Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 232238

Area Forecast Discussion
National Weather Service Portland OR
336 PM PDT Fri Mar 23 2018

.SYNOPSIS...Showers will continue through the weekend with snow
levels primarily above 1000 feet. However, these snow levels may drop
to around 500 feet during the morning hours each morning. There will
be a chance of thunder at times especially near the coast through
Saturday, and possibly inland at times in the afternoons and
evenings. An upper ridge will build in the NE Pacific early next week
with brief periods of light precipitation possible. The driest period
will be the middle portion of the week.

.SHORT TERM...Today through Monday...Snow levels have risen to 1000
to 1500 feet today as the March sun has warmed the boundary layer
sufficiently. However, once the sun goes down tonight, expect snow
levels to drop similar to this morning. The primary driver for the
dropping snow levels will be precipitation rates. 18Z NAM and GFS as
well as the 19Z HRRR all show heavier precipitation rates this
evening and overnight than previous runs. This is supported by the
current radar and satellite imagery which also shows these heavier
rates. The bulk of the heavier precipitation rates will be in the
south Willamette Valley and in the Cascades, but some guidance (19Z
HRRR) shows the precipitation further northwest which would impact
most of western Oregon. Therefore, elected not to end any of the
current advisories and extended them into the early morning hours.
Expect conditions late this evening and during the early morning
hours to be similar to this morning with all snow above 1000 feet,
some accumulating snow on grassy surfaces between 500 and 1000 feet,
and some snowflakes mixed with the rain below 500 feet, but any of
the heavier bands which set up could bring the snow level down to sea
level. Therefore, issued an SPS to highlight this potential, but do
not expect much impact for the lowlands, especially since the time of
greatest risk is during the late evening and overnight tonight when
travel should be limited. Expect snow levels to quickly rise above
1000 feet again by mid morning tomorrow which will eliminate any risk
for low level snow.

Showers will continue through the day Saturday and Saturday night and
Sunday morning will provide another opportunity for snow levels to
drop below 1000 feet. However, precipitation rates will be light
which may be an inhibiting factor for snow reaching to near
sea-level. However, any periods of clearing could also make lower
snow levels a possibility.

Showers will continue Sunday and Monday, but snow should be limited
to areas above 1500 feet as the cold upper trough starts to shift
east of the area. /Bentley

.LONG TERM...Monday night through Friday...An upper ridge will
dominate much of the long term period, however, our area will remain
on the southern edge of the "ridge runner" weak disturbances which
will be traversing the ridge. This will keep at least a slight chance
for precipitation most days, and the upper clouds will limit the
warming that may be seen otherwise. However, temperatures are still
expected to warm into the upper 50s to lower 60s. The best chance for
a completely dry day next week still appears to be Thursday. Beyond
this ridge, models are in agreement with a longwave trough
re-establishing across the Pacific NW. This would keep the cool
pattern well into April with additional chances for late season
foothill snow, and below normal temperatures. /Bentley

.AVIATION...An upper level low and associated cool and showery
air mass remain off the Pac NW coast through the weekend.
Current radar imagery shows plenty of showers across the region
this afternoon, with conditions a mix of VFR and MVFR. Expect
little change through the rest of the afternoon. A shortwave
trough embedded in the main low will rotate onto the southern
Oregon coast this evening, bringing a band of more organized
shower activity through NW Oregon and SW Washington from south to
north. Expect conditions will become predominantly MVFR this
evening with the steadier showers, with some localized IFR not
out of the question. Showers will continue into the overnight
hours, but should become more scattered towards Sat morning.
Expect MVFR cigs to remain at least into early Sat morning, and
potentially through most of the morning. There will remain a
chance of thunderstorms at the coast through the next 24 hrs,
with the best chances during the evening and early overnight

PDX AND APPROACHES...Expect off and on showers with mainly VFR
conditions through the afternoon. Steadier showers and MVFR
conditions likely to return between 00Z-03Z this evening and
continue through much of the night. The showers should taper off
somewhat by Sat morning, but expect MVFR cigs will linger through
at least mid-morning. Pyle


.MARINE...Broad and showery low pressure will remain off the Pac
NW coast through the weekend. Southwesterly winds should subside
to well below 20 kt this evening and through much of the
overnight period. However, a decaying surface low is modeled to
dive SE toward the southern OR coast overnight into Sat. There
has been some concern that this system will bring gale force
winds to the central OR waters. However, based on the latest
model runs, decided to drop the gale watch and go with a small
craft advisory for all of the waters starting early Sat morning.
The models are showing some boundary layer winds in the 30-35 kt
range in the far southern portions of PZZ255/PZZ275. So there
could be some isolated gale gusts south of Newport, but do not
think that they will be frequent enough for a gale warning. Also,
the small craft advisory winds will likely only extend to the
southern portions of PZZ250/PZZ270. North of around Tillamook
Head, it looks like winds will remain E-SE at 10-15 kt.

Winds will subside later Sat and Sat night as the low weakens and
moves onshore. Another weak surface trough is expected to move
through the waters Sun, but may not even be strong enough to
produce small craft winds. Then higher pressure builds over the
NE Pac for most of next week, which will lead to a period of much
more benign conditions. A thermal trough is expected to build
into northern CA towards the middle of next week, which will
bring northerly winds to the waters.

Seas will remain a mix of NW swell and fresh S swell through Sat.
They are expected to remain in the 10-12 ft range into this
evening. They may then briefly drop just below 10 ft overnight,
before increasing into the 10-12 ft range again for much of Sat.
They then subside well below 10 ft Sun and remain below 10 ft for
most of next week. Pyle


OR...Winter Weather Advisory until 5 AM PDT Saturday for Cascades in
     Lane County.

     Winter Weather Advisory until 1 AM PDT Saturday for Cascade
     Foothills in Lane County-Central Coast Range of Western
     Oregon-Coast Range of Northwest Oregon-Northern Oregon
     Cascade Foothills-Northern Oregon Cascades.

PZ...Small Craft Advisory for winds until 5 PM PDT this afternoon
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 NM.

     Small Craft Advisory for winds from 5 AM to 5 PM PDT Saturday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 NM.

     Small Craft Advisory for hazardous seas until 5 PM PDT Saturday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 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. is the U.S. government's official web portal to all federal, state and local government web resources and services.