Mesoscale Precipitation Discussion 0531
NWS Weather Prediction Center College Park MD
1245 PM EDT Tue Jul 24 2018

Areas affected...Northern Maryland/Delaware into central
Pennsylvania, far western New Jersey and southern New York

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

Valid 241643Z - 242130Z

SUMMARY...Flooding and flash flooding appear likely over the next
3-5 hours from northern Maryland/Delaware into portions of central
Pennsylvania, western New Jersey and southern New York. An
additional 2-4 inches of rain with rainfall rates of 1-2 in/hr
through 21Z can be expected.

DISCUSSION...Regional radar imagery at 16Z showed a NNW-SSE axis
of heavier rainfall in the vicinity of the northern Chesapeake Bay
with rainfall rates of ~1 inch in 15-20 minutes per
Wunderground.com observations. The broken line of heavy rain was
coincident with weak low level confluence beneath locally
diffluent flow aloft given the presence of a narrow upper level
ridge axis along the East Coast. The line has been gradually
shifting to the north and east which has prevented potentially
much higher rainfall totals over the past 2-3 hours but the
northern Mid-Atlantic region is saturated from a widespread 4-10
inches of rain which has fallen over the past few days.

CAPE values across the northern Mid-Atlantic into southern New
York are generally less than 1000 J/kg given cloud cover, but
given the moist environment, higher CAPE values are not needed to
support heavy rainfall rates. Nonetheless, clearing over eastern
Pennsylvania may allow for increasing instability through the
afternoon along with the development of new convection. Little
change to the synoptic environment is expected over the next
several hours which will keep precipitable water values over 2.25
inches and 30-40 kt 850 mb flow from the south helping to support
repeating and brief training of heavy rain cores. Given the
antecedent conditions, flooding and flash flooding are considered
likely.

Otto

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

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

LAT...LON   42377594 41437524 40337520 39307530 38617564
            38737623 39017666 39577699 40137715 40737744
            41337747 42157719 42377662