Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Thu Apr 12 2018


Prelim Day 1 QPF Discussion
Valid Apr 12/1200 UTC thru Apr 13/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Northern Rockies into the Plains and Upper MS Valley...

The trough over the west coast will move across the Great Basin
and Rockies through the day Thursday...with a strong area of low
pressure ejecting into the Central Plains Thursday night. Likely
to see a large area of snow/rain with this trough. Good model
agreement is seen across most of the terrain where orographic
effects will dominate...and thus here WPC QPF follows a multi
model blend weighted towards the higher res guidance. Likely to
see significant QPF north of the low across the MT plains into the
western Dakotas through the day 1 period. A corridor of increased
700 mb convergence, an inverted surface trough, and easterly
upslope flow in between the low to the south and high to the north
will all provide a favorable setup for this precipitation. Likely
to be a pretty tight northern gradient given the strength of the
high to the north pushing low level dry air south. Thus the
biggest model differences here seem to be with the northern extent
of the QPF axis across MT. Have noted a slight northward shift in
most of the new 0z guidance...including the high res models. WPC
thus trended a bit in this direction as well...with our axis
generally in between the furthest north GFS and south ECMWF.

On Thursday night the better mid/upper level forcing ejects into
the plains...with pieces of shortwave energy and dual left exit
and right entrance region upper jet dynamics. Thus expect to see
precipitation increase in coverage across the Northern Plains into
the Upper MS Valley. In fact, looks like we could see some
elevated convection form north of the surface warm front Thursday
night where elevated instability is forecast to develop. This
convective activity should impact portions of western NE north
into portions of central and eastern SD. Some differences with
exactly where this activity focuses Thursday night...and for row
WPC favored a middle ground consensus solution incorporating both
the global and high res guidance. This focuses some higher amounts
from north central NE into southeast SD east towards the MN/IA
border.


...Great Lakes into New England...

Isentropic lift ahead of a quick moving wave will continue to
produce an area of rain/snow from the Great Lakes into New England
through Thursday night. Generally favored a multi model blend to
handle QPF here...although think the GFS is probably too high
along the NY/PA border...being the only piece of guidance
suggesting higher amounts there.


...Pacific Northwest...

Onshore flow will continue showery conditions into
Thursday...although mid level ridging building into the region
should result in decreasing coverage and intensity. This lull is
short lived however, as another front will move into the Pacific
Northwest Thursday night. Models were in close enough agreement
that staying close to a blend should suffice here.


Chenard