Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Mon Apr 23 2018


Prelim Day 1 QPF Discussion
Valid Apr 24/0000 UTC thru Apr 25/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Eastern / Southeastern U.S...

A closed deep layer low continues to slowly move east across the
TN and OH Valley this afternoon....bringing with it a broad area
of showers and embedded thunderstorms. In general, there continues
to be a displacement of the better instability and the stronger
low level moisture transport axis. Thus the more organized area of
precipitation across SC/NC is more stratiform in nature...with
just localized heavier convective cores on the southwest flank
closer to the instability pool. Meanwhile, further southwest
within the instability pool weaker low level moisture transport,
lower PWATs, and less of a low level focus, is resulting in less
organized convection (although still locally and briefly heavy).
Thus not the best setup for flash flooding across the Southeast
and eastern Mid Atlantic...although some isolated issues could
still develop where any heavier convective cores are able to
briefly train.

Closer to the closed low we are seeing steep lapse rates and
diurnal heating result in the development of scattered
thunderstorms. PWATs are lower here, however cells will likely be
slow moving underneath the low, and storm motions are conducive
for some repeat south to north cell activity east of the low.
Localized 1-3" amounts are possible in the Marginal risk area that
was introduced here, and with FFGs lowered over portions of the
area due to recent rainfall, some localized flooding concerns
could arise.

Across the southern Appalachians, the favored upslope regions in
western SC/NC/VA will experience an extended period of strong
upslope southeasterly low level flow beneath difluent upper flow.
Rainfall rates will be lower here given little to no instability,
however the persistent strong southeasterly flow into the terrain
supports some 0.5" in an hour amounts in the more favored terrain,
which given the duration, could result in localized 5"+ totals.
Totals of this magnitude should result in some flooding concerns
developing with time. The day 1 Slight was trimmed some, as the
main threat should primarily be focused into and just east of the
terrain

Showers and embedded thunderstorms will continue into the day
Tuesday across portions of the OH/TN Valley into the Mid Atlantic.
Again, instability should be limited within the corridor of best
moisture transport and higher PWATS over the Mid Atlantic, thus in
general not expecting rates to be all that impressive. Some chance
we see instability work into coastal NC which could result in
heavier rates here, although also possible this instability and
heavier convective activity remains offshore. Over portions of
eastern TN/KY should see another round of diurnally driven
convection underneath the upper low. Could be some locally heavy
rain here, although not expecting much of a flash flood threat at
this point.

WPC QPF followed a multi model blend here, and ends up pretty
close to the 12z HREF mean. In general this results in a modest
increase in QPF amounts, with the biggest increases over the
eastern slope of the Appalachians, where orographic impacts will
be maximized, and underneath the closed low.


...Central and Northern Plains...

A wave will move out of the Rockies and into the Plains on
Tuesday, bringing a swath of rain/snow into the Plains. Again,
generally good model agreement allowed for a multi model blend to
be used, with the new WPC QPF looking similar to the 12z HREF mean.

Chenard