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.

SHORT TERM...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 =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 back 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. Recent high resolution models have reduced the precipitation amounts slightly as the warm front is shifting a bit further north. Rainfall totals between now and Wednesday at 5 PM, there is around a 50-70% chance of anywhere from 2-32 inches of additional rain in the south Washington Cascade foothills, 2.5-3.5 inches in the Coast Range north of Tillamook, and around 0.25-0.5 inch in the Willamette Valley.

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

The atmospheric river continues to push over the region though it targeting further north than previous forecasts suggested. Overall a mixture of MVFR and IFR CIGS/VIS with a few areas of low VFR in the southern Willamette Valley. Winds remain southerly which will create breezy conditions within the Willamette Valley and along the coast but will miss the Columbia River Gorge. One component to note though is the potential for LLWS at around 2000 ft. This LLWS will be isolated to those areas moreso sheltered from the surface winds.

Heaviest rain will ease after 12Z Wed however overcast skies still expected. Models are showing around a 20% chance of IFR conditions within the central and southern Willamette Valley, around 40% chance in the central and nearly 100% chance along the coast. Ultimately will be quite variable and difficult to track.

KPDX AND APPROACHES...Persistent stratiform rain through the next 10 hours with overcast skies through the next 24 hours. Will see minimal change in category levels though will see fluctuations within the flight categories. A component to be aware of is the potential for LLWS. While winds will be elevated around 2000 ft at KPDX and on the approaches, surface winds were elevated enough to omit LLWS from the TAF. -27

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

An atmospheric river remains in place across southwest Washington and northwest Oregon maintaining an elevated threat of river, and small stream flooding through at least Wednesday. Significant rainfall has already fallen across much of the region since early Monday, but the system is shifting slightly more north and thus rainfall has begun to ease. A second round of heavier rain though is expected early Wednesday morning bringing another surge of water to area rivers and streams. At this point, rain rates are not expected to be nearly as robust and thus urban flooding is less of a risk. 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. Currently, warnings are in effect for the Grays, Cowlitz, Nehalem, Wilson, Siletz and the Pudding. The Pudding is a slower responding river and thus may not see impacts until later Wednesday. Flood Advisories remain in effect for much of southwest Washington and the north Oregon Coast.

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. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...Wind Advisory until 10 PM PST Wednesday for ORZ104>115-123. WA...Wind Advisory until 10 PM PST Wednesday for WAZ202>208. Coastal Flood Advisory from 10 AM to 7 PM PST Wednesday for WAZ201. PZ...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.