textproduct: Portland

SYNOPSIS

A series of frontal systems will bring a period of wet weather Thursday through at least the middle of next week across southwest Washington and northwest Oregon. Ensemble guidance suggests a potentially impactful atmospheric river event next Monday into midweek, though uncertainty on exact impacts remains moderate to high.

DISCUSSION...Thursday afternoon through Wednesday

Radar imagery Thursday afternoon depicts rain moving southeast across SW Washington and NW Oregon as the first of many frontal systems pushes into the PacNW. Observations indicate a front has remained stalled just west of the coast through the morning and early afternoon hours with rain mainly focused in a band north of Astoria through Clackamas County. The front is beginning to slowly move southeast, allowing rain to begin spreading farther south into the area, continuing through the night. Due to the northwest orientation of winds with this front, the central and southern Willamette valley as well as Lane County likely won't see much rain with this first round. Another frontal system will move through the region tomorrow, accompanied by an influx of additional moisture, with ensemble guidance indicating IVT values peaking around 500-600 kg/m/s. The upper level flow becomes more zonal with this system tomorrow, allowing for more rain in the Willamette Valley and Lane County than the first system. Total forecast rain amounts for this evening through Friday night top out around 0.3-0.6 inches for the interior lowlands and 1.5-3.0 inches for the coast, Coast Range, and Cascades, with locally up to 4.0 inches possible over the Cascades. Breezy southwest winds will also be possible with the frontal passage on Friday, with gusts most likely peaking around 20-30 mph.

Showers are expected to linger through Saturday as zonal flow continues. Then, another frontal system moves through the area on Sunday. Ensembles indicate IVT values could peak around 300-500 kg/m/s with this frontal system, which would produce another round of widespread rainfall. Total rainfall amounts Saturday through Sunday are most likely to be around 0.4-1.0 inch for the interior lowlands, 0.75-2.0 inches at the coast and Coast Range, and 1.0-3.0 inches for the Cascades except for 0.4-0.8 inches over the Lane County Cascades. But, there remains a fair amount of uncertainty in both the low and high end potential. Impacts are not expected with rain amounts through Sunday, unless rain rates exceed 0.2-0.3 inches for multiple hours over certain watersheds leading to rivers that are prone to flooding quickly, such as the Grays at Rosburg and Willapa River. This will be something to monitor during heaviest periods of rain.

The main period of concern continues to focus on Monday into late next week as ensemble guidance continues to indicate the potential for an impactful atmospheric river event. Over the past 48 hours, GEFS and Euro Ensemble guidance have consistently suggested two rounds of elevated IVT plumes pushing into the PacNW next week, one generally on Monday and another sometime Tuesday into Wednesday, with IVT values likely remaining above at least 250 kg/m/s in between, likely producing a continuous rain event with two rounds of increased precipitation. However, significant uncertainty still remains in the timing of elevated IVT values, location of the moisture plumes along the WA and OR coast, and exact peaks of IVT values. For the first peak, the 12z GEFS mean is around 650 kg/m/s with +1 standard deviation around 850 kg/m/s, and the 12z Euro EPS mean is around 800 kg/m/s with +1 standard deviation around 900 kg/m/s. The second round still has much more variability with both ensembles indicating the mean IVT values around 500-600 kg/m/s with +1 standard deviation around 750-800 kg/m/s. What does this mean? All of these factors have fluctuated between forecast runs over the past 24 hours and will likely continue to fluctuate until we get closer to the event, though ultimately, confidence is increasing slightly in this being a multi-day atmospheric river event leading to at least minor flooding impacts.

The main period of concern for river flooding would be late Monday into Thursday as it takes time after rain begins for rivers to rise. HEFS guidance has introduced chances for a number of rivers to rise to Moderate flood stage (10-25%) and Major flood stage (8-15% chance) for rivers that drain from the Coast Range, Willapa Hills, and north Oregon and south Washington Cascades. The probabilities for specific river points can be found at the National Water Prediction Service website. Signals are also increasing for the potential of areal and small stream flood impacts for the Northern Willamette Valley north into the Washington lowlands (5-10% chance) as well as the Coast Range, Willapa Hills, and Cascades (30-50% chance), including the potential for landslides due to saturated soils. Additionally, there are signals for breezy winds with the frontal passage on Monday, although the ensemble spread remains very wide leading to low certainty in the peak wind forecast. If winds do end up even around 30-40 mph, saturated soils from nearly a week of rain could cause some downed trees, leading to impacts such as localized power outages. This is an event to keep your eye on, especially for those who live in flood prone regions. -03/DH

AVIATION

Radar imagery and terminal observations as of early Thursday afternoon depicts a mix of LIFR/IFR/MVFR CIGs and VIS across all terminals as a warm front lifts over the region and brings rain and mist. High confidence (>90% chance) that IFR/MVFR CIGs and VIS continue through the evening as this system progresses inland. Another frontal system will move in early tomorrow morning after 12-15z Fri, maintaining these low CIGs as the low-level atmosphere remains saturated. Guidance suggests potentially heavier rain with this second system, so there could be brief VIS reductions to IFR or lower for any terminal Friday morning. Winds generally southerly to southwesterly, remaining under 10 kt through late tonight. Southerly winds increase to 10-15 kt Friday morning for most terminals with gusts up to 20-25 kt along the coast.

KPDX AND APPROACHES...High confidence for IFR/MVFR CIGs and VIS through Friday morning as back-to-back frontal systems impact the area. East-southeasterly winds 4-6 kt increase and turn more southerly 8-10 kt after 12-15z Fri. -10

MARINE

Winds are forecast to gradually weaken to 10-15 kt and turn more westerly this afternoon and evening as the current front progresses inland. This lull will be short-lived as another frontal system will push across the waters tomorrow, returning southwesterly winds of 15-20 kt with gusts to 25-30 kt. Small Craft Advisories are in effect across all waters including the Columbia River Bar beginning 4 AM Friday, lasting through early Thursday morning. Seas of 4-5 ft this afternoon will build to around 7-9 ft at 10-11 sec as a northwesterly swell moves in with the next system. Will note that a very strong ebb may lead to higher seas of 10-11 ft in the Main Channel of the Columbia River Bar around 4-5 PM Friday. For the rest of the waters, seas likely (70-80% chance) build above 10 ft by Saturday morning. There is also a 30-50% chance that seas build above 13 ft, with the highest chances north of Cape Lookout and beyond 10 NM.

The parade of fronts continue through the weekend and into early next week. Chances for widespread and frequent Gale force wind gusts of 34 kt or greater remain under 15% through early next week; however, chances for brief and isolated Gale force wind gusts are around 40-50% on Sunday and 50-70% on Monday. Seas briefly subside below 10 ft on Saturday night and Sunday, before building again on Monday with the next system. -10

BEACH HAZARDS

Coastal Flood Advisories remain in effect from for minor tidal overflow during high tide from 9 AM to 3 PM Friday and Saturday for the south Washington coast and Clatsop County coast in Oregon. Minor flooding up to 1 foot above ground level is possible during high tide in low lying areas near bays, sloughs, and the low reaches of the coastal rivers. Coastal residents in the warned area should remain alert for rising water and take appropriate action to protect life and property. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory from 9 AM to 3 PM PST Friday for ORZ101.

Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for ORZ101.

WA...Coastal Flood Advisory from 9 AM to 3 PM PST Friday for WAZ201.

Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for WAZ201.

PZ...Small Craft Advisory until 6 PM PST this evening for PZZ210.

Small Craft Advisory from 4 AM Friday to 4 AM PST Saturday for PZZ210-251>253-271>273.