Quantitative Precipitation Forecast
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FXUS04 KWBC 201137

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

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 20/1200 UTC thru Mar 23/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...East Coast...

All the ingredients were coming into place for the latest in a
series of Nor`easters to produce a substantial swath of snowfall -
enough to disrupt life/commerce in the big cities.  Refer to the
WPC Heavy Snowfall Discussion for winter weather details. The
storm will also produce heavy rainfall over the coastal mid
Atlantic. The event will be stretched over two days, through
Wednesday. ng Long Island for QPF valid through 12z Wednesday.

The 00z upper air analysis depicted a very energetic and fairly
uncommon pattern over the southern states, with very short wave
spacing between a lead wave which drove a severe weather outbreak
over the South on Monday and an upstream wave that will amplify
quickly over the same region today. Rather than move out and
reinforce the offshore thermal gradients, the models now agree
that the movement of the lead short will be dictated by the
amplification of the trailing wave such that they are more in
phase with one another and become part of a relatively
consolidated upper air amplification over the Appalachians.  This
allows for cyclogenesis very near to the Mid-Atlantic coast,
peaking Tuesday night and Wednesday. Additionally, with the lead
wave now forecast to lift through the eastern Ohio Valley, it not
only induces a broad scale warm-advection / frontogenetically
forced precipitation event on Tuesday, but also acts to solidify
an elevated baroclinic zone over northern MD / southern PA and
across New Jersey. Strong lift during the initial setup phase,
followed by continual easterly low level flow off the ocean, into
the terrain, and then finally enhanced broad scale by the
developing cyclone and broad deep layer height falls going into
Wednesday, makes for a likely swath of hefty liquid equivalent
precipitation in this zone.

Other heavy amounts, although mainly in the form of rain, are
expected over the Virginia tidewater region and DELMARVA
peninsula, especially early Tuesday in association with the lead
wave. The system makes an attempt at forming a TROWAL back through
the Ohio Valley, and with nearly saturated air circulating around
the upper low, expect at least quarter to half inch liquid amounts
to occur back west.

The NAM sometimes does a good job on Day 1, especially along the
East Coast. Its QPF, at least through 12z Wed, was consistent with
global model mass fields / synoptic thinking. Given a decent model
consensus as well, we were able to use our in-house ensemble
products which had trended upwards with amounts. Generally the GFS
appears to have been too conservative leading into the event, and
the ECMWF has a known dry bias even in strongly forced events, but
its QPF did trend wetter. WPC used a 40/30/30 blend of our
ensemble / 00z NAM / 00z HREF blended mean.


A long advertised trough promises to bring heavy rain into
California, including an escalating risk of excessive rainfall /
flash flooding. The ECMWF evolution of the system had been slower,
but the 00z run started to catch up to the global consensus. With
good large scale agreement, a consensus approach to the QPF,
including bias-corrected tools, usually does well along the West
Coast. For simplicity, we used the same QPF blend here as we
employed elsewhere on Day 1, a 40/30/30 percent blend of our
ensemble / 00z NAM / 00z HREF blended mean. This may have produced
a result just slightly too heavy, given the WRF-ARW influence on
the HREF mean. The WRF-ARW is doing something we keep seeing east
of the Rockies, with a tendency to produce very heavy swaths of
convective precipitation along the southern edge of synoptic
forcing. This intensity, which would affect southern California on
Day 1 if the WRF-ARW were correct, seems unlikely given the 850 mb
flow really does not increase until Day 2. There is also a
pronounced lead shortwave passing by Southern California Tuesday
evening, with a brief period of subsidence in its wake. Therefore,
we will maintain only Slight Risk probabilities for excessive
rainfall on Day 1, leading into a more dangerous heavy rain event
on Day 2.

Day 2


There is a strong model signal day 2 for widespread heavy
precipitation across large portions of California from the
transverse range/southern California coast range--through the
Sierra and into the northwest California coast range.  The
atmospheric river event commencing 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.  Here pw anomalies of 4-5+
standard deviations above the mean and moisture flux anomalies of
5+ standard deviations above the 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 probability was added 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 during the second half of day
2---will likely produce life-threatening debris flow/mudslide

Pacific Northwest

Anomalous pw values will also be drawn northeastward into the
Pacific Northwest day 2 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.  .25-.50"+ areal average totals
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 is a lot of detail differences among the latest guidance
leading to lower confidence than other areas in the anomalous pw
plume.  The day 2 qpf did not deviate from the model consensus of
widespread moderate amounts.

Mid Atlantic into New England

The second and likely stronger of the two areas of low pressure
associated with the early spring winter storm affecting the
northeast will continue to deepen off the Mid Atlantic coast and
push toward southeast New England day 2.  There continues to be
model spread with the details of this cyclogenesis event.  The NAM
is on the western edge of the guidance and farther to the
northwest with heavy precip from northeast PA into southern NY
state and into central New England.  The NAM conest was farther to
the southeast with its heavy axis---closer to the axes of the 1200
utc EC---in house bias corrected mean and sref mean.  Heavy snows
possible in what will likely be a well defined comma
head/deformation precip band to the northwest of the deepening low
from the Mid Atlantic into central to southern New England.
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 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 continue into the
first half of day 3 before this axis sinks southeastward into the
northern Baja.  Model consensus is for additional heavy totals
through the Transverse range--with moderate to heavy precip values
extending southward through the Peninsular range day 3.  The
additional day 3 rains through the Transverse range will support
the continued significant debris flow/mudslide issues across the
numerous burn areas.  A moderate risk is depicted into the Thomas
burn area of the Transverse range---with a slight extended
eastward through the remainder of the Transverse range.  Precip
totals not expected to be as heavy as day 2 given the lessening
time of very anomalous pw values and moisture flux values into
this area.

Additional heavy precipitation and heavy snows likely through the
central to southern Sierra day 3 as persistent west southwesterly
upslope flow continues.

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.

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 greater totals day 3 compared to day 2 with
the heaviest amounts from the northern Wasatch into the far
eastern ID/northwest WY and across the Rockies of central to
northern ID.

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 from far northeast MT into
western ND and western SD.  At the moment---model consensus is for
light to moderate areal average amounts in the .10-.25" range.


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


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