Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1018 PM PDT Mon Mar 30 2026
Updated synopsis, aviation discussion, marine discussion, and short
term discussion.
.SYNOPSIS...Mostly dry weather through Tuesday, except for
chances for light rain/snow showers over the Cascades. A
stronger frontal system arrives Wednesday, returning widespread
rain, Cascade snow, and breezy southerly/southwesterly winds
through Thursday. Accumulating snow with hourly snowfall rates likely
over 1 in/hr at times along the Cascade passes Wednesday
evening/night will bring hazardous travel conditions, especially
along Santiam and Willamette Pass. High pressure re-builds Friday
into the weekend, returning dry and sunny weather with warming
temperatures.
&&
.SHORT TERM...Now through Wednesday night...Dry zonal flow
aloft will bring sunnier skies on Monday with dry weather
through Tuesday, mainly west of the Cascades. Surface high
pressure builds, resulting in northerly winds along the I-5
corridor. Weak thermal troughing will also set-up along the
coast and Cascades, leading to light offshore flow over the
Coast Range and Cascade crest. In addition, a weak low
pressure system over the southern Oregon and northern California
border will maintain a 40-60% chance for rain/snow showers
across the Lane County Cascades today, with chances spreading
into the north Oregon and south Washington Cascades on Tuesday.
Any accumulations will remain light with minimal impacts. Winds
turn more southerly/southwesterly ahead of the next system.
The next robust system arrives on Wednesday as an upper level
trough tracks southeastward from the Gulf of Alaska and swings a
frontal system across the Pacific Northwest. This will return
widespread rain, breezy southerly/southwesterly winds, and
Cascade snow across the region. The majority of ensemble members
from the high-resolution REFS suggests that precipitation from
the initial warm front will begin along the coast early
Wednesday morning (midnight-2 AM). Precipitation will gradually
spread inland and persist through Wednesday afternoon as the
trailing cold front follows.
Behind the cold front, precipitation turns showery and cooler
air will filter in the area. Snow levels will drop to 2000-3000
ft, resulting in accumulating snow along the Cascade passes and
potential travel impacts. A Winter Storm Watch has been issued
for the northern and central Oregon Cascades from 5 AM Wednesday
to 5 PM Thursday. Chances for 48-hour snowfall exceeding 1 foot
ending 5 PM Thursday having increased to around 90% along Santiam and
Willamette Passes, and 20-25% along Highway 26 at Government
Camp. Ensemble guidance continue to suggest that the bulk of the
precipitation will be south of Washington, with the heaviest
snowfall over the Cascades from Marion to Lane County between 8
AM-4 PM Wednesday, except between 4 PM Wednesday and 8 AM Thursday as
pass level as this is when snow levels will bottom out. This is when
REFS guidance is showing a 50-75% chance for hourly snowfall rates
exceeding 1 in/hr. Note if precipitation ends up tracking northward,
then Highway 26 could see greater snowfall and impacts. If you have
plans to travel through the Cascades, make sure to pack an emergency
supply kit and refer to ODOT/WSDOT for the latest road conditions and
restrictions.
West of the Cascades, minimal impacts expected from the rain and
wind. The highest chances for 48-hour rainfall exceeding 1 inch
between early Wednesday morning to early Friday morning are
around 40-60% across the southern Willamette Valley and 70-85%
for the coast south of Astoria. Southwesterly winds are forecast
to gust up to 20-25 mph, with stronger gusts of 30-35 mph for
higher terrain. Chances for stronger wind gusts along the coast
(45+ mph) and across the Willamette Valley/southwest Washington
lowlands (35+ mph) are around 10-25%. The main impact for the
lowlands is a 10-20% chance for thunderstorms Wednesday
afternoon/evening. Cooler air behind the cold front will result
in a more unstable atmosphere, and model soundings support this
with skinny CAPE profiles. Any passing thunderstorms may produce
lightning, brief heavy downpours that reduce visibility, small
hail, and erratic winds. -10/23
&&
.LONG TERM...Thursday to Sunday...Lowland rain showers and
Cascade snow showers continue on Thursday as the system weakens
and exits the region. Friday to Sunday, ensemble guidance is in
agreement that upper level ridging will build over the Pacific
Northwest, returning dry weather and sunnier skies. 500 mb
heights also increase during this time, suggesting warmer
temperatures and afternoon highs likely (70-90% chance) reaching
the mid 60s by the weekend. -10
&&
.AVIATION...High clouds were beginning to fill back in over
northwest Oregon and far southwest Washington from south to north
late Monday evening as a weak frontal boundary over southwest
Oregon continues pushing northward. Even with the increase in high
clouds tonight into Tuesday morning, VFR flight conditions will
likely prevail as cigs should generally stay above 6000-7000 ft.
As this front lifts northward, there is a 20-30% chance of light
rain at all terminals (except KAST) later tonight into Tuesday
morning, along with clouds obscuring the high Cascades. Towards
06z Wednesday, a Pacific frontal system will be approaching the
coast, bringing increasing chances for MVFR cigs (20% chance at
KAST, 50% chance at KONP). Chances for MVFR cigs increase to near
90% by 12z Wednesday. Note this is not reflected in the 06z TAF
cycle as conditions will most likely deteriorate shortly after 06z
Wednesday as rain begins moving in from the coastal waters.
KPDX AND APPROACHES...VFR conditions are expected to continue
through 06z Wednesday with increasing high clouds and cigs above
6000-7000 ft tonight into Tuesday morning. Light easterly winds
around 4-8 kt will most likely continue through at least 03z
Wednesday before becoming southerly around 5-10 kt by 06z
Wednesday. -23
&&
.MARINE...High pressure offshore will maintain breezy northerly
winds through this evening, gradually easing tonight as the
pressure gradient weakens. Will maintain the Small Craft Advisory
for the northern outer and central inner waters through midnight,
while the advisory for the central outer waters south of Cape
Falcon will continue through early Tue morning. Weak low pressure
along the coast will drift offshore on Tuesday, bringing more
benign conditions to the coastal waters. Seas of around 5 to 7 ft
at 8 seconds today are expected to subside to around 3 to 5 ft on
Tuesday.
A more robust frontal system is expected to move across the
waters late Tuesday through Wednesday, bringing elevated winds and
seas. Southerly winds begin increasing across the coastal waters
later Tuesday through Tuesday night as an occluding front
approaches. Guidance continues to suggest there is around a
70-90% chance of Gale Force wind gusts exceeding 34 kt with this
system, most likely Wednesday morning. Have issued a Gale Watch
from 2 AM through 2 PM Wednesday, as uncertainty remains in the
timing of the frontal passage. Wind waves are expected to build
on Wednesday becoming steep and choppy, with significant wave
heights likely building to around 12 to 15 ft, highest over the
southern waters.
Surface low pressure moves over the northern waters later
Wednesday bringing more westerly winds, along with an increasing
west swell which is expected to build seas into the lower to mid-
teens on Thursday. Marine conditions are expected to ease later
this week as high pressure returns northerly winds, and seas
subside into the weekend. -DH/23
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from late Tuesday night through Thursday
afternoon for ORZ126>128.
WA...None.
PZ...Gale Watch from late Tuesday night through Wednesday afternoon
for PZZ251>253-271>273.
Small Craft Advisory until midnight PDT tonight for PZZ252-253-
271.
Small Craft Advisory until 5 AM PDT Tuesday for PZZ272-273.
&&
$$
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