textproduct: Portland

SYNOPSIS

A prolonged wet pattern continues through early next week as multiple frontal systems push inland. The most notable period continues to focus on Monday through midweek, when a prolonged atmospheric river may bring significant rainfall and rising rivers across southwest Washington and northwest Oregon. Today and Sunday bring lighter rainfall, while Monday is expected to produce the highest totals across southwest Washington and northwest Oregon.

DISCUSSION...Now through Friday

Light to moderate showers persists across the area this morning as a steady southwesterly to westerly flow feeds moisture inland. Todays precipitation will be relatively modest with forecast totals from now to 5 AM Sunday near 0.10-0.25 inch across interior lowlands, 0.2-0.4 inch along the coast, 0.5-1.0 inch in the Coast Range, and 0.5-1.5 inches in the Cascades. Breezy southwest winds are expected at times with gusts typically 20-30 mph, strongest near exposed coastal and elevated terrain.

A more organized plume arrives Sunday with IVT values around 300-500 kg/ms. That wave increases rainfall efficiency, producing about 0.5-0.7 inch for interior lowlands, 0.7-1.5 inches along the coast, 1.5-2.5 inches in the Coast Range, 1.5-3.0 inches in the Cascades, and 0.5-1.5 inches in the Lane County Cascades. Widespread impacts are not anticipated through Sunday; however, if rainfall rates sustain around 0.2 to 0.3 inches per hour for several hours, a few faster-reponding basins, such as the Grays at Rosburg and the Willapa River, could rise quickly. Breezy southerly to southwesterly winds are expected at times with gusts typically 30-40 mph, strongest near exposed coastal and elevated terrain.

The first strong to moderate moisture surge is forecast for Monday afternoon. Ensemble clusters show concentrated IVT around 650-850 kg/ms for this plume, supporting prolonged, efficient rainfall. Expected Monday rainfall jumps markedly, with interior lowlands receiving roughly 1.25-2.50 inches (highest totals in southwest Washington; 1.8-2.0 inches in the Portland-Vancouver metro), coastal totals near 2.5-3.5 inches, the Coast Range and Cascades 3-5 inches, and the Lane County Cascades 0.75-1.75 inches. Given the duration and magnitude of this plume, faster- responding basins could see rapid rises and localized flooding; this period warrants close monitoring.

A second strong to moderate surge of atmospheric moisture is expected Tuesday into Wednesday, with ensemble means near 700-800 kg/ms. High-end ensemble solutions cluster around 850-950 kg/ms while lower-end members fall in the 550-650 kg/ms range. Even if the second surge trends a bit weaker, the combination of two consecutive high-IVT plumes and the residual moisture between them will sustain elevated rainfall totals and prolonged runoff. By midweek, soils will be saturated across much of the region and flood risk for rivers and small streams will increase. After the second surge, the core of the moisture feed is expected to diminish later in the week.

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. There is a 10-20% chance for max wind gusts over 40 mph for inland areas, and a 20-40% chance for gusts over 50 mph along beaches and headlands. If wind gusts of this magnitude do materialize, expect scattered downed trees and power outages.

Showery conditions will linger into Thursday and Friday as the flow gradually weakens and dries. Snow levels are expected to remain high (above 6000 ft) during this warm weather event, keeping most precipitation as rain in the Cascades. Forecast confidence in exact timing and magnitudes is still moderate given ensemble spread. Overall, the Monday through Wednesday time period remains the primary focus for potential high-impact hydrologic (see hydrology section below for more details) and wind concerns. ~12

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. Total rainfall amounts from 4am Monday to 4am Thursday (72 hour total) will most likely be around 4 to 6 inches at the coast and interior lowlands, and 5.5 to 9.0 inches in the Coast Range and Cascades. Lane County Cascades will be an exception, with 4-6.5 inches expected. There is a 10% chance for rain amounts up to one foot in the Cascades and Coast Range, up to 8-9 inches at the coast, and up to 7 inches in the Willamette Valley, Portland 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 5%). 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.

Considering the probabilities of flooding for rivers and urban areas, a Flood Watch has been issued for all of southwest Washington and northwest Oregon from 4 AM Monday to 4 AM Friday.

AVIATION

At 1030z Saturday, visibilities were in the VFR range with cigs in the high-end MVFR to low-end VFR range, with mainly high-end MVFR cigs at the coast and low-end VFR cigs inland. Moist westerly flow aloft is resulting in persistent rain showers in the Cascades and Coast Range, bringing frequent mountain obscuration. Expect a mix of MVFR/VFR flight conditions to continue through Saturday night before MVFR and IFR cigs become more widespread after 12z Sunday. This is when chances for IFR cigs increase to 20-30% for inland TAF sites and near 100% at the coast. Breezy southwest surface winds will continue throughout the TAF period with occasional wind gusts up to 20 kt. Steady stratiform rain will develop across the area between 06-12z Sunday.

KPDX AND APPROACHES...Expect a mix of high-end MVFR to low-end VFR cigs to continue through 12z Sunday, before trending towards low- end MVFR or even IFR after 12z as steady stratiform rain develops. Chances for MVFR cigs at that time peak near 90%, while chances for IFR cigs peak near 30%. Will note that at around 700 ft MSL, winds are SSW around 30 kt. Not nearly enough to add LLWS to the TAF, but for east approaches cannot rule out the potential for isolated periods of gusty crosswinds. -23/27

MARINE

Seas will remain between 11 and 16 ft around 11 to 13 seconds through today before lowering to 8 to 9 ft tonight. Seas are then forecast to remain steady until rising to 12 to 15 ft on Monday. With dominant wave periods around 11 seconds, seas will be steep and hazardous. In addition, expect west winds to continue today at 15 to 20 kt with gusts up to 25 kt, becoming west- southwest 10 to 15 kt with gusts up to 20 kt Saturday evening. Winds become south 20 to 25 kt with gusts up to 30 kt Sunday morning. Isolated and brief gale force gusts up to 35 kt are possible Sunday morning, especially over the inner waters. Have mentioned this possibility in the ongoing Small Craft Advisory that is currently in effect for the inner waters through Sunday. Decided to upgrade the outer waters to a Hazardous Seas Warning through Saturday evening to account for buoy observations reporting seas a few feet higher than previously expected, including a 17 foot northwest swell at buoy 089 that will progress through the coastal waters Saturday morning.

Gale force wind gusts may become more widespread on Monday when probabilities increase to 50-80% for max wind gusts over 34 kt, highest over the northern waters. These probabilities have trended upward since last night's update. Depending on how strong winds get on Monday, there is the potential for seas over 15 ft. Currently, there is a 10% chance for seas up to 18 ft. -23

BEACH HAZARDS

Coastal Flood Advisories remain in effect from for minor tidal overflow during high tide from 9 AM to 3 PM Saturday for the south Washington coast and Clatsop County coast in Oregon. Minor flooding up to 1 foot above ground level is possible during high tide in low lying areas near bays, sloughs, and the low reaches of the coastal rivers. Coastal residents in the warned area should remain alert for rising water and take appropriate action to protect life and property. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory until 3 PM PST this afternoon for ORZ101.

WA...Coastal Flood Advisory until 3 PM PST this afternoon for WAZ201.

PZ...Small Craft Advisory until 4 PM PST Sunday for PZZ210.

Small Craft Advisory until 7 PM PST Sunday for PZZ251>253.

Hazardous Seas Warning until 7 PM PST this evening for PZZ271>273.

Small Craft Advisory from 7 PM this evening to 7 PM PST Sunday for PZZ271>273.