textproduct: Portland

SYNOPSIS

A strong atmospheric river is underway across the Pacific Northwest, bringing heavy rainfall, breezy winds, and widespread hydrologic concerns. Widespread river flooding is likely across the region, 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.

LONG TERM...Wednesday Night through Sunday

The atmospheric river event weakens Wednesday afternoon into Thursday as moisture transport gradually decreases. Light residual showers continue, with some localized areas of moderate rain showers over higher terrain across southwest Washington and the north Oregon Coast Range. By Thursday, the pattern transitions toward a more typical post- frontal regime with shower activity gradually decreasing and winds weakening. Friday becomes drier as high pressure builds over the region. Snow levels will also gradually lower heading into the weekend, but still remain above pass-level at around 5500-6500 feet. Despite the weather becoming more tame late in the week, rivers and soils will continue responding to the cumulative rainfall load from the current atmospheric river event. Any slow-responding rivers within flood stage will need extra time to subside. -10/12

AVIATION

The strongest portion of the first stage of the atmospheric river continues to shift southward today with the frontal edge hitting the south Washington coast. This system has been exhibiting wind speeds around 30 kt at the surface with some exposed areas within the Columbia River Gorge above 3000 ft MSL seeing gusts up to 65 kt. The winds are beginning to ease inland though are continuing to blow along the coast. Widespread gusts up to 30 kt expected along coastal terminals. Winds inland remain gusty with speeds around 20-25 kt. We are beginning to see the shift in flight conditions as the system shifts southward. Northern runways are beginning to see CIGs lift to VFR while coastal and central runways are remaining IFR or MVFR. Through the night the front will bring the southern portions of the forecast area the MVFR conditions.

Through the day on Tuesday after 18Z, the atmospheric river will move north once again. Think of this system doing the "wave". As it does so, the areas that saw a brief reprieve will once again deteriorate. Overall, while a dynamic system with many different moving parts, the impacts will be similar through the next 24-hours.

Will add, that while it does not impact most of our terminals or runways, wind speeds at around 5000 ft MSL are gusting as high as 80 kt then decrease to around 70 kt after 21Z Tue. This is a very active jet stream.

KPDX AND APPROACHES...A mixture of VFR and MVFR conditions over the next 24 hours. Timing has been incredibly difficult due to the movement of the atmospheric river. Portland has been sitting in the bullseye of the system and therefore winds and rain have been strong. These conditions will continue to improve over the next few hours, but will deteriorate once again after 18Z Tue as the frontal system moves right back over. While winds will decrease through the day, winds aloft remain quite strong. -27

MARINE

Gales continue across all zones as a strong system passes over the region. Wind gusts 35-45 kt will continue through late tonight - strongest over the inner waters. The gale warning remains in effect until 1 AM Tuesday.

A lull is expected early Tuesday morning as the frontal boundary drops southward, decreasing winds from north to south. Wind gusts are expected to drop below 20 kt everywhere except zones PZZ273 and PZZ253. Seas will also remain steep during this time so a Small Craft Advisory will follow the gale warning from 1 AM to 7 PM Tuesday to cover these elevated conditions. Another front is expected to pass through the waters late Tuesday into Wednesday and will push wind gusts closer to 30-35 kt. There is increasing potential for gales Tuesday evening into the overnight and may require an upgrade to another Gale Warning. Seas will increase into the low to mid teens with this second system, reaming above 10 feet through Wednesday night. -19

BEACH HAZARDS

Tidal overflow flooding around high tide is 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 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 until 6 PM PST today 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

HYDROLOGY

A series of frontal systems will maintain very wet conditions through much of this week. The most notable period will extend through late Wednesday, when a long-duration atmospheric river will bring significant rainfall and the potential for widespread river flooding across southwestern Washington and northwestern Oregon. There is a 10% chance for 72-hour rainfall of up to 6-8.5 inches in the Cowlitz Valley, lower Columbia, Portland/Vancouver metro and northern Willamette Valley, 4-6 inches in the central Willamette Valley, 3-4 inches in the southern Willamette Valley, 7-11 inches in the Cascades, Cascade foothills, Coast Range, and the coast, except 3.5-7 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. Chances for high total rainfall amounts as well as brief periods of locally heavy rainfall in Lane County have continued to trend downward today. This suggests future river model suites will see similar downward trends in the likelihood of river 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, with a handful of rivers already observing flooding, namely the Grays River draining the Willapa Hills in Major flood and the Wilson River in Tillamook in Minor flood. 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. Flood Warnings have been issued for the Naselle River near Naselle and the Grays River near Rosburg draining the Willapa Hills in southwestern Washington, the Wilson and Trask Rivers near Tillamook, and Johnson Creek at Sycamore near Milwaukie and along the southern edge of the City of Portland.

There is also increasing confidence in the potential for flooding along small creeks and streams as well as urban flooding, including roadway flooding. Latest high-resolution guidance favors a band of heavier precipitation lingering over southwestern Washington and far northwestern Oregon late this evening, before sagging southward through the early morning hours. These potential 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. 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. Therefore, a Flood Watch remains in effect across all of southwest Washington and northwest Oregon from now until 4 AM Friday.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory from noon to 5 PM PST Tuesday for ORZ102. Wind Advisory until 4 AM PST Tuesday for ORZ104>115-123. WA...Wind Advisory until 4 AM PST Tuesday for WAZ202>208. PZ...Gale Warning until 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 until 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.