CNTRL CA ATTM.
DOWNSTREAM FROM THIS OVER MUCH OF THE GREAT BASIN IS A VERY
DYNAMIC SET-UP WITH VERY DIVERGENT FLOW SEEN ALOFT AND THUS A
STRONGLY FORCED DEEP LYR COLUMN WHICH IS SUPPORTING NUMEROUS
REPEATING CLUSTERS OF HVY SHWRS/TSTMS. CLOUD TOPS HAVE GENERALLY
BEEN COOLING OVER THE LAST COUPLE OF HOURS INDICATIVE OF AN UPTICK
IN DEEP LYR ASCENT. THE CONVECTION CONTINUES TO REDEVELOP IN
PARTICULAR OVER NWRN AZ...EXTREME SERN NV AND EXTREME SWRN UT
WHERE FLASH FLOODING HAS BEEN AN ONGOING PROBLEM AT LEAST LOCALLY
FOR SEVERAL HOURS NOW. SUPPORTING THE CONVECTIVE THREAT THIS
MORNING ASIDE FROM THE AFOREMENTIONED STRONG FORCING/DYNAMICS
ALOFT IS THE RATHER ANOMALOUS DEEP MOISTURE PLUME EXTENDING FROM
THE GULF OF CA NWD THROUGH NWRN AZ AND INTO SRN/CNTRL UT ATTM. THE
PWATS ARE IN THE 1.25 TO 1.5 INCH RANGE ALONG A RATHER NARROW N/S
AXIS...BUT THESE VALUES ARE LOCALLY OVER 200% OF CLIMATOLOGY.
SO...THE CONVECTION THAT HAS BEEN REDEVELOPING WILL CONTINUE TO
FOSTER HVY RAINFALL POTENTIAL WITH RATES UP TO OR LOCALLY
EXCEEDING 1 INCH/HR. 

STRONG DYNAMICS WILL PERSIST THROUGH THE MORNING HOURS AND DESPITE
THE RATHER MODEST INSTABILITY ENVIRONMENT...PERSISTENT DEEP LYR
ASCENT COUPLED WITH THE LOW/MID LVL MOISTURE TRANSPORT FROM THE
SOUTH SHOULD SUSTAIN SEVERAL MULTIPLE N/S ORIENTED BANDS OF
CONVECTION. 

THE LATEST HIRES MODELS INCLUDING THE 00Z ARW...00Z NMMB AND THE
00Z NSSL WRF SHOW A PATTERN OF REDEVELOPMENT ALL THE WAY THROUGH
MIDDAY AND JUST A GRADUAL EASTWARD ADVANCE TO THE OVERALL
ORIENTATION. SO THE FLASH FLOOD THREAT STANDS TO PERSIST FOR QUITE
A WHILE. WILL CONTINUE TO MONITOR.

ORRISON

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

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

LAT...LON   40361352 40041310 39611278 38811254 38601217 
            38681138 37701192 36711231 35551251 34851303 
            34711348 34791403 35241451 36081452 36951443 
            38301412 39351363 
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