Quantitative Precipitation Forecast
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FXUS04 KWBC 190705

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
304 AM EDT Mon Mar 19 2018

Prelim Day 1 QPF Discussion
Valid Mar 19/1200 UTC thru Mar 20/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern U.S...

A very dynamic southern stream trough will move through the
Tennessee Valley today, inducing a widespread precipitation event
from the Plains to the east coast - and some concentrated severe
weather per the Storm Prediction Center Day 1 Convective Outlook -
associated with a well defined frontal wave and the extensive
back-bent occluded front. The inherited WPC forecast served as a
good starting point. Areal average precipitation amounts will be
greater north of the warm front and along the occlusion, with the
aide of upglide over the frontal surface and deformation. Down
along the front and into the warm sector storms will be moving
quickly, with 70-80 knot westerly 500 mb flow. QPF will be less in
the warm sector, but we did attempt to be generous with coverage,
and carry at least tenth of an inch amounts down farther south
than some of the hi-res models owing to sufficient deep layer
height falls in a favorably unstable and uncapped environment.

WPC QPF was really a laborious manual effort given the level of
mesoscale influence expected today, but several pieces of guidance
were close to our expectations, especially the 00z NAM CONUS Nest
and GEM Regional. With the heavier local rain rates moving quickly
today, and less convection within the longer duration rainfall in
KS/MO/IL, there does not appear to be cause for more than a
Marginal Risk of excessive rainfall at any given location.

...Central and Northern Plains...

While low levels will feature dry ridging over the High Plains,
and northerly winds influenced in part by the mid Mississippi
Valley cyclone, there will be plenty of residual moisture in the
mid levels and cyclonic flow / digging northern stream features to
support light rain/snow from Kansas to North Dakota. With
precipitation forced primarily in the 700-500 mb layer, the SREF
6-hourly precip probabilities seemed to match well with the GFS
700-500 mb lift and saturation fields. We used these as a guide,
and came up with QPF resembling the 00z HREF mean.



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