Mesoscale Precipitation Discussion 0646...Corrected
NWS Weather Prediction Center College Park MD
230 AM EDT Mon Aug 13 2018

Corrected for change in header from flash flooding "possible" to
"likely"

Areas affected...northern MD into central PA

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

Valid 130602Z - 131202Z

Summary...Slow moving showers and thunderstorms are likely to
produce flash flooding through 12Z from portions of northern MD
into central PA. Additional rainfall totals of 2-4 inches are
expected.

Discussion...Regional radar through 0530Z showed an increase in
coverage of heavy rain over northeastern MD into portions of
central PA with a couple of cores nearly stationary, such as over
Baltimore, MD and roughly 20 miles southeast of Harrisburg, PA.
KLWX radar estimates were as high as 1-2 in/hr within the cell
over Baltimore city and county. The convection was warm topped
(tops of -40 to -50 C) with little in the way of lightning (except
over Baltimore). Precipitable water from blended TPW imagery, 00Z
soundings and GPS sites showed values ranged from near 1.5 inches
over central PA to near 2 inches along the northern Mid-Atlantic
coast, with an 850 mb low over VA keeping flow oriented from the
greater moisture source.

00Z RAOBs helped identify a 70-90 kt upper level jet max extending
from the VA/WV border into central New England with the right
entrance region and related upper divergence situated over
northern MD and central/eastern PA. Water vapor imagery showed an
upper low slowly pivoting east from the upper Ohio Valley with its
related trough axis becoming more neutrally oriented over the past
6-12 hours. This movement will help to increase upper level
diffluence across the northern Mid-Atlantic region helping to
support increased vertical motion amid tall/skinny CAPE profiles
seen on RAP analysis soundings from northern MD into PA.

The 00Z nmmb/arw/nam_nest and recent runs of the HRRR support the
idea of heavy rain spreading from northern MD into central PA over
the next 3-6 hours. Given above average rainfall across this
region over the past couple of weeks, flash flood guidance is
rather low (1 in/hr, 2 inches in 3 hours) helping increase the
likelihood of flash flooding.

Otto

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

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

LAT...LON   41857713 41697636 41327591 40677579 39787617
            39317663 39327720 39617759 40167798 40817820
            41287808 41577788