textproduct: Portland

SYNOPSIS

A prolonged, moisture-rich pattern will hold through midweek as two significant atmospheric river surges move across the region- first today, then a secondary and slightly weaker but still impactful round Tuesday night into Wednesday. The combination of high-integrated vapor transport, saturated soils, and periods of stronger winds will sustain the risk for hydrologic issues and localized wind impacts.

LONG TERM...Wednesday night through Saturday

The atmospheric river weakens late Wednesday night as the secondary plume shifts eastward and moisture transport diminishes. However, residual showers and pockets of moderate rain will persist through at least early Thursday, prolonging hydrologic sensitivity. By Thursday into early Friday, the pattern transitions toward a more typical post-frontal regime with cooler, showery conditions and decreasing winds. Light rain returns late Friday into Saturday. Snow levels will gradually lower into the weekend but remain high enough that little to no snow accumulation is expected for Cascades through the weekend. Although overall impacts lessen late week, rivers and soils will continue responding to the cumulative rainfall load from this Monday through Wednesday multi-day event. ~12

HYDROLOGY

A series of frontal systems will keep conditions very wet through much of this week. The most notable period will be late today 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 72 hour total rain amounts up to 6 to 8.5 inches in the Cowlitz Valley, lower Columbia, Portland/Vancouver metro and northern Willamette Valley, 4 to 6 inches in the central Willamette Valley, 3 to 4 inches in the southern Willamette Valley, 7 to 11 inches in the Cascades, Cascade foothills, Coast Range and the coast, except 3.5 to 7 inches from Florence to Newport and in areas of higher terrain in Lane County. 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 on Thursday, the threat of flooding will linger along slow responding rivers. Rivers draining the Coast Range and Willapa Hills have a 40-70% chance of reaching moderate flood stage and a 20-50% chance of reaching major flood stage. Additionally, many Willamette River tributaries that drain from the Coast Range and Cascades have anywhere from a 40-60% chance of reaching minor flood stage and a 20-40% chance of reaching moderate flood stage. Flooding of the Willamette mainstem rivers have a very low chance to occur (5-15%). 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 now until 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.

Grays River at Covered Bridge near Rosburg affecting Wahkiakum County, from this afternoon to late Wednesday evening. Moderate flooding is forecasted and there is a 50% chance of reaching major flooding. Wilson River near Tillamook affecting Tillamook County, from this evening to Wednesday evening. Minor flooding is forecasted and 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 Tuesday morning to late Wednesday evening. Minor flooding is forecasted and there is a 70% chance of reaching moderate flooding, and a 50% chance of reaching major flooding.

AVIATION

A moisture-rich frontal system will bring increasing south to southwest winds across the area today along with a band of steady rain that will slowly sag southward from northwest Washington this morning into southwest Washington and most of northwest Oregon by late morning. Rain will likely take a bit longer to begin at KEUG. Rain will be heavy at times from 18z Monday onward, except lighter in KEUG. This will result in persistent low-end MVFR cigs with brief periods of high-end IFR cigs for inland TAF sites, as well as surface visibility reductions down to 2-4 SM at times. Meanwhile, persistent LIFR to IFR flight conditions are favored to at the coast through 12z Tuesday, except through 09z Tuesday at KAST where cigs will begin lifting to MVFR or low- end VFR thereafter.

Surface wind gusts up to 35-40 kt are expected at the coast, and up to 25-35 kt within the Willamette Valley. In addition, low level wind shear will be on the increase at KPDX, KTTD and KAST Monday morning with 45-55 kt southwest winds at 2000 ft. Note the main concern is strong speed shear, as winds are nearly unidirectional from the surface up to 4000 ft, suggesting directional shear will be minimal.

KPDX AND APPROACHES...Expect mainly low-end MVFR cigs to continue through 12z Tuesday with a 20-30% chance for cigs to briefly fall below 1000 ft cigs Monday morning. Steady stratiform rain is set to begin by 18z Monday, becoming moderate to heavy at times Monday afternoon. It appears rain will briefly become lighter again Monday evening before intensifying again Monday night. Expect surface visibilities down to 2 to 4 SM when rain rates are highest. Light winds this morning will quickly increase out of the south by 16-17z Monday. Wind gusts up to around 30-35 kt are expected Monday afternoon through Monday night. Note there will also be a period of increased low-level wind shear Monday morning as winds at 2000 ft reach 45-55 kt out of the southwest. The main concern will be strong speed shear, as directional shear looks minimal. -23

MARINE

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 through 1 AM Tuesday as probabilities for gale force gusts are up to 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 widespread gale force gusts are not expected for a long enough period of time. One potential exception will be the inner waters, which have a 30-45% chance of marginal gale force wind gusts up to 35 kt. Seas also decrease slightly Tuesday into afternoon 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 remains in effect from 2 PM to 6 PM PST Monday for these coastal zones. It appears the threat for tidal overflow flooding is low on Tuesday, as total tide forecasts decrease considerably and are well below the 9.5 foot threshold. That said, minor tidal overflow flooding cannot be completely ruled out for a brief period of time around high tide on Tuesday given river levels will be running quite high at that time. Will continue to monitor the tidal and river forecasts 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 around 14-15 seconds with swell heights around 9-11 ft, with swell heights and swell periods 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. -23/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory from 2 PM this afternoon to 6 PM PST this evening for ORZ101-102.

Wind Advisory from 7 AM this morning to 4 AM PST Tuesday for ORZ104>115-123.

WA...Coastal Flood Advisory from 2 PM this afternoon to 6 PM PST this evening for WAZ201.

Wind Advisory from 7 AM this morning to 4 AM PST Tuesday for WAZ202>208.

PZ...Gale Warning from 7 AM this morning to 1 AM PST Tuesday for PZZ210-251-271.

Small Craft Advisory from 1 AM to 7 PM PST Tuesday for PZZ210- 251-271.

Gale Warning from 7 AM this morning to 7 AM PST Tuesday for PZZ252-253-272-273.

Small Craft Advisory from 7 AM to 7 PM PST Tuesday for PZZ252- 253-272-273.