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MESOSCALE PRECIPITATION DISCUSSION 0239
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1031 PM EDT WED MAY 30 2018

AREAS AFFECTED...WRN VA...BLUE RIDGE MTNS AND SHENANDOAH VALLEY

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 310228Z - 310630Z

SUMMARY...TROPICAL CONVECTIVE RAIN BANDS APPEAR LIKELY TO TRAIN
FOR SEVERAL MORE HOURS IN PORTIONS OF WESTERN VIRGINIA WITH RAIN
RATES APPROACHING 3 IN/HR IN SOME CASES. SEVERE FLASH FLOODING MAY
OCCUR WHERE TRAINING CAN PERSIST FOR SEVERAL HOURS...AND MAY BE
MOST LIKELY NEAR CHARLOTTESVILLE WHERE RADAR ESTIMATED RAINFALL
TOTALS ARE ALREADY EXCEEDING 8 INCHES.

DISCUSSION...BOTH KLWX AND KFCX RADARS ARE SHOWING TROPICAL
CONVECTIVE RAIN BANDS WITH LOW-ECHO CENTROIDS AND LIMITED
REFLECTIVITY ABOVE THE -10C LAYER. ALL THIS IS INDICATIVE OF
LIMITED ICE IN THE CLOUD LAYER AND DOMINANT WARM RAIN PROCESSES.
HIGH KDP VALUES AND RELATIVELY LOW ZDR VALUES ALSO TEND TO IMPLY
NUMEROUS SMALLER RAIN DROPLETS THAT IS MORE TYPICAL OF TROPICAL
RAINFALL. MRMS APPEARS TO BE UNDERESTIMATING RAIN RATES AS ITS
SURFACE PRECIP TYPE IS TAGGING THIS ACTIVITY AS CONVECTIVE RATHER
THAN TROPICAL. DUAL POL ESTIMATES APPEAR TO BE MORE
REASONABLE...AND SHOW RAIN RATES ALREADY REACHING CLOSE TO 3 IN/HR
IN SOME LOCALIZED AREAS...WITH TOTALS AS HIGH AS 8 INCHES ON THE
WEST SIDE OF CHARLOTTESVILLE.

THE SCATTERED RAIN BANDS ACROSS THE REGION SHOULD CONTINUE TO
BACKBUILD INTO SSWLY LOW LEVEL INFLOW AND PRECIPITABLE WATER
VALUES AROUND OR OVER 2 INCHES (ANALYZED BY THE RAP; GPS-PW OBS
AROUND 2.1 INCHES IN NRN VA) WILL SUPPORT CONTINUED EFFICIENT RAIN
PROCESSES. THIS SHOULD MAKE SCATTERED FLASH FLOODING A RISK INTO
THE OVERNIGHT HOURS. RADARS MAY UNDERESTIMATE RAINFALL FURTHER
FROM THE RADAR AS EVEN THE LOWER ELEVATION TILTS MAY OVERSHOOT
MUCH OF THE HEAVIEST RAINFALL GIVEN THE WARM RAIN PROCESSES.

PARTICULAR CONCERN EXISTS FOR THE CHARLOTTESVILLE AREA WHERE
MESONET SITES ARE SHOWING OVER 3 IN/HR RAIN RATES IN THE PAST HOUR
AND KLWX RADAR SHOWS CONTINUED BACKBUILDING AND TRAINING OVER
AREAS THAT HAVE ALREADY RECEIVED SIGNIFICANT RAINFALL. MAINTENANCE
OF THESE RAIN RATES FOR EVEN ANOTHER HOUR WOULD EXACERBATE ALREADY
REPORTED FLASH FLOODING AND POTENTIALLY LEAD TO SEVERE AND
LIFE-THREATENING FLASH FLOODING WITH DEEP INUNDATION IN LOW-LYING
AREAS.