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


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


Day 1

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

A well defined, slowly-moving closed low will bring a wet day to
parts of the southern and eastern United States. Models have done
well handling the spatial distribution of rainfall with this
system, but they have tended to over-forecast the magnitude. The
low itself, at 564 decameters at 500 mb, is barely 1.0 standard
deviation from climatology. Cloud cover hampering daytime heating,
and a mis-match between areas of strong forcing and areas of
larger CAPE, has also toned down the rainfall production. On
Monday there is seemingly a little greater chance for heavy and
excessive rainfall to occur, as the favored upslope regions in
western SC/NC will experience an extended period of strong
southeasterly low level flow beneath difluent upper flow. Models
also signal a heavy rainfall event along the Carolina coastline
where Gulf and Atlantic moisture streams will come into phase.

There are, however, still some uncertainties regarding instability
and event evolution. The NAM forecasts CAPE of greater than a few
hundred J/kg to develop not much farther north and west than Greer
and Charlotte. Sustained upslope will certainly boost rainfall
totals farther west, but the event in the mountains may be only
weakly convective. Meanwhile, at the coast, predicted cell motions
and model simulated reflectivity loops suggest that supercells
will move very slowly, and could contribute to locally intense
rainfall, but with widespred forcing leading to cell mergers - and
a slow but steady progression of the synoptic system may limit
duration of the heaviest rainfall at any given location.

For QPF, WPC favored the HREF Mean and NAM CONUS Nest which were
more tempered than the aggressive WRF-ARW solutions. The 03z
National Blend of Models also looked very good. Our forecast
trended upwards along the coast and in the mountains, still
maintaining a wet day, but also a relative minimum in the area
between. Ultimately, we may trim back amounts for the final
issuance, seeing as downscaling / post processing tends to go a
little too heavy in the Appalachians.


...Northern Rockies / Northern High Plains...

A relatively vigorous system with well defined mid level forcing
mechanisms had amplified over Idaho last Sunday / early Monday,
and will translate eastward today. The trough is forecast to
maintain a tight, closed mid level circulation along with a sense
of jet coupling and strong 700 mb frontogenesis, producing an
aggregate swath of forcing and associated rain/snow across
southern Montana and northern Wyoming - spilling into portions of
the Dakotas. Steep lapse rates hanging back in association with
the tail end of the trough will support some coverage of rain/snow
showers over areas farther west and south, and the arrival of deep
layer lift and a difluent upper level pattern onto the High Plains
should promote scattered convection from southeast Wyoming /
northeast Colorado into western Nebraska.

Model QPFs are clustered quite well. Therefore, WPC leaned toward
consensus and bias corrected tools such as our in-house ensemble
and the National Blend of Models. The previous forecast had a good
handle on this system, so overall change was minimal, but we did
increase amounts slightly along the swath of greatest deep layer
forcing affecting MT/WY/ND/SD.

Burke