MESOSCALE PRECIPITATION DISCUSSION 0155
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
302 PM EDT WED MAY 16 2018

AREAS AFFECTED...WV...SW/SC PA...WRN MD...NW VA...FAR SE OH

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

VALID 161900Z - 170100Z

SUMMARY...SCATTERED HEAVY RAIN SHOWERS AND THUNDERSTORMS MAY LEAD
TO FLASH FLOODING ACROSS PORTIONS OF THE CENTRAL APPALACHIANS
WHERE SOILS ARE NEARLY SATURATED ALREADY DUE TO RECENT RAINFALL.
HOURLY RAIN RATES COULD EXCEED 1 IN/HR IN THE HEAVIEST RAIN BANDS.

DISCUSSION...REGIONAL RADARS AT 1830Z SHOWED INDICATIONS OF
LOW-TOPPED CONVECTION BEGINNING TO INITIATE NEAR A STATIONARY
FRONT DRAPED ACROSS WEST VIRGINIA. MOST OF THIS CONVECTION HAD 30
DBZ ECHO TOPS BELOW 20000 FEET...AND THIS MAKES SENSE BASED ON RAP
FORECAST SOUNDINGS THAT SHOW MOST OF THE AVAILABLE CAPE (AROUND
500-1000 J/KG IN TOTAL) SITUATED BELOW THE -10C LAYER. THIS
INSTABILITY SHOULD BE JUST STRONG ENOUGH TO PROMOTE ORGANIZED
CONVECTIVE BANDS...BUT THE LIMITED CAPE ABOVE -10C MAY LIMIT
LIGHTNING ACTIVITY. NEVERTHELESS... WITH PRECIPITABLE WATER VALUES
AROUND 1.5 INCHES (PER CIRA BLENDED TPW AND GPS-PW OBSERVATIONS)
THE LOW-TOPPED CONVECTION SHOULD BE CAPABLE OF PRODUCING SOME
HEAVY RAINFALL. THESE PWATS ARE ABOVE THE 99TH PERCENTILE FOR MAY
IN THE CENTRAL APPALACHIANS.

IN ADDITION TO THE ANOMALOUSLY HIGH ATMOSPHERIC MOISTURE
CONTENT... THE COMBINATION OF WEAK STEERING FLOW AND RECENT HEAVY
RAINFALL SHOULD LEAD TO A THREAT OF FLASH FLOODING ACROSS THE
REGION. WINDS IN THE LAYER WITH THE MOST CAPE (850-500MB) ARE
RATHER WEAK ACROSS THE REGION AND ARE GENERALLY OUT OF THE
SOUTHWEST AT AROUND 5-10 KNOTS. THIS SUGGESTS THAT CONVECTION WILL
NOT ONLY BE SLOW-MOVING... BUT MAY BE ABLE TO MOVE NEARLY PARALLEL
TO PORTIONS OF THE MEANDERING STATIONARY FRONT (AND THUS SET UP
TRAINING). HOWEVER... EVEN IN THE ABSENCE OF SUBSTANTIAL
TRAINING... THE CONVECTION SHOULD BE MOVING SLOW ENOUGH THAT 1
IN/HR RAIN RATES WOULD STILL BE ACHIEVABLE. FURTHERMORE...MUCH OF
NORTHERN WEST VIRGINIA... SOUTHWEST PENNSYLVANIA... AND THE
PANHANDLE OF MARYLAND HAVE EXTREMELY LOW FLASH FLOOD GUIDANCE AND
VERY ELEVATED STREAMFLOW AND SOIL MOISTURE. THIS INDICATES A
FAVORABLE ANTECEDENT ENVIRONMENT FOR FLASH FLOODING... AND IT MAY
NOT TAKE SUSTAINED HEAVY RAIN OVER SEVERAL HOURS TO PRODUCE
FLOODING.

GIVEN THE EXPECTATION FOR LOW-ECHO CENTROID NATURE OF
CONVECTION...RADARS MAY HAVE A TENDENCY TO UNDERESTIMATE RAIN
RATES...PARTICULARLY MORE THAN 60NM FROM THE RADAR SITE WHERE THE
0.5 DEGREE BEAM HEIGHT WILL GENERALLY NOT SAMPLE BELOW 6000 FT.
THIS WOULD POTENTIALLY BE A GREATER ISSUE IN EASTERN WV... THE WV
PANHANDLE... AND EXTREME WESTERN MD.

LAMERS

ATTN...WFO...CTP...LWX...PBZ...RLX...RNK...

ATTN...RFC...MARFC...OHRFC...

LAT...LON   40617901 40427786 39927731 39427721 39037762
            38547825 38147891 37988013 38158135 38668197
            39158218 39668155 40128025