Quantitative Precipitation Forecast
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FXUS04 KWBC 202042

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
441 PM EDT Tue Mar 20 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 21/0000 UTC thru Mar 24/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...eastern Ohio Valley/Central Appalachians/Mid

All the ingredients were coming into place for a heavy snowfall
from Washington, DC to Philadelphia and New York City.  Refer to
the WPC Heavy Snowfall graphics and discussion for winter weather
amounts and probabilities. The storm will also produce heavy
rainfall over the coastal mid Atlantic, with rain changing to snow
in coastal areas of the northern mid Atlantic.

The models have come in better agreement with cyclogenesis in the
southern mid Atlantic with the low intensifying over the coastal
waters early Wed. Snow redevelops along the existing low level
frontal zone in southern PA and expands across NJ into the metro
New York City area.  The strong lift, followed by enhanced
moisture fluxes on easterly low level flow off the ocean, leads to
1.5 to 2 inches liquid equivalent precipitation in NJ, with over
an inch to Long Island and into southeast PA and northern MD and

The WPC QPF followed the model consensus, and blended the high
resolutions windows with the 12z NAM Conus Nest, and 12z Canadian
regional model.  The 12z ECMWF trended south with its QPF axis in
the mid Atlantic in response to its axis of low pressure shifting
south, so no the northern edge amounts remain uncertain in
northern PA/southwest NY/New England.

The high res windows and ensembles have concurred with the 12z nam
to produce one secondary maxima in the Ohio Valley centered on IN
and western Ohio as 850-700 mb convergence maxima develop west of
the circulation in the Oh Valley.  There is potential for banded
snow in this area.  Another area of heavier precip is where the
circulation produces 700 mb convergence in southern WV to adjacent
VA with lift aided by upslope flow. Given good multi-model
agreement, a blend of the models/ensembles was used.


The models continue to advertise widespread rain in California and
 heavy rain in portions of southern CA, particularly the coastal
ranges.  Secondary maxima develop in the interior mountains.
Please see the risks of excessive rainfall /
flash flooding graphics and discussion. The models show an
atmospheric river with moisture transport tonight increasing into
California, with the 12z nam showing precipitable water values
increasing to 1.25-1.5 inches in portions of coastal southern CA.
A well defined 870-700 mb convergence maxima moving onshore
between Santa Maria and Los Angeles,
This results in heavy rain developing Wed near Santa Barbara and
nearby ranges.
As the upper jet across north into northern CA, widespread rain
develops in northern CA. Lift from well defined upper divergence
maxima in central CA continue overnight and drift north into
northern CA in tandem with the upper jet maxima on Wed.

Due to good large scale agreement, a consensus approach to the QPF
was used, including the high resolution ensemble forecast mean for
the day 1 forecast. This approach usually does well along the West

Rainfall rates tonight gradually ramp up to half to three fourths
of an inch in six hours, increasing to 1-1.25 inches in 6 hours
during the day on Wed.

Day 2


There continues to be a strong model signal on day 2...Wednesday
evening through Thursday evening...for widespread heavy
precipitation across large portions of California from the
Transverse range/southern California coastal range--through the
Sierra and into the northwest California coast range.

The atmospheric river event which commences during the day 1
period will continue into the day 2 period as a mid to upper level
trof off the California coast continues to press eastward day 2.
This will sustain anomalously strong and moist onshore flow into
much of California with the emphasis for the strongest moisture
flux focused into the southern California coast range/Transverse
range and into large portions of the Sierra.

Precipitable water anomalies of 4 to 6+ standard deviations above
the climatological mean...and moisture flux anomalies of 5+
standard deviations above the climatological mean are expected
during day 2.   Model consensus is for very heavy precipitation
potential across these areas with a high likelihood of runoff
issues over the southern California coast range/Transverse range
and heavy snows into the central to southern Sierra.

With respect to the runoff issues over the southern California
coast range/Transverse range---a high risk area was maintained
with few changes from the overnight issuance...namely from near
Morro Pt---southeastward through the Transverse range to cover the
Canyon--Sherpa---Whittier---Rey and Thomas burn areas.  The
expected magnitude of the rainfall across these burn
areas---upwards to .25-50" per hour will likely produce
life-threatening debris flow/mudslide potential.

Pacific Northwest

Anomalously high precipitable water values will also be drawn
northeastward into the Pacific Northwest ahead of the strong
closed low amplifying and dropping southward to a position off the
B.C. coast.  While pw values will rise to 1 to 1.5 standard
deviations above the mean into the Pacific Northwest---the low
level flow will have a more terrain parallel
trajectory---supporting much lower precip totals than areas
farther to the south.

Areal average precipitation amounts in the range of .25-.50+
inches were depicted from the Washington-Oregon Cascades to the
Washington-Oregon coast ranges.   Heavier totals in the 1-2"+
range likely from far southwest Oregon into coastal northwest
California where the low level south southwesterly flow will have
a greater upslope component.

Inland into the Great Basin---widespread moderate precip totals
likely day 2 as the upstream anomalous pw axis pushes inland.
There remains a lot of detail differences among the latest
guidance leading to lower confidence than other areas in the
anomalous pw plume.

Mid Atlantic into New England

There should still be low pressure lingering off the coast of
southern New England by the time the day 2 forecast period begins
at 22/00Z.  Between the flow around the surface feature and the
cyclonic circulation aloft, there will still be a band of
precipitation along the coastline extending from southern New
England back towards the Mid Atlantic region in the first 6 to 12
hours of the forecast period.  Gradually, the system will ease far
enough out to sea where any of the dynamically forced
precipitation will come to an end.  Northwest flow behind the
system will draw colder air across the eastern Great Lakes
resulting in some lake enhanced precipitation before the wind
direction changes.

Mesoscale snow bands likely which will support potential for
enhanced snow totals depending upon where the bands set up.  See
the latest QPFHSD for additional winter weather information.

Southern to central Appalachians

Accumulating snows also likely from the central to southern
Appalachians early on day 2 where persistent northwesterly to
north northwesterly upslope flow is expected in the wake of the
deepening coastal low.

Day 3


The axis of much above average pw and moisture flux values
impacting southern California during day 2 will begin to shift
south of the area during the latter part of day 3 as mid level
heights associated with the main troughs pushes inland.  The
moderate to heavy rainfall occurring in and near the Tranverse
range and coastal ranges to the south will gradually push south
and eventually taper off.

Additional heavy precipitation and heavy snows likely through the
central to southern Sierra on day 3 as persistent west
southwesterly upslope flow continues, although this should also
gradually taper off during the day on Friday.

Pacific Northwest

The low level flow that had a significant terrain parallel
component day 2 over the Pacific Northwest will veer to a more
upslope direction day 3 as height falls from the strong closed low
off the Pacific Northwest/southwest Canada coast swing inland.
This will increase precipitation potential across the Pacific
Northwest from the Cascades to the coast ranges. As the core of
the upper trough makes its way eastward late on day 3, there
should be a general decrease in areal coverage and precipitation
rates late in the day 3 period.

Northern Rockies into the Great Basin

The inland push of height falls through the northern Rockies and
Great Basin will raise pw values to much above average levels day
3.  This will support additional widespread moderate to heavy
precip totals across the Great Basin into the northern Rockies.
Model consensus is for best 24-hour total precipitation amounts to
straddle the day 2/day 3 timeframes.  There should be a general
decrease in areal coverage and amounts by the end of day 3.

Northern plains

The inland moving height falls across the northern Rockies will
spread isentropic lift into the northern Plains early Friday
morning.  Strengthening southeasterly low level flow will support
increasing precipitation potential through broad isentropic
lift/warm advection pattern from far northeast MT into western ND
and western SD.  Precipitation amounts should generally be in the
0.10 inch to 0.25 inch range, although a few areas in the 0.25
inches to 0.50 inch range should be embedded within that area.


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


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