Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
301 AM EDT Sat Apr 14 2018


Prelim Day 1 QPF Discussion
Valid Apr 14/1200 UTC thru Apr 15/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Central to Northern Plains---Upper MS Valley---Great Lakes into
northern NY state and northern New England...

A surface low near the NE/MO/IA border will track east across IL
tonight. To the north and northwest of this low will see continued
wintry precipitation across SD/NE, pushing east into MN/IA/WI
through the day. Strong 850 mb moisture transport ahead of the low
will transport convection up and over the warm front into MI
tonight, although rainfall intensity north of the warm front
should be tempered by the lack of instability. Meanwhile, a lead
wave will push precipitation across the Great Lakes this morning
and into portions of upstate NY and northern New England this
afternoon into tonight.

In general, models are in pretty good agreement. Have noted some
south/north fluctuations with the deformation axis of wintry
precipitation. The 0z model guidance appears to be focusing in
with this axis...forecasting it to stretch across southern MN into
central and southern WI. This axis keep very good continuity from
our previous forecast...although the new WPC is higher with the
magnitude of amounts across this axis...and is more in line with
the 0z HREF mean. Across upstate NY and northern New England have
noted a northward shift in most of the new guidance...and thus WPC
trended in this direction as well.


...Lower MS Valley into TN and OH Valley and the Southeast...

As of 06z this morning a convective line stretches from IL south
into the Lower MS Valley and into east TX...with the heaviest
amounts focusing across southeast AR into northwest MS where some
training will continue into the early morning hours. Indications
are that this convective line will become more progressive by 12z
as it continues to push eastward. In general the line is forecast
to remain progressive through the day 1 period...likely limiting
the potential for any extreme rainfall amounts. With that said,
still think some slowing of the line is likely by this afternoon,
resulting in a corridor of enhanced flash flood potential. As the
shortwave energy over the Plains digs into the MS Valley this
afternoon the entire trough will take on more of a negative tilt.
This should act to slow the forward progression of the line across
portions of southeast LA into southern MS and much of AL. The
expectation is that the digging wave and negatively tilted trough
will aid in upstream convective development that will expand along
the already present eastward moving convective line. Thus, even
though the line will maintain some movement east, the slowing of
it and upstream development will allow for some southwest to
northeast training of the cells embedded within the line. Moisture
parameters will remain impressive...with strong 850 mb moisture
transport and PWATs approaching climatological max values. Thus
any training would present a flash flood threat.

WPC QPF for the warm sector of this system generally blended the
HRW ARW, ARW2 and the HREF Mean. The HRRR was also heavily used
early in the period, as it was the best match of the high res
guidance to the current radar. The ARW was also a good match,
although feel like it may become a bit too progressive by later
today. Thus incorporated some of the slower ARW2 and HREF mean to
account for this. Given that organized convection is ongoing and
will maintain through the day...generally stuck with the high res
guidance for this warm sector QPF...as it is situations such as
this when they typically are much more useful than the global
guidance.

Overall the new WPC QPF is a bit more progressive than our
previous forecast...which fits in line with current observational
trends and the high res guidance. However, overall the axis of
highest amounts remains very similar to our previous forecast
stretching from southeast LA into southern MS and into AL. Did
shrink the areal extent of the higher amounts to better fit the
model consensus. Amounts were decreased some from northern AL into
the OH Valley. Instability will be less here and the orientation
of convection further south may help cut back on totals over this
area as well. Again, think this is a case where the high res
guidance is much more useful than the global models. Did however
keep amounts slightly higher than some of the CAMs show...as
sometimes they do under do QPF some on the northern edge of
convective lines.


...Pacific Northwest into the Northern Rockies...

An additional 1-2" of QPF expected across the coastal ranges of
WA/OR and the WA Cascades. WPC QPF follows close to the HREF mean.

Chenard