Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Sat May 12 2018


Prelim Day 1 QPF Discussion
Valid May 12/1200 UTC thru May 13/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...Mid Mississippi Valley to the Northern Mid-Atlantic and
Southern New England...

As a deep mid-upper level low remains centered over the Great
Basin, broad cyclonic flow is expected to persist from the upper
Midwest to the Northeast this period.  At the surface, a wavy
slow-moving boundary will extend from the northern Mid-Atlantic
coast back into the central Plains.  Showers and storms will
continue to develop along and north of the boundary as a series of
mid-upper level shortwaves moving along the base of the trough
interact with a relatively deep moisture pool, fed by broad
southwesterly flow into the boundary.  A mid level shortwave
associated with ongoing convection across Iowa is expected to
reach the upper Ohio valley by the afternoon hours.  This energy
along with daytime heating and deepening moisture along the
boundary, is expected to support shower and storm development from
the upper Ohio valley to the northern Mid-Atlantic and southern
New England coasts Sat afternoon-evening.  While locally heavy
amounts are possible, the anticipated progressive nature of these
storms is expected to limit the threat for widespread heavy
accumulations. 

Further west, another round of showers and storms are expected to
develop and move east from the mid Mississippi valley into the
southern Great Lakes region during the evening into the overnight
hours Saturday into Sunday.  Upstream energy interacting with a
deepening moisture pool on the nose of a strengthening
southwesterly low level jet (PWs increasing to over 1.5 inches) is
expected to support moderate to heavy amounts from east-central
Iowa through northern Illinois and Indiana into northwest Ohio
late in the period.  Guidance suggests a period of training storms
may bolster the potential for heavy amounts across this region.

Overall, WPC QPF axis of heavier amounts remains similar to
previous forecasts, but with an increase in areal average amounts
across northern Illinois to northwest Ohio.  This was supported by
a majority of the hi-res guidance and reflected in the overnight
HREF mean.

...Great Basin/Northern Rockies and High Plains...

The previously noted upper low is expected to continue to support
widespread precipitation from the northern Great Basin into the
northern Rockies and High Plains.  Enhanced upper divergence along
the left-exit region of an upper jet streak is forecast to support
some moderate to locally heavy accumulations (including some
significant mountain snows) across portions of northern Utah and
Nevada, western Wyoming and southern Idaho.

Pereira