Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Tue May 01 2018


Prelim Day 1 QPF Discussion
Valid May 01/1200 UTC thru May 02/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1


...Lower Missouri Valley into the Upper Midwest...

Shortwave energy over the eastern Pacific will pivot southeastward
toward the base of the longwave western U.S. trough, and in doing
will further amplify the trough as height falls push southward
across Baja and northwest Mexico while a mid-upper low closes off
across the desert SW. While there continues to be consensus among
the models with respect to the longwave pattern, spatial and
timing differences continue with respect to the convectively-aided
shortwaves ejecting northeast of the trough and along the northern
periphery of the downstream upper ridge. Meanwhile, as separate
northern stream trough across western Canada begins to impinge the
the aforementioned upper ridge late in the period (Tue night)
across the upper MS Valley, the increased confluence aloft will
generate a 90-100+ kt upper level jet streak across far northern
portions of the Upper Midwest into southern Canada. All of the
models are picking up on these favorable dynamical signals for
heavy rainfall ahead of the MCVs and into the favorable right
entrance region of the aforementioned jet streak; however, the
bi-modal distribution of QPF maxima from some of the models, along
with the typical warm-season latitudinal differences among the
operational guidance (especially global vs. high res) is once
again making for a low-confidence QPF in terms of the
timing/location details.

Believe the bi-modal distribution per much of the guidance is
indicative of the MCVs not entirely coupled within the most
favorable right entrance region upper jet forcing. Thus a rather
broad upper divergence signature depicted, with two distinct areas
of stronger deep-layer ascent noted -- one farther south across
the lower MO Valley associated on the southern periphery of the
MCV and resultant outflow boundaries displaced farther s-se of the
main surface front, with the other one farther north over portions
of the upper Midwest/MS Valley where the favorable upper jet
forcing will more of a factor. WPC noted the heaviest QPF along
the former (southern) axis -- at the apex of a 50+ kt low-level
jet where greater (untapped) deep-layer instability will be
present.

...Western U.S....

The amplifying upper trough will generate a broad area of
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. Highest areal-average totals
(0.50+ inch) were noted across the southern NV mountains into the
Wasatch and Uintas, as well as along the San Juan and Laramie
Mtns. 

Hurley