Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 200954

Area Forecast Discussion
National Weather Service Portland OR
254 AM PDT Tue Mar 20 2018

.SYNOPSIS...Rather quiet weather with morning clouds and fog in the
valleys today under weak high pres. But, a low off the coast will
spread clouds back across the region later today and tonight, with
rain for Wed into Wed night. Then cool and showery weather for Thu
through at least Sat, with snow levels down at 1500 to 2000 feet.

.SHORT TERM...Today through Thursday...Overall, fairly quiet early
morning across the region. Mid and high clouds continue to skirt
across Lane and Linn counties, along with occasional flurries over
the Cascades. Elsewhere, mostly clear skies. Will have areas of low
clouds and fog this am, with locally dense fog at times. Temperatures
are in the lower to middle 30s, with coldest areas in the south
Willamette Valley and in the Cowlitz River Valley. WIll keep mention
of freezing fog in most areas up and down the I-5 corridor this am,
though heart of Portland/Vancouver metro should stay above freezing.

Most of the day will be partly to mostly sunny, that is after the
morning fog/low clouds.  Active flow of moisture off the Pac will
maintain variable mid/high clouds over Lane County and southward.

Models continue to show a gradual shift of clouds northward across
the region tonight. This mainly due to approaching low pres that is
currently well offshore from northern California. Upper flow will
becoming more south to southwesterly tonight as the low approaches.
As such, clouds will spread north tonight, with some light rain as
well late tonight, and mainly Wed am.

This low will pivot a deformation zone northward on Wed, and lie
nearly SW to NW across western Oregon. This will bring occasional
rain to most of the region by Wed afternoon. But, does still appear
the main rainy period will be Wed night as the low lifts into nw
Calif and sw Oregon. With the southerly flow aloft ahead of the low,
will see decent chunk of warmer air across the region, with snow
levels over the Cascades rising to 5500 to 6000 feet Wed night.  Have
boost QPF for Wed night, Generally a 0.40 to 0.60 inch of rain for
lowlands, but 0.75 to 1.00 inch for mountains. With snow levels
rising, any accumulations at the passes will be brief, as snow will
mainly be on the higher peaks/ridges.

Things get more interesting later Wed night into Thu am. Moisture and
clouds will still be over the region at that time, but a cold front
dropping south from the Gulf of Alaska will swing inland early Thu.
This will bring much colder air aloft into the region, with snow
levels plummeting to 2000 feet during the day on Thu. Rain will end
early in the morning, then will see showers increasing as the colder
unstable air shifts inland. Also, could see a few thunderstorms Thu
afternoon, especially along the coast. With snow levels dropping
during the day, moderate potential for small hail.

.LONG TERM...Thursday night through Sunday...Models in good
agreement moving an upper low, with origins in the Gulf of Alaska,
down the BC coast Thu night and Fri, then inland across the region
Sat. This brings a showery and quite cool air mass across the
region. Thickness values down into the range of 522 to 525 dm Fri
and Sat morning suggest snow levels well down into the Coast Range
and Cascade Foothills, with maybe some flakes seen in the higher
hills of the low elevation valley zones. Sun the upper trough is
expected to move east with weak ridging following. A weak short wave
is hinted at Mon in both the GFS and EC, so will need to hang on to
some low pops Sun and Mon, with temps warming a bit, but still
likely cooler than seasonal normals.


.AVIATION...12z TAF expectations: Am expecting a near repeat of
the last 24 hours with fog largely covering the inland terminals
today. Winds have eased with temperatures hovering near to or at
dew point temperatures at this hour. KEUG has dipped below
freezing so have amended for FZFG there. May get FZFG at KSLE but
will wait for the whites of its eyes and keep it as FG. Expect
most areas will also burn off by mid-day, with KEUG being the
last again finally breaking free 20-21z. Expect VFR for all areas
thereafter under increasing high clouds with only limited and
low confidence fog/low stratus late tonight and probably after

KPDX AND APPROACHES...VFR conditions will likely give way to IFR
or LIFR fog/stratus by 13z Tue morning. Fog is expected to slowly
lift during the mid to late morning hours Tuesday, then scatter
out to VFR conditions with onshore flow by 19z. JBonk/Weagle


.MARINE...Winds over achieved expectations a little today with
gusts hovering around 21kt through the evening. Have issued a
SCA for winds for today based on that performance and with models
showing N-S pressure gradients a little stronger. It appears the
winds will mainly stay from about Tillamook Head southward but
extend quite a ways offshore, making this a different style of
northerly than a typical summertime event. Seas are unlikely to
exceed 6 feet but will remain choppy and wind wave driven today.

Wednesday will see a closed low, currently well off the central
California coast drift northeast and phase in with a upper trough
deepening off the BC Coast. Winds appear to remain well within
SCA criteria of 21-33 kts, but this will introduce a mixed swell
with combined seas reaching into the lower teens. The deepening
trough will close off in to an upper low and send periodic
SCA wind events and potential gales across the waters through
the weekend. Best chance for gales currently appears to be
Friday, but again, details are sketchy at best for the following
several days. Seas will have potential to build into the mid
teens with any of the aforementioned wind events. /JBonk


PZ...Small Craft Advisory for winds from 1 PM this afternoon to 11
     PM PDT this evening 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.