Area Forecast Discussion
Issued by NWS Portland, OR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
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
000
FXUS66 KPQR 170542
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
941 PM PST Mon Jan 16 2017

.SYNOPSIS...Brisk east winds continue to supply the Willamette Valley
with very cold Columbia Basin air. Snow cover remains several inches
deep in the Portland metro area and vicinity, adding
to the persistence of the cold air. The low-level cold air will come
into play as a very moist Pacific frontal system spreads rain across
the forecast area Tuesday and Wednesday. This will likely result in a
major ice storm in the west and central Columbia Gorge, with
significant icing also likely Tuesday for much of the Portland and
Vancouver metro areas. Icing is also likely in some Coast
Range and Cascade valleys, and may extend as far south as Salem. This
system is expected to bring very windy conditions to the coast and
higher terrain in the Coast Range. Occasionally heavy rain may also
cause some flooding, especially in areas impacted by last week`s
snowstorm. A return to colder weather is expected by the end of the
week, with additional systems bringing the threat of snow again at
least down into the hills.

&&

.SHORT TERM...Tonight through Thursday...The main update to the fcst
this evening was to upgrade the Freezing Rain Advisory for the
Portland/Vancouver metro areas to an Ice Storm Warning. The fcst
models continue to suggest that strong southerly winds aloft will
erode the cold air at the surface by the afternoon in the metro area.
However, as anyone who has paid any attention to the weather this
winter could tell you, the models notoriously scour out the cold air
too quickly in an offshore flow event. In this case, the models seem
to be having a particularly difficult time handling the strength of
the Columbia basin cold pool and surface high and the easterly
gradient through the gorge. The past few days, the models have been
showing the showing the surface high and gradient weakening, with
observations stubbornly refusing to go in this direction. Think that
the models are again overdoing the weakening of the cold pool and the
easterly gradient during the day tomorrow. Have delayed the
transition from rain to freezing rain until late in the afternoon for
most of the metro area, and kept freezing rain for eastern portions
of the metro area well into tomorrow night. The eastern metro looks
likely to see significant ice accumulations Confidence in timing is
still not high, and wouldn`t be surprised to see freezing rain
continue through a good portion of the metro into tomorrow evening,
which could mean a major ice event for portions of downtown Portland
and Vancouver.

The fcst models seem to be in agreement that there will be a period
of light precipitation early tomorrow morning that likely falls as
freezing rain for most of the lowlands. Surface temps for most are
already below freezing, so confidence is precip type to start is
high. It looks like there will then be a break during the late
morning and early afternoon before a period of much heavier
precipitation arrives during the late afternoon and evening.

Previous short term forecast from 326 PM remains unchanged...Here we
go again. Ice instead of snow this time.  Easterly flow coming out of
the gorge will feed sub freezing air into the interior lowlands while
snow covered valleys in the Cascades and coast range have remained
cold under temperature inversions. A warm and moist system will
overrun this cold air to generate freezing rain under the shallow
temperature inversion.Areas impacted will be a majority of the
Willamette Valley and interior lowland zones in southwest Washington,
as well as the deeper valleys of the coast range and Cascade
foothills. Greatest impacts will be in the west and central Gorge and
Upper Hood River Valley where around an inch and possibly 1.5" to 2"
inches ice Tuesday and Wednesday.

Model timing of precipitation is a bit slower than 24 hours ago and
this has been a trend in the past few days. The first slug of
moisture is carried toward Vancouver Island Tuesday morning with
generally light amounts over our area, the better surge of moisture
arrives late Tue afternoon and evening.  This delay will allow
central and south Willamette Valley zones moderate before too much
ice accumulate.  However closer to the west entrance of the Gorge
persistent east winds will keep parts of the north Willamette Valley
sub-freezing through the day Tuesday, with some moderation above
freezing in areas away from the Gorge Tue afternoon.  The
Portland/Vancouver metro area, especially toward the gorge may
experience continued ice accumulation into Tuesday evening which and
have greater ice accumulation and resulting impacts in that area.

With easterly pressure gradients continuing to resupply the Gorge
with cold air, see no reason temps will climb above freezing in Hood
River until later Wed afternoon, despite most model soundings
suggesting it occurs much sooner than that. That means nearly this
entire storm - which has enough rainfall to spark flood concerns -
will add about an inch of ice to the already astounding 18-24 inches
of snow and ice on the ground in Hood River. This brings up concerns
about weaker roofs caving in due to the extreme buildup of snow and
ice. The usual power outages and travel concerns are expected with
this event as well.

While the flood threat is still very real across SW WA/NW OR Tue-Wed,
QPF appears a little lighter than it did 24-48 hours ago. One reason
for this is that the vast majority of the forecast area will probably
miss out entirely on any rainfall from the first strong wave...which
is also the system slowing down the overall progress of the front.
This system is expected to intensify rapidly but curl north, dumping
the heaviest rain on Vancouver Island and the Olympic Peninsula. The
next two shortwaves still look like formidable rain-producers, as
this system taps into abundant subtropical moisture. Overall Tue-Wed
rain totals of 3-7 inches are still likely for the coast, Coast
Range, and S WA Cascades, with general 1.50 to 2.50 inch totals
elsewhere. With plenty of snow still on the ground across the inland
valleys north of Salem, this will likely still be enough to cause
urban and small stream flooding issues as heavy rain and melting snow
combine to overwhelm drainage systems. Icing from the beginning of
the event and a frozen ground will exacerbate the situation as
well...with much of the Portland and Vancouver metro area expected to
be a slushy, sloppy mess by the afternoon commute Tuesday. The last
shortwave and cold front will bring one last dose of heavy rain
Wednesday before precip tapers to showers behind the cold
front...which ironically will likely be the mechanism which finally
scours out all the low-level cold air across the region.

One last item of concern...wind along the coast and in the higher
terrain of the Coast Range. A High Wind warning has been issued
primarily for the beaches and headlands and higher terrain of the
Coast mountains, for gusts 60 mph to 70 mph Tuesday afternoon into
Wednesday morning. This looks to be in good shape, as
models have actually increased 850 mb winds a bit over the past few
runs.

Eventually this strong wind aloft will chip away at the low-level
cold air despite easterly pressure gradients through the Columbia
Gorge. Strong westerly flow aloft will induce lee-side troughing over
the Columbia Basin; the resulting Chinook effect will put a big dent
in the cold air over the basin. Hood River will probably be one of
the last spots to lose the low-level cold air...sometime around late
Wed afternoon or evening and just in time for it to be replaced by
the next batch of cold air behind the front Wednesday night and
Thursday.  /26

.LONG TERM...Thursday night through Monday...Models remain in decent
agreement that the progressive weather pattern will continue through
the extended forecast, albeit with some slight timing differences. A
deepening upper level low looks to approach the Pac NW late Thursday
and swing a cold front across the forecast area on Friday which will
bring another round of gusty winds and more rain. Conditions look to
become more showery Friday night as the trough shifts inland. Colder
air aloft will filter in over the region during this time with snow
levels lowering to around 2000 ft by Saturday morning. Showers look
to continue on Saturday as a weak shortwave trough moves across the
region. Another upper level trough will approach the area Sunday and
swing another frontal system across the Pac NW. Post frontal showers
look to persist into early next week under northwesterly flow aloft,
with snow levels remaining below 2000 ft. There is still a chance of
low elevation snow during the extended forecast, but confidence is
extremely low. Nonetheless, keep an eye on the forecast as things
may change in the coming days. /64
&&

.AVIATION...VFR conditions continue across most of the region this
evening with a high overcast deck remaining in place. Increasing
mid clouds may help limit any low stratus/fog development, but
will maintain some hint of development around KHIO and KEUG.
Strong offshore gradient will continue to produce 35-45 kt gusts
at the west end of the Columbia Gorge, and threat for LLWS near
Gorge increases after 10-12z through Tue. Initial precipitation
expected to reach the coast around 12Z Tue, with MVFR conditions
likely by 18Z. Initial precipitation reaches the interior between
15Z and 17Z, likely as -FZRA. More steady precip then arrives in
afternoon, with -FZRA becoming increasingly confined to KPDX
through gorge and rain elsewhere. MVFR expected for the
coast inland.

KPDX AND APPROACHES...Low level offshore flow will produce VFR
conditions through at least 18z Tuesday. East wind gusts 35-45 kt
will continue near the west end of the Gorge, including KTTD
through the morning, then gradually decrease late in the
afternoon. Light precipitation expected to begin around 16Z Tue,
as -FZRA. After brief lull, more significant moisture arrives in
afternoon with FZRA at the terminal toward the west end of the
Gorge through at least late Tue afternoon.   Weishaar/Cullen

&&

.MARINE...Latest guidance shows boundary layer wind speeds
increasing to 20 to 30 knots between 03Z and 08Z Tue. Solid gales
continue over all waters overnight through about midday Tue. 12Z
NAM shows 35 to 45 kt boundary layer wind speeds between 18Z Tue
and 00Z Wed. The GFS is a bit stronger with solid 40-45 knots
across all waters with coastal jet enhancement over the inner
waters. Will maintain the current Storm Watch. Models suggest wind
speeds start to diminish Wed afternoon, with the NAM and GFS
showing sub-gale force wind speeds by 00Z Thu. However, another
deep low pres area moves through the NE Pac Fri for potential
strong gales. The GFS is weaker with this low than the ECMWF and
lifts it north toward Vancouver Island sooner than the ECMWF, but
would still result in gale force wind for the waters. More
substantial model differences Sat night and beyond. The ECMWF
wants to develop another deep low near 42N 132W 12Z Sun while the
GFS has no hint of this feature.

Seas remain below 10 ft today. By 12Z Wed combined seas to 25 feet
can be expected. Given these seas will be produced by wind moving
parallel to the coast, do not foresee any issues with high surf
criteria. However, high surf conditions likely to develop late in
the week as a potentially strong westerly wind fetch sets up over
the waters. It appears the main energy will be directed at the
central and south Oregon waters but it is still too early to
pinpoint the details. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Ice Storm Warning from 6 AM Tuesday to 6 PM PST Wednesday for
     Central Columbia River Gorge-Upper Hood River Valley-
     Western Columbia River Gorge.

     Flood Watch from Tuesday evening through late Wednesday night
     for Cascade Foothills in Lane County-Central Coast Range
     of Western Oregon-Central Oregon Coast-Central Willamette
     Valley-Northern Oregon Cascade Foothills-South Willamette
     Valley.

     Freezing Rain Advisory from 6 AM to 6 PM PST Tuesday for
     Northern Oregon Cascade Foothills-Northern Oregon Cascades.

     Flood Watch from Tuesday morning through late Wednesday night
     for Coast Range of Northwest Oregon-Greater Portland Metro
     Area-Lower Columbia-North Oregon Coast.

     Ice Storm Warning from 4 AM to 10 PM PST Tuesday for Greater
     Portland Metro Area.

     Freezing Rain Advisory from 2 AM to 2 PM PST Tuesday for Coast
     Range of Northwest Oregon-Lower Columbia.

     High Wind Warning from noon Tuesday to 4 PM PST Wednesday for
     Central Coast Range of Western Oregon-Coast Range of
     Northwest Oregon.

     High Wind Warning from noon Tuesday to 10 AM PST Wednesday for
     Central Oregon Coast-North Oregon Coast.

     Freezing Rain Advisory from 4 AM to 10 AM PST Tuesday for South
     Willamette Valley.

     Freezing Rain Advisory from 4 AM to 2 PM PST Tuesday for
     Central Willamette Valley.

WA...Ice Storm Warning from 6 AM Tuesday to 6 PM PST Wednesday for
     Central Columbia River Gorge-South Washington Cascades-
     Western Columbia River Gorge.

     Flood Watch from Tuesday morning through late Wednesday night
     for Greater Vancouver Area-I-5 Corridor in Cowlitz County-
     South Washington Cascade Foothills-South Washington Coast-
     Willapa Hills.

     Ice Storm Warning from 4 AM to 10 PM PST Tuesday for Greater
     Vancouver Area.

     Freezing Rain Advisory from 2 AM to 2 PM PST Tuesday for I-5
     Corridor in Cowlitz County.

     High Wind Warning from noon Tuesday to 10 AM PST Wednesday for
     South Washington Coast.

     Freezing Rain Advisory from 4 AM to 6 PM PST Tuesday for South
     Washington Cascade Foothills.

PZ...Storm Watch from Tuesday afternoon through Wednesday morning
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Gale Warning until 1 PM PST Tuesday for Coastal Waters from
     Cape Shoalwater WA to Cascade Head OR out 60 nm.

     Small Craft Advisory for winds until 10 PM PST this evening for
     Coastal Waters from Cascade Head OR to Florence OR out 60
     nm.

     Gale Warning until 1 PM PST Tuesday for Coastal Waters from
     Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 10 PM
     PST Tuesday.

&&

$$

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.



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.