Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
1004 am PST Sat Dec 20 2014
Synopsis...mild and very wet weather has developed over southwest
Washington and northwest Oregon and is expected to continue through
the weekend as a strong westerly flow transports an atmospheric river
or plume of anomalously high moisture into the area. The heaviest
rain will be over the higher terrain...but all areas will see
substantial rains. The heavy rain should ease Sunday night into
Monday. There is the potential for some flooding late this weekend
into early next week. Longer range model output is suggesting a
colder air mass will settle over the Pacific northwest late next week
with snow down into at least the Cascade foothills and Coast Range.
Morning update...rainfall rates have picked up significantly this
morning as a warm front streams copious amounts of sub-tropical
moisture onshore. AMSU derived satellite imagery is showing a plume
of precipitable water around 1.25+ inches feeding into the system. So
far the heaviest rain has been along the Oregon coast and Coast
Range...where in general one to two inches of rain have fallen so
far. There is a pocket of even higher amounts up to 3 inches in the
Wilson and nestucca river basins east of Tillamook. Did not make too
many changes to the predicted rainfall totals this morning as things
generally seem to be on track. The main forecast update was to decrease
the quantitative precipitation forecast for the Willamette Valley tonight and sun. After the warm
front moves through this afternoon...we will lose some of the forcing
we are currently receiving from isentropic lift with the warm front.
The winds will also switch from southwesterly to westerly. With the
850 mb westerly wind speeds expected to be around 40 to 50 knots
overnight through much of sun...there will be very strong lift along
the Coast Range and downward motion on the Lee side...which should
contribute to a significant rain shadow effect. The latest total quantitative precipitation forecast
for the valley has been decreased to around 1 to 4 inches...with the
higher totals being on the east side of the valley closer to the
Will hang on the Winter Weather Advisory for the South Washington
Cascades into the early afternoon. The colder air over the Cascades
has been stubborn to leave...and current temperatures are still right
around freezing as low as 4000 to 4500 feet according to several of the
S Washington Cascade snotels. With radar showing fairly heavy precipitation now over
the Cascades...locations the higher Cascade passes may pick up
several additional inches of snowfall over the next few hours.
However...with the warm front moving through the region
today...expect temperatures to rise above freezing sooner rather than
later...with snow levels remaining well above the Cascade passes
through the remainder of the atmospheric river event. Pyle
Also...individuals planning to be along the coast today and tomorrow
should continue to closely monitor the forecast. The large...long
period seas will bring dangerous conditions to the surf zone. Anyone
on beaches should exercise extreme caution and remain alert as large
waves may wash ashore with little warning. Large waves may also
crash over jetties and sweep away unsuspecting beachgoers and storm
Previous discussion issued 258 am Sat Dec 20...
Short term...our well advertised atmospheric river event has begun
over the Pacific northwest early this morning...with overrunning rain
having spread across our entire forecast area ahead of a strong warm
front. The models show a couple of warm frontal waves moving through
today that will keep today very wet...with a decent surface trough
moving by just to our north middle day that will produce some rather
strong winds along the coast. We may get close to high wind values at
the beaches and headlands...but will stay with the current forecast
of just below high wind criteria.
The main story though will be the rain. Once the warm front moves
through later today...several more surges of rain follow tonight into
Sunday in the continued strong westerly flow aloft. The lower level
flow will turn more westerly as well tonight and Sunday...and we may
start to see a bit of rain shadowing to the Lee of the Coast Range.
But the coast and higher terrain will continued to hit hard by the
rain with significant orographic enhancement.
As a result...the current Flood Watch looks good as we should start
to see significant rises on many area rivers by tonight and Sunday.
The biggest question in my mind right now is if there will be any let
up or breaks in the heavy rain after the warm front moves through
later today and between any of the moisture surges coming in tonight
and Sunday. This will ultimately control how many rivers flood and
how high they get. Nevertheless...the Hydro section of this
discussion will not be updated this morning as it still looks
The strong westerly flow starts to buckle late Sunday as an upper
ridge starts to develop in the northeast Pacific...which continues to
amplify Sunday night and Monday. The main effect in our area is that
it will the precipitation and moisture coming into the Pacific
northwest will decrease as the onshore flow abates...though it will
take some time for any rivers that flood to react and recede.
Will continue the Snow Advisory for snow this morning for the South
Washington Cascades ahead of the incoming warm front. Snow levels
should rise and the snow turn to mostly rain this afternoon. The
Oregon Cascades will see a few inches of snow this morning as well.
Long term...no changes. Previous discussion follows...
Monday night through Friday...an upper level ridge will amplify over
the region late Monday into Tuesday. Extended models agree on a weak
shortwave attempting to flatten the ridge Tuesday morning as an upper
level trough begins to deepen in the Gulf of Alaska. Cloudy and rainy
conditions will linger into the middle of next week as this dirty
ridge remains over the area. The next more significant system arrives
late Wednesday afternoon as the upper level trough axis becomes more
negatively tilted and starts to shift inland. This Marks the next
best shot for the Cascades to finally see some snow
accumulations...as snow levels drop to 2000 to 2500 feet through
Christmas day. Holiday travelers headed over the Cascade passes will
certainly need to monitor the forecast. Colder north flow will follow
and allow temperatures to cool well below normal as conditions dry
out next Friday. /27
Hydrology...a classic atmospheric river type precipitation event is
underway over the region today...which will continue to impact
the region into early Monday. A very long fetch of moisture with
origins west to the South China Sea is feeding into this
system...bringing the potential for very large rainfall totals
across the region. A deep and moist westerly flow will develop
today and continue through Sunday. With the flow forecasted to
remain perpendicular to the Coast Range and Cascades for an
extended period of time...expect strong orographic enhancement to
lead to the highest rainfall totals over the mountains. The Coast
Range...foothills and Cascades are expected to see in the range of
6 to 12 inches of rain...with locally higher amounts possible.
Lower but still significant totals are expected for the lower
terrain. Projected rainfall along the coast is forecasted to be
around 4 to 8 inches. The interior lowlands should see 1 to 4
inches...with the highest amounts closer to the Cascade foothills.
Given the latest forecasted rainfall totals...it appears that
there is the strong potential for flooding over the weekend on
numerous rivers and tributaries draining the Coast Range and
Cascades. A few rivers may even reach major flood stage. The
Current River stage forecasts indicate that the faster responding
rivers could reach flood stage as early as tonight. There are also
likely to be impacts from Urban and Small Stream flooding and
flooding along low lying pasture land.
There is also potential for some tidally influence flooding concerns
over the next couple of days along the coast. High tide this
afternoon near Toke Point will likely touch just above 11 feet due to a
tidal anomaly around 1.5 feet. This may be sufficient to provide some
minor tidal overflow issues for the South Washington coast.
However...concern remains greater for high tide on Sunday afternoon
for both the Oregon and South Washington coasts given that abundant
rainfall today and tonight will likely push flows on coastal rivers
significantly higher. Several coastal rivers will be approaching or
exceeding flood stage by midday Sunday...so suspect that tidal
overflow concerns may encompass many coastal spots. Rivers likely
remain high into Monday...so there may be impacts again during high
tides. Any tidal overflow issues that materialize will likely be most
prevalent between 9 am and 3 PM on Sunday and Monday.
In addition to the flooding concerns...excessive rainfall has the
potential to create landslides...debris flows...and excessive runoff
in the vicinity of recent wildfire burn scars. Burn scars from two
large fires this Summer...the 36 pit fire in the Clackamas river
basin near Estacada and the deception complex near Oakridge...will
be particularly vulnerable.
Please continue to follow the latest forecasts. River forecasts are
available on our web Page at:
http://water.Weather.Gov/ahps2/index.Php?Wfo=pqr (all lower case).
Aviation...mix of flight conditions across the region...with
MVFR/VFR across the interior and IFR/MVFR at the coast. Trend of
increasingly MVFR over most of the area will continue with
approaching frontal boundary and continued rain...with pockets of
IFR likely at times along the coast and Coast Range. Expect higher
terrain to be obscured in clouds and precipitation through the day. Gusty
S winds along the coast to 35-45 knots through this evening...then
Kpdx and approaches...MVFR ceilings prevail this evening...likely
remaining near 2000 feet. Visible will also become restricted...down to
3-4 sm by 19z through this evening. May see brief periods of IFR
with heavier rain continuing this afternoon through the overnight.
With increasing west-SW flow aloft this afternoon...some potential
for low level wind shear 21z-03z this evening. Cullen
Marine...southerly gales on the waters this morning...with
another burst of stronger gale gusts this afternoon. Cannot rule
out a few isolated low end storm force gusts...particularly near
the coast...but think these will be local and infrequent at most.
Winds will ease gradually into this evening...with solid small
craft level winds over the waters by midnight. Winds continue to
ease through day sun...with winds generally 15 knots or less for late
Sun night through Wednesday...trending to west or northwest.
Seas remain around 19 to 21 feet over the waters this morning and
should stay in the range through much of the day...perhaps running
slightly higher over the far outer waters. Seas will then subside
to near 15 feet by early Sunday...but remain above 10 feet through at
least the first half of the week. Cullen
or...Flood Watch from 4 PM PST this afternoon through Monday
afternoon for northwest Oregon.
High surf advisory until 7 PM PST this evening for central
Oregon coast-north Oregon coast.
Washington...Flood Watch from 4 PM PST this afternoon through Monday
afternoon for southwest Washington.
High surf advisory until 7 PM PST this evening for South
Winter Weather Advisory until 1 PM PST this afternoon for South
Pz...Gale Warning until midnight PST tonight for coastal waters from
Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until 6 PM
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This discussion is for northwest Oregon and southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.