Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 240012
AFDPQR

Area Forecast Discussion...
National Weather Service Portland OR
411 PM PST Fri Feb 23 2018

.SYNOPSIS...Chilly air mass will remain entrenched over the region
today into early next week. Another disturbance over the Gulf of
Alaska will spread precipitation across the region later this
afternoon and tonight, with snow levels still rather low. But, much
stronger system arrives on Sunday, with rain for the lowlands, and
very heavy snow and winds in the Cascades. This cool and active
weather continues next week with more chances for low elevation snow.

&&

.SHORT TERM...Tonight through Saturday...Mid and high level clouds
have been thickening up today in advance of the next weather
disturbance headed our way from the northwest. The very chilly start
to the day, combined with the increasing cloud cover has kept high
temps today in the mid to upper 30s through most of the interior
lowlands, with low 40s at the coast.

A surface low along the British Columbia coast this afternoon is
bringing a warm front toward the Pac NW coast. Isentropic lift
associated with this front has been spreading increasing light
precipitation into the region from the NW over the past several
hours. Even though surface temps are generally well above freezing,
the airmass over the region is still quite cold, in the -7 to -9 C
range. This should ensure that precipitation will start out as snow
or a rain/snow mix down to the lowest elevations. We expect some
light accumulations, maybe an inch or two, for portions of the
lowlands through this evening as precipitation intensity increases
and evaporational cooling brings surface temps down closer to
freezing. Accumulations should be mainly confined to grassy surfaces
and at elevations above 500 feet. For elevations below 500 feet,
accumulations look unlikely at this time. Southerly winds of 10 to 15
kt are expected to continue through the Willamette Valley tonight,
which will prevent much in the way of diurnal cooling. Evaporational
cooling may cool things enough for a dusting at the lowest
elevations, but not much more. Snow levels are expected to rise up
closer to 1500 feet after midnight as the warm front aloft moves
through and moderates the air mass somewhat. This will end the
threat for snow for the lowlands for the weekend. Expect significant
mountain snowfall overnight into Sat morning as a sharp upper level
shortwave moves through the region. Winter weather advisories are in
effect for the Cascades, Cascade Foothills, and coastal mountains for
this time period.

Precipitation will taper off Sat afternoon and evening as the upper
level trough moves out of the CWA. However, orographic snow will
continue in the Cascades during this time. Another cold trough will
dive south into the Pac NW Sat night and Sun. This will bring another
round of significant mountain snowfall, with snow levels generally
hovering around 1500 to 2500 ft during this time. We have issued
winter storm watches for the Cascades and Cascade foothills. The
coastal mountains may also eventually require a warning depending on
what snow levels turn out to be. Snow totals of
1 to 2 feet are expected for the Cascades and higher elevations of
the foothills during the 24-hr period from Sat night through Sun
night. Strong westerly winds will bring gusts 40 to 50 kt or higher
above the treeline, leading to very poor visibilities traveling over
some of the mountain passes.

Precipitation begins to taper off Sun night into Mon morning. Colder
air arriving with the trough may again bring snow down into the
lowlands going for Mon morning. At this time, precipitation amounts
during this time look to be quite light. Significant impacts are not
currently expected, but some light accumulations may be possible
into the some of the hills around the lowlands by Mon morning.
Showers will continue to decrease through the day Mon, and clouds may
try to clear from north to south in the afternoon. but it will be
another chilly day. Pyle

.LONG TERM...Monday night through Friday...Fairly active pattern
will continue through the long range. Operational 12Z GFS and ECMWF,
when compared to the ensembles, are in fairly good agreement
regarding the overall large-scale details. Another short-wave drops
into the area from the northwest Mon night and Tue. This system will
have a more maritime influence. Snow levels Mon night will generally
be 1000 to 1500 feet, although down to 500 feet in the Gorge and
East Skamania county. Model 1000-500 mb forecast thickness values
will be in the 525-530 dm range with 850 mb temps around -5C. This
system slides through the forecast area Tue. QPF amounts do not look
all that impressive, but could be enough for snow advisory amounts
in the S Washington and N Oregon Cascades.

GFS 500 mb spaghetti plots start to show more variation beginning
Wed. A deepening upper level trough along the British Columbia Coast
will allow a more moderate west to northwest 500 mb flow to impact
the area. A leading short-wave is expected to reach the area Wed
night. The ECMWF indicates a complex surface low pressure off
Vancouver Island 12Z Thu with an 8-10 mb KAST-KOTH south gradient.
The operational 12Z GFS is much different, with a closed low off the
Central Oregon coast and offshore low-level flow through the Gorge.
The NCEP GFS ensembles valid 00Z Thu are all over the place,
suggesting low confidence in the general forecast. Would tend to
favor the ECMWF at this time and have trended the forecast in that
direction. Eventually, the large upper level trough takes up
residence over the NE Pac 00Z Fri. This would maintain an
unseasonably cool air mass over the region, with snow levels
generally 1000-1500 ft. There is better agreement between the GFS
and ECMWF Thu night and Fri, leading to better forecast confidence.
All in all, the extended period will continue the Cascade snowpack
improvement, but not expecting any additional valley floor snowfall
during the period.  Weishaar
&&

.AVIATION...A cold and dry air mass currently in place as an
incoming front and attendant band of precipitation spreads across
the forecast area this afternoon and evening. This will result
in VFR cigs this afternoon deteriorating into MVFR tonight and
Saturday, possibly even locally IFR. Precipitation inland will
likely start as flurries this afternoon, with snow increasing
this evening, turning to rain by Saturday morning. Coastal
locations may see a rain/snow mix this afternoon and evening
before changing to rain. Best chance of snow accumulations
tonight will be KSLE northward.

PDX AND APPROACHES...VFR cigs this afternoon and possibly some
flurries. The snow will increase this evening with 1-2 inches
possible before changing to rain by sunrise Saturday. Conditions
will lower to MVFR and possibly IFR tonight, then back to MVFR
Saturday and possibly even VFR cigs later Sat. pt
&&

.MARINE...The next system was dropping southward from the north
across the waters this afternoon and will continue to do so
tonight. The main low pressure area stays north of the waters
near Vancouver Island through Saturday with Small Craft Advisory
threshold winds over our waters, except gales in the northern
waters late tonight and Saturday morning, especially over the
outer waters. There is a bit of a lull in the winds later
Saturday and Saturday evening before another system drops south
and brings more Small Craft Advisory threshold winds Sunday
except gales are again pretty likely over the northern waters
later Saturday night and Sunday morning.

This series of systems from today through the weekend produces a
nice northwesterly fetch off the B.C. coast that will likely lead
to seas in our coastal waters being well up in the teens through
the weekend. pt
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 8 AM PST Saturday for Central
     Columbia River Gorge-Western Columbia River Gorge.

     Winter Weather Advisory until 4 PM PST Saturday for Northern
     Oregon Cascade Foothills.

     Winter Weather Advisory from 6 PM this evening to 4 AM PST
     Sunday for Cascades in Lane County-Northern Oregon
     Cascades.

     Winter Storm Watch from late Saturday night through Sunday
     evening for Cascades in Lane County-Northern Oregon
     Cascades.

     Winter Weather Advisory from 6 PM this evening to 4 PM PST
     Saturday for Cascade Foothills in Lane County.

     Winter Weather Advisory until 4 AM PST Saturday for Greater
     Portland Metro Area.

     Winter Weather Advisory until 4 AM PST Saturday for Lower
     Columbia.

     Winter Weather Advisory until noon PST Saturday for Coast Range
     of Northwest Oregon.

     Winter Weather Advisory from 6 PM this evening to 4 AM PST
     Saturday for Central Willamette Valley.

WA...Winter Weather Advisory until 8 AM PST Saturday for Central
     Columbia River Gorge-Western Columbia River Gorge.

     Winter Weather Advisory until 4 PM PST Saturday for South
     Washington Cascade Foothills.

     Winter Weather Advisory from 6 PM this evening to 4 AM PST
     Sunday for South Washington Cascades.

     Winter Storm Watch from late Saturday night through Sunday
     evening for South Washington Cascades.

     Winter Weather Advisory until 4 AM PST Saturday for Greater
     Vancouver Area.

     Winter Weather Advisory until 4 AM PST Saturday for I-5
     Corridor in Cowlitz County.

     Winter Weather Advisory until noon PST Saturday for Willapa
     Hills.

PZ...Small Craft Advisory for winds until 1 AM PST Saturday for
     Waters from Cape Shoalwater WA to Cascade Head OR from 10
     to 60 NM.

     Gale Warning from 1 AM to 10 AM PST Saturday for Waters from
     Cape Shoalwater WA to Cascade Head OR from 10 to 60 NM.

     Small Craft Advisory for winds until 3 PM PST Saturday for
     Coastal Waters from Cascade Head OR to Florence OR out 60
     NM.

     Small Craft Advisory for hazardous seas from 1 AM to 3 PM PST
     Saturday 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.

     Small Craft Advisory for Rough Columbia River Bar from 9 PM
     this evening to 6 PM PST Saturday.

     Small Craft Advisory for winds until 3 PM PST Saturday for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 NM.

&&

$$

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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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