Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Wed May 02 2018


Prelim Day 1 QPF Discussion
Valid May 02/1200 UTC thru May 03/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1


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

Two discrete clusters of convection were prevalent early this
morning per the mosaic radar loops. One was draped across the
upper Midwest within an area of favorable dynamics south of the
entrance region of an upper level jet streak, ahead of an old MCV,
and along/se of the quasi-stationary surface front. Elevated
instability across this region is modest -- with mucapes around
1000 j/kg supporting pockets of 1+ inch/hour rainfall rates. The
other area is farther south across portions of the lower MO Valley
from northeast KS into northern MO -- where the convection has
effectively grown upscale into an MCS overnight. Over this region
short-term rainfall rates were more robust (pockets of
1.5-2.0"/hr), owing to the stronger deep-layer instability
(mucapes closer to 2000 j/kg), strong low-level moist inflow (50+
kts), and favorable alignment of the low-level jet (llj) to the
mean wind in favoring upwind propagation and cell training. The
high-res guidance (including the more recent HRRR and experimental
HRRR runs) continue to show a diminishing trend with the MCS later
this am with the dissipation of the llj. However, with a new quasi
w-e outflow boundary laid down s-se of the main frontal boundary,
the concern is that the new convection firing later in the day
with the peak heating -- and overnight as the activity once again
grows upscale into an MCS -- will again be displaced farther south
of the main frontal boundary. Some model differences on exactly
where this convection initiates during the afternoon hours. The
00z ECMWF and UKMET were in good agreement focusing on central KS.
Given some consistency with this location, WPC QPF is weighted
towards these solutions Wednesday afternoon. The high res models
can sometimes struggle with this more synoptically driven
initiation at longer lead times. Thus, leaning towards the global
UKMET/ECMWF is sometimes better in these situations, with the high
res becoming more useful as we get closer to the event and/or
convection is being driven by more mesoscale processes.


...Western U.S....

The amplifying upper trough will generate a broad area of
generally light-moderate QPF -- which given the degree of forcing
(broad area of favorable upper level difluence) -- will not be
linked exclusively to the elevated terrain. Some locally heavy
rain is possible across the Southwest, although limited
instability should generally cap the rates. WPC generally followed
a multi model blend weighted towards the 12z HREF mean across this
area.

Hurley/Chenard