Quantitative Precipitation Forecast
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FXUS04 KWBC 222009

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

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 23/0000 UTC thru Mar 26/0000 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

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.

Days 2/3

WPC QPF was based on a blend of the 12Z ECMWF/GFS (which had good
agreement) with a little 12Z NAM included.

No excessive rainfall is expected in this period.

...Pacific Northwest to the Northern Rockies...

Occluded and weakening low pressure comes ashore near the OR/CA
border late Saturday reaching the Great Basin as a weakened open
wave Sunday. Short wave energy ahead of the low taps Pacific
moisture (though only half inch PW) to produce locally heavy QPF
over higher terrain of coastal OR/CA Friday night into Saturday.
Local areal averages around one inch can be expected in the CA
coast ranges and Cascades for Day 2 (00Z Sat-00Z Sun) and the
Sierra Nevada for Day 3 (00Z Sun-00Z Mon). Associated height falls
allow mainly higher terrain QPF of half inch or less across the
northwestern CONUS.

...Northern Plains/Upper MS Valley/OH Valley/Carolinas...

Short wave energy tracking over the CO Rockies Friday evening with
a surface low tracking east across the central Great Plains Friday
night. In doing so the wave runs into the central CONUS upper
level ridge and weakens. Before the low can weaken too much Friday
night a baroclinic zone develops north/east of the low with
increasing southeasterly low level flow. One inch PW from the
western Gulf spreads north ahead of the low and into the Great
Plains where a swath of rain to the southwest and snow to the
northeast sets up. Moderate intensity can be expected in this
frontal zone over MN/IA/IL/MO and into southern IN/KY.

The low weakens to a remnant trough which drifts across the
Carolinas Saturday night. Rates will decrease as it moves on,
though locally enhanced precip can be expected over the southern
Appalachians/Smokies late Saturday. 12Z guidance took a southward
jog with precip over southern VA and over NC Saturday night.
Considered continuity a little bit with this southward shift to
slightly limit the southward movement of the QPF.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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