MESOSCALE PRECIPITATION DISCUSSION 0913
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
223 AM EDT WED OCT 25 2017

AREAS AFFECTED...LONG ISLAND...CT...RI...MA...NH...WESTERN ME

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

VALID 250620Z - 251220Z

SUMMARY...PERSISTENT BANDS OF HEAVY RAIN ARE EXPECTED TO CONTINUE
TO LIFT NORTH ACROSS PARTS OF LONG ISLAND AND NEW ENGLAND THROUGH
THE EARLY MORNING HOURS. FLASH FLOODING IS LIKELY.

DISCUSSION...THE LATEST GOES-EAST IR SATELLITE IMAGERY LOOP
CONTINUES TO SHOW AN ELONGATED N/S AXIS OF HEAVY SHOWERS AND
EMBEDDED THUNDERSTORMS IMPACTING A LARGE AREA OF INTERIOR NEW
ENGLAND. SOME OF THE COLDEST CONVECTIVE TOPS ARE CURRENTLY NOTED
OVER PARTS OF CENTRAL AND NORTHERN NH AND ALSO SOUTH OF LONG
ISLAND. THE ACTIVITY CONTINUES TO FOCUS ALONG AND JUST AHEAD OF A
QUASI-STATIONARY FRONTAL ZONE THAT IS ALIGNED GENERALLY PARALLEL
TO THE DEEPER LAYER SOUTHERLY FLOW AROUND THE EASTERN FLANK OF A
DEEP LAYER TROUGH OVER THE GREAT LAKES AND OH VALLEY. THE LATEST
RAP ANALYSIS IS SHOWING A PERSISTENT AND STRONG DEGREE OF LOW
LEVEL CONVERGENCE WITH ENHANCED MOISTURE TRANSPORT AIDED BY A 50
KT SOUTHERLY LOW LEVEL JET.

ATMOSPHERIC RIVER CONDITIONS ARE WELL ESTABLISHED ACROSS THE
REGION WITH A LONG-FETCH AXIS OF PWATS EXCEEDING 1.75 INCHES OFF
THE EAST COAST AIMED INTO SOUTHERN NEW ENGLAND ALONG WITH 850 MB
MOISTURE FLUX ANOMALIES OF A HIGHLY IMPRESSIVE 4 TO 5 SIGMAS ABOVE
NORMAL. THIS COUPLED WITH RELATIVELY CONCENTRATED MOISTURE IN THE
500/300 MB LAYER AS PER 03Z CIRA-LPW DATA IS RESULTING IN
EFFICIENT RAINFALL PROCESSES FOR ENHANCED RAIN RATES THAT ARE
OCCASIONALLY REACHING 2 INCHES/HR. LOCALLY ENHANCED OROGRAPHIC
ASCENT OVER THE HIGHER TERRAIN IS ALSO PROMOTING HEAVIER RATES.

THERE WILL LIKELY CONTINUE TO BE A GENERAL N/S AXIS OF HEAVY AND
OCCASIONALLY TRAINING SHOWERS AND THUNDERSTORMS OVERNIGHT AND
THROUGH DAWN THAT WILL IMPACT A LARGER AREA OF INTERIOR NEW
ENGLAND ALTHOUGH WITH A GRADUAL EASTWARD SHIFT OF THE CONVECTIVE
AXIS AS PER THE 00Z HIRES ARW/NMMB AND NSSL-WRF SOLUTIONS AND
RECENT HRRR TRENDS. THIS WILL BE A RESULT OF THE LARGER SCALE
TROUGH AXIS GRADUALLY SHIFT FARTHER EAST.

THE MOISTURE TRANSPORT VALUES ALSO SHOULD TEND TO SLACKEN IN THE
09Z TO 12Z TIME PERIOD WHICH SHOULD PROMOTE A BIT OF SLACKENING OF
THE RAIN RATES GOING TOWARD DAWN. VERY WEAK INSTABILITY PARAMETERS
WILL ALSO SUPPORT THIS TREND.

NEVERTHELESS...ADDITIONAL RAINFALL AMOUNTS OF AS MUCH AS 2 TO 3
INCHES CAN BE EXPECTED...WITH ISOLATED HEAVIER AMOUNTS OVER THE
HIGHER TERRAIN WHERE STRONGER OROGRAPHICS WILL BE IN PLACE. THIS
WILL LIKELY PROMOTE MORE FLASH FLOODING AND ESPECIALLY OVER AREAS
THAT HAVE ALREADY BEEN SEEING TRAINING BANDS OF HEAVY RAINFALL
THROUGH THE EVENING. WILL CONTINUE TO MONITOR...BUT IT IS AGAIN
EXPECTED THAT CONDITIONS WILL IMPROVE A BIT TOWARD DAWN.

ORRISON

ATTN...WFO...ALY...BOX...BTV...CAR...GYX...OKX...

ATTN...RFC...NERFC...

LAT...LON   46027015 45396985 44447012 42857072 41527098
            41017143 40817205 40737285 41077315 41577313
            42197296 43007269 43837231 44917185 45717122