Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
321 AM EDT Sun Apr 22 2018


Prelim Day 1 QPF Discussion
Valid Apr 22/1200 UTC thru Apr 23/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Southeast U.S...

A closed low of moderate strength and maintaining itself along its
slow eastward march - characteristically a very April system -
will spread rain and thunderstorms throughout areas from the lower
Ohio Valley to the Gulf Coast. In Florida easterly onshore flow,
heating, and gradually increasing deep layer ascent will also
promote widespread coverage of rainfall.

Model guidance is in decent agreement, and the notion of
widespread 1 to 3 inch rainfall is supported even by the GEFS
Reforecast data based on analog events. The main question is
whether rain rates will be intense enough or persistent enough
over any given location to yield much in the way of flash
flooding. Generally the most persistent forcing and better
alignment of lower and upper level flow should occur during the
day from northern Mississippi into western Tennessee. This is
perhaps where training will be most prevalent, but instability
will be more marginal. Areas farther south should see somewhat
greater instability, but with mid level flow cutting across from
west to east, the mean 0-6 km winds are at a sharp angle to
Corfidi vectors, suggesting training will not be too common. Given
flash flood guidance values are also much higher, and will be
difficult to surpass with southward extent, WPC trimmed back a bit
on the inherited Slight Risk area for the 12z Sun- 12z Mon period.
Otherwise, Slight Risk is maintained in areas of lower FFG across
northern MS/AL into southern TN, and including the Atlanta Metro
to the east, as well as the more favorable training environment in
western TN.

Throughout these areas areal average precipitation is forecast at
1.5 to around 3.0 inches for the 24-hour period, and judging by
moisture availability and hi-res model solutions, hourly rates
could peak around 1.50 inches, leading to at least some risk of
flash flooding. For QPF there was no ideal solution, but the
models were well clustered. WPC began with a blend of the 00z GFS,
WRF-NMM, and the non bias corrected version of our in-house
ensemble (more generous with areal coverage). We expanded coverage
northward, resulting in quite a bit more QPF over northern TN into
KY compared to the previous cycle, and toned down QPF along the
east coast of Florida, as low level convergence and cell motions
appear erratic enough to spread the convection out.


...Northern Rockies...

In the immediate wake of a lead shortwave - which deposited a
frontal zone into the northern Rockies - a stronger shortwave
trough will amplify Sunday over Idaho and Montana, bringing a
period of strengthened frontogenesis and well defined upper jet
forcing. Weakly convective rain/snow will likely result. The
previous WPC forecast had a great handle on this event, and we
made only minor tweaks using the 00z GFS and our in-house ensemble
tools.

Burke