MESOSCALE PRECIPITATION DISCUSSION 0108
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1159 PM EDT SUN APR 15 2018

AREAS AFFECTED... CENTRAL VA INTO NORTHERN MD

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

VALID 160327Z - 160727Z

SUMMARY..RAINFALL AMOUNTS OF 2 TO 3 INCHES COULD RESULT IN A
LOCALIZED FLASH FLOOD THREAT THROUGH THE EARLY MORNING HOURS.

DISCUSSION...REGIONAL RADARS CONTINUE TO SHOW CELLS MOVING
NORTHWARD ALONG A CONVECTIVE LINE MOVING SLOWLY EASTWARD ACROSS
CENTRAL VA INTO NORTH CENTRAL MD. THE CONVECTION LINES UP WITH THE
WESTERN EDGE OF A MUCAPE GRADIENT...WITH VALUES GENERALLY BETWEEN
5000/1000 J/KG. ALONG AND AHEAD OF THE FRONT...A 60 TO 70 KNOT LOW
LEVEL JET CONTINUES TO PUMP 1.50 INCH PRECIPITABLE WATER AIR INTO
THE CENTRAL AND NORTHERN MID ATLANTIC STATES. 

THE COMBINATION OF MOISTURE AND INSTABILITY IS FEEDING THE LOW
TOPPED FORCED CONVECTION. REGIONAL RADARS INDICATED HOURLY
RAINFALL RATES GENERALLY BETWEEN 1.00/1.25 INCHES...THOUGH MESONET
HOURLY READINGS ARE CLOSER TO AN INCH. AS THE INDIVIDUAL CELLS
TRACK NORTH ALONG THE SLOWLY MOVING FRONT...A SHORT TERM TRAINING
THREAT REMAINS.

THE MOST RECENT HRRR/00Z WRF ARW/00Z WRF NMMB SHOWED LOCAL
RAINFALL AMOUNTS OF 2.00 TO 3.00 INCHES...WITH THE HIGHEST AMOUNTS
ACROSS CENTRAL VA. THE RAP SHOWED THAT INSTABILITY BECOMES A
LIMITING FACTOR FOR FLASH FLOODING MOVING FORWARD...WITH THE AXIS
OF BEST MUCAPE SLIDING TOWARD COASTAL NC/VA.

HOWEVER...THE LOW LEVEL JET BECOMES MORE CONVERGENT WITH
TIME...AND REACHES NEARLY 80 KNOTS ACROSS NORTHERN MD TOWARD
16/08Z. THE COMBINATION OF STRONG MOISTURE FLUX AND DYNAMIC
FORCING WITH A NEGATIVELY TILTED LONG WAVE TROUGH OVER THE OH
VALLEY MAY BE ABLE TO COMPENSATE FOR THE DWINDLING INSTABILITY TO
POSE A FLASH FLOOD THREAT.

TRAINING IS REQUIRED FOR FLASH FLOODING TO OCCUR EARLY THIS
MORNING, AS FLASH FLOOD GUIDANCE VALUES GENERALLY INCREASE OVER
CENTRAL VA AND NORTHERN MD. THUS...FLASH FLOODING IS ONLY
CONSIDERED POSSIBLE.

HAYES

ATTN...WFO...AKQ...CTP...LWX...RNK...

ATTN...RFC...MARFC...SERFC...

LAT...LON   39997751 39997679 39877630 39397626 38057661
            37247688 36687730 36697810 37187878 38947864
            39717825 39737824