MESOSCALE PRECIPITATION DISCUSSION 0006
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
222 PM EST FRI JAN 12 2018

AREAS AFFECTED...MUCH OF THE NORTHEAST CORRIDOR

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

VALID 121921Z - 130500Z

SUMMARY...MODERATE TO OCCASIONALLY HEAVY RAINFALL WILL CONTINUE TO
PUSH THROUGH THE NORTHEAST CORRIDOR...WITH AN INCREASE IN RAINFALL
RATES EXPECTED FROM NYC METRO INTO SRN NEW ENGLAND AROUND 4-7 PM
EST. THE COMBINATION OF SNOW MELT AND RAINFALL SHOULD PRODUCE
FLOODING ACROSS THE AREA.

DISCUSSION...A BAND OF RAIN CONTINUES TO AFFECT THE
NORTHEAST...AND STRETCHES FROM DOWNEAST MAINE...ARCING TO THE
SOUTHWEST THROUGH THE SOUTHERN WHITE MTNS...BERKSHIRES...AND TO
THE PHILADELPHIA METRO AREA AS OF 19Z. MODELS PREDICT THAT THIS
SHOULD LARGELY CONTINUE INTO THE EVENING HOURS WITH A SLIGHT DRIFT
TO THE EAST AND NORTHEAST...ASSOCIATED WITH A SHIFT IN THE AXIS OF
STRONGEST LOW-LEVEL MOISTURE TRANSPORT WHICH WILL MOVE CLOSER TO
THE COAST AND LIFT UP THE COAST WITH TIME. AS STRONGER PVA
ASSOCIATED WITH THE PRIMARY SHORTWAVE SPREADS NORTHEAST
OVERNIGHT...THE RAIN MAY BEGIN TO FILL IN ONCE AGAIN AFTER 00Z.

OVER THE NEXT SEVERAL HOURS AND PARTICULARLY FROM 21-00Z...MORE
FOCUSED BANDS OF RAIN MAY DEVELOP FROM THE NYC METRO AREA INTO
SOUTHERN NEW ENGLAND...AS WELL AS COASTAL MAINE. THESE WOULD BE
ASSOCIATED WITH A PROJECTED INCREASE IN BOUNDARY LAYER MOISTURE
CONVERGENCE...WHICH SHOULD ALLOW FOR MORE ORGANIZED BANDING AND
THE POSSIBILITY OF NARROW FOCUSED BANDS OF HEAVIER RAINFALL. THIS
IS ESPECIALLY TRUE FROM NYC INTO CT/RI...AND PORTIONS OF MA WHERE
MARGINAL INSTABILITY IS ALSO EXPECTED. 18Z RAP ANALYSIS SHOWED
SEVERAL HUNDRED J/KG OF MUCAPE NOSING INTO THIS REGION IN THE
CHANNEL OF DEEPER MOISTURE. THE MOST RECENT HRRR RUNS SHOW HOURLY
RAIN RATES PEAKING CLOSE TO 1 IN/HR IN NARROW BANDS BETWEEN 21-00Z
IN THIS HIGHLIGHTED AREA AND THE 12Z HREF HAS THE HIGHEST
PROBABILITIES OF 1 INCH OF RAIN CENTERED ON CENTRAL CT IN THE SAME
TIME FRAME. THIS SIGNAL ON HI-RES MODEL GUIDANCE ALSO FAVORABLY
ALIGNS WITH EXTRAPOLATED TIMING (PER GOES-16 AND REGIONAL RADARS)
OF CONVECTION OFFSHORE OF SRN COASTAL NEW JERSEY.

THE ANOMALOUS MOISTURE LEVELS WILL CONTINUE TO SUPPORT
OCCASIONALLY HEAVY RAIN INTO THE EVENING HOURS...WITH PRECIPITABLE
WATER VALUES NEAR OR IN EXCESS OF JANUARY MONTHLY RECORD LEVELS.
FOR PORTLAND/GREY MAINE...THE HIGHEST PWAT ON RECORD IS 1.31
INCHES (SIMILAR FOR THE WINTER SEASON OVERALL)...BUT THE RAP IS
FORECASTING VALUES TO REACH AROUND 1.6 INCHES. THIS IS INDICATIVE
OF JUST HOW ANOMALOUS THE DEEP MOISTURE PLUME IS. WHEN PAIRED WITH
FROZEN GROUND ACROSS MUCH OF THE REGION...AND AN ABUNDANCE OF SNOW
EAST OF THE HUDSON RIVER...THE INGREDIENTS ARE CERTAINLY IN PLACE
FOR FLOODING. THE FROZEN GROUND SUGGESTS THAT MORE RUNOFF IS
LIKELY FOR A GIVEN RAIN RATE THAN WOULD NORMALLY BE
ANTICIPATED...AND THE ENHANCED RUNOFF WOULD BE LIKELY TO OCCUR IN
CONCERT WITH MELTING SNOW. THEREFORE...EVEN RAIN RATES THAT SEEM
RELATIVELY MODEST COULD PRODUCE FLOODING ISSUES. MORE RAPID ONSET
FLASH FLOODING WILL BE POSSIBLE IN MORE FOCUSED BANDS OF HEAVY
RAINFALL...PARTICULARLY WHERE THEY ALIGN WITH FROZEN GROUND OR
URBAN AREAS.

LAMERS

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

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

LAT...LON   46416733 44996658 43266908 41867041 40437225
            39647394 40327531 41807441 42977298 44087157
            45936895