Mesoscale Precipitation Discussion 0637
NWS Weather Prediction Center College Park MD
537 PM EDT Sat Aug 11 2018

Areas affected...Northeast Corridor

Concerning...Heavy rainfall...Flash flooding likely

Valid 112136Z - 120330Z

Summary...Heavy rainfall was already leading to flash flooding
across parts of far eastern Pennsylvania and New Jersey as of 5 PM
EDT. Additional storms were developing in those areas, and could
worsen ongoing flooding in locations that have already received
heavy rainfall today. Additionally, some heavy rain bands may
develop further northeast into southern New England into the
evening hours, potentially expanding the threat of flash flooding
further north in the region.

Discussion...Numerous Flash Flood Warnings were in effect as of
21Z in far eastern Pennsylvania and into New Jersey, and multiple
flooding reports have been received from these areas through the
afternoon hours. A large portion of the area has received over an
inch of rainfall today, per NSSL QPE. 21Z RAP analysis placed a
corridor of enhanced low-level frontogenesis in a WNW-ESE
orientation from southeast Pennsylvania into southern New Jersey.
This was leading to another uptick in convective activity in the
same area, including the Philadelphia metro area. KDIX dual pol
radar and MRMS estimated rain rates of 1-2 in/hr with these
developing storms, and that should be sufficient to cause flash
flooding to redevelop in some locations in SE PA and S NJ, or
exacerbate ongoing flooding and lead to more significant flooding
or inundation. Additionally, some hi-res model guidance indicates
that the rain rates in this area could increase above 2 in/hr over
the next several hours as storm mergers and collisions take place.
This seems plausible given analyzed PWs around 2 inches and access
to around 1500 j/kg MLCAPE -- convective clusters should be
relatively organized.

Further north, some rain bands were persisting in northern New
Jersey, where many areas have also received over an inch of rain
so far today. Therefore, flash flooding appears likely to continue
in some locations in northern New Jersey as well for at least the
next couple hours.  Of particular concern is a convective cluster
slowly drifting east, and approaching KTEB as of 2130Z. KDP values
off KOKX radar were elevated into the 2-3 deg/km range and ZDR was
generally around 1.5 dB or less -- indicative of numerous small
water droplets, extremely heavy rain and highly efficient rainfall
production. Dual pol estimates and MRMS put the rain rates with
this cluster as high as 2.5 in/hr, and the rain appeared to be
falling on areas that received heavy rainfall earlier in the
morning. Therefore, the CREST hydrologic model was showing
significant response and very elevated values of max unit
streamflow over northern New Jersey -- very likely indicating
significant flash flooding near the I-80 corridor. This may
continue to spread slowly east over the next hour or two towards
Yonkers.

Yet further northeast along the I-95 corridor, additional rain
bands may develop into the early evening hours along a band of
slowly increasing low-level convergence through Connecticut and
into eastern Massachusetts. Precipitable water values remain
elevated in this region, and although the area has not received
heavy rainfall yet today (as compared to places further south),
flash flooding will be possible. Hi-res models do show the
potential for some 1-2 in/hr rain rates pushing into southern New
England.

Lamers

ATTN...WFO...ALY...BGM...BOX...CTP...GYX...LWX...OKX...PHI...

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

LAT...LON   43007093 42267049 41057156 40177342 38427498
            39127620 40017636 41257549 42397378 42997237