NWS Portland, OR wrote its last area forecast discussion before the first event at Nov 3, 7:40 PM PST, 4 hours 20 minutes earlier.
Area forecast discussion, Nov 3, 7:40 PM PST, 4 hours 20 minutes before
Area Forecast Discussion...UPDATED
National Weather Service Portland OR
840 PM PDT Thu Nov 3 2022
Updated Aviation and Marine Sections
.SYNOPSIS...Atmospheric river sets up Friday afternoon with
heavy rain and rising snow levels through early Saturday
morning, along with windy conditions. Cool and showery weather
then expected to continue into the middle of next week.
&&
.SHORT TERM /THROUGH SATURDAY NIGHT/...
Light rain has returned to the northern coastal areas as a warm
front pushes its way east. A strong area of frontogenesis will
produce a strong southerly jet over the nearshore waters and
along beaches and headlands. However, it will not be strong
enough for a high wind advisory at this time. Rainfall amounts
through this evening should be limited to tenth of an inch or
less in the Willamette Valley and the high Cascades, with around
a quarter to a half inch in northern coastal areas and the
Willapa Hills. There will be an odd transition period with
freezing temperatures near and at the surface with light rain
which could produce some very light ice accumulations in the
high Cascades during the overnight hours. This freezing rain
window will be incredibly brief as the warm air quickly
approaches restricting ice accumulations during the commuting
hours.
These light precipitation accumulations during the overnight
hours will preface an approaching atmospheric river event on
Friday with the arrival of an intense 160+ kt jet streak
spanning the NE Pacific and taking aim on the Washington and
Oregon coasts.
Forecast details have not changed much with regard to the
pending atmospheric river event as all metrics continue to
indicate a significant precipitation event unfolding across the
area. A strong to possibly extreme atmospheric river will impact
the region beginning along the WA Coast on Friday morning and
shifting southward along the OR Coast Friday evening into early
Saturday morning. ECMWF and GFS ensemble model suites have both
trended slightly upward in depicting integrated vapor transport
values inching closer to 1000 kg/ms and straddling the strong to
extreme threshold over the area Friday afternoon into early
evening. This slight upward trend is also reflected in the the
ECMWF Extreme Forecast Index, which now places QPF values for
this event close to the 100th percentile across southwest WA
while keeping the bulk of northwest OR above the 90th percentile
relative to late October/early November forecasts from the past
20 years. Fortunately, guidance continues to show strong
agreement in dropping the AR south of the area relatively
quickly as an amplifying upper level trough approaches the
region on Saturday, which should limit the residence time and
keep the AR more solidly in the strong as opposed to the extreme
category and hopefully minimize hydrologic impacts as a result.
All told, QPF amounts for this event are expected to range from
6 to 8 inches across the northern Coast Range, the Willapa
Hills, and the Cascades, with the highest amounts favoring
southwest exposed terrain. Meanwhile, rainfall amounts in the
Willamette Valley will likely end up in the 2-3 inch range
through Saturday morning. Expect quick responses on the flashier
coastal rivers such as the Grays and Willapa in SW WA, with
nuisance flooding and ponding of water in urban areas also a
high likelihood through Saturday morning.
One wrinkle will be along the S WA coast with regards to
minor flooding. The tidal station at Toke Point will reach total
tide of 9.5 to 10 ft during the 1130 AM high tide. If the
atmospheric river stalls or overperforms and the Willapa or
Grays reaches higher discharge than currently forecasted then
minor flooding due to tidal overflow is possible for a 6-7 hour
period surrounding high tide. Minor tidal up to one foot is
possible in low lying areas.
Winter impacts will be minimal with the initial swath of
precipitation as snow levels quickly climb above 7000 ft Friday
and Friday night.
In addition to precipitation impacts, wind will be a concern
going into the overnight hours Friday and through the day
Saturday. The strong jet aloft will translate down towards the
surface. A low to mid level jet that will impact the higher
terrain. A High Wind Warning has been issued for the Coast Range
and Cascade foothills. Wind gust reaching 50 to 60 mph likely
with the highest terrain possibly reaching 70 mph. The Wind
Advisory for the Willamette Valley has been left in place with
minor timing adjustments with models coming in a little faster
than previously expected. Expect these higher winds Friday
morning and peak early evening as a surface front tracks through
the valley with sustained winds in the 20-30 mph range and
gusts to around 40 mph. Even if winds are borderline to meet
strict advisory criteria, the early season nature of the event
with leaves still on the trees could lead to more widespread
impacts with minor tree damage and a maybe few power outages
possible. -BMuhlestein/Burling
.LONG TERM...SATURDAY NIGHT THROUGH THURSDAY...
Longer range forecast models and their ensembles show excellent
agreement in unseasonably cool temperatures moving into the Pac
NW by this weekend. Snow levels will fall below the Cascade
passes behind our cold front by Saturday. There will probably be
a brief lull in the precipitation after the cold front passes,
but models are increasingly agreeing on another significant slug
of moisture moving onshore with a vigorous shortwave and
reinforcing cold front later Sat night/Sun morning.
As this next system approaches, 12z EC ensemble data show good
clustering around -2 deg C for 850 mb temps, which would correspond
to a snow level of around 3000-3500 ft in onshore flow. However, the
intensity some models are showing in precip suggest dynamic cooling
could drag the snow level considerably lower, but not all the way
down to the valley floors. That said, this may be sufficient to
bring snow levels down into the foothill and Coast Range elevations
if the precipitation is as heavy and persistent as latest guidance
shows. Will likely need Winter Storm Watches or at least Winter
Weather Advisories for the Cascades - and possibly the foothills -
for this next event.
Given the excellent forcing shown by all deterministic models
early Sunday morning, will also need to watch rainfall totals in the
lowlands for hydrologic issues, but the colder temperatures should
help mitigate any new hydro issues from this second surge of
precipitation.
Beyond Sunday the upper trough will linger over the region, but with
decreasing moisture as next week progresses. Enough moisture lingers
Monday for scattered showers, with snow levels generally around 1500-
2000 feet. Cannot completely rule out wet snowflakes down almost as
low as the valley floors in any heavier shower Monday, but sticking
snow does not yet look to be in the cards. The earliest measurable
snowfall in a season at Portland Airport is November 11, 1955. It is
a bit earlier for downtown, where records go back further. For the
Portland Downtown station, the earliest measurable snowfall in a
season was October 29, set in 1935.
For Monday night through Wednesday, the main weather issue will be
our cold temperatures, which should get below freezing for the vast
majority of our forecast area for overnight lows. Most outlying
areas will see a hard freeze with lows in the 20s Mon and/or Tue
nights. Depending on how stout this cold air mass becomes - and it
is awfully stout for early November - we will need to watch the next
Pacific frontal system for potential mixed precip issues in and near
the Columbia Gorge. Weagle
&&
.AVIATION...06Z TAFs: NOTE: KONP communications issues continue to
be sporadic. Observations have been missing from 2140Z through at
least 0330Z Friday. We will wait to see if observations return and
continue without significant interruptions for 24 hours before
removing the AMD NOT SKED entry on the KONP TAF.
As of 0330Z satellite imagery showed a warm front draped over
western Washington and extreme northwest Oregon. Flight conditions
were mainly MVFR along the coast and VFR inland, but cigs
generally 035-050. Expect MVFR to become more IFR along the coast
overnight. Inland areas likely to remain generally low-end VFR
early in the 06 TAF period, but become more MVFR overnight. Areas
of IFR can be expected after 18Z Friday, especially in the
southwest Washington interior. The cold front is expected to move
across southwest Washington between 03Z and 06Z Saturday, with
corresponding improvement to at least an MVFR to VFR mix.
South wind gusts will increase to 25-30 kts along the coast
tonight, then ramp up to 35-45 kt by 18Z Friday. Significant wind
speed shear is likely Friday. Model soundings for the coast show
near-surface wind speeds around 20-25 kt 18Z Friday, with 50-60
kt at FL020.
For detailed Pac NW aviation weather information, go online to:
http://weather.gov/zse
KPDX AND APPROACHES...VFR expected in the early portion of the TAF
period, but becoming predominant MVFR overnight. Rainfall intensity
expected to increase Friday afternoon. Southeast surface wind
increases to 15-20 kt overnight, with southwest wind 40-55 kt from
FL015 to FL020. This will result in LLWS and have included it for
the terminal and at KTTD. Weishaar
&&
.MARINE...As of 0330Z gale force wind gusts were noted at buoy
46029 and near the Columbia River bar. The 12Z model guidance shows
boundary layer wind speeds generally 25-30 kt through around 12Z
Friday, then increasing to 30-40 kt. The 975 mb wind speed guidance
indicates speeds 45-50 kt 18Z Friday through around 03Z Saturday.
The 975 mb values can be used as a proxy for peak wind gusts.
Thus, there is the potential for isolated storm-force gusts, but
not expecting enough coverage or duration for a storm warning.
Models also imply some semblance of coastal jet development this
evening, with gusts to 40 kt out to around 20 nm and mainly over
the Oregon waters.
Wind becomes west to northwest late Friday afternoon over the
south Washington and north Oregon waters, spreading southward
Friday night. Will maintain gale force gusts through 12Z Saturday,
even during the post-frontal west to northwest wind. Solid small
craft advisory wind speeds occur Saturday through Saturday night,
then falling below 15 kt Sunday night.
At 03Z buoy 46248 reported 13 ft seas, but with a dominant 7-8
second south wind wave period. However, the combined 13 foot seas
were already 2.5 ft higher than model guidance. Thus, the under-
forecast model bias continues. Will continue with the 2-4 foot added
bias to the wave heights Friday through Saturday night to account
for the persistent underforecast model values. The extra 2-4 ft
will result in significant wave heights 20-22 ft, but would not be
surprised if there were some 25-foot waves. Seas are expected to
fall below 15 ft Monday afternoon, then below 10 ft late Tuesday
or Tuesday night. Weishaar
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 11 AM to 11 PM PDT Friday for Coast
Range of Northwest Oregon-Northern Oregon Cascade
Foothills-Upper Hood River Valley.
High Wind Warning from 2 PM Friday to 1 AM PDT Saturday for
Cascade Foothills in Lane County-Central Coast Range of
Western Oregon.
Wind Advisory from 8 AM Friday to 1 AM PDT Saturday for Central
Willamette Valley-Greater Portland Metro Area-South
Willamette Valley.
WA...High Wind Warning from 11 AM to 11 PM PDT Friday for South
Washington Cascade Foothills-Willapa Hills.
Wind Advisory from 8 AM Friday to 1 AM PDT Saturday for Greater
Vancouver Area.
PZ...Gale Warning until 11 PM PDT Friday for Columbia River Bar.
Gale Warning until 5 AM PDT Saturday for coastal waters from
Cape Shoalwater WA to Florence OR out 60 nm.
&&
www.weather.gov/portland
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