Quantitative Precipitation Forecast
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FXUS04 KWBC 212052

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

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 22/0000 UTC thru Mar 25/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Western U.S...

On Day 1, the models target California for the heaviest
precipitation with
several inches of liquid equivalent precipitation in the coastal
ranges of southern CA and along the spine of the CA Sierra Nevada
range. One distinct area is within the plume of deepest moisture
streaming onshore into southern CA into the southern CA Sierra
Nevada ranges. The integrated water vapor imagery is well above
normal and the slow movement of the max precipitable water values
in the streaming streaming onshore in the counties north of Los
Angeles combines with 850 mb convergence and 300 mb divergence to
produce areas of heavy rain, with ascent aided by windward
The continuation o9f the high moisture and water vapor transport
into interior southern Ca leads to maxima along the
foothills and mountains of interior CA with heavy Sierra Nevada
snows, with several feet likely over the next day and a half.

A second area covers northern CA to the OR border as the northeast
Pacific upper low and trough drift towards
the coast...with a jet max streaming onshore and inland into the
ranges of ID and adjacent western MT.
The upper jet moving into NV Thu afternoon supports upper
divergence maxima streaming northeast from the CA Sierra across NV
into UT with a maxima near the northern Wasatch/Uintas of UT,
driving precipitation in windward terrain.

QPF was derived from a combination of the 06z high resolution
ensemble forecast mean along with the 12z runs of the wrf
ARW/nmmb/arw 2/nam/gfs. See the winter weather forecast amounts
and snow probabilities for heavy snow potential in the mountains
of CA

...Northeastern U.S...

The model indicate that as the low pressure off the coast moves
slowly northeastward overnight. the heavy snow event in the mid
Atlantic states crosses Long Island New York and southeast New
England before heading north into the Canadian maritimes.  The
850-700 mb front and convergence along it should persist as a
focus for heavier, banded precipitation, creating a swath of
maximum QPF along its path in Long Island to eastern MA this
evening and then tapering in Long Island as the front departs.
An enhancement develops as easterly flow onshore into eastern MA
produce coastal convergence and aids synoptic scale lift tonight
in producing ascent and snow.

As the circulation continues northeast Thursday, the winds back
and then the onshore flow of mid level moisture and lift over the
850-700 mb front occurs up the coast in downeast Maine.
Soundings and surface observations in interior New York and New
England indicated very dry air in place,
which has provided for a sharp northern edge to the snow.  The dry
air counter acts the moisture fluxes to prevent heavy snow from
extending further inland into Maine.

The models are not in total agreement yet, as the 12z nam was
heavier than other forecasts.  Given the WRF
ARW/ARW2/NMMB/Canadian regional GEM are drier, manual QPF cuts
back on NAM amounts in eastern Maine on Thu. WPC favored a QPF
blend of the HREF mean with the global model consensus.  See WPC
Heavy Snow graphics and discussion for potential amounts and

Day 2


The axis of much above average pw values---4 to 5+ standard
deviations above the mean---will gradually be forced to the south
and east of California from late Thursday into Friday.  Before its
departure, moderate to heavy rainfall will be ongoing at the start
of the forecast period. Precipitation rates will be lessening by
the start of the day 2 forecast period at 23/00Z as the axis of
the anomalous moisture flux and PW axis pushes eastward.

Runoff issues remain a great concern into Thursday evening with
any additional rain falling over the recent burn areas across the
Transverse range including the
and Pilot Burn area.  The day 2 excessive rainfall
outlook--including the high risk area was maintained over the
western portions of the Transverse range---with the moderate area
covering 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 forecast
to veer and take on an 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.

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.  A general
model compromise worked well here given comparable placement and
precipitation forecasts from the NCEP and non-NCEP global models.

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 finding no reason to deviating from this.

Eastern New England

The comma head/deformation precip band with the storm storm will
be pulling away from the coast with a corresponding decrease in
areal coverage and intensity of precipitation.  Some light
precipitation will be lingering in the cold, cyclonic flow around
the storm that should result in some enhancement by the Great

Day 3

Northern California into the Pacific Northwest and Great Basin to
the Central Rockies

The strong closed low off the B.C. coast will sink slowly
southeastward to off the Pacific Northwest coast from late day 2
into 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
parts of the 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.

Precipitation will be waning during the latter part of the day 2
time period from the Southwest---Great Basin into the central
Rockies as height falls press 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---TN 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.  WPC continued to favored a slightly farther south
qpf axis---especially once the system reaches the Tennessee Valley
late on day 3.  The GFS fit in between the slower/drier ECMWF and
the much faster 21/12Z runs of the NAM.  Both the GFS and ECMWF
continue to show 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.  Accumulating snows
possible on the northern portion of this broad precipitation
shield--see the latest QPFHSD for additional winter weather


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


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