Quantitative Precipitation Forecast
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FXUS04 KWBC 210936

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
535 AM EDT Wed Mar 21 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 21/1200 UTC thru Mar 24/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern U.S...

On the synoptic scale it is becoming less common in the modern age
to reach Day 1 and have trouble finding a QPF consensus. Yet this
early spring Nor`easter remains an enigma. The forecast leading
into this storm has shown remarkable sensitivity to small scale
interaction among shortwaves embedded in the unusually energetic
yet low of amplitude - flow over the southern U.S. The overall
trend had been toward more of a major snowstorm with 1 to 2 inch
liquid equivalent amounts from Baltimore northeastward. The trends
from one forecast cycle to the next, however, have been so
inconsistent whether considering one model or all of them, that we
are largely left with a manual forecast effort, relying on
conceptual models more than any specific piece of guidance
(especially where QPF is concerned).

The model mass fields seem to have finally settled down, with a
nicely depicted track of a low deepening off the Mid-Atlantic
coast, down to about 990 mb as it then slowly lifts northeast. The
850-700 mb front / inverted trough structure north of the low
track would appear to have set up as was anticipated from the
MD/PA border region up through NYC / Long Island and beyond. This
should become a focus for heavier, banded precipitation, creating
a swath of maximum QPF in the vicinity of this region. Farther
south the easterly onshore flow and deep layer ascent will be less
long lived, but still substantial in the early hours of the day.

Watching animations of the model fields, the focus and pivot in
the 850-700 mb layer should occur near NYC / Long Island, acting
to maximize snow and liquid equivalent amounts there. Most
guidance actually does agree to this concept, all but the 00z
ECMWF whose QPF axis was inexplicably offshore despite little
change to the mass field solution.

An additional concern, however, is that the evening soundings
indicated very dry air in the mid levels, and despite height falls
overnight, snowfall has been a bit lazy to fill in over the
Mid-Atlantic. The strongest height falls also do not extend up
into the baroclinic zone over southern New England until evening
when the pivot and eastward progression should be kicking in, thus
limiting the residence time of the *best* deep layer forcing. All
this being said, the WPC QPF may end up being slightly heavy
handed, but we are confident that it in the right place, hopefully
an accurate depiction of the pattern.

Working initially with the trends in NCEP guidance, and supported
a fair amount by the UKMET, WPC favored a QPF blend of the HREF
mean and a version of our in-house ensemble which gives
significant weight to the hi-res model suite. This yielded a
similar precip pattern to the previous NWS forecast, but with
heavier amounts, up to a half inch heavier liquid amount in the
vicinity of Long Island. The forecast does still support heavy
snowfall along the east coast - see WPC Heavy Snow Discussion.

...Western U.S...

On Day 1 into Day 2, the closed low well offshore of California
phases with and is kicked by the northern stream flow, creating a
full latitude trough with a pronounced southern stream component
at its base. The pattern as a whole makes some progress into the
Western U.S. on Wednesday, bringing widespread rain/snow as far
inland as the northern Rockies down to the lower Great Basin /
lower Colorado River basin. By far, however, this event will be
known as a California storm, with heavy Sierra snows and heavy
rainfall along the coast. The inherited WPC forecast was ratcheted
up about as high was was necessary, especially as some of the
heavy coastal rain into southern California will spread over into
the Day 2 period. Therefore, we started with the previous
forecast, and looked to just make minor adjustments to keep up
with trends. For one, we trimmed back on the coverage of quarter
inch amounts in the rain shadow areas of eastern WA/OR, based on
hi-res model consensus, NDFD, and the GEFS Re-forecast data based
on analog events. Secondly, there has been a continued trend for
the heaviest Day 1 rainfall (through 12z Thursday) to back up
northward through San Luis Obispo County and Monterey County.
There will certainly be heavy rainfall reaching down into Ventura
County and possibly the L.A. basin, but overall we nudged the Day
1 centroid farther north up the coast. This will mean that the
risk of flash flooding and debris flows associated with some of
the largest fire burn scars will straddle the Day 1 into Day 2
periods, with the full brunt of the moist plume and deep layer
forcing not arriving until early Thursday.

QPF was derived from a high percentage previous WPC, combined with
influence from the 00z NAM CONUS Nest and our in-house ensemble.
The WRF-ARW did not follow the trend, and was more aggressive with
heavy rain extending into Los Angeles suburbs. This is its
tendency nationwide (south and east of consensus), but it is
sometimes right. The Excessive Rainfall Outlook will remain
generous with its coverage given this possibility.

Day 2


The axis of much above average pw values---4 to 5+ standard
deviations above the mean---in the ongoing atmospheric river event
will persist into the southern California coast range---Transverse
range and Peninsular range during the first half of the upcoming
day 2 time period. This anomalous pw axis will be shifting
eastward later during day 2 out of southern California and into
the northern Baja and northeastward into the Great Basin---central
to northern Rockies and eventually into the northern to central
high plains.  For California---there is good agreement for
additional heavy precipitation through the southern California
coast range/Transverse range and through the Sierra.  Hourly
rainfall rates of .25-.50" possible during Thursday
morning---beginning to lessen in intensity by early afternoon as
the axis of the anomalous moisture flux and PW axis pushes
eastward.  Runoff issues will remain a great concern into Thursday
morning over the recent burn areas across the Transverse range
including the
and Pilot Burn area.  The high risk area during the day 1 period
was maintained over the western portions of the Transverse
range---with the moderate region expanded eastward from the
previous day 3 outlook to cover the above mentioned burn scars.
Heavy snows likely through the Sierra.  See the latest QPFHSD for
additional winter weather information.

Great Basin---Northern Rockies

The inland moving height falls through the Great Basin---central
to northern Rockies will spread the above mentioned axis of much
above average pw values northeastward day 2.  This will support
widespread moderate to locally heavy precipitation through the
Great Basin and into the Northern Rockies.  Heaviest totals
expected from the Wasatch/Uintas into the northern Rockies from
far eastern ID/western WY into central to northern ID.

Pacific Northwest

The low level flow that is forecast to have a mostly terrain
parallel trajectory day 1 across the Pacific northwest is expected
to veer to a more onshore direction as height falls rotate inland
on the south side of the south and southeast side of the strong
closed low forecast off the B.C. coast day 2.  Widespread moderate
to locally heavy precip likely from the Washington-Oregon Cascades
west to the northwest California coast range---Oregon coast range
into the Olympic range.

Northern plains

Strong isentropic lift will be spreading eastward into the
Northern Plains by the end of the day 2 time period associated
with the above mentioned height falls pushing through the Great
Basin.  PW values will be rising to much above average levels at
this time---supporting increasing precipitation coverage from
northeast MT into western portions of ND and SD.  Model consensus
is for moderate totals across these areas during the day 2
period---with the WPC qpf not deviating from this.

Eastern New England

The comma head/deformation precip band with the developing storm
off the Mid Atlantic Wednesday will continue to affect eastern New
England at the beginning of the day 2 period before lifting
northeastward into the Canadian Maritimes.  At the moment---any
significant precip for the day 2 time period likely to be along
the immediate coastal sections from Cape Cod into coastal Maine.

Day 3

Northern California into the Pacific Northwest

The strong closed low off the B.C. coast will sink slowly
southeastward to off the Pacific Northwest coast day 3.  This will
maintain deep layered cyclonic flow into the Pacific Northwest and
northern California.  PW values not expected to be anomalous in
this deep layered cyclonic flow---with values below seasonal
norms.  Heaviest precip totals expected over the northern
Sierra---northwest California coast range into the southwest
Oregon coast range  where PW values will be greatest and the
upslope component the strongest.  Aerial average 1-2"+ totals
expected across this area--with aerial average amounts in the
.25-50"+ range north of this into the Pacific Northwest.

Southwest---Great Basin---Central Rockies

Precipitation will be waning during the first half of the day 3
time period from the Southwest---Great Basin into the central
Rockies as height falls will be pressing northeastward from these
areas and into the Central to Northern Plains.  For the day 3
period---additional light to moderate totals possible

Northern-Central Plains---Lower MO Valley---Mid to Upper MS Valley

The above mentioned height falls exiting the Great Basin/central
Rockies region and pushing into the central to northern Plains
will support an expanding area of precipitation across portions of
the Northern to Central Plains into the lower MO---mid to upper MS
valley region.  Favored the GFS/EC solution of a slightly farther
south qpf axis---especially across portion of the lower MO Valley
into the mid to upper MS Valley region.   Both the GFS and ECMWF
are showing convection developing in an axis of instability Friday
night/early Saturday over eastern portions of the Central
Plains---Lower MO Valley-Mid MS Valley region.  The NAM does
indicate this area of instability but seems underdone with qpf
potential here.  Accumulating snows possible on the northern
portion of this broad precipitation region from large portions of
ND---into western to southern MN--northeast IA and northwest IL.
See the latest QPFHSD for additional winter weather information.


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


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