Mesoscale Precipitation Discussion 0632
NWS Weather Prediction Center College Park MD
122 PM EDT Sat Aug 11 2018

Areas affected...Eastern Pennsylvania, New Jersey, Southern New
York

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

Valid 111720Z - 112300Z

Summary...Redevelopment of thunderstorms is expected this
afternoon in parts of the Mid Atlantic, particularly from eastern
Pennsylvania into New Jersey. These storms should become more
numerous by mid-afternoon with the potential for locally heavy
rainfall and flash flooding. Rain rates may reach 2-3 in/hr with
the strongest storms.

Discussion...Skies have generally cleared across eastern
Pennsylvania and northern New Jersey in the wake of some
convective activity overnight and into early this morning. GOES-16
visible satellite indicated an increasingly well-developed cumulus
field as of 17Z, with several more enhanced cumulus lines (and
incipient convective clusters) initiating along two
quasi-stationary fronts draped across the region. A more
concentrated area of convective development was in southern New
Jersey, along a boundary that was likely some combination of an
existing surface front and outflow from morning thunderstorms to
the north. Other than perhaps in the lowest 0.5km or so (surface
winds were somewhat chaotic and light), forecast soundings and
area radar VWPs indicated increasingly aligned southwesterly flow
through much of the depth of the troposphere, and this coincided
with a ribbon of enhanced low-level moisture transport along and
just west of the I-95 corridor from SE PA into S NY. The alignment
of the flow should favor small convective clusters and lines that
are oriented roughly parallel to the mean flow and could lead to
some training and enhancement of heavy rain durations.

The environment will support very heavy rain rates with moderate
instability (MLCAPE around 1500 j/kg) and deep moisture with PWATs
around 1.8 to 1.9 inches per GPS-PW observations. While PWATs of
that magnitude are not necessarily records for this time of year,
they are above the 90th percentile on the OKX sounding
climatology. Several convection-allowing models explicitly
forecast localized rain rates peaking in the 2-3 in/hr range, and
it seems reasonable to expect 2+ in/hr rain rates given the
overall environment. The most favorable region for flash flooding
seems to be from southeast Pennsylvania into northern New Jersey.
This is an area that has cleared out on visible satellite and thus
instability should build, but the flash flood guidance is also
reduced (lowest near and to the northwest of I-95). The chance of
a localized more significant event cannot be discounted either.
The 12Z HREF does show some small (10-15%) neighborhood
probabilities of 6hr rainfall reaching 1% annual exceedance
probability thresholds, and similar neighborhood probabilities of
receiving upwards of 5 inches of rain. Therefore, while a
localized more significant rainfall and flash flood event is far
from certain, it can't be ruled out.

Lamers

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

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

LAT...LON   41907495 41577390 41077364 40027376 39347447
            39247532 39767663 40777754 41637635