Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
142 PM EDT Thu Mar 22 2018


Prelim Day 1 QPF Discussion
Valid Mar 23/0000 UTC thru Mar 24/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Western U.S...

A deep trough/closed low digging south offshore the Pacific
Northwest will begin to gradually shift inland tonight through
Friday across the West Coast which will maintain deep and moist
cyclonic flow across the coastal ranges of WA, OR and northern CA
and into the Cascades. This will maintain numerous shower activity
which will be aided by slowly steepening mid level lapse rates and
favorable onshore/upslope flow over the terrain. Expecting locally
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.

Meanwhile, the lead shortwave currently coming into CA out ahead
of the aforementioned trough/closed low offshore the Pacific
Northwest will eject rapidly inland across the Intermountain
region tonight and then across the Rockies on Friday. This will
take the remnants of a well-defined atmospheric river in across
the region which is characterized by 850/700 mb moisture flux
anomalies of over 5 standard deviations above normal, and
precipitable water values of locally over 4 standard deviations
above normal. This will result in a highly efficient precipitation
processes over the higher terrain with west and southwest facing
slope of the Wasatch, Tetons and high country of western CO seeing
moderate to heavy precipitation. Water equivalents will locally be
over 1 inch through late Friday as the energy traverses the
region. WPC leaned toward a multi-model consensus for QPF with
generally a blend of the GFS, ECMWF and hires CAM guidance used.


...Northern/Central Plains southeast to the Middle Mississippi
Valley...

The aforementioned shortwave crossing through the Intermountain
region through tonight will ultimate encroach on the High Plains
by late Friday. The strong influx of Pacific moisture within the
deep layer warm conveyor belt will be encountering a much colder,
drier air mass out across the northern Plains in particular. As
low pressure develops in the lee of the Rockies, expect an axis of
frontogenetically enhanced precipitation to break out over
northeast MT and the Dakotas late tonight and early Friday which
will ultimate begin to streak southeastward toward the middle MS
Valley by Friday evening. The enhanced Pacific moisture transport
will likely result in moderate to heavy precipitation, including
heavy snowfall across portions of North and South Dakota. Some
relatively strong frontogenetical banding of snowfall will be
likely which will enhanced the snowfall rates. Please consult the
latest QPFHSD for more details on the expected winter weather
impacts. In general, WPC leaned toward a consensus of the GFS,
ECMWF and the hires models including the ARW, ARW2 and NMMB.

Orrison