MESOSCALE PRECIPITATION DISCUSSION 0204
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
201 AM EDT SUN MAY 27 2018

AREAS AFFECTED...NORTHERN NJ...NEW YORK CITY

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

VALID 270600Z - 271200Z

SUMMARY...WARM ADVECTION OVER STATIONARY BOUNDARY ACROSS NJ TO
PRODUCE CONVECTION CAPABLE OF FLASH FLOODING THROUGH EARLY MORNING
HOURS.  WEAK WINDS AND WEST OF MOISTURE/LLJ AXIS MAY BE A LIMITING
FACTOR FOR OVERALL COVERAGE/MAGNITUDE.

DISCUSSION...05Z SURFACE ANALYSIS DEPICTS A WEAKLY DEFINED WARM
FRONT THAT BISECTS NJ FROM BLM TO TTN BEFORE ANGLING NW TO WEAK
SURFACE WAVE NEAR IPT.   WHILE NOT TOO STRONG AT THE SURFACE...RAP
ANALYSIS DEPICTS NARROW CORRIDOR OF EASTERLY 850-7H FLOW IS
EXPECTED TO DEVELOP IN THE NEXT FEW HOURS FURTHER STRENGTHENING
LOW TO MID-LEVEL CONVERGENCE.  CURRENT VAD WINDS SUPPORT THIS WITH
REDUCTION OF SWLY FLOW ACROSS LONG ISLAND AND NEWARK.  GOES-WV AND
RADAR MOSAIC SHOW MCV CURRENTLY OVER WESTERN PA WHICH WILL AID IN
INCREASED SWLY BOUNDARY LAYER TO 7H FLOW.  Q-AXIS WITH TPWS UP TO
2.0" WITHIN THE SWLY FLOW IS DIRECTED FROM THE NECKS OF VA ACROSS
S DE TOWARD STRONGER DEEP CONVECTION SEEN ALONG THE WARM FRONT
SOUTH OF LI ATTM (GOES-16 AND KOKX RADAR).  STILL WEST OF THE AXIS
MOISTURE IS STILL FAIRLY DEEP AND PROFILES ARE FAIRLY SATURATED
THROUGH THE LOWER PORTIONS OF UPDRAFT WITH TPWS IN THE 1.75"
RANGE.  RAP ANALYSIS ALSO DEPICTS A WELL OF UNTAPPED INSTABILITY
ACROSS NE MD WITH MUCAPES UP 2000+ J/KG THAT SHOULD SUPPORT STRONG
UPDRAFTS.  UPPER LEVEL DIVERGENCE IS NOT AS STRONG AS FURTHER
NORTH WHERE RIGHT ENTRANCE TO 50-60KT 3H JET IS PROGGED TO SETUP
OVER NEW ENGLAND BY 12Z.  PERHAPS PERIPHERAL ASCENT OR ENTRANCE
EVACUATION MAY HELP UPDRAFT STRENGTH.  AT PEAK CONVECTION EXPECTED
AROUND 8-9Z...RAIN RATES UP TO 2"/HR ARE POSSIBLE. 

VAD WINDS ALONG WITH RAP FORECAST SUGGEST LOW LEVEL CLOUD BARING
FLOW IS NOT IDEALLY SET UP FOR WEST TO EAST TRAINING...HOWEVER
WITH LEFT NOSE OF LLJ SUPPORTING 20 MAYBE AS HIGH AS 30 KTS
LATER...ADDITIONAL REDEVELOPMENT ALONG THE FRONT IS POSSIBLE FOR A
SMALL AREA OF REPEAT RAINFALL MAINLY JUST NORTH OF THE FRONTAL
ZONE FROM TTN TO SANDY HOOK/BLM.  WITH WEAKER INFLOW...THREAT FOR
MULTIPLE ROUNDS OR EVEN STRONG MST FLUX SHOULD LIMIT ANY FLOODING
THREAT TO MAINLY NJ AND THE DELAWARE VALLEY OF PA AND PERHAPS
STATEN ISLAND AND NYC IF THE FRONTAL BOUNDARY DOES NOT SAG
SOUTHWARD WITH OUTFLOW FURTHER DISTANCING THE SOURCE OF NEW
DEVELOPMENT.  MPD AREA IS ALSO RESTRICTED FROM THE PINE BARRENS OF
NJ DUE TO THE MUCH HIGHER FFG IN THE AREA. 

HI-RES CAMS SUCH A RECENT HRRR AND HRRRV3 SEEM TO LIMIT THE
OVERALL WESTERN COVERAGE A BIT BETTER (GIVEN THE BACKED LOW LEVEL
FLOW).  HERE TOTALS OF 2-3" WITH ISOLATED 4" NOT OUT OF THE REALM
OF POSSIBLY MATCH CURRENT TRENDS...THOUGH BELIEVE BEST
CONVECTION/RAINFALL WILL BE OVER THE HUDSON CANYON REGIONS
OFFSHORE DUE TO BETTER LLJ MST AND FORCING/UPGLIDE ASCENT.  STILL
GIVEN THE LOWER FFG FLASH FLOODING IS CONSIDERED POSSIBLE THROUGH
12Z. 

GALLINA

ATTN...WFO...OKX...PHI...

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

LAT...LON   41237473 41227457 41117417 40947387 40667378
            40427384 40007392 39957406 40087431 40187468
            40377510 40537526 41017503