Quantitative Precipitation Forecast
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FXUS04 KWBC 162120

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
519 PM EDT Fri Mar 16 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 17/0000 UTC thru Mar 20/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...21Z Update...

There were a few tweaks to the placement of the precipitation over
the central Plains to better fit short-term radar imagery and
satellite imagery.  Also made a couple of changes in the 06Z to
12Z time frame across parts of the southeast to better fit the
high-resolution guidance.  Overall, the changes were minor and did
not reflect a major shift in the forecast reasoning.

...West of the Continental Divide...

Deep low pressure will slowly nudge into the central Great Basin
through the day 1 period, with spokes of vorticity pinwheeling
cyclonically on all sides. One particular wave -- currently off
the British Columbia coast -- will pivot south then southeast
toward southern CA on Sat. This vort lobe and attendant compact
upper jet streak will allow for modest (and short-lived)
amplification on the southern periphery of the trough -- with more
pronounced height falls toward late Sat and Sat night. As the
trough buckles, while still largely suppressed, some subtropical
moisture will be drawn northward as the warm conveyor belt (WCB)
too becomes more meridional with time. PW values peaking around
0.50-0.60 inches is only marginally anomalous for the time of year
(0.5-1.0 standard deviation above normal), however with the
strengthening (albeit short-lived) 850-700 mb moist flow directed
largely orthogonal to the southern Sierra and San Gabriel-San
Bernardino Mtns into the Peninsular Ranges, some
orographically-driven higher QPF totals above 1 inch are expected.
 Elsewhere throughout the West, cold air aloft and the broad
scale, deep-layer cyclonic flow will support a showery regime over
much of Oregon, and some semblance of a renewed frontal zone /
secondary Pacific front behind the one which entered the Great
Plains - will help to focus precipitation in Nevada and Idaho. WPC
QPF here was derived from a blend of the 12Z HREF Mean, WRF-ARW2,
our in-house ensemble summarizing the larger scale guidance, while
also maintaining a degree of continuity from the previous forecast.

...Central Plains - mid-Mississippi Valley into the Ohio and
Tennessee Valleys and Deep South...

A compact cyclone over the central Plains will encounter a highly
amplified shortwave ridge downstream, and in-turn open with time
as it approaches the mid MS Valley and Ohio Valley. Thus the area
of mod-heavy precipitation will shrink with time as per the model
progs; however, as the wave opens, the uptick in forcing via the
northern stream jet axis (right-entrance region) will favor an
increase in low-level frontogenesis north of the surface boundary.
As a result, expect to see a narrower (banded) structure with the
higher totals -- averaging aoa 0.50 inch -- focused in a NW-SE
fashion from eastern IA through central IL-IN. Farther east, warm
advection and upglide on the 295-300 K surfaces will support rain
arcing out through the lower Ohio Valley / Tennessee Valley and
lower Mississippi Valley. A consistent signal has been for a more
concentrated rainfall over parts of LA/MS, supported by a southern
stream jet streak and proximity to deeper moisture seen on the
evening soundings over south Texas. The SREF 6-hourly rainfall
probabilities were useful, as was the previous WPC forecast which
seemed to have a good handle.

Days 2/3...

Day 2 Northern Rockies/Northern Plains/Pacific Northwest...
A closed mid level low near the ID/MT border early on day 2
focuses low-mid level convergence and lift, producing on the order
of 0.5-0.75 inches liquid equivalent over the Bitterroot range
according to the multi-model consensus. Given there was generally
good model agreement with the evolution of the mid level system,
the WPC QPF was based on a blend of continuity and the 12z

A second closed mid level low drifts south just off the Pacific
Northwest coast on day 2.  Upslope flow ahead of the mid level
system, is expected to combine with the moisture advection and
lift to produce areas of 0.25 inches of qpf over the OR Cascades.

Day 3 Northern Rockies/Northern Plains/Pacific Northwest...
As a long wave trough crosses the Northern High Plains on day 3,
moisture is drawn northward ahead of the mid level system on an
increasing low level southeast flow along an inverted surface
trough. The resultant QPF could be influenced by reductions in
moisture fluxes by the heavier QPF to the south in the central
There was generally good model agreement with the slow forward
movement of the mid level trough, so the WPC QPF was based on a
blend of continuity and the 12z ECMWF/GFS/NAM.

Day 2 Great Basin/Southern Rockies/Southwest...

Short wave energy exits southern CA and NV on day 2 and moves east
in tandem with the low level front across the ranges of UT and AZ
into the ranges of western CO and northwest NM.
Combined mid level convergence/upper divergence maxima in advance
of the frontal bndry produce a period of lift and higher elevation
snow and lower elevation rains in UT/AZ/CO/NM with higher amounts
where ascent is aided by terrain such as the Wasatch in UT and
Mogollon Rim and AZ, extending into he ranges of western
CO/northwest NM.
The combination of moisture, lift and marginal instability is
expected to produce axes of 0.25/0.50 inches of qpf across the
aforementioned locations.

Day 3 CO Rockies...

The forming lee low towards the end of day 2 into early day 3 in
eastern CO drifts east on day 3.  Precipitation is focused on the
CO front range early day 3 but tapers as the low moves east of the
region and ascent declines.

Day 3 Central Plains to mid MS Valley...

As a 700 mb low moves east out of CO across the central Plains,
moisture is drawn northward ahead of the system on an increasing
low level southeast flow. Strong low level convergence developing
in the theta-e advection zone across the central Plains results
in widespread rainfall. The resultant QPF could be locally heavy
over the Central Plains.

The 12z GFS and 18z NAM showed high amounts northwest of the
circulation center in either southern NE or northern KS, and these
solutions were given less weight in favor of either the slightly
faster or slightly further south UKMET/Canadian, and/or ECMWF
Southern Plains. The combination of moisture and instability feeds
low topped convection that produces an axis of 0.75 to 1.50 inches
of qpf stretching from north central KS into west central NE.

The 00z GFS had qpf amounts in excess of 2.00 inches over portions
of western NE, but most other 00z solutions has values closer to
the WPC QPF amounts. The highest qpf amounts appear as though they
will occur over the higher flash flood values of the Sandhills,
and some of the qpf will fall in the form of snow. For now, no
excessive area was placed over west central NE, but if later runs
consistently place qpf over lower flash flood guidance values, a
Marginal Risk area could be needed in later forecasts.

...Southern Plains/Gulf Coast states/Southeast

The models show a couple of different clusters of showers and
storms forming in the eastern portions of the southern Plains and
moving east into the lower MS Valley and southeast. A wide spread
of model/ensemble solutions leads to low confidence in these

The fist break out of convection occurs in the vicinity of the low
level warm front in northeast TX/northern LA on day 2 that moves
east in tandem with the warm front on the nose of the low level
jet and moisture/warm advection.  The models have typical timing
and latitude differences with more weighting given to the heavier
GFS/Canadian global/UKMET solutions. The 12z NAM likely continue
to be far north with its QPF placement in association with the
front.  The WPC QPF blended the 06-12z GFS/12z ukmet/12z Canadian
global on both days 2-3.

The possible redevelopment of the showers/storms in the lower MS
Valley and southeast occurs Sun evening as the southern stream jet
emerges across the southern Plains, driving an upper divergence
maxima across northeast TX into LA.  Convection in this favored
region occurs into the expected deep moisture (with precipitable
water values of 1.50/1.75 inches, which is about two standard
deviations above the mean) and moderate instability.

The QPF continues south of the NAM and closer to the
GFS/UKMET/Canadian, which are  closer to the greatest instability
and available moisture emerging north from the Gulf of Mexico.
A  Slight Risk area of excessive rain continues across southeast
AR/northeast LA east through central MS, central AL, and adjacent
west central GA, where training could result in locally higher qpf
amounts. Confidence remains below normal on the timing of the
clusters of showers/storms.


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


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