Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
231 AM EDT Tue Apr 17 2018


Prelim Day 1 QPF Discussion
Valid Apr 17/1200 UTC thru Apr 18/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Northeast and Great Lakes...

Snow showers will continue today off the Great lakes and into the
terrain of northern New England underneath the closed mid/upper
level low. Given the small scale features of lake effect and
orographic enhancement...WPC QPF relied solely on a blend of the
0z HREF members across this region.


...Northwest into northern Rockies...

Lingering onshore flow will result in showers into the the terrain
of western WA/OR and the northern Rockies. Shortwave ridging
building in later today into tonight should result in decreasing
coverage of showers. WPC QPF blended the 0z HREF members.


...Central and northern Plains into the Mid and Upper MS Valley...

Shortwave energy will push east out of the Rockies and into the
Plains by Tuesday night, with a surface low ejecting across the
Plains and into the MS Valley. Precipitation will move across
portions of WY/MT early today, and into the western Plains this
afternoon, ahead of the shortwave trough along a low level
convergence axis. Through this period utilized a multi model blend
for QPF, with the high res and global models showing some
usefulness. Would appear like the 12z ECMWF may be too dry here,
with the GFS/UKMET and HREF members generally wetter than it. At
the same time, concerned the GFS and HREF members may be too wet
over western ND, with the non NCEP models generally lighter here.
Thus the multi model blend mentioned above seemed to result in a
good middle ground solution.

By later Tuesday night things get a little less certain as the low
moves into the MS Valley. Given the strong dynamics in play (well
defined shortwave and strongly divergent flow in the upper levels)
would think we should see a compact but briefly intense area of
precipitation northwest of the low. By this time the 0z HREF
members generally seem too dry and quite a bit different than the
global model consensus...resulting in lower confidence by this
period. Often the high res models can struggle with non convective
precipitation by the end of day 1 into the day 2 period. Thus
given the more consistent global model signal, stayed away from
the HREF by this period. WPC QPF is thus closer to a blend of the
0z GFS/UKMET and 12z ECMWF by this time...placing a max corridor
from eastern NE into western IA. This is a bit south of our
previous forecast. The GFS has shown a consistent trend south over
the last several cycles, with the ECMWF and UKMET showing decent
consistency further south. Thus the southward shift seems to be
the way to go at this time.

Chenard