Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
314 AM EDT Fri Mar 23 2018


Prelim Day 1 QPF Discussion
Valid Mar 23/1200 UTC thru Mar 24/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1


...Northern Plains southeast to the middle Mississippi Valley and
western Ohio Valley...

A very dynamic shortwave over southern CA at 0600Z will undergo a
bit of shear over the next 24 or so hours as it encounters the
broad central U.S. ridge. At the same time however, the upper jet
streak maintaining strength as it progresses eastward into the
Central Plains by Friday night will continue to support a vigorous
(albeit more compact) vort max on the northern flank. Narrow
left-exit region forcing (upper difluence/DPVA and associated
ribbon of deep-layer WAA/isentropic lift) ahead of this shortwave
will push east across the northern Plains toward 00Z, while
pushing into the mid and (southern) portions of the Upper MS
Valley overnight Fri into early Sat. Meanwhile, as the NW-SE
oriented upper jet streak in the lee of the upper ridge pushes
east across the upper Midwest-Upper Great Lakes and Ohio Valley,
the ensuing right entrance region forcing will augment the upper
divergence/large-scale ascent (steepen the S-N isentropic lift)
while also enhancing the low-mid layer frontogenesis. The models
continue to depict this "coupled" dynamical setup in between the
approaching (southern) and receding (northern) upper jet streaks
-- though differ in terms of location of best deep-layer lift,
low-level frontogenesis, and thus the axis of heaviest QPF.

In terms of the 00Z model guidance, the NAM remains the farthest
northeast with the QPF axis, with the ECMWF on the SW flank of the
the guidance spread. The latest WPCQPF comprised a blend of the
GFS and UKMET -- both solutions serving as a middle ground between
the NAM and ECMWF, while getting support from both the 00Z HREF
mean and latest NBM (which were also utilized in the multi-model
blend). Am a bit concerned that we could see a s-sw trend with the
heaviest QPF given the weaker static stability profile farther
south (toward the northern periphery of the forecast MUCAPE axis).
Model progged 700-500 mb theta-e lapse rates in fact show zero to
slightly negative lapse rates lifting into southeast NE and
southern IA.


...Western U.S...

A deep trough/closed low digging south offshore the Pacific
Northwest will begin to gradually shift inland today across the
West Coast and maintain deep, moist cyclonic flow across the
coastal ranges of WA, OR and northern CA and into the Cascades.
This will sustain numerous shower activity, which will be aided by
slowly steepening mid level lapse rates and favorable
onshore/upslope flow over the terrain. Expecting an additional 1
to 3+ inches of rain for the coastal ranges and especially far
northwest CA and southwest OR. Lesser amounts are expected over
the Cascades. WPC leaned toward a multi-model consensus for QPF,
with generally a blend of the GFS, ECMWF and hires CAM guidance.

Hurley