MESOSCALE PRECIPITATION DISCUSSION 0188
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
806 PM EDT THU MAY 04 2017

AREAS AFFECTED...PORTIONS OF WESTERN & SOUTH-CENTRAL WA

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

VALID 050005Z - 050405Z

SUMMARY...THUNDERSTORMS ARE EXPANDING IN COVERAGE ACROSS WESTERN
WA AND SHOWING OCCASIONAL MERGERS.  HOURLY RAIN RATES UP TO 1.5",
WITH LOCAL AMOUNTS TO 3", COULD LEAD TO HEAVY RAIN-RELATED ISSUES.

DISCUSSION...A SURFACE WAVE HAS BEEN MOVING EASTWARD ACROSS
NORTHWEST OR OVER THE PAST FEW HOURS AND AT 23Z WAS NEAR THE
DALLES IN OR.  THE CLIMATOLOGICAL CIRCULATION IN EASTERN WA
APPEARS TO BE MIGRATING SOUTH TO JOIN WITH A ZONE OF WIND SHEAR. 
DEW POINTS NORTH OF THE BOUNDARY ARE QUITE HIGH, WITH 50S AND 60S
NOTED IN RECENT OBSERVATIONS.  WITHIN ITS INVERTED TROUGH/TROWAL
FEATURE, THUNDERSTORMS ARE CONGREGATING NEAR THE CASCADES. 
PRECIPITABLE WATER VALUES ARE ESTIMATED TO BE 1-1.25" FROM 18Z
RADIOSONDES AND THE BLENDED TPW PRODUCT.  INFLOW AT 700 HPA IS
20-30 KTS, IMPORTING MUCAPES OF 2000+ J/KG INTO THE REGION FROM
WESTERN OR.  ALONG WITH THE 0-6 KM BULK SHEAR OF 35-50 KTS IS
HELPING TO ORGANIZE CONVECTION, WITH THE THREAT OF SUPERCELLS. 
HOURLY RAIN RATES ON RECENT RADAR IMAGERY HAVE OCCASIONALLY RANGED
UP TO 1.5" WHEN CELL MERGERS HAVE OCCURRED.

THE EXPECTATION IS FOR THE HEAVY RAIN THREAT TO MOVE ACROSS THE
CASCADES INTO THE WESTERN COLUMBIA BASIN WITH TIME, ELEVATED OVER
A BOUNDARY JUST SOUTH OF THE OR/WA BORDER.  GFS-BASED GDI
INSTABILITY INDEX VALUES INDICATE THAT CELL COVERAGE SHOULD REMAIN
THIS EXTENSIVE PAST 03Z, PERHAPS AS LATE AS 06Z.  THE WIND FLOW
WITH HEIGHT IS NEARLY UNIDIRECTIONAL, RAISING THE SPECTER OF CELL
TRAINING AS WELL.  HEAVY RAIN CORES SHOULD TEND TO MOVE NORTHEAST
AT 25-30 KTS.  THE WETTER MESOSCALE GUIDANCE, SUCH AS THE 18Z NAM
CONEST AND 12Z SPC WRF, HAVE BEEN DOING WELL IN DEPICTING THE
THREAT AREA SO FAR.  HOWEVER, AS THE STORMS MOVE OFF THE CASCADES
INTO THE COLUMBIA BASIN, IT IS POSSIBLE THAT CELL COVERAGE COULD
DECREASE, INCREASING UNCERTAINTY IN FUTURE EXPECTATIONS, SO USED A
FOUR HOUR WINDOW.  HOURLY RAIN RATES UP TO 1.5", WITH LOCAL
AMOUNTS TO 3", COULD LEAD TO HEAVY RAIN ISSUES.

ROTH

ATTN...WFO...OTX...PDT...PQR...SEW...

ATTN...RFC...NWRFC...

LAT...LON   48332182 48292087 48032017 47291944 46471908
            45541909 45381984 45422150 45502170 46402248
            47192281 47662230 48292213