MESOSCALE PRECIPITATION DISCUSSION 0661
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
526 PM EDT WED AUG 09 2017

AREAS AFFECTED...CENTRAL/NORTHERN CA...SOUTHERN OR...SOUTHERN
ID...CENTRAL/NORTHERN NV...NORTHWEST UT

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

VALID 092125Z - 100325Z

SUMMARY...SLOW-MOVING SHOWERS AND THUNDERSTORMS WITH RAINFALL
AMOUNTS OF UP TO 1.5 INCHES/HR ARE EXPECTED THIS AFTERNOON AND
INTO THE EARLY EVENING HOURS. SOME FLASH FLOODING IS POSSIBLE.

DISCUSSION...A COMPACT MID LEVEL VORT CENTER IS SEEN IN GOES-WEST
WV IMAGERY ALONG NEAR THE INTERSECTION OF THE BORDERS OF CA/NV/OR
AND IS WITHIN A SOMEWHAT BROADER UPPER TROUGH THAT IS STRETCHED
OUT ACROSS THE GREAT BASIN. THIS ENERGY IS COUPLING WITH
SUFFICIENT BOUNDARY LAYER INSTABILITY AND MOISTURE PROFILES FOR
SOME SCATTERED CLUSTERS OF HEAVY SHOWERS AND THUNDERSTORMS. IN
FACT...THE PWATS ARE ACTUALLY QUITE ANOMALOUS OVER NORTHERN CA AND
NORTHERN NV WHERE THEY ARE 2 TO 3 SIGMAS ABOVE NORMAL. THIS SHOULD
TEND TO SUPPORT RAINFALL AMOUNTS THAT MAY EXCEED 1.5 INCHES PER
HOUR. ALREADY SOME OF THE STRONGEST ACTIVITY APPEARS TO BE OVER
THE OROGRAPHICALLY FAVORED SHASTA/SISKIYOU RANGES AND THE
CENTRAL/NORTHERN SIERRA-NEVADA.

THE CONTINUATION OF STRONGER DIURNAL HEATING SHOULD ENHANCE
SURFACE BASED INSTABILITY FURTHER THROUGH THE AFTERNOON AND THIS
COUPLED MORE FAVORABLE MID LEVEL DYNAMICS/FORCING...AND ABOVE
NORMAL PWATS SHOULD RESULT IN AN INCREASE IN CONVECTIVE COVERAGE
THROUGH THE AFTERNOON AND INTO THE EARLY EVENING HOURS ACROSS MUCH
OF THE GREAT BASIN AND ADJACENT HIGH TERRAIN.

THE 12Z NSSL-WRF APPEARS TO HAVE THE BETTER HANDLE ON THE CURRENT
LEVEL OF CONVECTION AND IT WOULD SUGGEST RAINFALL AMOUNTS OF
LOCALLY AS MUCH AS 2 INCHES WITH THE STRONGER THUNDERSTORM CORES.
HOWEVER...SOME ISOLATED HEAVIER AMOUNTS TO OVER 3 INCHES CANNOT BE
RULED OUT GIVEN THE SLOW CELL MOTION. SOME FLASH FLOODING WILL BE
POSSIBLE AS A RESULT.

ORRISON

ATTN...WFO...BOI...EKA...HNX...LKN...MFR...PIH...REV...SLC...
STO...

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

LAT...LON   42711701 42701533 42621336 42451210 42011158
            41511155 40721179 39931229 39521325 39251418
            39251615 39181767 38031850 37861922 38772014
            39472056 40242118 40492180 40102226 39462233
            39182270 39842328 40662355 41712331 42342244
            42562093 42571914