Area Forecast Discussion
Issued by NWS Portland, OR

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121
FXUS66 KPQR 221113
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
313 AM PST Mon Jan 22 2018

.SYNOPSIS...Cool onshore flow continues to spread showers across much
of southwest Washington and northwest Oregon this morning. Snow
levels will remain below the Cascade passes today, with a few
additional inches of snow expected. Showers will decrease later this
afternoon and evening, but the break will be brief as our next potent
Pacific frontal system arrives Tuesday for more rain, coastal wind,
and plenty of snow for the higher Cascades. The front is expected to
stall Tuesday night and Wednesday, leading to a prolonged period of
fairly steady rain and Cascades snow. The front will finally be
pushed east as a strong and cold upper trough moves into the Pac NW.
Numerous showers and a few thunderstorms are expected later Wednesday
into Thursday, with snow levels lowering into the foothills. Showers
linger into Friday, then another potent system appears likely to
arrive during the weekend.

&&

.SHORT TERM...Today through Wednesday...KLGX and KRTX radars show
showers filling in again this morning across southwest Washington and
northwest Oregon. The enhancement is likely due to a shortwave trough
moving onshore into Washington, and will last through mid morning
until warmer air aloft gradually stabilizes the air mass. Another 2
to 5 inches of snow are expected in the S WA/N OR Cascades by 10 AM.
Albeit marginal, the Winter Weather Advisory for those areas this
morning looks to be in good shape and will let it ride. Cancelled the
Lane County advisory a bit early since the main band of snow
associated yesterday`s frontal system pushed south of the area.

Weak, transitory ridging will slide across the Pac NW for a break in
the precip tonight. Given the wet weather we have had lately, we may
need to watch for fog tonight if any clearing materializes. Decided
against adding fog to the forecast as it appears high clouds will be
spreading onshore in advance of a strong frontal system expected to
impact the area Tuesday and Wednesday. The main low pressure system
associated with this front is just now taking shape out near
40N/150W, and is expected to rapidly strengthen and curl north toward
Haida Gwaii Tuesday, meanwhile pushing its occluding cold front
toward the coast.

Models have sped this system up a bit over the past several runs,
with precip now expected to spread across the forecast area Tuesday
morning. This has implications for the Hood River Valley and eastern
Skamania County, where easterly gradients ahead of the incoming
system will tend to lock cold air in place as precip arrives. While
surface temperatures are expected to hover near freezing, the column
appears cool enough to support snow levels around 1000 feet around
Hood River and Mount Adams Tuesday morning. Luckily, precip is
expected to start off light, so it appears the upper Hood River
Valley will only get an inch or two of snow before snow levels jump
to 3000-4000 feet Tuesday afternoon. A few wet snowflakes may mix
down to Hood River proper Tuesday morning, but no impacts are
expected.

Models are showing good agreement now that this system will have
decent access to subtropical moisture. The front is also expected to
stall over our forecast area through Wed as additional waves of low
pressure develop along it and slow the progress of the front. QPF is
expected to be in the 2 to 4 inch range for the higher terrain Tue
through Wed, with 0.75 to 1.5 inches for the inland valleys. While
rainfall doesn`t appear sufficient for significant hydro issues, it
does look to be enough to produce heavy snow for especially the
higher Cascades. Snow levels will likely rise above the Cascade
passes ahead of the front Tuesday, but the higher ski resort
elevations should see mainly snow, especially Santiam Pass northward.
Therefore 1 to perhaps 2 feet of snow may fall at these higher
elevations. We strongly considered a Winter Storm Watch for these
areas, but after some collaboration we and WFO Seattle decided to
hold off to avoid confusion with the event ongoing this morning.

Winds along the coast will be strong and gusty, but at this point it
appears most of the beaches and headlands will see gusts 45-55 mph;
just below warning criteria. Timing and location of above-mentioned
waves of low pressure along the front will be key to whether or not
winds are stronger. If low pressure develops along the front and
heads into southern Oregon before the front moves onshore, that would
cut down on the wind threat. On the other hand, if a low develops
along the front and strengthens while tracking up the Oregon coast
Tuesday (or Wednesday), winds would be stronger and may extend
further inland than currently expected. Models offer a variety of
solutions on where/when said waves of low pressure will develop and
track along the front, so we`re keeping the forecast less specific
for now.  Weagle

.LONG TERM...No Changes. Previous discussion follows...Wednesday
night through Sunday...Active weather pattern to continue through
Friday as a slow-moving trough keeps swinging moisture into the
Pacific Northwest.

Snow levels drop Wednesday night into Thursday to around 1500 ft,
then possibly lower by Friday morning.With 850-mb temps lowering
to around -6C on both the GFS and ECMWF, straight model guidance is
trying to bring snow levels down to 500-800 ft, but the pattern
doesn`t seem conducive for low-elevation snow (southerly winds and
onshore pressure gradient). Looking at the 1000-500-mb thicknesses,
under onshore flow, we typically need thicknesses below 522 dam for
snow levels below 1000 ft (local study). Both the GFS and ECMWF have
thickness around 522-524 dam. The ECMWF is the lower of the two,
with lowest thickness around 521.6 dam Friday morning. Because of
the pattern and the uncertainty with it being 6 days out, for now I
have limited snow levels to 1000 ft. "IF" this does pans out we may
see some snow mix down to around 800 ft; however, with precipitation
fairly light on Friday and ground temperatures above freezing, we
won`t see a lot of snow and it shouldn`t accumulate below 1000 ft.

Snow levels start to come back up Friday evening as a milder system
starts to move in with another surge of moisture. Saturday morning
will be pretty wet, but ridging starts to build Saturday afternoon,
pushing the moisture plume north by late Saturday night or early
Sunday. There is some uncertainty with how far north this ridge will
build. This will determine whether we dry out on Sunday. Both the
GFS and ECMWF send the plume of moisture north into northern
Washington and British Columbia, while the Canadian CMC sags the
moisture plume farther south over our area. Have leaned toward drier
pops, while keeping a chance due to uncertainty in how far north the
ridge will build over the West Coast. -McCoy

&&

.AVIATION...VFR conditions prevailing with isolated MVFR at the
coast and inland. Expect more MVFR through 20Z as a weak trough
moves from the coast to inland areas. VFR should prevail in the
afternoon and evening. Next front spreading rain to the coast
late this evening.


PDX AND APPROACHES...VFR prevailing but occasional MVFR can be
expected through about 20Z as a weak trough moves through. Also a
some cigs around 1500 ft developed under a break in higher cloud
cover. Do not this this will last long, but will have to monitor.
/mh

&&

.MARINE...A relatively quiet period today as weak high pressure
builds over the waters. But the next gales arrive to the outer
waters late tonight then closer to the coastline Tue morning. A
Gale Warning has been issued for this event. Seas do not look
quite as high on Tuesday as this past system with seas likely
peaking at or below 20 feet.

The next storm system of concern arrives Wednesday night into
Thursday. There is now a bit better model agreement now with an
occluding surface low tracking towards Vancouver Island. Gale
force winds are likely again, Seas will likely be a bit higher
and may exceed 20 feet given the track and strength of this low.

There is surprisingly good model agreement into the weekend
with enough confidence to expect another round of gale force
winds and high seas next weekend. The active pattern appears to
continues into early next week. /mh /Bentley

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 AM PST this morning for
     Northern Oregon Cascades.

WA...Winter Weather Advisory until 10 AM PST this morning for South
     Washington Cascades.

PZ...Small Craft Advisory for winds until 7 AM PST this morning for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 NM.

     Gale Warning from 7 AM to 10 PM PST Tuesday for Coastal Waters
     from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory for hazardous seas until 7 AM PST Tuesday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 10 NM.

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

     Small Craft Advisory for hazardous seas until 1 AM PST Tuesday
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 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. This area is
commonly referred to as the CWA or forecast area.



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