textproduct: Portland

SYNOPSIS

A relatively quiet stretch of weather comes to an end late tonight as a series of frontal systems bring periods of rain and breezy conditions through the weekend and into early next week. The first system arrives Friday, followed by another round of rain Sunday. High surf will also be a concern along the coast heading into the weekend.

DISCUSSION...Now through Tuesday

Upper-level ridging will continue to influence the Pacific Northwest today, keeping most of northwest Oregon and southwest Washington dry. The ridge is gradually weakening as an upper-level trough approaches the northeast Pacific, but the associated moisture will largely remain north of the forecast area. A few showers could still reach the north Oregon and south Washington coasts, particularly near Clatsop and Pacific Counties, where probabilities remain around 15-30%. Interior temperatures will trend slightly cooler today, with highs generally in the upper 50s to low 60s, though still above seasonal averages.

The first in a series of frontal systems is expected to reach the region late tonight into Friday. This will bring a return to widespread rainfall and cooler temperatures, with a greater than 70% chance for most locations to receive at least 0.25 inches of rain by early Saturday morning. The potential for heavier rainfall is greatest along the coast, particularly from Tillamook County southward, where probabilities of 24-hour totals exceeding 1 inch range from 40-70%. Probabilities along the I-5 corridor are lower, generally around 5-20%.

Southerly winds will increase along the coast as the front moves through, with gusts generally reaching 30-40 mph. While there is only a 10-20% chance of coastal gusts exceeding 55 mph, isolated tree damage and power disruptions cannot be completely ruled out. Inland wind impacts appear more limited, with only a 5-10% chance of gusts exceeding 40 mph along the I-5 corridor. Securing loose outdoor items ahead of the incoming system remains a good precaution.

Snow levels will remain relatively high Friday, around 5500-6500 feet, keeping the Cascade passes free of accumulating snow. As the front moves east Friday afternoon, snow levels lower to around 5000-5500 feet through Saturday morning before rising to above 6000 ft by Saturday afternoon. This could allow for some light snowfall near Willamette Pass, while significant impacts to the passes are not expected. Precipitation should gradually diminish Saturday as the first system exits, providing a brief break before the next round of rain arrives.

Another frontal system is forecast to move into the region Saturday night into Sunday, bringing a renewed widespread rainfall and breezy conditions, especially along the coast. There remains some uncertainty regarding precipitation amounts, but current guidance indicates a 60-70% chance of 24 hour rainfall totals exceeding 1 inch along the coast from Saturday evening through Sunday evening. Probabilities decrease considerably farther inland, with only a 5-15% chance along the I-5 corridor. Snow levels are expected to stay above 5500 feet, keeping the Cascade passes primarily rain.

Beyond Sunday, ensemble guidance continues to favor upper-level troughing across the West Coast into early next week. While the timing and strength of individual systems remain uncertain, the broader pattern supports continues cool and unsettled weather with additional opportunities for precipitation.

The repeated rounds of rainfall through the weekend and into next week could gradually increase soil saturation and runoff into area rivers and streams. The potential for widespread river flooding remains low, currently below 5%, though isolated flooding cannot be ruled out. More localized issues may develop if heavier rainfall rates occur, including ponding on roadways and flooding in urban areas, small streams, and other locations with poor drainage. Specific rainfall rates remain uncertain at this range, but residents can take simple precautions by clearing leaves and debris from gutters and storm drains ahead of the wetter pattern. ~12

AVIATION...Through 12Z Friday

Southwest flow aloft will continue with mostly dry conditions across the area. The only exception is along the northern OR and southwest WA coast where light rain is possible through the morning. Thinning high clouds across the airspace has allowed some fog to develop in the central and southern Willamette Valley and the western metro around KHIO. Expect terminals with these deteriorated conditions to improve back to VFR by 18-21Z Thu. Along the coast, a mix of MVFR/VFR conditions will gradually clear and improve to predominantly VFR by 18Z Thu.

Expect mostly light and variable winds within the central/southern Willamette Valley, easterly winds around the Portland Metro, and southerly winds along the coast. Winds along the coast will increase this afternoon as another front approaches, with gusts up to 25-30 kt expected. A few inland terminals will see winds approach 10 kt around 06Z Fri as the front moves inland.

KPDX AND APPROACHES...Broken high clouds continue to stream overhead with VFR conditions expected through the TAF period. There is a low 20-30% chance for MVFR CIGs from now until 17Z Thu. Easterly winds around KTTD should inhibit any low cloud cover. Light easterly winds around 5 kt will increase towards 10 kt around 06Z Fri. ~12

MARINE

A stalled frontal boundary offshore will continue to weaken through this morning. Seas of around 12 to 16 ft with a period of 13-15 seconds lingers this morning, maintain marginal hazardous seas across the coastal waters. By this afternoon, southerlies again ramp up ahead of the next low pressure system that will approach the area later Thursday into Friday. A strong Gale Force front is expected to peak early Friday morning, with south to southeasterly winds gusting over 34 kt for potentially over 24 hours. There is also a 40-50% chance for Storm Force wind gusts to occur across the outer coastal waters during the peak timing, but probabilities remain too low to upgrade to a Storm Watch. Therefore, the Gale Watch has been upgraded to a Gale Warning, and is in effect for all coastal marine zones from this evening through Friday afternoon.

Seas are expected to become very steep and hazardous by Friday as a fresh south-southwesterly swell likely develops, building seas to around 14 to 18 ft with a dominant period of around 10-11 seconds. This will be followed by a strong, long-period westerly swell that is expected to build seas above 20 ft Friday afternoon, with a 40-70% chance of seas greater than 25 ft. There is also a 10-30% chance for significant wave heights reaching 30 ft in the outer waters, with highest chances to the south. Another strong front is expected to move across the waters later Saturday into Sunday with high chances for another round of strong southerly Gales, while seas remain elevated through the weekend. ~12/DH

BEACH HAZARDS

A moderate to high sneaker wave threat continues along the coast through this week as a westerly swell of 12 to 16 ft at 14 seconds continues today. This will result in waves running up significantly farther on a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning.

Additionally, confidence is increasing that a stronger westerly swell of 18 to 24 ft with a dominant period at 16-18 seconds will impact the coast later Friday into Saturday for dangerous surf conditions. Destructive waves may wash over beaches, jetties, and other structures unexpectedly.

Extreme caution should be used if you plan to visit beaches this week. The large wave runup may dislodge logs on area beaches and result in them rolling up and down area beaches, which have historically killed or seriously injured unsuspecting people, so there will be much more of a threat than just getting wet from a rogue wave.

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 10 PM PST this evening for PZZ210.

Gale Warning from 10 PM this evening to 4 PM PST Friday for PZZ210.

Hazardous Seas Warning until 7 AM PST this morning for PZZ251>253-271>273.

Small Craft Advisory from 7 AM this morning to 7 PM PST this evening for PZZ251-272-273.

Gale Warning from 7 PM this evening to 4 PM PST Friday for PZZ251-272-273.

Small Craft Advisory from 7 AM this morning to 1 AM PST Friday for PZZ252-253.

Gale Warning from 1 AM to 4 PM PST Friday for PZZ252-253.

Small Craft Advisory from 7 AM this morning to 4 PM PST this afternoon for PZZ271.

Gale Warning from 4 PM this afternoon to 4 PM PST Friday for PZZ271.