Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Mon Apr 23 2018
Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 24/0000 UTC thru Apr 27/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr
...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
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
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
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
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.
...Mid-Atlantic and Ohio Valley to the Northeast/Great Lakes...
The mid-upper level low moving slowly through the Tennessee Valley
today and Tuesday will weaken as it ejects off the Mid-Atlantic
coast on Wednesday and interacts with a northern stream trough
amplifying over the Great Lakes. Ongoing heavy rainfall along and
ahead of the attendant cold front on Day 1, will continue into the
very early periods of Day 2, but as the low level inflow weakens
along with the upper level forcing, widespread heavy rains should
also lesson. Given the day 2 excessive period (valid 12z Tues to
12z Wed) spans the last half of the day 1 qpf period, a marginal
risk across the Mid-Atlantic was left basically untouched from the
On Wednesday, a surface low develops along the Mid-Atlantic coast
which should still promote the continuation of mainly moderate to
locally heavy rains from the northern Mid-Atlantic to the
Northeast Wednesday and into Thursday. Light to moderate
precipitation should also develop farther north and west of the
low in a deformation zone which is forecast to set up over the
Great Lakes. Models continue to diverge somewhat on this, with the
NCEP models slightly faster with the northern stream trough than
the non-NCEP models. Although the newest model runs seem to be
trending faster, so the WPC QPF was based on a compromise of the
GFS/ECMWF, with a slight lean towards the faster solutions. The
result is a QPF depiction very close to that of the previous shift.
Upper troughing moving out of the Rockies on Tuesday, will drop
southeastward across the central Great Plains on Wednesday and
into the lower Mississippi Valley on Thursday. Moisture along the
associated frontal boundary, in combination with favorable upper
dynamics, should allow for light to moderate rainfall across the
central and southern Plains on Tuesday into Wednesday, which moves
into the lower Mississippi Valley and Deep South by Thursday. The
slightly faster previous run of the ECMWF has come into better
agreement with the slower GFS, so the WPC QPF was based on a blend
of these two models.
Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml