Area Forecast Discussion
Issued by NWS Portland, OR

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325
FXUS66 KPQR 060503
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
903 PM PST Mon Dec  5 2016

.SYNOPSIS...A surface low offshore of the south Washington coast will
move south along the Oregon coast tonight and continue showers for
southwest Washington and northwest Oregon through Tuesday morning
with snow levels remaining very low. Dry and cool north winds are
expected Tuesday and Tuesday night as an upper ridge builds over the
area and interior valleys may have freezing fog early Wednesday
morning. Strong cold east winds Wednesday will set the stage for a
winter weather event Wednesday night and Thursday as a front moves
across the area. Active weather continues into early next week.
&&

.SHORT TERM...Tonight through Wednesday...Satellite imagery shows a
surface low around 190 miles offshore of Astoria this evening.This
low will continue to feed showers across the region tonight as a
cold upper level shortwave with 500mb temps near -38 deg C (-36degF)
also moves over the area. Overnight temperatures will be near
freezing in most locations and snow levels will remain very low (less
or equal to 1000 feet).

The showers should be fairly light though and do not expect
accumulations below 700 feet. The showers should decrease from north
to south late tonight through Tuesday morning. The showers will
likely linger across the Lane County Cascades and possibly the
Central Oregon coast range into Tuesday afternoon, and have extended
the winter weather advisory for the Lane Cascades through noon
Tuesday. Have made a few other minor changes to tonight`s winter
weather advisories. See PDXWSWPQR for further details on the expected
snow amounts, timing, and areas of snow.

If temperatures dip to or below freezing with any clearing between
showers, moisture on surfaces may freeze and patchy black ice may
form on roadways and sidewalks. This could result in unexpected slick
surfaces. TJ

The remainder from the previous discussion(318 PM Monday)...Drier
northerly flow will spread in from the north on Tuesday as the
surface low moves south with drying. A few showers may linger in Lane
County until late in the day.

An upper ridge will build in Tuesday night and Wednesday with strong
offshore flow developing especially on Wednesday. The winds will be
quite strong especially along the Columbia River and through the
Portland and Vancouver metro areas. After some areas of morning
valley freezing fog, the day will remain on the cold side in the
afternoon with some wind chill affecting comfort. This will also set
things up for the winter weather mix expected Wednesday night and
Thursday that has been moved to the next section for this issuance.
Tolleson

.LONG TERM...No Changes. Previous discussion follows...Thursday
through Monday...Precipitation will begin as snow for most areas
Wednesday night with a few inches of accumulation expected before the
transition to ice begins. Currently, have precipitation all rain
along the immediate coast on Wednesday night, but given the strength
of the offshore flow, it is possible that the column will remain cool
enough for a quick round of snow even at the beaches. Will wait until
the shorter term period with higher resolution models to better
diagnose the possibility of coastal snow.

The Thursday morning commute is looking quite treacherous as an
atmospheric river event overruns the cold air in place over the
area. Precipitation will change to freezing rain during the late
morning to early afternoon. Don`t anticipate freezing rain sticking
around too long for the south and central valley as winds switch
southerly pretty quickly. Despite the short duration, a few tenths
of an inch are possible on top of the inch or 2 of snow which will
cause significant issues. The higher end impacts will be in the
Columbia River Gorge and eastern Portland metro areas. This is where
there is still a lot of uncertainty with regard to the duration of
the freezing rain. Models have trended later into the day Thursday
with the east winds continuing. It is hard to latch on to a specific
solution at this time given the model uncertainty about the
cyclogenesis offshore.

The ECMWF ends the east winds through the Gorge quicker as it has a
weak surface low which comes onshore near Astoria at 00Z Friday.
This surface low coming on the coast will displace the Columbia
Basin high pressure and end the supply of cold air. The GFS, GEM,
and NAM all have a stronger surface low tracking somewhere towards
the Vancouver Island/Olympic Peninsula area. This is the preferred
solution at this time, especially since yesterday`s 12Z ECMWF showed
a similar solution. Even though there are 3 models in agreement with
a stronger surface low, the exact track of this system is still very
important. The GEM and NAM track the surface low further northwest
than the GFS. As a result, the high pressure in the Columbia basin
would be able to sustain itself much longer without a low pressure
center or strong front kicking it out. At this time, have kept the
ongoing forecast more in line with the GFS for the Portland metro
area which has snow through late morning followed by freezing rain
through the afternoon and changing over to all rain sometime during
the evening Thursday. Am not necessarily convinced the GFS is the
correct one, however, this solution is a good compromise between the
possible solutions at this time.

It is worth mentioning that if the more pessimistic solutions
verify, NAM or GEM, the Columbia River Gorge and even the eastern
Portland metro could see a major ice storm. It is still a bit early
to jump on that solution, but it is important to mention that
possibility as agencies prepare for this winter event. Especially
given the trend in the models towards longer duration east winds and
the tendency for east winds in the Gorge to sustain longer
than models forecast.

While there is plenty of potential for frozen precipitation impacts,
another impact which confidence is higher for is strong east winds
through the Gorge Wednesday Night. The DLS to PDX gradient is
modeled between 8 and 10 mb. Models tend to under forecast this
pressure gradient by a few mb, therefore expect the gradient to top
out over 10mb. This type of gradient would bring mid 40 mph wind
gusts to the eastern Portland metro area with winds likely
approaching 100 mph at Crown Point. As if these winds weren`t
impactful enough, as ice begins to accumulate on branches and
powerlines, impacts could quickly snowball. To reduce confusion with
an ongoing event and winter weather advisories currently out, will
not issue a high wind watch at this time. A high wind watch will
likely be needed for at least the Gorge at some point given our
fairly high confidence about the strong winds.

Beyond Thursday, the big story will be continuing snow in the
Cascades. Several more feet of snow are likely between Thursday and
Sunday as freezing levels stay below 3000 feet and fronts continue
to move on shore into the weekend. Snow may even impact a few of the
Coast Range passes as snow levels dip to near 1500 feet at times.

/Bentley
&&

.AVIATION...As of 0430z...Numerous showers continue to stream
across the region. They are bringing a mix of conditions ranging
from patchy IFR cigs in the upper Tualatin Valley near KHIO to
patchy VFR. Largely MVFR Cigs are the norm, however. Will need to
watch a more organized batch of showers just off the coast
associated with a weak low pressure system. At the moment, it
appears these will fall primarily as rain or a rain/wet snow mix
as they move through later tonight/early Tue morning. However,
heavier showers may briefly bring the snow level down to the
terminals, with the best chance being south of a KUAO-KAST line.
Any snow accumulation would likely be less than 1 inch. The cloud
cover from these showers may actually help keep temps slightly
above freezing more than anything. Areas north of KUAO-KAST
should see some clearing after about 10z, which could help temps
fall below freezing by sunrise...especially for the more outlying
terminals like KHIO and KKLS. Conditions will slowly improve
Tuesday, with forecast models suggesting stubborn IFR lasting well
into the day at KHIO...but somewhat faster improvement at other
terminals.

KPDX AND APPROACHES...Temperatures remain in the mid to upper 30s
this evening, with primarily rain showers continuing to move
across the PDX metro. Cigs/Vsbys likely to be close to 1000 ft
much of tonight, tending toward IFR in showers and MVFR outside of
showers. More organized batch of showers just offshore appears to
be headed just south of the metro area; mainly south of a KUAO-
KAST line. Enough cloud cover will likely remain intact to hold
temps above freezing for much of the night. However, some gradual
clearing after 10z may allow temps to briefly dip below freezing
by sunrise. Given dewpoints in the mid 30s and a fairly warm
ground...would only give about a 20% chance of temps falling below
32 deg F long enough to cause runway icing issues tonight/Tue
morning. Trending toward VFR Tue morning with the first hard
freeze of the season expected Tue night. Weagle

&&

.MARINE...Some gusts 20-25 kt still occasionally being reported by
coastal waters buoys, but these are primarily outflow from showers
moving across the waters. Thus we allowed the SCA for winds expire
earlier in the shift. There will then be a relative lull in the
winds for a couple of days. A low will slide by just outside the
waters and into S Oregon tonight into Tue without having much
impact for our waters. The next significant frontal system will be
approaching Wed night and moving through early Thu. Models have
diverged again but have some agreement with bringing a strong
frontal system and a period of very gusty offshore winds
Wednesday. It`s then followed be the cold front late Wednesday and
early Thursday. The offshore winds could be strong enough to
produce low-end gales in portions of the coastal waters. Gusty
post-frontal winds likely continue into early Fri.

A westerly swell train will continue to impact the waters, keeping
seas around 11 to 13 ft before dropping below 10 ft Tue evening.
The late week frontal system should push seas at least back into
the mid teens, with the potential for 20 ft seas or higher
depending on the track of the low and how strong the winds end up
being.Weagle/JBonk/Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 9 PM PST this evening for Central
     Columbia River Gorge-Upper Hood River Valley-Western
     Columbia River Gorge.

     Winter Weather Advisory until 3 AM PST Tuesday above 700 feet
     elevation for Central Willamette Valley-Coast Range of Northwest

     Oregon-Greater Portland Metro Area-Northern Oregon Cascade
     Foothills-Northern Oregon Cascades-South Willamette Valley.

     Winter Weather Advisory until 10 AM PST Tuesday for Cascade
     Foothills in Lane County-Cascades in Lane County.

     Winter Weather Advisory until 6 AM PST Tuesday for Central
     Coast Range of Western Oregon.

WA...Winter Weather Advisory until 9 PM PST this evening for Central
     Columbia River Gorge-South Washington Cascade Foothills-
     Western Columbia River Gorge-Willapa Hills.

     Winter Weather Advisory until 3 AM PST Tuesday for Greater
     Vancouver Area above 700 feet elevation.

PZ...Small Craft Advisory for hazardous seas until 4 PM PST Tuesday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

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

     Small Craft Advisory for Rough Columbia River Bar from 7 AM to
     11 AM PST Tuesday.
&&
$$

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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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