MESOSCALE PRECIPITATION DISCUSSION 0236
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
558 PM EDT WED MAY 30 2018

AREAS AFFECTED...WESTERN INTO NORTHERN VA

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

VALID 302200Z - 310300Z

SUMMARY...CONVECTION FORMING AND TRAINING IN A HIGH MOISTURE
ENVIRONMENT IN PLACE OVER PORTIONS OF WESTERN INTO NORTHERN VA
COULD POSE A FLASH FLOOD THREAT INTO THE EVENING HOURS.

DISCUSSION...REGIONAL RADARS INDICATED AN INCREASING AREA OF
CONVECTION FORMING OVER PORTIONS OF WESTERN AND CENTRAL VA LATE
THIS AFTERNOON. THE CONVECTION IS FORMING IN RESPONSE TO
INSTABILITY (WITH MLCAPE VALUES GENERALLY BETWEEN 1000-1500 J/KG)
DUE PRIMARILY TO DIURNAL HEATING. A LOW LEVEL SOUTH SOUTHEAST
UPSLOPE FLOW CONTINUES TO FEED RICH MOISTURE (WITH PRECIPITABLE
WATER VALUES NEAR 2.00 INCHES ACROSS CENTRAL VA...WHICH IS ABOUT
TWO STANDARD DEVIATIONS ABOVE THE MEAN).

THUS FAR...THE CONVECTION HAS BEEN FAIRLY SCATTERED...DUE MOSTLY
TO THE LACK OF SYNOPTIC SCALE FORCING. AS A NEGATIVELY TILTED LONG
WAVE TROUGH APPROACHES FROM THE OHIO VALLEY DURING THE EARLY
EVENING HOURS...INCREASING LIFT COULD ALLOW THE CONVECTIVE
COVERAGE TO INCREASE. IN THE  MID LEVEL FLOW SOUTHWEST
FLOW...CELLS ARE EXPECTED TO KEEP MOVING NORTHEAST...AND THE DEEP
QUASI UNIDIRECTIONAL FLOW IS EXPECTED TO ALLOW CONVECTION TO
TRAINING IN THE AXIS OF BEST INSTABILITY AND DEEPEST MOISTURE.

THERE HAS BEEN A FAIRLY STRONG HIGH RESOLUTION MODEL SIGNAL (LED
BY THE MOST RECENT HRRR/EXPERIMENTAL HRRR) FOR 3.00+ INCH RAINFALL
AMOUNTS OVER PORTIONS OF CENTRAL INTO NORTHERN VA. THE TWO MOST
RECENT HRRR RUNS HAVE BOTH HAD 6.00+ INCH RAINFALL AMOUNTS
CENTERED BETWEEN KSHD AND KLKU THROUGH 31/02Z. WHILE THESE VALUES
COULD BE ON THE HIGH SIDE...GIVEN THE DEPTH OF THE MOISTURE IN
PLACE...AND THE POTENTIAL FOR TRAINING...THESE VALUES COULD END UP
SERVING AS HIGH END RAINFALL AMOUNTS.

AS MENTIONED EARLIER...IT IS NOT CLEAR JUST HOW WIDESPREAD THE
CONVECTION BECOMES BEFORE THE INSTABILITY IN PLACE STARTS WANING
WITH THE LOSS OF DAY TIME HEATING. BECAUSE OF THAT...FLASH
FLOODING IS ONLY CONSIDERED POSSIBLE...DESPITE THE HRRR SIGNAL FOR
VERY HEAVY RAINFALL.

HAYES

ATTN...WFO...AKQ...LWX...RLX...RNK...

ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...

LAT...LON   38997875 38897779 38517730 37937720 37557742
            37277774 37067820 36907896 36528097 36988140
            37548089 37858021 38337971 38857932