Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
312 AM EDT Fri Apr 13 2018


Prelim Day 1 QPF Discussion
Valid Apr 13/1200 UTC thru Apr 14/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Northern and Central Plains east into the Mid/Upper MS Valley
and Great Lakes...

A large and energetic system will bring a swath of moderate to
locally heavy QPF amounts from portions of the Central and
Northern Plains east across the Mid/Upper MS into the Great Lakes.
While models are in fairly good agreement...some important
differences remain. Fist off, it would appear that precipitation
early Friday will focus a bit further west than previously
forecast...with the trough ejecting east a bit stronger and more
neutral to negatively tilted as it moves into the Plains. This is
supported by the 0z HREF components and recent HRRR runs. How the
system evolves through the day is a bit less clear...and thus a
general multi model blend seemed fine. A concern will be the
northern gradient...with a strong high to north supplying low
level dry air. Thus likely to be a pretty tight gradient. In
general have noted a southward shift over the past several cycles
of the global models...with the 0z HREF components also a bit
south. Overall using a blend of the global and high res models
seemed to produce a good compromise for this northern
gradient...with the new WPC forecast pretty similar to our
previous forecast with this aspect. The 0z HREF mean and UKMET are
the closest individual solutions to our QPF preference over this
region.

Another area of interest will be near and just north of the strong
warm front forecast to stretch from IA east into IL and southern
MI. The combination of favorable mid/upper level dynamics, strong
850 mb moisture transport into and over the boundary, and decent
elevated instability....should all support a favorable environment
for convective development by Friday afternoon into the overnight.
Initiation should be over IA with activity moving northeastward
into portions of southern MN/WI/MI. The threat for the heaviest
rainfall rates will be closest to the front...with the activity
becoming increasingly elevated as you head north of the front. Did
nudge our QPF a bit closer to the ECMWF/UKMET for this
activity...which have been more consistent showing some locally
heavier totals. The 0z HREF mean seemed a bit light across
portions of IA into northern IL and southern WI.

...Eastern portions of the Southern Plains toward the Lower MS
Valley and TN Valley...

Convection will initiate Friday afternoon in a line from northeast
TX into northwest LA into western AR. Moisture parameters will be
high enough to support heavy rainfall...with PWATs approaching the
climatological 95th percentile to go along with strong 850 mb
moisture transport. Thus heavy rates are likely...and so the
determining factor for flash flooding concerns will be the
duration. Convection will become more organized as we head into
Friday evening and the better forcing moves into the area. Overall
the system as a whole is progressive in nature...although there
may be a period where training is possible. Would appear that
storms will initially form out ahead of the surface cold front in
a region of confluent 850mb flow and enhanced moisture transport.
As the wave to the west digs southeast into the area, storms may
tend to form further southwest closer to the surface front...with
this activity then merging into the prefrontal storms. If this
were to occur then we could end up with a corridor of heavier
rainfall totals and a higher flash flood risk. Thus in general
looks like a broad region where some flash flood concerns could
arise given the favorable environment for high rates...and a
smaller region where potential training could cause a more
significant threat.

Some model differences remain with the axis and magnitude of
rainfall totals. While good to start, thought the GFS probably
becomes a bit too progressive by the end of the period. Thus
preferred a blend of the 0z HREF, UKMET and 12z ECWMF...which
seemed to produce a good consensus solution. This results in a
broad 1-3" area from northeast TX into western TN. The consensus
region for the best training threat is from northern LA into
southern AR and western MS...where a swath of 3-5" (locally
higher) is expected. This is slightly north of our previous
forecast...with a cap likely defining the southern edge of the
threat through day 1. The new WPC forecast is also slightly more
progressive than our previous forecast to take into account the
ARW and ARW2.


...Pacific Northwest...

Onshore west southwest low level flow will be re-strengthening
into the Pacific Northwest tonight into Friday to the southeast of
a developing closed low just to the south of the Gulf of AK. This
will support moderate to heavy precip values from the
Washington-Oregon Cascades---west to the northern Oregon coast
range into the Olympic Range.

Chenard