Quantitative Precipitation Forecast
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FXUS04 KWBC 230700

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
259 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 (i.e. into
eastern NE, northern MO).

...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.



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