Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 181706
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
906 AM PST Mon Dec 18 2017

.SYNOPSIS...Not much change today, as generally cloudy and dry, with
rain at times north of a Tillamook to Portland line. A strong cold
front will arrive on Tue, spreading rain and mountain snow into the
region, along with breezy south winds. Snow levels drop dramatically
later Tue afternoon and Tue evening, with heavy snow in the Cascades.
This will bring occasional snow down to the foothills and even the
Coast Range as well. Then cool and showery for Wed. Turns dry on
Thursday, but will be colder. Cold and generally dry weather
continues through the coming weekend.

&&

.SHORT TERM...Today through Wednesday...Made only minor changes with
the morning update. The changes being increased today`s temperatures
a couple of degrees for the interior valleys, increased todays QPF
for extreme SW Washington, and reduced sky cover slightly to the
south and the Willamette Valley. ~TJ

The rest of the discussion is from the previous issuance sent at 323
AM. Rather persistent baroclinic zone remains nearly stationary over
western Washington this am. This has been bringing occasional light
rain to areas north of a Tillamook to Mt St Helens line, with just
spotty light rain at times just to south of that line. Should stay
dry, but still lots of clouds for areas to the south, such as Newport
to Salem and southward. Not much change in this pattern today into
this evening

Now, low pressure will start strengthening well offshore tonight, and
will head northeast tonight and Tue. Models fairly good agreement
that this low will move onto the north Washington coast and across
the Olympics Tue am. This low will flex a warm front across the
region later tonight and early Tue am. But, it will also push a very
strong cold front to the coast tomorrow morning. This front will
push east to the Cascades by mid to late afternoon. Appears pres
gradients increase late tonight into Tue am, but do not think will
get warning type winds. Rather, would expect south to southwesterly
winds will gust of 45 to 55 mph for the coastal areas, with gusts 30
to 35 for the interior lowlands.

Rainfall will be increasing later tonight into Tue am, with fairly
decent shot of rain for most areas. Generally, 1 to 3 inches of rain
for the coast/Coast Range/Willapa Hills and the South Washington
Cascades. Bit less to the south over the higher terrain, and
generally 0.75 to 1.00 inch for the interior lowlands.

Unlike many fronts, appears that bulk of the precipitation will be
along and behind the front. This has some concern as think about
snow. Snow levels will start Tue up around 5000 to 6000 feet, but
drop rapidly behind the front. Snow levels down around 2500 to 3000
feet by late afternoon, and lower down to near 1500 feet later Tue
evening. With the heavier precipitation arriving, should see heavy
snow Tue into Tue evening for the Cascades. Snow accumulations will
slow down later Tue night as precipitation turns more showery, or
more hit and miss. With high confidence in snowfall potential, will
issue Winter Storm Warning for the Cascades, and make it time
staggered, with warning starting later tonight for the south
Washington Cascades, and Tue afternoon for Lane County Cascades.

As snow levels lower, should get some snow accumulations across the
foothills of the Cascades, Willapa Hills, and Coast Range Tue night.
At this time, now watches or warnings for those areas, but suspect
will need a snow advisory, as those areas will likely get some
accumulations Tue night into early Wed am.

Cool Wed, with snow levels 1500 to 2000 feet. Showers will be
decreasing in the afternoon. Again, will get few more inches of snow
in the Cascades, with minor accumulations for other higher terrain.
Even areas down as low as 1000 feet may see some wet snowflakes in
the am, if a heavier shower passes overhead.       Rockey.


.LONG TERM...Wednesday Night through Sunday...Made significant
changes in the long term forecast to account for much colder airmass
due to retrogression of the longwave ridge which has dominated our
weather for the past 2 weeks. This will allow several reinforcing
shots of cold air to move into the Columbia Basin and lead to an
extended period of colder than normal temperatures.

The long term period will begin with dry conditions as building
heights will quickly bring any rain and snow showers to an end
Wednesday evening. This dry period will continue through Thursday
night before a reinforcing shot of cold air arrives on Friday. While
models have been showing this shortwave for several days, today`s 12Z
model runs are all more consolidated with the upper energy and dig
the shortwave further west than previous runs. Therefore, despite the
surface low having a similar track to the weak surface wave on
Wednesday, this wave will be bringing colder air aloft. While current
guidance is still keeping snow levels just above the valley floor
(500 to 1000 feet), this front will bring a much better chance for
the lowlands to see a few snowflakes mixing in than the Wednesday
system, especially once the winds become northeast as the surface low
moves by to the west. Beyond Friday, dry continental air will end any
precipitation chances and the primary focus will shift toward an
increasingly arctic airmass settling into the area.

While operational models have only recently shifted the upper ridge
westward and hinted at the cold air outbreak making it west of the
Rockies, ensembles had been showing this solution for several days
now which gave enough confidence to significantly lower temperatures
next weekend. While the forecast low for next Saturday is now near
the GFS ensemble mean, it is still warmer than many of the 12Z
ensemble members. 2/3rds of the members have overnight lows between
15F and 18F while the ensemble mean is elevated due to 2 extremely
warm members (34F and 40F). Further retrogression of this ridge could
lead to even colder temperatures than currently forecast, especially
given the projected magnitude of this arctic high. While a 1060 mb +
(Or 1070 mb if you believe the GEM or JMA) high pressure center seems
unlikely, it is worth noting that ECMWF also has a 1063 mb high and
the GFS only a few mb shy of 1060 mb. The exact magnitude of this
high is not important, but it does show the intensity of the arctic
air moving south along the front range of the Rockies. In addition,
given the extensive snow cover over most of British Columbia, there
will be very little modification of the airmass before it arrives in
the Pacific NW. Given the model trend for retrogression of the upper
ridge across the NE Pacific, would not be surprised if the Columbia
Basin high ends up being as strong or stronger than the surface high
east of the Rockies in Montana. In addition to the cold dry air this
weather pattern would bring to our area, this will bring concerns for
strong gap winds by early next week as cross Cascade gradients
increase. /Bentley

&&

.AVIATION...A weakening front lay over the north Oregon coast
early this morning and will likely stall some where between
Garibaldi and Lincoln City. Expect that this will result in
continuing MVFR cigs over the interior, especially for areas
north of KSLE. South of KSLE may more of a mix of MVFR and VFR.
The coast should remain a mix of MVFR/IFR. The stalled front will
lift north as a warm front beginning this evening, which will
likely result in conditions being slow to improve. There should
be some lifting of the cigs from south to north during the late
afternoon and evening, then lowering late tonight as the cold
front approaches. Coastal areas will become windy late tonight
with southerly gusts 30 to 40 kt possible along the coastline.

PDX AND APPROACHES...MVFR cigs likely to persist into the
afternoon, then gradually improve to VFR. Occasional light rain
is expected through the next 24 hrs. MVFR possible late tonight.
/mh

&&

.MARINE...A weakening front lay west to east between Garibaldi
and Cascade Head early this morning. The front will remain nearly
stationary today then a low will develop on the front tonight and
Tue. The front is forecast to track towards the north or central
WA coast making landfall as a 996-998 mb low Tue morning. The
stalled front will lift north as a warm front tonight. A strong
cold front follows early Tue which will bring solid gales to the
waters. Somewhat favorable surface pressure pattern for a
coastal jet but model soundings show lower layers remain mixed.
However with this mixing winds near 1500 ft of 45 to 50 kt may
surface as the front nears. Peak winds for the coast line most
likely between 7 AM and noon. Should see gales end for all water
by early to mid afternoon Tue, but will have the warning through
4 PM to account for potential delay. A secondary low is modeled
to swing down Tue night and Wed, which might NW post frontal
winds on the gusty side. Then winds decrease considerably later
Wed through Fri as higher pres builds into the waters.

Seas beginning to subside and are in the 7 to 9 ft range early
this morning. The strong front tonight and Tue is expected to
push seas into the mid teens, where they will likely remain
through Wed. Seas subside below 10 ft for Thu and Fri. Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning for...
      North Oregon Cascades for Tuesday through Tue night.
      Lane County Cascades for Tue afternoon through Tue night.

WA...Winter Storm Warning for the south Washington Cascades from
     midnight tonight through Tue evening.

PZ...Gale Warning from 1 AM to 4 PM PST Tuesday for Waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 NM.

     Gale Warning from 4 AM to 4 PM PST Tuesday for Coastal Waters
     from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 AM
     PST early this morning.

     Small Craft Advisory for Rough Columbia River Bar from 3 PM
     this afternoon to 7 PM PST this evening.

&&

$$

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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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