textproduct: Portland

SYNOPSIS

Today is the last dry day before a series of fronts 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 is very high.

DISCUSSION...Wednesday afternoon through Tuesday

Today is the last dry day in at least the next week before a series of weather systems associated with Pacific moisture produce multiple rounds of widespread rain over NW Oregon and SW Washington. Although today remains dry, satellite imagery indicates mid- to high-level clouds streaming over the region Wednesday afternoon ahead of the first frontal system. These clouds are keeping temperatures on the cooler side with 2 PM PST temperature observations in the low to mid 40s across much of the interior except for the 30s over the Cascades. Areas of patchy fog and frost are possible for interior valleys once again late tonight through early tomorrow morning before rain begins spreading inland.

Upper level high pressure over the region and into the eastern Pacific will become more zonal over the next couple of days as the first weather system moves over the ridge. A warm front associated with this weather system will approach the coast from the northwest late tonight, then stall just off of the coast through tomorrow. This will bring the first round of widespread rain, beginning around midnight to 3AM along the northern coast then quickly spreading inland through the early morning hours with showers continuing through the night. Due to the northwest orientation of winds, the Willamette Valley will likely be rain shadowed and see lower rain accumulations. By Friday morning, the cold front begins moving closer to the coast, pushing the warm front inland. This cold front will bring a stronger surge of moisture with IVT values peaking around 500-700 kg/m/s, and round two of widespread heavier rain impacts the region through Friday afternoon. The flow will be more westerly at this point, allowing more precipitation to fall in the Willamette Valley. The precipitation amounts through Saturday night are expected to be around 0.4-0.6 inches for the central and southern Willamette Valley, 0.6-1 inch from the Portland area north through the I-5 corridor, 0.9-2 inches along the coast and Coast Range, and 0.9-3 inches along the Cascades. Some elevated winds are likely as the cold front passes through on Friday with a 20-40% chance of wind gusts of at least 30 mph for inland valleys and a 50-75% chance along the coast. There's a 20-50% chance of gusts of 40 mph or higher along the Cascades.

Guidance suggests the cold front might stall and weaken over the PacNW into Saturday, with showers lingering through Saturday. Then, another frontal system is slated to move through the area on Sunday. Ensembles indicate IVT values could peak around 400-500 kg/m/s with this frontal system which would produce another surge of widespread rainfall. A bit uncertainty exists in rain amounts during this 48 hour period. Looking at the 25th to 75th percentiles, the interior lowlands could receive anywhere from 0.3-1 inch of rain, lowest amounts in the southern Willamette Valley. The coast, Coast Range, and Cascades could see anywhere from 0.5-2.5 inches, lowest in Lane and Lincoln counties. Impacts are not expected with rain amounts Thursday 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 at Willapa. This will be something to monitor during heaviest periods of rain.

The main period of concern for the forecast period is next week as ensemble guidance continues to indicate the potential for an impactful atmospheric river event Monday through Wednesday. Significant uncertainty still remains in the length of elevated IVT values, location of the moisture along the WA and OR coast, and exact peak of IVT values (the 12z GEFS mean is around 550 kg/m/s with +1 standard deviation around 800 kg/m/s and the 12z Euro EPS mean is around 700 kg/m/s with +1 standard deviation around 900 kg/m/s). These factors have jumped around between forecast runs over the past 24 hours and will likely continue to fluctuate until we get closer to the event. There is the potential that this ends up being another moderate atmospheric river event, similar to the preceding events. If this is the case, impacts are likely to be minimal once again. However, if a strong to extreme atmospheric river event materializes, there is the potential for widespread river flooding and/or wind damage as soils will already be saturated and river levels higher from the rain from the 4 days prior. The main period for river flood concerns is Tuesday into Wednesday as it takes time after rain begins for rivers to rise. HEFS guidance indicates a 10-25% chance of rivers reaching Minor flooding for Coast Range and Coastal rivers and 5-20% chance for Willamette River tributaries, mainly focused on Tuesday into Wednesday, possibly beyond depending on the rain forecast and/or response times of the rivers. Additionally, there are signals for breezy winds with the frontal passage, although the ensemble spread remains very wide leading to low certainty. If winds do end up even around 30-40 mph, saturated soils could cause some localized impacts due to downed trees. Keep an eye on this forecast, especially if you live in flood prone regions. -03

AVIATION

Terminal observations as of early Wednesday afternoon depict a mix of MVFR and VFR CIGs across the Willamette Valley while the coast maintains VFR conditions due to light offshore flow. Expect these CIG trends to continue through the rest of the afternoon and evening. Tonight, light winds and a moist low-level atmosphere will bring a 40-60% chance of IFR/MVFR stratus re- developing in the Willamette Valley after 06-09z Thu. There is also moderate to high confidence (60-80% chance) for LIFR conditions due to fog potential in the south Valley including KEUG, and the Tualatin Valley including KHIO.

After 12-15z Thu, any LIFR/IFR CIGs or VIS in the Valley will trend toward MVFR as the next system approaches the area and returns rain and increased mixing. Meanwhile, CIGs along the coast will begin to lower, with high confidence in IFR and low-end MVFR CIGs (>80% chance) by 21z Thu. Winds generally remain variable and under 5 kt through tonight, turning more southerly in the Valley after 12z Thu with the next system.

KPDX AND APPROACHES...Mix of MVFR and VFR CIGs this afternoon and evening. Rain and a 30-50% chance for MVFR CIGs return after 10-12z Thu as the next system approaches. High confidence for MVFR CIGs after 18-21z Thu. Variable winds around 5 kt or less through the TAF period, becoming more southeasterly Thursday afternoon. -10

MARINE

High pressure over the waters today will maintain northerly winds 10 kt or less through tonight. Seas around 6 to 7 ft continue to subside as well.

Active weather returns as winds turn southerly on Thursday ahead of the first in a series of fronts. Chances for frequent wind gusts greater than 21 kt remain below 15% on Thursday, with winds turning more westerly behind the front. However, another slightly stronger front moving through on Friday will bring a 50-70% chance for wind gusts greater than 21 kt, with the highest chances north of Cape Foulweather. Will likely need to issue a Small Craft Advisory for Friday with the upcoming forecast package. The only Small Craft Advisories in effect right now are for the Columbia River Bar this afternoon and Thursday afternoon due to very strong ebbs bringing seas in the Main Channel up to 9-10 ft.

A series of fronts continue through the weekend and into early next week. There is a 50-60% chance for isolated gale force wind gusts on Monday, but chances for frequent and widespread Gales are only 15-30%. Seas remain around 6-8 ft at 11-12 seconds through Friday, building on Saturday due to an increasing west-northwesterly swell. There is a 60-80% chance of seas exceeding 10 feet by late Friday night into Saturday morning. -10

BEACH HAZARDS

A long-period westerly swell is expected to continue along the northwestern Oregon and southwestern Washington coasts through tonight, bringing a moderate risk for sneaker waves. Waves may run up farther than normal on beaches, including over rocks, logs, and jetties. In addition, a period of perigean spring or "king" tides means these waves will have an even easier time reaching high up onto beaches, limiting the areas which may be safe from wave action. A Beach Hazards Statement therefore remains in effect through 10 PM tonight. Sneaker waves can create potentially life-threatening conditions in the surf zone where beachgoers can be unexpectedly knocked off their feet and pulled out into cold ocean waters. Caution should be used when in or near the water, and those with children should be especially watchful. Never turn your back on the ocean, and be sure to keep an eye on the waves. Those participating in razor clam digs should additionally exercise caution.

Additionally, there is a Coastal Flood Advisory for minor tidal overflow flooding during high tide from 9 AM to 3 PM Thursday and Friday for the South Washington Coast and the 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. Remain out of the water to avoid hazardous swimming conditions. -10

PQR WATCHES/WARNINGS/ADVISORIES

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

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

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

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

PZ...Small Craft Advisory until 5 PM PST this afternoon for PZZ210.

Small Craft Advisory from 2 PM to 6 PM PST Thursday for PZZ210.