Mesoscale Precipitation Discussion 0829
NWS Weather Prediction Center College Park MD
318 AM EDT Thu Sep 13 2018

Areas affected...far northern MD into central/eastern PA and NJ

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

Valid 130715Z - 131300Z

Summary...Highly efficient rainfall could lead to localized flash
flooding from northern MD into central/eastern PA and NJ through
13Z. 3-5 inch totals will be possible in a couple of locations.

Discussion...As of 07Z, localized heavy rainfall was occurring
within an axis of 700 mb confluence from CT to northern NJ into
south-central PA, focused south of a departing New England
shortwave. While low level flow is weak at less than or equal to
10 kt below 700 mb, the environment is very moist with
precipitable water values within a tenth of an inch of 2 inches
per recent GPS observations across the area. Isothermal
reflectivity values show 40-50 dBZ occurring primarily at and
below 0 C, which when combined with warm topped IR satellite
imagery and limited lightning, is indicative of efficient warm
rainfall processes underway. Recent Wunderground.com rainfall
observations ending 06-07Z from northern NJ confirmed rainfall
rates of 1-2 in/hr with slow moving rainfall in northern Middlesex
and eastern Somerset counties, leading to 3-5 inches of rain since
04Z. The convection appears to be somewhat elevated via RAP
analysis soundings, rooted between 850-700 mb, with convection
located on the cool side of a stationary front extending northeast
to southwest through the Mid-Atlantic region.

Flow will remain weak through the remainder of the morning which
will limit forcing for ascent, which when combined with low CAPE
values should keep rainfall cores limited in coverage. However,
the environment is supportive of efficient 1-2 in/hr rainfall
rates on top of an anomalously wet week of rainfall from northern
MD into much of PA and portions of NJ, which has increased the
susceptibility to flash flooding. The expectation is for localized
3-5 inch rainfall totals through 13Z along with possible
flooding/flash flooding.

Otto

ATTN...WFO...BGM...CTP...LWX...OKX...PHI...

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

LAT...LON   41047482 40497429 40047527 39657662 39467753
            39687821 40277831 40707811 40967744