Mesoscale Precipitation Discussion 0896
NWS Weather Prediction Center College Park MD
857 PM EDT Tue Sep 25 2018

Areas affected...Long Island, Connecticut, Rhode Island, Ern
Massachusetts

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

Valid 260056Z - 260500Z

Summary...A band of heavy rain has been repeatedly affecting
portions of Long Island and Connecticut already early this
evening, producing flash flooding. This band of heavy rain is
likely to persist for at least another hour or two, and should be
able to (1) sustain ongoing flash flooding in portions of Long
Island and Connecticut, and (2) perhaps expand the flash flood
threat into Rhode Island and eastern Massachusetts. Rain rates up
to 2-3 in/hr will be possible.

Discussion...KOKX radar shows a roughly 100mi long convective rain
band as of 01Z, extending from Milford MA, to Chester CT, to Port
Jefferson NY. The band was effectively training, only exhibiting a
few miles of lateral movement in the past hour and oriented
roughly parallel to the 850-700mb wind. The band was likely
embedded more in the low-mid level flow, particularly around that
850-700mb layer, as the vast majority of higher reflectivity
values were concentrated below 15,000 feet AGL, or close to the
melting layer. This implies that rainfall production should be
very efficient, and also suggests that training should continue
for the next few hours as the low-mid level mean winds should be
fairly consistent in that same time period.

RAP analysis showed very strong boundary layer moisture
convergence right along the main axis of the convective rain band,
and extending northeast to the Boston metro area and toward the
southern coast of Maine. It also showed an instability axis
extending along and just east of this convergence zone, nosing
into Long Island and Connecticut. MUCAPE above 500 j/kg was
generally concentrated south of Massachusetts as of 01Z. However,
over the next few hours, the RAP forecasts a gradual
destabilization into eastern Massachusetts due to increasing
low-level thetae. This suggests that convective bands could begin
to expand or develop further northeast over the next few hours. A
competing factor, though, will be a forecasted decrease in the
low-level convergence, which would tend to lead to less organized
convective banding. Therefore, although the destabilization could
allow for convection to exist further east and northeast than it
does now, decreasing organization may limit the effectiveness of
training and rain rates over 1+ hr time periods.

Overall, it appears flash flooding is likely to continue in
portions of Long Island and Connecticut for another couple hours
where the convective rain band has already produced significant
rainfall. This threat may expand further east and northeast into
Rhode Island and eastern Massachusetts, but the rain bands may
become less organized after 03Z, with the flash flood threat
becoming increasingly localized in the 03-06Z time frame.

Lamers

ATTN...WFO...BOX...OKX...

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

LAT...LON   42687104 42297031 41437090 40977173 40527297
            40837339 41967232