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.

SHORT TERM...Today through Wednesday

The strong atmospheric river (AR) event has begun across the Pacific Northwest, with radar imagery showing moderate to locally heavy rain across northwest Oregon and southwest Washington as of early Monday afternoon. Satellite imagery clearly shows a long, narrow band of moisture extending from the Pacific Northwest out 3000+ miles offshore, and about 1200 miles northwest of Hawaii. This atmospheric river is being driven by a sub-tropical ridge centered just offshore of Southern California and Baja California (~32.05N/130.94W), and a trough centered south of the Aleutian Islands of Alaska (~45.79N/164.89W). Moisture is being advected along the northwestern periphery of the ridge and the southeastern periphery of the trough, leading to the extended band of moisture progressing toward the Pacific Northwest. Ensemble guidance suggests minimal change in the synoptic pattern over the next 48 hours; therefore, this atmospheric river event and associated impacts are expected to last through at least Wednesday.

There are one of two surges of moisture coming into the Pacific Northwest with this AR. The first is coming in right now, which is associated with integrated vapor transport (IVT) values of 750-900 kg/ms based off GFS and Euro ensemble guidance. This will maintain steady rainfall and lead to substantial rainfall amounts across western Washington and far northwest Oregon (especially Salem northward). Lower rainfall amounts are forecast for the southern Willamette Valley (Eugene) as most of the moisture is being directed further north. Urban and small stream flooding is possible throughout this AR event, especially in areas that are low-lying and have poor-drainage. If you will be commuting during periods of heavy rain, make sure to slow down, use your headlights, and leave plenty of distance ahead of you.

Since 12 AM Monday, weather stations have already reported 1.5-2.25 inches of rain across the north Oregon and south Washington coasts, Willapa Hills, north Oregon Coast Range, and south Washington Cascades and foothills. These amounts were achieved by heavier rainfall rates of 0.25-0.50" per hour. Based off the latest REFS guidance, most ensemble members suggest that these rainfall rates will continue through late Monday evening into early Tuesday morning over the aforementioned areas. Rainfall rates this high will result in rivers and creeks that drain off the Willapa Hills and Clatsop/Tillamook Counties to start rising sharply through the evening (see hydrology section for more details on river flooding). Elsewhere, rainfall amounts since midnight generally range from 0.25-1", with the highest amounts further north you go. South of Albany, OR and across the Lane County Cascades, amounts have been around 0.10" or less, which is expected given the more northward track of the AR.

This plume of AR moisture will briefly track southward tonight into early Tuesday morning, bringing increased rainfall toward Lane County. Will see a very brief break in rainfall rates across far northwest Oregon and southwest Washington Tuesday morning, but that doesn't mean we'll be clear of any impacts. Local rivers tend to lag a few hours following heavy rainfall, so will most likely see rivers continuing to rise and potentially hit flood stage even when rain has decreased. Again, more details in the hydrology section on this.

The second surge of AR moisture will hit western Washington and far northwest Oregon Tuesday afternoon into Wednesday, bringing another round of moderate to locally heavy rainfall to these areas. It will be a similar set-up as today, but this time ensemble guidance suggests IVT values will be around 600-700 kg/ms. Given the amount of rain that will have fallen before this second round, local rivers will continue to rise and concerns for urban and small stream flooding will continue. For snow and winter sport lovers, unfortunately this AR will be very warm and snow levels will remain above 8000 feet through Wednesday.

Wind will also potentially bring impacts during this AR event. Wind Advisories remain in effect through 4 AM Tuesday across the central/north Oregon Coast Range, Willapa Hills, Willamette Valley from Salem northward, and southwest Washington lowlands. Southerly winds are forecast to gust up to 30-40 mph tonight. Saturated soils will increase the likelihood that even moderate gusts could cause uprooting of trees and lead to damage. While there is moderate to high confidence in potential impacts due to wind, there is low confidence on exactly where impacts may occur. If you see a downed power line, move away from it. Do not drive over downed power lines. If you come into contact with a downed power line, stay inside the car and call for help. -10

LONG TERM

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

Messy conditions are expected to persist through the TAF period and beyond as westerly flow brings moisture laden air into the region. A steady stream of rain will continue through the afternoon with a short lull this evening. CIGs inland are expected to maintain MVFR but could see brief drops to IFR in the heaviest rainfall. KEUG will be a potential exception as rain is generally expected to be lighter over the southern Willamette Valley and may not see persistent VIS or CIG reductions. CIGs along the coast remain IFR or LIFR through most of the period. Could see a brief period of improvement for KAST and KPDX/KTTD as the stream of moisture shifts south late tonight into early Tuesday morning before lifting back northward Tuesday afternoon.

Winds will be gusty through much of the period. Expect wind gust of 35-45 kt along the coast up to 25-35 kt within the Willamette Valley.

KPDX AND APPROACHES...MVFR conditions are expected to persist through much of tonight. A steady stream of rain will keep low CIGs and moderately impacted VIS in place. Moderate to heavy rain through the afternoon will lighten up a bit this evening before increasing again overnight. Guidance suggests a short period of improving CIGs early Tuesday morning as the stream of moisture shifts slightly to the south. Could see a brief period of clouds lifting above 3000 feet before dropping back below that mark by the afternoon. Winds will be southerly around 20 kt with gust to 25-35 kt through around 12z Tuesday. /19

MARINE

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

A lull is expected late tonight 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 keep conditions very wet through much of this week. The most notable period will be 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. Therefore, a Flood Watch remains in effect across 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, Flood Warnings have been issued for the Grays River at Covered Bridge in Wahkiakum County, the Wilson River near Tillamook in Tillamook County, and Johnson Creek at Sycamore in Clackamas and Multnomah Counties.

Major flood stage at the Grays River is forecast, which will lead to flooding of lowland areas and roads in the vicinity of Highway 4 and Rosburg. there will be two peaks for this river. There is high confidence that the first peak will occur between 1 AM to 4 AM Tuesday. The river will briefly subside between 4 AM to 10 AM Tuesday, but a second round of heavy rain will result in another peak late Tuesday into Wednesday. For the Wilson River, Minor flood stage is forecast from late tonight to late Wednesday morning, and for Johnson Creek, Minor flood stage is forecast from Tuesday morning to Wednesday evening. Both the Wilson River and Johnson Creek have a 50% chance of reaching Major flood stage. If you know you live in a flood- prone area, make sure to monitor the latest weather and river forecasts, and make a plan for potential impacts.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory until 6 PM PST this evening for ORZ101- 102.

Wind Advisory until 4 AM PST Tuesday for ORZ104>115-123.

WA...Coastal Flood Advisory until 6 PM PST this evening for WAZ201.

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.