Mesoscale Precipitation Discussion 1023
NWS Weather Prediction Center College Park MD
111 PM EST Mon Nov 26 2018

Areas affected...NYC Metro Area, New Jersey, SE Pennsylvania

Concerning...Heavy rainfall...Flash flooding possible

Valid 261810Z - 270000Z

Summary...A period of moderate, to occasionally heavy, rain is
likely across the I-95 corridor from Philadelphia to New York City
this afternoon and early this evening. Over 1 inch of rain is
possible in under 6 hours, which could lead to new or worsening
flooding across the area. Most of the rivers in the area are
running higher than average with some places of ongoing flooding
already.

Discussion...Surface observations suggest a weak surface wave or
surface low is trying to form near the Lower Chesapeake Bay. This
low should continue developing in the next few hours and lift into
New Jersey by late afternoon, per the 12Z HREF. Enhanced low-level
convergence is likely near and just ahead of the low, where strong
south-southeast winds (advecting moisture northward) will begin to
meet a cooler, continental air mass and weaker northeasterly flow
just inland from the coast. The expected configuration of the
developing low and associated coastal front should focus an area
of heavier rainfall in the I-95 corridor this afternoon and early
this evening. The surface low will be lifting north steadily, so
the rain should not linger for a very long time. However, a 3-6
hour period of moderate to heavy rain appears likely.

12Z HREF 40km neighborhood probabilities of over 1 inch of rain
are as high as 70-80 percent in this area, indicating relatively
high confidence that some location(s) in this relatively confined
region will receive that amount of rainfall. However,
probabilities using a variable radius method (that accounts for
ensemble agreement on the precise placement of those amounts) are
much lower. This suggests that 1 inch amounts are unlikely to be
widespread, and there is not great model agreement on where they
will occur.

6-hour flash flood guidance in portions of the area is around 1
inch, so localized 1 inch amounts would be likely to at least
approach FFG and perhaps exceed it. Observed streamflow across the
region is generally above the 95th percentile, and some rivers are
already above flood stage. The elevated soil moisture and high
river levels should reduce infiltration and could lead to general
runoff issues across the area, as will the large amount of
impervious surfaces in the urban corridor. In summary, despite a
lack of exceptional rain rates, antecedent conditions would favor
some localized new flooding issues, or worsening of existing
flooding, this afternoon and evening.

Lamers

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

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

LAT...LON   41247436 41217374 40847336 40467349 40037392
            39797471 39737532 40007578 40377574 40717534
            41107483