textproduct: Portland

SYNOPSIS

A steady, moisture-rich pattern continues through midweek as two strong atmospheric river surges arrive Monday and again Tuesday night to Wednesday. Conditions become increasingly impactful Monday and Tuesday as deep subtropical moisture and stronger winds move inland. River rises and localized flooding will need close attention through the week. Gusty winds are also expected and could bring down trees and cause power outages.

DISCUSSION...Today through Saturday

Scattered showers perish across the area as a weak disturbance continues to move across the region, a quiet precursor to the more impactful weather system that moves in overnight.

The overall pattern remains basically the same from previous forecasts with slight changes in rainfall totals and where impacts are expected. The synoptic set-up for the week starts with a ridge of high pressure centered over the Pacific between Hawaii and southern California. Clockwise circulation will advect sub-tropical moisture northward toward the PacNW in several round between Monday and Wednesday night. Mid level flow will predominantly be zonal as the top of the ridge extends northward along the West Coast. At the same time, several troughs will support surface lows and cold fronts that will provide lift over the region. The first slug of AR moisture enters the picture late tonight into Monday with IVT values along the coast peaking between 750-1000 kg/ms and values inland peaking between 500-750 kg/ms. This round of rain will be falling on already saturated soil, WPC has maintained the Slight/Marginal ERO risk over the CWA. The slight risk covers all of SW WA, down the coast range in OR and along the Cascades down to around Santiam pass. A couple of changes to note with this first round are the slight northward shift in the highest IVT values, leading to changes in the total QPF Monday through Tuesday. Totals across the southern Willamette Valley and Lane County Cascades and Coast Range have dropped slightly while totals over southwest Washington areas generally north of Salem have increased slightly. While this has not caused major changes in expected impacts, several river forecasts have seen increases in flooding potential. More details are included in the hydrology section below.

The second round of high IVT values is expected to impact the region Tuesday night into Wednesday. IVT Values look similar to the first round, maybe a touch lower which will lead to slightly lower rainfall totals. Rainfall is still expected to be heavy with this second round, regardless of whether it reaches the same level as the fist round. Expect continued urban and river flooding issues at least through Wednesday.

Winds will be a concern Monday through Wednesday, with gusts up to at least 30 mph inland and up to at least 40 mph at the coast. Given the soils are already saturate and heavy rain with gusty winds is expected, a Wind Advisory has been issued for potential impacts. The advisory covers the OR and WA Coast Range, Willamette Valley from Salem northward through the Portland/Vancouver Metro, I-5 corridor in WA, and the Cascade Foothills north of Salem. Saturated soils will see persistent rain and eventually gusty winds, resulting in downed trees and possibly power outages. Confidence in impacts occurring is high but confidence in exact location and extent of impacts is low.

Snow levels will remain well above 6000 ft through midweek due to the warm subtropical air mass, ensuring that nearly all precipitation in the Cascades falls as rain. By Thursday and Friday, the moisture plume weakens and transitions toward a showery pattern. Conditions will gradually ease, but rivers and soils will remain sensitive due to cumulative rainfall. -19/12

AVIATION

The warm frontal boundary continues to shift towards the region overnight. Will see an increase of cloud cover and more widespread MVFR conditions. Showers will begin to intensify by 15Z Mon which will persist for a prolonged period of time. With the heavy rain and reduced CIGs, expecting widespread MVFR conditions if not IFR along the coast. With the frontal system will see strong south to southwesterly winds. Gusts as high as 40 kt possible along the coast with increasing chances for gusts around 30 kt within the Willamette Valley. The Columbia River Gorge and foothills will see less wind though will remain elevated. These winds will be coupled through the atmosphere so approaches too will be impacted. At around 2000 ft, wind speeds will be southwest with speeds up to 50 kt. Some models are suggesting gusts as high as 65 kt but those would be isolated. Therefore, have included the mention of LLWS in some area TAFs.

KPDX AND APPROACHES...VFR conditions prevail until around 09-12z when MVFR ceilings are expected to return. Scattered showers are possible through the night. A frontal system will bring increasing rain after 14z Monday with lowered visibilities. South winds will increase with gusts up to 25-30 kts after 14z. -27/03

MARINE

Winds have decreased substantially this afternoon behind this morning's frontal system, and will continue to decrease into the evening. There will be a brief period of benign conditions tonight with wind gusts below 20 kts and seas under 8-9 ft.

A stronger frontal system is set to arrive early Monday, bringing gale force wind gusts up to 35-45 kt, strongest over the inner waters and near the Columbia River Bar. A Gale Warning is in effect from 3 AM Monday to 1 AM Tuesday as probabilities for gale force gusts are up to 80-90%. With the increasing winds, seas will become steep and hazardous Monday afternoon into Monday night. Expect seas to peak somewhere between 14 to 17 ft.

Winds decrease from north to south late Monday night into Tuesday morning with gusts falling below 20 kts everywhere except for portions of zones PZZ273 and PZZ253 briefly on Tuesday morning. Then another frontal system moves through the waters Tuesday afternoon into Wednesday, causing increasing southwesterly winds with gusts up to 25-30 kt likely. There's around a 20% chance of occasional gale force wind gusts over 34 kts across the waters Tuesday night, though it is not expected to have widespread gale force gusts for a long enough period of time at this moment. Seas also decrease slightly Tuesday into Wednesday but remain above 10 ft. Seas and winds subside late in the week. -23/03

BEACH HAZARDS

Tidal overflow flooding around high tide is likely on Monday, December 8 and possible on Tuesday, December 9. Despite total tide forecasts decreasing over the coming days, river levels will be rapidly rising due to a prolonged period of heavy rain Monday through Wednesday. Confidence has increased that several rivers along the Oregon Coast north of Lincoln City and the SW Washington Coast will reach at least 80% of flood flow by high tide Monday afternoon, when total tide forecasts are expected to peak near or slightly above 9.5 ft. Therefore, a Coastal Flood Advisory for tidal overflow has been issued from 2 PM to 6 PM PST Monday. Uncertainty remains on whether a Coastal Flood Advisory will be needed on Tuesday because even though there is high confidence that coastal rivers will be above 80% of flood flow during high tide, the total tide forecast is below 9.5 ft at most tidal points. Will continue to monitor the tidal forecast for Tuesday.

Additionally, there is an elevated threat for sneaker waves late Wednesday through Thursday along the SW Washington coast south through the central Oregon coast. The swell period increases to 14-15 seconds with swell heights around 9-11 ft with both decreasing late Thursday night. This will create energetic waves that will present a moderate to high risk for sneaker waves, waves which can run significantly farther up a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock beachgoers off if their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning. Caution should be used when in or near the water. Always keep an eye on the waves, and be especially watchful of children. -03

HYDROLOGY

A series of frontal systems will keep conditions very wet through the weekend and much of next week. The most notable period will be late Monday through late Wednesday, when a prolonged atmospheric river will bring significant rainfall and the potential for widespread river flooding across southwest Washington and northwest Oregon. There is a 10% chance for rain amounts up to 10 inches in the Cascades and Coast Range, up to 8-10 inches at the coast, and up to 7.5-8.5 inches in the Willamette Valley, Portland/Vancouver metro and Cowlitz Valley. While rain amounts this high are unlikely to occur, they represent a reasonable worst case scenario. This outcome would result in widespread major flooding.

Although rain will likely become lighter Thursday into Friday, the threat of flooding will linger along slow responding rivers. Rivers draining the Coast Range and Willapa Hills have a 30-60% chance of reaching moderate flood stage and a 20-40% chance of reaching major flood stage. Additionally, many Willamette River tributaries that drain from the Coast Range and Cascades have anywhere from a 40-70% chance of reaching minor flood stage and a 20-50% chance of reaching moderate flood stage. Flooding of the Willamette mainstem rivers is unlikely to occur (less than 10%). Probabilities for specific river points can be found at the National Water Prediction Service website.

There is also increasing confidence in the potential for flooding along small creeks and streams as well as urban flooding, including roadway flooding. Periods of heavy rain will also increase the risk for landslides in areas of steep terrain and debris flows over recently burned areas. People, structures, and roads located below steep slopes, in canyons, and near the mouths of canyons may be at serious risk from rapidly moving landslides.

A Flood Watch remains for all of southwest Washington and northwest Oregon from 4 AM Monday to 4 AM Friday. Considering increased confidence and probabilities for flooding of rivers and urban areas as the forecasted event approaches, a Flood Warning has been issued for the following rivers.

Wilson River near Tillamook affecting Tillamook County, from Monday evening to late Tuesday morning. There is a 75% chance of reaching moderate flooding and a 50% chance of reaching major flooding. Grays River at Covered Bridge near Rosburg affecting Wahkiakum County, from Monday evening to early Thursday morning. There is a 70% chance of reaching moderate flooding and a 50% chance of reaching major flooding. Johnson Creek at Sycamore affecting Clackamas and Multnomah Counties, from Monday evening to late Wednesday evening. There is a 70% chance of reaching moderate flooding, and a 60% chance of reaching major flooding.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory from 2 PM to 6 PM PST Monday for ORZ101- 102. Wind Advisory from 7 AM Monday to 4 AM PST Tuesday for ORZ104>115-123. WA...Coastal Flood Advisory from 2 PM to 6 PM PST Monday for WAZ201. Wind Advisory from 7 AM Monday to 4 AM PST Tuesday for WAZ202>208. PZ...Gale Warning from 3 AM Monday to 1 AM PST Tuesday for PZZ210- 251-271. Gale Warning from 3 AM Monday to 7 AM PST Tuesday for PZZ252- 253-272-273.