textproduct: Portland

SYNOPSIS

A prolonged atmospheric river continues to influence the Pacific Northwest, sustaining periods of heavy rainfall, breezy winds, and widespread hydrologic concerns through late Wednesday, possibly as late as early Thursday. Widespread river flooding remains likely, along with urban and small stream flooding due to persistent heavy rain. Wind may lead to downed trees and power outages due to saturated soils.

DISCUSSION...Now through Monday

The strong atmospheric river (AR) event continues across northwest Oregon and southwest Washington. Significant rainfall will continue to bring flooding of rivers, creeks, streams, and other low-lying and flood-prone locations through at least Wednesday. Concerns for landslides and debris flows remain as soils remain saturated.

Since midnight on Mon, Dec 8th (the beginning of the event), weather stations have reported rainfall totals of 4 to 7 inches across the the north Oregon Coast Range, Willapa Hills, south Washington Cascades, and north Oregon Cascades, 3 to 5 inches across the Cowlitz Valley, Clark County lowlands, north Oregon coast, and south Washington coast, 1 to 2.25 inches across the central and northern Willamette Valley, and 0.50 to 1 inch in the southern Willamette Valley.

Moisture from this AR has briefly shifted southward today, with much of the rain falling across northwest Oregon. Mesoanalysis as of Tuesday afternoon shows a warm front approaching the Pacific Northwest, which will gradually shift rain northward into southwest Washington and far northwest Oregon. This will be the second round of impactful rain from this AR. The heaviest rain tonight into early Wednesday morning will be over the Coast Range, Willapa Hills, south Washington Cascades, and north Oregon Cascades. Moderate to high confidence (50-70% chance) that these areas will receive an additional 4 to 6 inches of rain from this afternoon to 4 AM Thursday, with a 10-20% chance for locally higher amounts of 7+ inches. This will further exacerbate flood impacts considering the amount of rain that has already fallen.

Meanwhile, high resolution guidance suggests a rain shadowing effect will keep rainfall amounts lower for interior lowland valleys. Forecast amounts are around 0.10 to 0.25 inches across the Willamette Valley and most southwest Washington lowlands. Based on REFS guidance, chances for 0.5 inch of rain or more across interior valleys are only 5-15%, highest across the Portland/Vancouver Metro and northern Clark County lowlands. An exception is the far northern Cowlitz Valley where there is a 50-60% chance for an additional 1 inch of rain of greater north of Kelso/Longview through Wednesday afternoon.

Snow levels remain high, holding near or above 8000 ft through at least Wednesday, maintaining rain in the Cascades and further contributing to elevated runoff and river responses. A gradual downward trend in snow levels is likely late Wednesday night.

Winds remain a secondary hazard and continue to carry relevance given soil saturation. Winds will gradually increase overnight as this next front moves in. Southwesterly winds will be breezy with gusts up to 25 to 30 mph for interior valleys, and 35 to 45 mph for across the Coast Range, Willapa Hills, south Washington Cascades, north Oregon Cascades, and ridgetops surrounding the Columbia River Gorge. Due to saturated soils, even moderate wind gusts may be capable of uprooting trees or power lines. Therefore, a Wind Advisory has been issued for much of northwest Oregon through Wednesday evening.

The atmospheric river weakens late Wednesday as moisture transport decreases and the plume begins to dissipate. Showers will persist into Thursday, though rainfall intensity will be much lighter and increasingly confined to higher terrain across southwest Washington and the north Oregon Coast Range. A more stable pattern develops Friday into Saturday as high pressure builds, leading to a notable drying trend. Snow levels gradually lower to around 5500 to 6500 ft through the weekend. Despite the improving weather, hydrologic impacts for slow-responding rivers may linger a bit longer. Soils will remain saturated, particularly across the Coast Range and Cascades, leaving the region sensitive to additional rainfall. Additional frontal system look to arrive early next week. -10/12

AVIATION

A mixed bag of LIFR/IFR/MVFR cigs and IFR/MVFR/VFR visibilities continues across northwest Oregon and southwest Washington, with cigs and visibilities being lowest from KONP to KSLE and points north where a large band of steady stratiform rain continues. Rain will be heavy at times through Tuesday afternoon, especially during the morning and early afternoon hours north of KEUG. Rain should finally begin to taper off from south to north between 06-12z Wednesday as the aforementioned band of stratiform rain lifts northward over western Washington.

Although winds have become less gusty, occasional wind gusts up to 20-25 kt are still being observed inland and up to 30 kt at KONP. Winds will increase along the coast through the day, especially at KONP where wind gusts will likely approach 35-40 kt. The remaining TAF sites will see wind gusts increasing late this afternoon into tonight.

KPDX AND APPROACHES...Expect steady stratiform rain to persist through at least 00z Wednesday, heavy at times this morning before rain attempts becoming relatively light this evening. This will result in development of MVFR cigs between 18-21Z, then persisting throughout the TAF period, with occasional dips to IFR possible (20-35% chance) through 00z Wednesday, and then to 60-70% by 01z. Although winds have weakened this morning, expect winds to increase again towards 02-06z Wednesday when southerly to southwesterly wind gusts up to 25-30 kt are likely.

MARINE

Seas are in the 12-13 ft range this afternoon. Winds, which died off a bit this morning, are ramping up again this afternoon and will continue to do so tonight as a stronger frontal system moves in. This will cause gales (gusts to 40 kt) and steep to very steep seas over the inner waters into Wednesday. Gusts of 30 kt are expected over the outer waters with a low chance (30%) of gale force gusts (35 kt). This increase in wind could result in significant wave heights peaking around 14 feet through Wednesday evening. Gales end by Wednesday evening, but steep seas likely continue, so we'll likely need to extend the small craft advisory into Thursday morning. Seas gradually fall below 10 feet Thursday afternoon with relatively quiet conditions expected over the coastal waters Friday and most the weekend. Latest guidance is showing the next chance for stronger winds and higher seas late Sunday night into Monday. -Spilde

BEACH HAZARDS

Tidal overflow continues to be a threat to the coastal communities of south Washington, north Oregon and central Oregon. While tidal stages may be below typical markers for traditional tidal overflow, the increase in flow of area rivers increases the probability for overflow. Tidal overflow concerns remain for this afternoon's high tide, then will surge again during Wednesday afternoon's high tide. The Toke Point tidal gauge is expected to peak near 9.5 ft around 3-4 PM Wednesday and the Willapa River will likely remain above 80% of flood flow during high tide. Therefore, the Coastal Flood Advisory has been extended for the high tide on Wednesday.

There remains 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, which are waves that can run significantly farther up a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock beachgoers off of 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. -27

HYDROLOGY

A strong atmospheric river remains in place across southwest Washington and northwest Oregon this morning, maintaining an elevated threat of urban, river, and small stream flooding through at least late Wednesday. Significant rainfall has already fallen across much of the region since early Monday, and river responses are now well underway. Additional rainfall today through Wednesday from the second moisture surge will prolong elevated flows, with many rivers already exceeding flood stage, or forecast to exceed flood stage within the next 24-36 hours.

Rainfall over the past 24 to 36 hours has already pushed several rivers into flood stage, including record flooding on the Grays River (the old record height was 33.15 ft, which was broken around 10 PM Monday when river levels peaked around 33.4 ft), major flooding on the Naselle River, moderate flooding on the Willapa River, and minor flooding for Nehalem River, Wilson River, Trask River, Beaver Creek, East Fork Lewis River, Miami River, and Washougal River. Elsewhere, many basins continue rising and are expected to respond sharply again as periods of heavy rain continue. While the Siletz River, Cowlitz River (Kelso and Castle Rock), and Johnson Creek are currently not in flood stage or action stage, these rivers are expected to reach minor flood stage late this morning. Expect the Pudding River to reach minor flood stage by late Wednesday.

The threat for flooding along small creeks and streams as well as urban flooding, including roadway flooding, will continue through Wednesday. Latest high-resolution guidance favors a band of heavier precipitation lingering over southwestern Washington and far northwestern Oregon this morning, with rates around 0.25 inch per hour or higher. These heavy rainfall rates could make for a hazardous morning commute along the I-5 corridor from Salem north through Portland/Vancouver to Kelso/Longview with abundant water atop area roadways. Ponding of water on roads will increase risk of hydroplaning and car accidents.

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 in effect across all of southwest Washington and northwest Oregon from now until 4 PM Thursday, except for most of Lane County which has been removed from the Flood Watch.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory until 5 PM PST this afternoon for ORZ101- 102.

Wind Advisory until 10 PM PST Wednesday for ORZ104>115-123.

WA...Coastal Flood Advisory until 5 PM PST this afternoon for WAZ201.

Wind Advisory until 10 PM PST Wednesday for WAZ202>208.

PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210- 251-252.

Gale Warning until 10 PM PST Wednesday for PZZ210-251-252.

Gale Warning until 4 AM PST Wednesday for PZZ253.

Small Craft Advisory from 4 AM to 10 PM PST Wednesday for PZZ253.

Small Craft Advisory until 10 PM PST Wednesday for PZZ271>273.