Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 242220
AFDPQR

Area Forecast Discussion
National Weather Service
220 PM PST Sat Feb 24 2018

.SYNOPSIS...Showers decreasing this evening. A cold front will
produce rain and higher elevation snow late tonight through Sunday
evening. The Cascades can expect a lot of snow during this period and
expect winter traveling conditions if planning on travelling over the
Cascade passes. Snow levels lower to near the valley floor Sunday
night and Monday, as showers decrease. Dry weather is expected Monday
night with additional rain and low elevation snow possible Tuesday.
An active weather pattern continues through next weekend.

&&

.SHORT TERM...Tonight through Tuesday. Showers will decrease this
evening, before a precipitation ahead of an approaching cold front
produces rain and higher elevation snow across SW Washington and NW
Oregon late tonight through Sunday night. Snow levels are expected to
be around 1500 feet this evening then rising to 2000-2500 feet for
the coastal mountains tonight, and for the Oregon Cascade Foothills
Sunday morning. The parent surface low associated with the front is
currently over the Gulf of Alaska. The models are fairly
consistent on the strength and position of this low and are
initializing well with satellite imagery. This allows for good
forecast confidence on the timing and probability of the
precipitation.

Snow levels are expected to remain around 1500 feet for the SW
Washington Cascades as the precipitation arrives late tonight (around
midnight), and will likely remain around 1500 feet through Sunday.
Have therefore issued an elevation based (above 1500 feet) Winter
Weather Advisory for the South Washington Cascades early Sunday
morning through Sunday afternoon. Have otherwise made no changes to
the Winter Weather Warnings for the Washington and Oregon Cascades as
1 to 2 feet of snow is still expected.

Onshore winds will favor higher precipitation totals for the higher
elevations, and expect around 0.5 inch of rain for the Willamette
Valley and interior low lands rainshadowed by the coast range from
late tonight through Sunday afternoon.

Colder air moves in behind the cold front Sunday night, and snow
levels will lower to near the valley floor Sunday night and remain
low through Monday morning. The showers will dramatically decrease as
the colder air moves in, and although snow showers are likely down to
the valley floor, do not expect snow accumulations below 500 feet.
Less than an inch of snow is expected between 500 and 1000 feet with
an exception for the Coastal Mountains and Cascade Foothills where
orographic lift will result in slightly higher accumulations (up to 2
inches).

Dry weather is expected Monday evening with clearing skies. THis will
favor radiation cooling and maintain temperatures near or below
freezing Monday night. A weaker cold front is expected on Tuesday.
The snow levels will start out low (500-1000 feet) Tuesday morning,
but south winds should rise them up to around 1500 feet in the
afternoon. QPF is fairly light with this front, and do not expect
much if any accumulations at the valley floor. Up to 2 inches of snow
is possible for the coast range and the Cascade foothills ,with
slightly higher amounts possible for the Cascades Tuesday morning
through Tuesday evening. ~TJ

.LONG TERM...Tuesday night through Saturday. A rather active weather
pattern looks to continue through the long term forecast period with
a longwave trough remaining across the western portion of the U.S.
through the period. Outside of the broad pattern, however, specific
details remain uncertain given large spread in the model guidance.
While the 12z GFS depicts a rather deep low pressure system just off
the Oregon/Washington coasts by Wednesday evening, there is
considerable spread in NAEFS members. It should be noted that the
ECMWF does show a somewhat similar low developing but with much less
impressive structure. Therefore, will maintain the forecast along a
compromise solution. Eventually, the large upper level trough
becomes more solidly centered over the far NE Pacific by late
Thursday. This would maintain an unseasonably cool air mass over the
region with snow levels again lowering to around 1000 feet. Will
need to continue to monitor the potential for some snow at least
mixing in below 1000 feet early next weekend as the cold air mass
looks to remain in place with at least enough moisture to support
showers. At this point, however, expect that the extended period
will feature continued enhancement of the Cascade snowpack but not
any additional low elevation snowfall. Cullen
&&

.AVIATION...Forecast area remains in a post-frontal unstable air
mass. Shower activity becoming more tied to terrain, as evidenced
in satellite and radar data. Predominantly VFR across the coast
and interior lowlands this afternoon, except for brief periods
of MVFR in showers. This pattern continues through about 10Z Sun.
West to Northwest 850 mb flow will keep MVFR or worse conditions
for the higher terrain, such as the Oregon Coast Range, Cascades
and SW Washington Willapa Hills. Gradual improvement in these
areas after 02Z as shower activity wanes. Next warm front reaches
the area after 10Z Sun. Expect increasing MVFR after 12Z,
especially for the coastal areas and SW Washington interior.
Widespread MVFR or worse through Sun morning, followed by
increasing VFR for the interior valleys Sun afternoon.

PDX AND APPROACHES...Post-frontal air mass over the terminal and
vicinity through this evening. This will bring the typical VFR
with brief MVFR in heavier showers, although shower activity will
diminish through the evening. Precipitation returns by 12Z Sun,
with MVFR conditions likely by 15Z. West wind at the terminal
will back to south this evening. Weishaar

&&

.MARINE...Solid small craft advisory wind continues over the
waters this afternoon. Latest model guidance shows boundary layer
wind speeds peaking at 30-35 kt across PZZ270 shortly after
midnight. Have opted to upgrade the gale watch to a warning valid
through 18Z Sun, although it will be a marginal event. Extended
the existing small craft wind advisories for PZZ255 and PZZ275
through 06Z Mon. Wind speeds start to diminish Mon morning and
are expected to fall below small craft advisory criteria by Mon
afternoon. Another frontal system Mon night and Tue will likely
produce small craft advisory wind.

Forecast confidence suffers a large drop Wed night through the
end of the week as models show a large variance regarding a
developing low pressure. For example, the ECMWF valid 06Z Thu
shows a 984 mb surface low at the north end of Vancouver Island
with a trailing frontal system over the waters. This would result
in possible low-end gale force wind. The GFS, on the other hand,
has a sub-980 mb low near 45N 127W, then deepens it to around
975 mb near Cape Disappointment 12Z Thu. This would bring the
potential for storm force wind. Have trended the forecast more
toward the ECMWF for this time frame.

Seas have peaked in the mid teens and will slowly subside a
little this evening and tonight. Seas increase into the mid and
upper teens late Sun into Mon as the fetch remains directed at
WA & OR. Latest ENP guidance shows seas falling just under 10 ft
Tue morning. The latest ENP model develops a core of 30 ft seas
off the South Oregon coast 12Z Thu, in response to the deep low
pressure as seen in the GFS. The ECMWF model has 12 to 15 ft
seas over the waters 12Z Thu and is the favored solution. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from 4 AM to 10 PM PST Sunday for Cascades
     in Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory from 4 AM to 4 PM PST Sunday for South
     Washington Cascade Foothills for elevations less than 1500 feet.

     Winter Storm Warning from 1 AM to 7 PM PST Sunday for South
     Washington Cascades.

PZ...Small Craft Advisory for winds until 10 PM PST Sunday for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 NM-Coastal Waters from Cascade Head OR to Florence
     OR out 60 NM.

     Small Craft Advisory for hazardous seas until 6 AM PST Monday
     for Coastal waters from Cape Shoalwater WA to Cascade Head
     OR out 10 NM-Coastal Waters from Cascade Head OR to
     Florence OR out 60 NM.

     Small Craft Advisory for winds until 1 AM PST Sunday for Waters
     from Cape Shoalwater WA to Cascade Head OR from 10 to 60
     NM.

     Gale Warning from 1 AM to 10 AM PST Sunday for Waters from Cape
     Shoalwater WA to Cascade Head OR from 10 to 60 NM.

     Small Craft Advisory for hazardous seas until 1 AM PST Sunday
     for Waters from Cape Shoalwater WA to Cascade Head OR from
     10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 4 PM
     PST Sunday.

&&
$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area.



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