MESOSCALE PRECIPITATION DISCUSSION 0274
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1102 PM EDT WED AUG 13 2014
AREAS AFFECTED...SOUTHERN TO EASTERN MAINE...
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY
VALID 140302Z - 140732Z
SUMMARY...AN AREA OF TRAINING CONVECTION HAS BEEN OBSERVED NEAR
PORTLAND...MAINE WITH 4" OF RAINFALL IN ONLY 90 MINUTES. FLASH
FLOODING IS BEING REPORTED ACROSS THE REGION WITH NUMEROUS
UNDERPASSES FLOODED AND VEHICLES STRANDED. THIS MESOSCALE BAND
SHOULD MAINTAIN A LIKELY THREAT FOR FLASH FLOODING DURING THE NEXT
FEW HOURS.
DISCUSSION...A SYSTEM WITH A LONG HISTORY OF PRODUCING FLASH
FLOODS FROM MD UP THROUGH NY...LONG ISLAND...AND SOUTHERN NEW
ENGLAND HAS SEEN THE DEVELOPMENT OF ANOTHER IMPRESSIVE MESOSCALE
RAIN BAND. SURFACE OBSERVATIONS INDICATE HOURLY RAINFALL RATES
HAVE REACHED THE 2 TO 3 INCH RANGE DURING THE PAST COUPLE HOURS.
MUCH OF THIS ACTIVITY HAS CONGREGATED ALONG A BAROCLINIC ZONE
WHICH HAS SET UP ALONG THE MAINE COAST. WHILE PWATS ACROSS COASTAL
MAINE ARE NOT AS IMPRESSIVE AS VALUES SEEN EARLIER IN THE
EVENT...00Z RAOBS INDICATE PWATS OF 1.75-2.00 INCHES HAVE INVADED
THE REGION. THIS RANKS APPROXIMATELY TWO STANDARD DEVIATIONS ABOVE
NORMAL FOR MID-AUGUST. WHAT IS MORE IMPRESSIVE IS THE FORECAST
850-MB WINDS WHICH ARE TO BE IN THE 50 TO 55 KNOT RANGE DURING THE
NEXT SEVERAL HOURS. INSTABILITY REMAINS LOW WITH MUCAPE VALUES IN
THE 100-500 J/KG RANGE PER THE 02Z RAP-BASED OBJECTIVE ANALYSIS.
IN SPITE OF THE LOW AMOUNT OF INSTABILITY IN PLACE...WARM RAIN
PROCESSES HAVE PROVEN TO BE QUITE EFFICIENT IN HEAVY RAINFALL
PRODUCTION.
HIGH-RESOLUTION GUIDANCE HAS NOT HANDLED THIS MESOSCALE FEATURE
WELL AS THE MODELS HAVE FEATURED A BROADER AXIS OF MODERATE TO
HEAVY PRECIPITATION. THEIR FORECAST DURING THE NEXT 6 HOURS
SUGGESTS ONLY 1 TO 2 INCH AMOUNTS. GIVEN THE CURRENT STATE OF THE
CONVECTION...THIS MAY BE ACCURATE FOR AERIAL COVERAGE BUT EXPECT
POCKETS OF 1-2 INCH HOURLY RAINFALL RATES TO CONTINUE WITH
OCCASIONALLY HIGHER RATES THROUGH AT LEAST 06Z.
RUBIN-OSTER
ATTN...WFO...CAR...GYX...
ATTN...RFC...NERFC...
LAT...LON 44857002 45166911 45176802 44736741 44336757 43996844
43696898 43486921 42846975 42877042 43387063 43927043
44857002