textproduct: Portland

SYNOPSIS

A steady, moisture-rich pattern continues through midweek as two strong atmospheric river surges arrive Monday and again Tuesday night to Wednesday. Today will see comparatively lighter rainfall, but conditions become increasingly impactful Monday and Tuesday as deep subtropical moisture and stronger winds move inland. River rises and localized flooding will need close attention through the week.

DISCUSSION...Today through Friday

Light to moderate rainfall will continue at intervals today under upper level zonal flow. A passing disturbance this evening will maintain steady but manageable rain rates, with amounts today lower than what is expected early in the week. Rainfall totals through tonight are projected to reach 0.2 to 0.4 inch across the interior lowlands, 0.4 to 0.8 inch along the coast, 0.8 to 2.0 inch in the Coast Range, and 1 to 2.5 inches in the Cascades. These totals alone are not expected to produce widespread impacts, but they will further saturate soils ahead of the first major surge of moisture on Monday.

Sunday night into Monday marks the transition to a significantly wetter regime as the first atmospheric river strengthens offshore and moves inland. IVT values Monday are expected to rise sharply, reaching 600 to 800 kg/ms in many ensemble members. Confidence is high that this surge will produce widespread heavy rain beginning Monday morning, with notably heavier rain amounts beginning Monday evening and continuing overnight into early Tuesday. Rainfall amounts for Monday continue to increase each model cycle. Current expectations from 4 AM Monday to 4 AM Tuesday place 1.25 to 2.50 inches across the interior lowlands, with the highest amounts centered over southwest Washington, where 1.9 to 2.2 inches are likely in the Portland-Vancouver corridor. Coastal areas will see 3 to 4.5 inches, the Coast Range 4.5 to 7 inches, and the Cascades 4 to 7 inches. The Lane County Cascades will be lower, generally 0.75 to 1.75 inches. These amounts will occur on already softened ground, prompting notable river responses and elevated hydrologic concerns (see hydrology section below for more details).

Winds will likely be a significant impact on Monday through Wednesday, notably on Monday. Confidence is high that inland locations will experience gusts of at least 30 mph on Monday, while coastal communities and headlands will see at least 40 mph. There is a 5 to 10% chance of advisory-level southwesterly gusts of 45 mph or greater within the Willamette Valley on Monday. Along the coast and the Coast Range, there is a 40 to 60% probability of gusts reaching or exceeding 50 mph. Should gusts of this magnitude occur, scattered downed trees and power outages will be possible. Given that soils are already wet and will continue to saturate further, root stability will be compromised, raising the risk of tree damage and associated impacts.

The second atmospheric river surge arrives Tuesday night into Wednesday and will closely follow the substantial rainfall produced on Monday. IVT values with this second wave generally center around 700 to 800 kg/ms, with ensemble solutions allowing for a high-end scenario of 850 to 950 kg/ms and a lower scenario of 550 to 650 kg/ms. The duration of this second round may be longer than the first surge, maintaining high confidence in additional heavy rainfall. Rainfall on Tuesday will be somewhat moderated during the day, but heavier rates redevelop Tuesday night and persist through Wednesday. Overall from 4 AM Tuesday to 4 AM Thursday, expect 3.5 to 5 inches across the interior lowlands, with the highest amounts over southwest Washington and lowest amounts in the southern Willamette Valley. The Portland/Vancouver corridor will likely receive 4.5 to 5 inches. Coastal areas will see 5.5 to 8.25 inches (highest amounts in Tillamook), the Coast Range 8.25 to 12 inches, and the Cascades 7.5 to 11 inches. Similar to Monday, the Lane County Cascades will be lower, generally 4 to 7.5 inches. Combined totals from both surges will be substantial and will almost certainly produce localized flooding somewhere in the forecast area. Rivers draining the Coast Range remain the most sensitive to rapid rises.

Winds will stay elevated Tuesday and Wednesday, though Monday remains the peak wind day. Inland gusts around 30 mph and coastal gusts around 40 mph are expected during this period, with the potential for isolated stronger gusts depending on the timing of each frontal passage. Again, due to saturated soils and compromised root stability, scattered downed trees and power outages will be possible.

Snow levels will remain well above 6000 ft through midweek due to the warm subtropical air mass, ensuring that nearly all precipitation in the Cascades falls as rain. By Thursday and Friday, the moisture plume weakens and transitions toward a showery pattern. Conditions will gradually ease, but rivers and soils will remain sensitive due to cumulative rainfall.

Forecast confidence is moderate regarding the timing and intensity of Mondays surge and moderate for the Tuesday to Wednesday period due to spread within the ensemble clusters. Overall, the early to midweek timeframe remains the primary focus for hydrologic and wind-related impacts. See the hydrology section below for discussion related to the Flood Warnings/Watches and river forecasts. ~12

HYDROLOGY

A series of frontal systems will keep conditions very wet through the weekend and much of next week. The most notable period will be late Monday 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 rain amounts (from 4 AM Monday through 4 AM Thursday) up to one foot in the Cascades and Coast Range, up to 8-10 inches at the coast, and up to 7.5-8.5 inches in the Willamette Valley, Portland/Vancouver metro and Cowlitz Valley. While rain amounts this high are unlikely to occur, they represent a reasonable worst case scenario, and are only 1-3 inches higher than the official forecast. This worst case scenario would result in widespread moderate to major flooding.

Although rain will likely become lighter Thursday into Friday, the threat of flooding will linger along slow responding rivers. Most rivers draining the Coast Range and Willapa Hills have a 30-75% chance of reaching moderate flood stage and a 20-50% chance of reaching major flood stage. Additionally, several Willamette River tributaries that drain from the Coast Range and Cascades have anywhere from a 60-80% chance of reaching minor flood stage and a 20-40% chance of reaching moderate flood stage. Flooding of the Willamette mainstem rivers is unlikely to occur (less than 10%). Probabilities for specific river points can be found at the National Water Prediction Service website, which do vary quite a bit from river to river.

There is also increasing confidence in the potential for flooding along small creeks and streams as well as urban flooding, including roadway flooding. Water pooling on roadways will increase the 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 for all of southwest Washington and northwest Oregon from 4 AM Monday to 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, which currently have the highest chance of reaching at least minor flood stage within the next 36-48 hours:

Wilson River near Tillamook affecting Tillamook County, from Monday evening to late Tuesday morning. This river has a 75% chance of reaching moderate flooding and a 50% chance of reaching major flooding. Grays River at Covered Bridge near Rosburg affecting Wahkiakum County, from Monday evening to early Thursday morning. This river has 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 Monday evening to late Wednesday evening. This creek has a 70% chance of reaching moderate flooding, and a 60% chance of reaching major flooding.

AVIATION

At 1130z Sunday, a mix of VFR/MVFR cigs and visibilities were being observed across the area, with predominately VFR cigs inland and predominately MVFR at the coast. That said, still expecting MVFR cigs to become more widespread for inland areas through 18z Sunday as persistent stratiform rain continues across northwest Oregon and southwest Washington and probabilities for cigs below 3000 ft increase to 60-80%. There is also a 15-25% chance for periods of IFR cigs Sunday morning and afternoon, except 50-60% at KAST and 80-90% at KONP. By 00z Monday, expect persistent stratiform rain to transition over to light rain showers.

KPDX AND APPROACHES...Persistent light rain will continue through 19-20z Sunday before transitioning to off and on light rain showers. Expect VFR cigs to eventually fall to high-end MVFR thresholds by approximately 14z Sunday. This is also when low level wind shear will increase. With SW winds around 30-35 kt at 2000 ft and 5-10 kt winds out of the SE to E at the surface, there will be a period of modest low level wind shear at KTTD and KPDX Sunday morning. -23

MARINE

A frontal system moving over the coastal waters this morning will bring the continuation of south to southwest winds with wind gusts upwards of 20-35 kt, strongest over the central and southern waters where a marginal Gale Warning remains in effect. Winds will decrease substantially Sunday afternoon and evening, resulting in a brief period of benign conditions with seas under 8-9 ft and wind gusts falling below 20 kt.

However, this break will be short-lived. A relatively stronger frontal system is set to arrive on Monday, bringing gale force wind gusts up to 35-45 kt, strongest over the inner waters and near the Columbia River Bar. Gale Watches have been upgraded to Gale Warnings now that confidence levels are high and probabilities for gale force gusts are up to 80-90%. 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. Seas decrease slightly Tuesday into Wednesday but remain above 10 ft, with a period of breezy westerly winds up to 25-30 kt likely Tuesday night (70-80% chance). Seas and winds subside late in the week. -23

BEACH HAZARDS

Tidal overflow flooding is possible around high tide on Monday and Tuesday, December 8-9. 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. With several coastal rivers likely to reach at least 80% of flood flow and total tide forecasts peaking near or slightly above 9.5 ft, Coastal Flood Advisories may need to be issued along all of portions of the coast if confidence increases. Since the high tide alone will not be enough to cause tidal overflow flooding, the outcome will be highly dependent on which watersheds receive the heaviest rainfall and thus which rivers reach at least 80% of flood flow. The timing of river level rises will matter too, as it will need to coincide with high tide. On Monday, river levels will be rising quickly, but it appears rivers may stay below 80% of flood flow during high tide before climbing above 80% Monday evening/night. If the timing is correct, this should mitigate flood concerns around high tide. On Tuesday, confidence is higher coastal rivers will reach at least 80% of flood flow during high tide, but that is when the total tide generally dips below 9.5 ft. Will need to monitor both days closely and see how the river forecasts evolve. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210- 251-271.

Gale Warning from 7 AM to 10 PM PST Monday for PZZ210-251-271.

Gale Warning until noon PST today for PZZ252-253-272-273.

Gale Warning from 7 AM Monday to 1 AM PST Tuesday for PZZ252-253- 272-273.