Area Forecast Discussion
Issued by NWS Portland, OR

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891
FXUS66 KPQR 201114
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
312 AM PST Fri Jan 20 2017

.SYNOPSIS...Rather persistent upper low will sit over the northeast
Pacific through Sunday, sending fronts across the region about every
18 to 24 hours. Most areas will see rain, with snow above 3000 feet.
But cold air in the Columbia Gorge and Hood River Valley will ensure
periods of light snow for that area. Showers decrease on Mon as the
upper low moves south into Calif. Then will see dry and seasonable
temperatures for Tue through end of next week.
&&

.SHORT TERM...Today through Sunday...Broad area low pres sits
offshore over the northeast Pacific, with west to southwesterly flow
aloft over our area. This will send a series of fronts into the
region, each bringing a period of steady rain, with showers between
the fronts. First front now over offshore and into far sw Oregon
early this am, but will lift northward today. over northwest Oregon
today. Will see rain spreading over all of the area this am, with
snow above 3000 feet. Generally, only expecting 2 to 5 inches in the
Cascades today, which is not enough snow to support any advisories.
Even so, travelers should be prepared for snowy conditions on the
higher elevations roads today and tonight.

At 2 am, the offshore pres gradients across the Cascades was at 5 mb,
and will strengthen more today. Though winds/flow will not be as
strong as earlier in the week, it will be enough to deepen the cold
air pool over east slopes and into the Columbia Gorge. Will be enough
to support some brief sleet or snow early this am for that area, but
should turn to snow quickly by mid-morning. Not a major precipitation
event, but enough to see 1 to 3 inches of snow for those areas. Will
maintain Winter Weather Advisory for the Columbia Gorge and Hood
River Valley.

Once this front shifts north later today, will see showers tonight
into Sat am, but next front will lift north across the region on Sat.
Overall, not much change to the air mass, so will see chilly rain for
the lowlands, and snow above 2500 or 3000 feet. Again, will expect to
see occasional snow in the Columbia Gorge and Hood River Valley, so
will extend the Winter Weather Advisory through Sunday.

Another likely sub-970 mb low sets up off the coast near bouy 5 again
Sat night and Sun, displacing the previous low. This new low will
send another front into the region on Sunday, with nearly a repeat of
what occurred on Sat, with chilly lowland rain and mountain snow.

When all said and done, seems reasonable to see total snowfall for
today through Sunday of 2 to 6 inches in the Columbia Gorge (heaviest
east of Cascade Locks), and 4 to 8 inches in the Hood Valley, as well
as higher elevations of the Coast Range. For the Cascades, generally
12 to 15 inches of new snow on the passes, with up to 2 ft of new
snow on high peaks.

Only issue for the weekend seems to be snow levels in Clark County
and North Willamette Valley as east winds will continue. Have lowered
snow levels a bit more, but will keep any accumulating snow above
2000 feet, meaning only potential would be ridges on the foothills
closer to the Cascades. But again, this bears watching. If offshore
is stronger than anticipated, could see snow levels down to 1000 to
1500 feet later Sat and Sun.       Rockey


.LONG TERM...Sunday night through Thursday...A low pressure system
moves into the Northern California Coast early next week. As it
skirts the Oregon Coast Sunday night, will probably see windy
conditions mainly along Oregon`s Central Coast. Showers will be
decreasing on Monday, with a decent chance Monday is dry for most of
the area. Left a slight chance of showers, though, in the event this
system tracks a little farther north than currently forecast. Upper-
level ridge moves over the area midweek, bringing dry conditions and
offshore winds starting Tuesday and continuing into Wednesday. Next
chance of precipitation may be on Thursday with a weak shortwave
moving up into Vancouver Island, but confidence is very low at this
time. -McCoy
&&

.AVIATION...VFR conditions continue through sunrise at most TAF
sites with occasional MVFR cigs arriving during the morning hours.
In addition, MVFR visibilities are possible in the heavier
showers. Winds are increasing at KTTD as offshore flow through the
Gorge strengthens. Elsewhere, current light and variable winds
will turn southeasterly and strengthen to near 10 knots.

KPDX AND APPROACHES...VFR conditions continue through sunrise.
Occasional MVFR cigs/vsby possible afterwards, especially during
stronger rain showers. /Bentley

&&

.MARINE...Extended period of active marine weather from this
morning through the weekend. An occluded front is moving through
the waters this morning. This front will increase winds to low
end gale force for a few hours between 3 AM and 7 AM. Strongest
winds will be beyond 30 nm offshore, so the gale force winds likely
won`t be seen at Buoy 50. Winds will remain southeasterly through
the day which will protect nearshore waters from small craft
winds. Therefore, kept the noon expiration for the small craft
advisory for winds in the nearshore waters. At the same time, the
gale warning will expire for the outer waters and will need to be
replaced
by a small craft advisory for winds and seas. With the
southeasterly winds continuing, a small craft advisory for winds
likely won`t be needed for the nearshore waters until Saturday
morning. At that time, a front will swing through the waters as
the surface low retreats northwestward.

Beyond Saturday, the forecast is more complicated as a seasonably
strong surface low moves up from the south and settles off the
Oregon shore through Monday. This system is quite complicated with
several pieces of energy phasing as the surface low strengthens
and eventually occludes off the Oregon coast. There is good model
consistency in a strong (sub 970mb) surface low in the general
area of the Oregon coast, but the exact proximity and strength
remain uncertain. With a surface low of this strength, storm force
winds are likely, but the location and timing of these winds won`t
become more clear until the aforementioned uncertainties are
ironed out.

Seas will remain elevated through early next week with two
distinct maxima on Saturday and Sunday. The long period swell
generated from the 975-980 surface low located near 46N and 143W
will arrive Saturday morning and elevated seas will remain through
Saturday evening. While the higher (>30 knot seas) will head
towards California, seas will likely exceed 20 feet for all of the
Oregon and south Washington waters. With a period of 18 seconds,
there will be a lot of force behind the seas Saturday. As a
result, wave energy flux values were plenty high enough to warrant
a high surf advisory for all Oregon and southwest Washington
beaches for the entire day Saturday. Even if periods come in a bit
shorter (16 to 17 seconds) or seas remain in the upper teens, wave
energy will still be strong enough that High Surf conditions will
likely be observed.

On Sunday, ENP guidance has now brought seas up close to 20 knots
as the strong surface low occludes offshore. These seas seem a bit
low considering how strong the surface low will be and the fact
that some locations will likely observe hurricane force wind gusts
in close proximity to the surface low. However, given the nearly
stationary nature of the surface low, swell will be mostly fresh
and there won`t be any dynamic fetch to help build the higher
seas, so decided to keep seas in line with guidance at this time.
/Bentley

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 4 AM to 7 PM PST Saturday for Central
     Oregon Coast-North Oregon Coast.

     Winter Weather Advisory until 6 pm Sunday for...
 Central Columbia River Gorge
 Western Columbia River Gorge
 Upper Hood River Valley.

WA...High Surf Advisory from 4 AM to 7 PM PST Saturday for South
     Washington Coast.

     Winter Weather Advisory until 6 pm Sunday for...
 Central Columbia River Gorge
 Western Columbia River Gorge
 Upper Hood River Valley.

PZ...Small Craft Advisory for Rough Columbia River Bar until 4 AM
     PST Saturday.

     Gale Warning until noon PST today for Waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 nm.

     Small Craft Advisory for winds until noon PST today for Coastal
     Waters from Cape Shoalwater WA to Florence OR out 10 nm.

     Small Craft Advisory for hazardous seas until 4 AM PST Saturday
     for Coastal Waters from Cape Shoalwater WA to Florence 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. The area is
commonly referred to as the forecast area.



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