Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 172327
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
327 PM PST Tue Jan 17 2017

.SYNOPSIS...A moist front will bring widespread heavy rain, localized
freezing rain (Hood River Valley, Gorge, and Portland/Vancouver), and
high winds to the coast and coast range tonight and Wednesday. The
rain will turn to showers Wednesday night and Thursday as snow levels
lower down to the Cascade passes. An other moist front is expected
Thursday night and Friday with a series of lows maintaining showers
through next weekend.
&&

.SHORT TERM...Tonight through Friday...Alot of areas have warmed
above freezing before the heavier rain has arrived this afternoon,
and have cancelled the freezing rain advisory for the Central
Willamette Valley and the North Oregon coast range. The I-5 corridor
in Cowlitz County and the lower Columbia River valley have been
slower to scour out the shallow colder air, and have extended the
freezing rain advisory until 6 pm there. Not all areas along the I-5
Corridor will see freezing rain, but there are enough surface
observations reporting below freezing temperatures to warrant
extending the advisory.

Reports of liquid vs. freezing precipitation have varied greatly
across the Portland and Vancouver metro area late this morning. Many
areas warmed above freezing during the break earlier this morning,
and we have received reports of liquid rain for north Clark County in
Washington...and for south and southwest Portland areas. However,
many of these areas saw temperatures cool back to near or just below
freezing as the heavier rain began moving in early this afternoon. We
have accumulating ice here at the NWS Portland office in the Parkrose
area, and have been getting reports of accumulating ice in Camas,
Troutdale, and other areas close to the Columbia River Gorge
entrance. The roads quickly slicked up as the freezing rain began
with accidents quickly following. Freezing rain is also impacting the
Columbia River Gorge resulting in the closure of I-84 between
Troutdale and Hood River due to multiple accidents.

When the colder air will scour out of these areas and freezing rain
turns to rain depends on when the east surface pressure gradient
through the Columbia River Gorge weakens. The gradient has weakened a
couple of mb since this morning, but will remain above 6 mb into
Wednesday morning. This will likely keep the cold air (and freezing
rain) for the lower areas of the Portland metro area influenced by
the Columbia Gorge going through late tonight and possibly into
Wednesday morning. The Gorge and Upper Hood River Valley areas will
continue freezing rain through early Wednesday evening. Have
increased qpf and ice accumulation a bit for tonight and Wednesday to
better represent the high moisture observed offshore this afternoon.


What is causing this rain? An occluded front which is forming nicely
off of the Washington and Oregon coast this afternoon. This front has
quite a bit of moisture with it offshore with satellite derived
precipitable water shows a swath of precipitable water values  between
1.25 and 2.2 inches. The air mass will lose some of this moisture as
the front begins to move inland this evening, but the precipitable
water will be much higher than the climatological daily normal. For
perspective, the seasonal daily normal precipitable water for Salem
today is 0.58 inch. A low moving along the front overnight tonight
will slow the inland progress of the front, and maintain rain through
late Wednesday evening.

Due to the high moisture observed offshore and the prolonged period
of rain, have increased qpf for tonight and Wednesday morning. From
tonight through Wednesday evening, expect 3 to 7 inches of rain for
the coast range, 2 to 4 inches for the Cascades and the Cascade
foothills, 2 to 4 inches for the coast, and 1 to 2 inches for the
Willamette Valley. This rain will likely result in flooding of some
of the coastal rivers, small streams, and possibly urban flooding in
low-lying areas or areas with clogged drains.

South winds are increasing along the coast ahead of the front with
Cape Disappointment reporting gusts around 60 mph, Abernathy
reporting gusts around 75 mph, and Clatsop Spit around 45 mph. These
winds will continue to increase this evening and remain strong into
Wednesday morning, and am not going to make any changes to the High
Wind Warning. Valley winds will increase this evening too with south
gusts up to 35 mph possible for the South and Central Willamette
Valley. The east winds through the Gorge will delay the south winds
for the North Willamette Valley, and do not expect as noticeably
gusty south winds for the Portland/Vancouver area.  However, the east
winds will be quite gusty for East Portland, Fairview, and Troutdale.


Rain will turn to showers behind the front Wednesday night with an
upper level trough continuing showers through Thursday. Snow levels
which will be high (above 8000 feet) tonight will slowly lower
Wednesday and be down to 4000 feet by Thursday morning for some light
snow accumulations for the Cascade passes. Another potentially moist
front arrives early Friday morning. The airmass will be colder with
this front and snow levels will be between 2500 and 3000 feet. ~TJ

.LONG TERM...No Changes. Previous discussion follows...Friday night
through Monday...Models in good general agreement on keeping a closed
low off the WA and southern BC coast from Fri night through Sun
night, rotating a series of shortwaves through the region. The
shortwaves however show a tendency to split...with some energy
lifting ne through the Pacific NW and some energy headed down into
northern CA. identity of the waves quickly becomes difficult to
correlate between the models, so will generally have to limit pops to
high chance to low likely range at any given time. Models, EC in
particular, showing a tendency to hang onto an offshore flow through
the gorge, and it is not clear that the cold air ever truly gets
scoured out of the basin east of the Cascades. As such, will keep a
chance of freezing in for the gorge and hood river valley. Mon and
Tue models trend towards ridging aloft approaching the coast,
resulting in decreasing pops. EC is drier of the models, and as a
result tends to colder temps, and as such will still need to hang
onto some chance for freezing rain in the Gorge and Hood River Valley
all the way into Tue.

&&

.AVIATION...A very moist frontal system is bringing heavy rain
across SW Washington and NW Oregon this afternoon. Expect steady
precipitation to continue through the rest of today and tonight.
Also expect this system to bring gusty southerly winds to the
coastal TAF sites later this afternoon through Wed morning. Gusts
of 45 to 50 kt are possible at times. Flight conditions have
deteriorated to MVFR or IFR at most locations over the past
several hours as the rain intensity has picked up. Vsbys and cigs
have been restricted at the coast, and expect this to continue
through tonight. So far, Vsbys have been the restriction for the
interior TAF sites. Expect MVFR cigs to eventually move into the
interior later this evening or overnight.

Gusty easterly wind through the Columbia River Gorge will keep
temperatures at or below freezing well into tonight for KPDX and
KTTD. This will lead to a period of freezing rain, with
substantial accumulations possible. KPDX will be the first to warm
up and switch over to plain rain, likely sometime overnight
tonight. KTTD may keep freezing rain well into Wed morning. Also,
with the gusty easterly winds at the surface and strong southerly
winds aloft, low-level wind shear will be a concern through
tonight.

KPDX AND APPROACHES...Precipitation has picked up steadily this
afternoon. Expect it to continue, heavy at times, through
tonight. Gusty easterly winds through the gorge are keeping temps
below freezing, so precipitation is falling as freezing rain.
Expect freezing rain to continue through much of tonight before
finally switching over to regular rain late tonight. Significant
ice accumulation is likely by the time of switchover. Expect MVFR
vsbys to continue through tonight, becoming IFR at times. Also
expect cigs to lower to MVFR later this evening or overnight.
Finally, with the gusty east winds at the surface and strong
southerly winds aloft, low level wind shear will be a concern
through tonight. Pyle

&&

.MARINE...Southerly winds have gradually picked up through the day
in response to a strong warm front moving through the waters. The
main frontal boundary will remain stalled to the northwest of our
coastal waters through the early Wed, which will lead to an
extended period of strong southerly winds. The buoy obs are
showing solid gale force winds as of 2 PM. Expect them to increase
further later this afternoon and this evening, with gusts of 45 to
50 kt likely over much of the waters. This is a setup that would
support some coastal jet enhancement, and expect localized gusts
to 55 kt or higher within 20 nm of the coast. The cold front will
push onshore Wed afternoon, allowing winds to subside from north
to south during the day. Expect winds to generally be 25 kt or
less by late Wed afternoon. Winds then remain much lighter Wed
night and Thu, although a surface trough moving through Thu may
require a small craft advisory for winds. Another system on Fri
may bring the potential for additional gales. The active weather
pattern continues through the weekend and into early next week,
although fcst details remain cloudy that far out.

Seas are steadily increasing today in response to the
strengthening southerly winds. The latest buoy obs show seas
around 16 to 19 ft. They should increase above 20 ft later this
afternoon, and peak at 22 to 26 ft this evening and overnight.
After a prolonged period of 20 ft, seas will gradually subside
along with the decreasing winds during the day Wed. Expect that
they will be in the low teens by Wed night or early Thu. Seas will
increase to near 20 ft again with the Fri system. The model
guidance then suggest that a large long-period westerly swell
arrives over the weekend. If things continue to track as expected,
high surf conditions will likely meet warning criteria over the
weekend. Pyle

&&

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

     Flood Watch from 7 PM PST this 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.

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

     Ice Storm Warning until 10 PM PST this evening for Greater
     Portland Metro Area.

     Freezing Rain Advisory until 6 PM PST this evening for Lower
     Columbia.

     High Wind Warning until 4 PM PST Wednesday for Central Coast
     Range of Western Oregon-Coast Range of Northwest Oregon.

     High Wind Warning until 10 AM PST Wednesday for Central Oregon
     Coast-North Oregon Coast.

WA...Ice Storm Warning until 6 PM PST Wednesday for Central Columbia
     River Gorge-South Washington Cascades-Western Columbia
     River Gorge.

     Flood Watch 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 until 10 PM PST this evening for Greater
     Vancouver Area.

     Freezing Rain Advisory until 6 PM PST this evening for I-5
     Corridor in Cowlitz County.

     High Wind Warning until 10 AM PST Wednesday for South
     Washington Coast-Willapa Hills.

PZ...Storm Warning until 10 AM PST Wednesday for Coastal Waters from
     Cape Shoalwater WA to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 4 PM
     PST Wednesday.
&&
$$

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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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