National Weather Service Text Product Display

Area Forecast Discussion
Issued by NWS Portland, OR
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KPQR 010504
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
904 PM PST Tue Jan 31 2017

.SYNOPSIS...A rather moist well developed low remains parked to our
southwest off the southern Oregon and northern California coast. A
weak front will remain stalled over Lane County tonight, bringing
light snow to the Lane County Cascades. The front will then slowly
drift northward again on Wednesday as a warm front. East winds will
increase dramatically tonight and Wednesday with snow levels lowering
to near the valley floor but only light accumulations
are expected most inland areas. A stronger front is forecast to lift
north from the low into Lane County Thursday afternoon and the
remainder of the area Thursday night and Friday for increasing snow,
sleet, and freezing rain. The precipitation will turn to mostly
freezing rain through the day Friday except snow will continue in the
Gorge. The precipitation west of the Gorge will turn to rain from
south to north late Friday and Friday night. Active weather continues
through early next week, with more low elevation snow possible.
&&

.SHORT TERM...Tonight through Friday...A baroclinic zone pushed south
through the forecast area today as a cold front. The front is now
stalled over the far southern portions of the fcst area, and will
remain nearly stationary overnight. Not much is showing up on radar
this evening, but ODOT webcams show steady snow falling at Willamette
Pass and Santiam Pass. So will keep likely PoPs with light
accumulations in the Lane County Cascades through this evening. Will
retain low chances of rain over the southern Willamette Valley and
central Oregon Coast Range. Precipitation is modeled to drop off
overnight, but think light snow will continue at times in the
Cascades into Wed morning. Cold high pres is now settling over the
Columbia Basin as expected. Colder and drier air is also beginning to
work in Cascades in the northerly flow behind the cold front, with
the cloud cover decreasing considerably over the northern half of the
CWA. Temps have already dropped into the mid 30s for most of the
lowlands. Decided to knock fcst overnight lows down a few degrees,
with most of the lowlands dropping below freezing.

The models show east winds dramatically increasing through the Gorge
overnight and Wednesday as the Columbia Basin high becomes more
entrenched. This will drop snow levels close to the valley floor late
tonight and Wednesday, with wind gusts in the Portland metro area to
wind advisory values. The weak front in our area is forecast to
slowly drift north on Wednesday but any accumulations will tend to be
light, mainly a dusting, though an isolated inch value could happen.
The east winds will tend to dry out any moisture through
sublimation/evaporation as the band lifts north toward Portland.
Expect only minor impacts Wednesday, though we have seen it does not
take much snow to snarl traffic in our area.

Of more significance is the frontal band or deformation zone that the
models lift north into our southern forecast zones Thursday afternoon
and the remainder of the forecast area Thursday night and Friday.
Model soundings indicate that the precipitation type near Portland
will start out as snow with perhaps up to a couple of inches, but
then transition to sleet and then freezing later Thursday night and
especially Friday. Near Eugene, the precipitation type may briefly be
snow or sleet Thursday afternoon but the bulk of the precipitation
will be freezing rain into Friday. Salem will be in between these two
extremes.

On Friday the precipitation will be mostly freezing rain in the
valley in the morning, though Portland northward may still see some
snow or sleet. All valleys should be freezing rain Friday afternoon,
though the models soundings suggest that the freezing rain may start
to be just rain near Eugene later in the day. The models show a good
southerly surge of wind up the valley Friday night that should end
the freezing rain west of the Gorge.

The Gorge will see snow into Friday, with freezing rain spreading in
Friday and Friday night, lingering into Saturday.

The coast, mainly the south Washington and parts of the north Oregon
coasts may see a little snow or sleet before moderation spreads in
Friday.

Ice accumulations are going to be tricky, but a tenth or two to
around a half inch are possible depending on location. Pyle/Tolleson

.LONG TERM...Friday night through Tuesday. Surface low pressure
located offshore will drift north along the coast, or just offshore,
late Friday, which will allow southerly winds to continue to move
through the central and northern Willamette Valley, progressively
bringing an end to freezing rain as temperatures warm above
freezing. However, the easterly surface gradient through the
Columbia River Gorge is quite slow to ease, so expect freezing rain
to hang on through Friday night for the far east Portland metro area
(including Camas, Troutdale, and Gresham), and into Saturday for
much of the gorge and the lower elevations of the Hood River Valley
into Saturday. As a result, have continued to hold temperatures for
the gorge and Hood River Valley at the low end of the guidance
envelope through Sunday. Regular rain then continues for most of the
region into the weekend in a fairly active pattern, with periods of
heavy precipitation at times as reinforcing disturbances rotate
around the broad upper low that remains firmly in place off the
northern end of Vancouver Island through the weekend. Meanwhile,
snow will continue in the Cascades, with snow levels climbing
slightly but remaining below about 5,000 feet through the entire
weekend.

Details in the models become a bit less consistent by the end of the
weekend into early next week as the next week, partially due to the
influence of a more progressive upper low undercutting the northern
stream low that remains in more of a blocking position downstream of
the strong ridge over Alaska. While the latest ECMWF suggests more
interaction and a well-defined surface low moving up the coast,
these two systems remain somewhat out of phase in the latest
deterministic GFS run, with the bulk of the energy and significant
moisture heading south into California. Thus, due to the significant
spread among forecast models, have for now maintained a more
conservative approach to the forecast for the start of next week,
with snow levels lowering only to around 1,500 feet on Monday as
enhanced moisture moves into the region. However, given both the low
confidence in the overall evolution of the pattern and some support
among several of the GEFS ensemble members, can not rule out the
possibility of low-elevation snow again around Monday of next week.
However, by midweek, expect a trend for increasingly dry and warmer
conditions as the persistent upper low finally moves out of the
region and heights begin to rise.   Cullen
&&

.AVIATION...Conditions have improved to VFR for the entire
forecast area. The area of question in the south valley is now
under the influence of a strong enough north wind that it appears
dry air advection will keep any lower ceilings at bay tonight.
Increasing winds through the gorge will lead go increasing gusty
east winds at KTTD and KPDX starting in the next few hours.

KPDX AND APPROACHES...VFR conditions through the period. Gusty
east winds will pick up through the gorge later tonight. A few
gusts may approach 40 knots by this time tomorrow evening.
/Bentley

&&

.MARINE...Issued gale warning for northern outer waters as 00Z
guidance continued the stronger trend with the winds starting
tomorrow. Now expect gusts as high as 40 knots by this time
tomorrow evening in areas further than 30 miles offshore. In
addition, these stronger winds have confirmed the likelihood of
square seas for the gale warning area during the same timeframe.
Now have wind waves approaching 9 to 10 feet with periods near 6
seconds.

Strongest offshore flow will be Wed through Thu. Expect east wind
gusting to 20 to 30 kt over much of the region, but gusts to 40
kt likely on outer waters north of Tillamook. Wind eventually
shifts to the south late Fri or Fri night with decreasing wind
speeds. Latest models in agreement through Sat night suggesting
small craft advisory wind speeds. Model variability increases Sat
night through Mon with regard to a low pressure area that models
show developing near 38N 135W. The GFS keeps this low well to
the south, making landfall near KSFO. The ECMWF tracks the low
northeast, reaching buoy 029 between 06Z and 12Z Mon. Per OPC
have trended toward the ECMWF Sun through Tue.

Seas 5 to 7 ft through Wed. Increasing wind will result in a
building wind wave component. Likely to see wind waves up to 9 ft
over the n waters. Although the longer-period background swell is
expected to be around 2 to 4 ft Wed, the shorter-period larger
wind waves will create steep and choppy sea conditions. Total
seas likely to reach 10 ft later over the weekend.
Weishaar/Bentley

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from Thursday evening through Saturday
     afternoon for Central Columbia River Gorge-Upper Hood
     River Valley-Western Columbia River Gorge.

     Winter Storm Watch from Thursday afternoon through Friday
     evening for Coast Range of Northwest Oregon-Northern
     Oregon Cascade Foothills.

     Winter Storm Watch from Thursday morning through Friday
     afternoon for Cascade Foothills in Lane County-Central
     Coast Range of Western Oregon-South Willamette Valley.

     Wind Advisory from 4 AM Wednesday to 4 AM PST Friday for
     Greater Portland Metro Area.

     Winter Storm Watch from Thursday evening through late Friday
     night for Greater Portland Metro Area-Lower Columbia.

     Winter Storm Watch from Thursday afternoon through Friday
     evening for Central Willamette Valley.

WA...Winter Storm Watch from Thursday evening through Saturday
     afternoon for Central Columbia River Gorge-Western
     Columbia River Gorge.

     Wind Advisory from 4 AM Wednesday to 4 AM PST Friday for
     Greater Vancouver Area.

     Winter Storm Watch from Thursday evening through late Friday
     night for Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Cascade Foothills-Willapa Hills.

PZ...Small Craft Advisory for winds until 7 AM PST Wednesday for
     Waters from Cape Shoalwater WA to Cascade Head OR from 10
     to 60 nm.

     Gale Warning from 7 AM Wednesday to 4 PM PST Thursday for
     Waters from Cape Shoalwater WA to Cascade Head OR from 10
     to 60 nm.

     Small Craft Advisory for winds until 6 PM PST Thursday 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.

&&


$$

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland

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.
National Weather Service Regional Headquarters
