Mesoscale Precipitation Discussion 0540...Corrected
NWS Weather Prediction Center College Park MD
630 PM EDT Wed Jul 25 2018

Corrected for change header info to flash flooding "likely"

Areas affected...portions of the northern Mid-Atlantic region into
southern/eastern NY/southern New England

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

Valid 252220Z - 260400Z

Summary...Flash flooding is likely to continue for portions of the
northern Mid-Atlantic region into southern portions of the
Northeast through 04Z. Additional rainfall of 2-4 inches will be
possible where cell training is greatest, with isolated higher
totals possible.

Discussion...Regional radar imagery at 2145Z showed a NNE-SSW line
of heavy rain oriented from near Scranton, PA through Harrisburg
to near Westminster, MD. A few reports ending 2145Z indicated
rainfall rates of 2-3 in/hr within this line where
training/repeating was greatest, just southwest of Harrisburg,
with the rest of the line peaking in the 1-2 in/hr range. This
line is the greatest near term concern given its orientation
aligning with the mean low-mid level flow and its location
overlapping with 5-10 inches of rain which has fallen over the
past 4 days from portions of northern MD into eastern PA.

As a slow moving 850-700 mb trough axis, currently stretching from
central NY into WV, continues toward the east, the line of
convection moving through eastern PA should continue to slide east
as well with possible weakening beyond 00Z with the onset of the
nocturnal stabilizing boundary layer.

Farther to the east, visible satellite imagery showed a low level
circulation roughly 60 miles east of the DelMarVa peninsula, out
ahead of a larger scale, negatively tilted upper trough axis over
the Mid-Atlantic region. Flow along the east coast is already
strongly diffluent given the placement of an upper ridge axis just
offshore, but enhanced lift tied to the mesoscale low east of the
DelMarVa is expected to reach near New York City between 00-03Z.
Some concerns if instability will be high enough to support large
scale heavy rainfall rates, but given the high precipitable water
environment, added forcing for ascent and the HRRR simulated
reflectivity forecasts of a mesoscale deformation axis stretching
from NJ into the NYC metro, flash flood concerns are elevated in
the 00-03Z time frame.

MD/PA/NJ and NYC metro regions all have the potential of seeing
2-4 inches of rain through 04Z, locally higher, which would cause
flash flooding.

Otto

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

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

LAT...LON   42477477 42447382 42327279 41847232 41147267
            40567347 39587433 38957541 38717664 39137725
            40297721 41807624 42257559