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Area forecast discussion 
National Weather Service Portland or
910 PM PST Friday Dec 19 2014

Synopsis...mild and very wet weather is expected this weekend with
a strong westerly flow over the Pacific resulting in heavy rain
across much of the region...especially mountainous terrain. 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.

&&

Short term...surface observations and radar indicated some light
rain scattered over the area this evening. Moist isentropic lift is
expected to pick up overnight...and is seen lowering in model forecasts
from near the 300k isentrope down to 290k and below after
midnight...which should result in most areas seeing rain developing
overnight. Low level pressure gradients increase rapidly overnight and
into Sat morning as a warm front approaches the coast. Flow is
initially out of the S to SW Sat morning...but turns to the west during
the day. Surface pressure gradients in models continue to suggest
strongest winds along the coast to run from late tonight through Sat
morning...but likely to be a little below high wind criteria for
most locations. Will continue to mention gusts to around 55 miles per hour
mainly for The Headlands and beaches. The strong SW flow in the
morning coupled with the arrival of the tropical moisture coming
across the Pacific suggests some heavy rainfall developing over much
of the forecast area Sat morning. As the flow turns more westerly at
850 mb later in the day...orographic effects will enhance with
windward terrain catching the heaviest rain...and a bit of a rain
shadowing effect beginning to show to the leeward sides of the
mountains. Snow levels will rise during the day Sat with warm air
advection...but not before some snow falls in the Cascades. The SW
facing Cascades of S Washington will be the most orographically favored in
the morning when snow levels remain low...and so are likely to see
the most significant snowfall. On the coast...seas expected to begin
subsiding by early afternoon which coupled with shortening wave
periods will bring an and to high surf threat.

Remainder of short term discussion unchanged...both GFS and NAM are
trending down with quantitative precipitation forecast on Sunday so decreased it to some extent...
but since that looks like when the best orographic enhancement will
occur...did not want to limit it too much. Did bring inland valley
quantitative precipitation forecast down a bit though to reflect likely strong rain shadow effect
from the Coast Range. Still think we could see rain rates in the
higher elevations of 1.5 to 2 inches per 6 hours for as many as 24
hours in a row though. A few inches of snow fell today on Mount Hood
and probably some of the other area peaks but snow pack is generally
minimal right now. Despite low snow pack...latest Hydro guidance
suggests some rivers will reach flood stage. Please refer to
hydrology section and current Flood Watch for more details.

The atmospheric river spigot looks like it finally shuts off Sunday
night with decreasing probability of precipitation into Monday as high pressure pushes into
the area from off the California coast. However river flooding may
continue to be a threat as the significant weekend precipitation drains down
from the higher terrain. Bowen/wishaar

&&

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 2000-2500 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...confidence continues to increase that a significant
atmospheric river type precipitation event will impact southwest
Washington and northwest Oregon over the weekend and into early next
week. Precipitation will start late tonight into early Sat as a warm
front pushes onshore. The baroclinic zone will then stall over the
region later Sat and remain nearly stationary into early Monday.
Significant sub-tropical moisture with a long fetch that stretches
all the way to the South China Sea will feed into this frontal
system...bringing the likelihood of very large rainfall totals
across the region. Low and middle level winds will switch from
southerly to westerly Sat afternoon. These strong westerly winds
will continue into early Monday...which will create significant
orographic enhancement to the precipitation along the higher terrain
of the Coast Range...foothills...and Cascades. These areas are
expected to see around 6 to 12 inches over rain through early
Monday...with localized areas seeing even higher amounts in the north
Oregon Cascades. The lower terrain is forecasted to see
lower...although still impressive totals. We expect 4 to 8 inches to
fall along the coast...and 2 to 5 inches over the interior lowlands.

Based on the latest forecast rainfall totals...numerous area rivers
are now projected to reach flood stage. A few locations are even
projected to reach major flood stage. A Flood Watch has been issued
for all of SW Washington and northwest Oregon starting late Sat afternoon
and continuing through Monday. Some of the faster responding rivers
draining the Coast Range and Cascades could reach flood stage as
early as Sat evening. Slower responding and main Stem rivers would
likely not reach flood until later Sunday or early next week.

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.

As is usually the case with these atmospheric river heavy
precipitation events...a lot depends on exactly where the band of
subtropical moisture sets up. The forecast models have been consistently
highlighting our region for some of the most significant impacts for
the last several days. However...there could still be shifts in the
exact positioning of the system...which would effect precipitation
estimates and river forecasts. 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).
Pyle

&&

Aviation...rain beginning to filter into the area with conditions
slowly lowering as a warm front approaches the area. MVFR is
increasing on the coast and should be widespread overnight with
areas of IFR developing and continuing through Saturday in
moderate to heavy rain. Inland areas generally VFR but expect
conditions to head to low MVFR around 12z Sat or so with the warm
front closer to the area. Moderate to heavy rain at times will
also reduce visibilities to 1 to 2 miles a times. The warm front
lifts north of the area midday Sat but continued rain as the
following front remain stalled over northwest Oregon and SW Washington.

Kpdx and approaches...current VFR conditions with light rain is
expected to yield to low MVFR around 12z Sat as a warm front
approaches. IFR conditions also possible especially late tonight
and Sat morning. /Mh

&&

Marine...variety of marine impacts over the next 24 hours. Larger
long period westerly swell reached buoy 46089 about 80 nm offshore
late this afternoon and had been fluctuating between 16 and 19 feet
but has now settled around 20 feet. Near shore buoys are around 16
feet and should climb to near 20 feet by midnight and continue through
Saturday afternoon then begin to subside. Current buoy
observations compare favorably to model output so confidence in
forecast is good.

Southwesterly to westerly winds will continue into
Sunday...keeping seas well in the teens through Sunday night. Seas
may temporarily bottom out near 10 feet early next week before a
weak front...and accompanying larger westerly swell around 15 feet
slides into the waters towards midweek.

Winds with the next frontal storm system is still on track to
impact the waters late tonight and Saturday morning. Expect winds
to gradually increase with solid gale force winds of 30 to 35 knots
with gusts of 40 knots expected across the waters by Saturday
morning. 00z model runs a touch weaker so possibility of
widespread storm force winds has diminished. There may be an hour
or two between 4 am and 7 am Sat where a coastal jet develops and
push gusts up closer to 50 knots. Additional gusts of 35 knots may
linger across the waters into Saturday evening as another weak
surge of winds enters the waters.

People should avoid walking and climbing on jetties along the
Oregon and Washington coasts through Saturday...but especially
this afternoon and evening. There is considerable concern that
sneaker waves will run up much farther up the beach than waves
observed over the previous 15 to 30 minutes. Under similar
scenarios...people have been killed by being swept off their feet
and dragged out to sea by the receding water or by the water
dislodging stuck Driftwood that subsequently roles over an
unsuspecting person. Please be cognizant of this when recreating
on the beaches of the Oregon and Washington coasts through
Saturday.

High tide near Toke Point is expected to flirt with 11 feet on
Saturday due to a tidal anomaly near 1 feet. However...the heaviest
rain will not have arrived to allow the flows on the Willapa and
Naselle rivers to increase enough that minor tidal overflow
flooding will be a concern. While the tidal anomaly will likely be
slightly less on Sunday...the substantial increase in river flows
expected on the Willapa and Naselle rivers increase our concern on
minor tidal overflow flooding occurring around the towns of
Raymond...Naselle and other low lying spots around Willapa Bay
Washington. In fact...by midday Sunday...several Oregon coastal
rivers will likely be near flood stage or exceeding flood stage so
tidal overflow concerns will likely encompass a larger portion of
our coastline. Due to the rivers remaining relatively high into
Monday...tidal overflow concerns could certainly linger with the
high tide midday Monday. Expect these issues to be most prevalent
between 9 am and 3 PM on Sunday/Monday. /Neuman

&&

Pqr watches/warnings/advisories...
or...Flood Watch from Saturday afternoon through Monday afternoon
for Cascade foothills in Lane County-Cascades in Lane County-
central Coast Range of western Oregon-central Columbia
River gorge-central Oregon coast-central Willamette Valley-
Coast Range of northwest Oregon-greater Portland metropolitan area-
lower Columbia-north Oregon coast-northern Oregon Cascade
foothills-northern Oregon Cascades-south Willamette Valley-
Upper Hood River Valley-western Columbia River gorge.

High surf advisory until 4 PM PST Saturday for central Oregon
coast-north Oregon coast.

Washington...Flood Watch from Saturday afternoon through Monday afternoon
for central Columbia River gorge-Greater Vancouver area-I-5
corridor in Cowlitz County-South Washington Cascade
foothills-South Washington Cascades-South Washington coast-
western Columbia River gorge-Willapa Hills.

High surf advisory until 4 PM PST Saturday for South
Washington coast.

Winter Weather Advisory until 1 PM PST Saturday for South
Washington Cascades.

Pz...Small Craft Advisory for winds until 1 am PST Saturday for
coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.

Gale Warning from 1 am to 9 PM PST Saturday for coastal waters
from Cape Shoalwater Washington to Florence or out 60 nm.

Small Craft Advisory for hazardous seas until 1 am PST Saturday
for coastal waters from Cape Shoalwater Washington to Florence or out
60 nm.

Small Craft Advisory for rough Columbia River bar until 4 am PST
Sunday.

&&



$$

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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.

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