Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
313 PM EDT Thu May 10 2018


Prelim Day 1 QPF Discussion
Valid May 11/0000 UTC thru May 12/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...Eastern U.S...

A trough encountering seasonably steep lapse rates and sufficient
moisture will yield scattered strong but quick moving
thunderstorms this afternoon across the Carolinas / Mid-Atlantic
States / interior New England. Predictability of the individual
thunderstorm swaths is not high given this particular convective
mode, so we generally followed observational trends and hi-res
ensemble means, yielding no more than a half to one inch areal
average precipitation (heaviest swaths in the mid Atlantic).


...Central Plains to the Great Lakes...

A Great Plains style precipitation event is expected to evolve
tonight, with quasi-surface-based convection growing upscale and
heading east across Nebraska and western Iowa. Models are strongly
suggestive of a second precip maximum to occur overnight near the
IA/MN border in a zone of upglide focused to the north of a
frontal zone that had dipped into the area. There is some
potential for these events to overlap somewhat, and to boost the
event totals over parts of IA/MN, if the convection coming out of
Nebraska can be maintained to some extent by the low level jet.
Moisture availability is not optimum, with PW values climbing just
above 1.00 inch in the pre convective environment. The
thermodynamic environment may hamper rainfall efficiency and
promote forward motion through strong cold pools. Deep layer
forcing with the low amplitude upper wave is also expected to
remain farther north, just off the instability axis. So there are
some negative factors regarding any truly focused potential for
excessive rainfall.

Still, the model QPFs actually converged a bit toward a better
consensus, and they do favor areal average 1 to 2 inch amounts
from near Sioux Falls eastward overnight, with locally higher
amounts possible, although extreme amounts are unlikely given the
transient nature of the forcing. The previous WPC forecast had
seemingly done a good job handling wide model spread. The 12Z
cycle saw models trend toward the WPC QPF placement, but with
somewhat lesser amounts. Using the HREF Mean and WRF-NMM (which
has been confusingly good at QPF this spring) we trended toward
more confined maxima and just a slight reduction to QPF magnitude,
while keeping placement nearly the same as before. These thoughts
were coordinated with local offices and NCRFC.


...Rockies / northern Intermountain West...

A well-defined shortwave will continue to dig toward the Great
Basin, with a closed mid level center forming near Idaho during
Day 1. A surface high settling toward the international border out
on the plains will help to focus easterly low level flow into the
foothills and mountains. As a difluent upper flow pattern develops
east / northeast of the low center, there should be some pockets
of enhanced vertical motion and precipitation. One such event is
expected tonight in south-central Montana, and another is expected
Friday in southeast Wyoming. This is evident on somewhat larger
scales, while there may be other local mountain enhancement
scattered around this system as well. WPC started with a blend
that included our in-house ensemble as influenced by the hi-res
models, along with the GFS and NAM CONUS Nest. Continuity was also
considered. Although not very anomalous in the height field, this
system should produce widespread quarter inch and greater
precipitation, with the more favored areas receiving 0.50 to 1.25
inches liquid through Friday afternoon. This may compound spring
snowmelt flooding in the long run, and wherever pockets of
instability develop there may be some convectively enhanced
precipitation leading to at least a low percentage chance of some
flash flooding.

Burke