Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
343 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.

Thus not surprisingly -- the 00Z model suite continues to show
quite a bit of discrepancy with the QPF details from late Wed into
early Thu morning. There is consensus among the guidance with the
axis of heavier rainfall farther north, i.e. from IA to northern
IL-IN and southern MI, associated along and south of the
quasi-stationary surface front and closer to the aforementioned
upper level jet streak (enhanced right entrance region forcing).
However, outside of the 00z ECMWF and UKMET across south-central
and southeast KS, the global guidance continues to struggle with
the likely MCS evolution Wed night-early Thu farther south along
the aforementioned outflow boundaries and toward the (weakening)
mid-level cap -- i.e. across southern MO and potentially into
northeast OK-far northern AR). WPC opted to some extent
incorporate the ECMWF and UKMET idea with the QPF across KS --
however lean a bit toward the WRF-ARW and NMMB blend from 00-12Z
Thu across southern MO. As is often the case with more
convectively-aided dynamical forcing in the warm season, well
south of the main surface front with deep-layer ridging in place,
the question becomes "how far south" will the convection align. In
such cases it seems that the ARW seems to do a good job leading
the way with the southern trend.


...Central TX...

The models, particularly the high-res CAMs, again show a cluster
of heavier QPF east of the Big Bend initiating along the dry line
late in the afternoon and growing upscale (somewhat) overnight Tue
with the uptick in low-level inflow. The global models in fact all
show a small MCV developing as a result. WPC noted a small pocket
with areal-average totals between 0.50-1.0"; however, most of the
high-res CAMs show isolated max amounts closer to 1.5-2.0". The
strong low-mid layer shear (0-6 km bulk shear over 50 kts) would
favor more forward or downwind propagation than otherwise, which
would limit the threat for training convection. However,
well-organized multi-cellular clusters (supercells especially)
would favor isolated excessive short term rainfall rates. As a
result, WPC will continue with the MARGINAL risk in the new day 1
ERO.

Hurley/Chenard