Quantitative Precipitation Forecast
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FXUS04 KWBC 230755

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

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 23/1200 UTC thru Mar 26/1200 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.

Days 2/3

...Pacific Northwest/northern California into the Intermountain
West and northern Rockies...

A broad area of lower heights across the western U.S. will feature
multiple embedded shortwaves which will focus periods of organized
precipitation over the region. More specifically, one such
disturbance is forecast to lift out of far western Montana into
southern Alberta/Saskatchewan early Sunday. To the south, another
shortwave will dig through northern California/Nevada, albeit with
a rather limited reservoir of moisture. Strong mid-level lift
combined with upslope augmentation will bring locally heavy
precipitation to the Shasta, Siskiyou, and Sierra Nevada ranges
through early Sunday morning. Such amounts were quite comparable
among the 00Z NAM/GFS/ECMWF output which bolstered confidence in
the forecast. General lighter amounts were painted downstream as
the upper trough becomes more stretched out across the central
Great Basin. While much of the West Coast eastward into the
northern Rockies should be on the wet side, amounts are quite

...Central U.S./MS River Valley into the OH/TN Valleys and
southeastern states...

On Saturday morning, modest mid-level height falls ejecting from
the middle Mississippi Valley toward the lower Ohio Valley will be
in the process of becoming more stretched out in time. The
attendant surface cyclone tracking from Missouri toward the upper
Tennessee Valley should continue to weaken given the shearing
nature of the pattern aloft. In spite of this, expect a healthy
swath of precipitation spreading across southeastern
Kentucky/northeastern Tennessee/far western North Carolina and
Virginia. Models show sufficient 850-mb flow in excess of 40 to 45
knots advecting 1 to 1.25 inch precipitable water values toward
the southern Appalachians. This would allow for additional upslope
forcing to the QPF amounts with areal averages on the order of 1
to 1.50 inches, primarily through early Sunday. Much of the action
should shift toward the southeastern U.S. by the end of the
weekend although the activity may become more elevated in nature
as surface ridging extends down into the eastern Gulf of Mexico.
The overall model trend on Day 2 has been to stay the course or
shift southward compared to recent days. As such, followed a
consensus of the 00Z GFS and 12Z ECMWF/CMC/UKMET with a primary
focus on the higher resolution output of the GFS/ECMWF solutions.
By Day 3, reverted back to the 18Z GFS given the most recent run
showed spurious amounts over the southeastern U.S. Its output
looked unreasonable as mid-level heights gradually rise and the
better surface convergence sits well upstream.

Back over the center of the country, a few areas of focus are
noted in the guidance. Initially, a light swath of precipitation
across the northern-central U.S. will give way to a brief drying
period. With broad southwesterly flow stretching over much of the
western half of the country, weak to moderate perturbations will
help increase the coverage of precipitation over the
Northern/Central Plains. Models are quite abrupt in return flow
advecting moisture back up the Plains with 850-mb winds shifting
to at least 30 to 35 knots by Sunday morning. This will occur in
response to increasing warm advection with areas of lower pressure
shifting up the higher terrain of the Northern Rockies. With the
better forcing lurking well upstream, amounts remain modest at

Across the Southern/Central Plains into the Ozarks, convection
should spread over the region primarily on Day 3. The primary
drivers appear to be a multitude of weak impulses pushing out of
northern Mexico and the Four Corners region. The better focus
would take place along a poleward lifting warm front although an
additional area of interest would occur back along the dryline.
Among the latest guidance, the 00Z GFS is the wettest while the
00Z/12Z ECMWF are a bit more disorganized with the activity. Given
some potential capping issues, the latter solutions may be a more
accurate portrayal of the situation. The manual QPF did take a
blend of these two solutions but did downplay the amounts somewhat.

...New England...

Within a region of deep cyclonic flow, a pronounced shortwave will
dig through the northeastern U.S. early in the weekend. With the
best moisture well offshore, much of the activity will be driven
by instability forced showers underneath the cooling mid-level
temperatures. A consensus approach was utilized which kept general
amounts under 0.15 inches for the most part during the period.


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


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