textproduct: Portland

SYNOPSIS

An active weather pattern will continue into next week and produce periods of rain and windy conditions, especially along the coast. A large westerly swell will bring high surf Sunday and an elevated threat of sneaker waves through the weekend.

DISCUSSION...Now through Friday

A progressive and active pattern will continue to support repeated chances for rain as system after system affects the Pacific Northwest through the next week. The first vertically-stacked upper and surface low are centered some 500 miles offshore as of early Saturday afternoon, while an associated frontal boundary is oriented SSW-NNE and is expected to reach the coast by 3-5 PM this afternoon. The front will steadily track inland, reaching the Willamette Valley by 7-9 PM this evening. While overall rainfall from the fast-moving system looks unlikely to yield impacts, a brief period of heavy rainfall rates beneath the narrow cold frontal rain band has a 10-20% chance of producing 0.25" or more per hour, which could be sufficient to yield nuisance flooding of urban, low-lying, or other poor-drainage areas, especially where leaf litter has blocked storm drains. These rainfall rates are most likely from 7-9 PM along the coast and within the Coast Range, and 9-11 PM along the I-5 corridor, per short-range ensembles. Behind the frontal boundary, rain will transition to hit-or-miss rain showers which linger through much of Sunday. Possible clearing overnight following the evening rainfall may allow for areas of patchy dense fog, most likely in sheltered valleys of Lane County, where winds will be lighter.

Strong, gusty winds also remain a potential hazard as the front moves onshore. Winds out of the south will gradually build through this evening, peaking near the time of the cold frontal passage. Peak gusts have a 50-60% chance of exceeding 58 mph along the coasts of Tillamook, Clatsop, and Pacific Counties, most likely between 6-8 PM. A High Wind Warning therefore remains in effect from 3-11 PM this afternoon and evening in those areas. Inland, peak gusts will most likely be lower, near 30-35 mph in the 8-10 PM window, but still have a 10% chance of reaching 40 mph anywhere along the I-5 corridor. While gusty winds will ease overnight, strong winds this evening could down trees and limbs and yield at least isolated power outages.

Of note, winds observed at offshore buoys as of 3 PM Sat show the front has been progressed more slowly than expected early this afternoon; if this slower motion persists, the timing of peak winds and rain rates could end up being a few hours later.

There remains strong consensus that the next weather system will move into the region Monday into Tuesday, followed by relatively drier weather Wednesday into Thursday as upper-level ridging quickly amplifies over the Rockies. The amplified pattern will also favor troughing over the Northeast Pacific, allowing for persistent north-northeastward moisture transport within an atmospheric river on its eastern flank. There is substantial uncertainty in the position of this moisture stream at this lead time, although the majority of long-range ensembles place the peak moisture flux to the north along the coast of the Olympic Peninsula as the front nears the coast Friday into next weekend. Chances for 2" or more of rain in 48 hours reaches 40-50% along the coast from Friday through next Saturday night, and 10-25% along the I-5 corridor. Given repeated rounds of antecedent rain, increasingly saturated soils could allow for greater runoff into area rivers, and the forecast will continue to be monitored for potential flooding concerns. -36

AVIATION...Through 18Z Sunday

Satellite imagery and terminal observations as of late Saturday morning depicts a mix of of LIFR, IFR, MVFR, and VFR conditions across the region along with high clouds ahead of the next system. The coast remains predominately MVFR, while terminals across the Willamette Valley are gradually clearing out of the fog and low stratus from early this morning. KUAO and KSLE remain LIFR, however, expect improving conditions to VFR by 19-20z Sat.

Tonight, a cold front will push through the region, returning rain and a brief period of IFR/MVFR CIGs and VIS. Expect these lower CIGs and VIS between 02-05z Sun along the coast, and 05-08z Sun across the Valley during the frontal passage. Behind the front, conditions will quickly improve as rain transitions into showers and CIGs improve to a mix of MVFR/VFR. Winds generally under 10 kt across the area through 21z Sat. After 21z Sat-00z Sun, will see southerly winds gradually increasing ahead of the front. Tonight, expect southerly wind gusts to peak around 35-40 kt along the coast, and 20- 25 kt across the Valley.

KPDX AND APPROACHES...Expect predominately VFR conditions with high clouds through the early evening. Tonight after 04-05z Sun, the next front will push through and return moderate to locally heavy rain along with IFR/MVFR CIGs and VIS through 07-09z Sun. Conditions improve to VFR behind the front with showers. Southerly winds increase after 00z Sun with gusts up to 25 kt through late tonight. Winds gradually decrease after 08-10z Sun. -10

MARINE

An active weather pattern will continue across the waters into next week. The next strong front moves eastward across the waters tonight, bringing strong southerly winds with widespread gales and storm-force wind gusts up to 50-55 kt. Buoy observations as of early Saturday afternoon are showing wind gusts up to 30 kt. The front is moving slower than expected, but winds will gradually increase throughout the afternoon/evening, and peak around 1900-2200 this evening. As the front moves through, southerly winds will shift more southwesterly. Storm Warnings remain in effect for all waters north of Cape Foulweather including the Columbia River Bar through 0400 Sunday, along with Gale Warnings for the waters between Florence, OR and Cape Foulweather. If it appears that storm force wind gusts are coming much later, then the Storm Warning may need to be extended.

Following the frontal passage, a steep, long period westerly swell will push into the waters early Sunday morning. There is high confidence for seas climbing above 20 ft for a 12-18 hour stretch. At this point, it appears seas will likely reach 20 ft between 0100-0400 on Sunday and not drop below 20 ft until after 2200 Sunday. Seas are forecast to peak around 22-26 ft across the northern waters including the Columbia River Bar, and 19-23 ft across the southern waters. This is a forecast that was nudged about 1-3 feet higher than model guidance, as we have seen these large westerly swell events overperform in the past.

Seas gradually subside late Sunday night into Monday. Additional storm systems will bring additional rounds of Gale Force winds to the waters with seas generally hovering between 10-16 ft for much of next week. The next round of Gale Force winds appears most likely (60-80% chance) to arrive Monday into Tuesday, with the highest chances north of Cape Falcon. -10

BEACH HAZARDS

A westerly swell of 10-15 ft at 12-13 seconds this afternoon will bring a moderate to high threat of sneaker waves along the coast. A much larger westerly swell arrives overnight and builds Sunday morning peaking somewhere between 20-27 ft with a dominant period of 15-16 seconds. This will result in beach erosion and bring an even higher threat of sneaker waves along the coast on Sunday. High Surf Warnings and Advisories are in effect from 4 AM to 10 PM Sunday along the entire coast.

Extreme caution should be used if you plan to visit beaches on Sunday. The large wave runup will 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. Given the momentum behind these large longer period waves, it's not out of the question that we receive a report or two of damage to infrastructure along the coast, particularly around high tide midday Sunday. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...High Wind Warning until 11 PM PST this evening for ORZ101-102.

High Surf Warning from 4 AM to 10 PM PST Sunday for ORZ101-102.

High Surf Advisory from 4 AM to 10 PM PST Sunday for ORZ103.

WA...High Wind Warning until 11 PM PST this evening for WAZ201.

High Surf Warning from 4 AM to 10 PM PST Sunday for WAZ201.

PZ...Storm Warning until 4 AM PST Sunday for PZZ210-251-252-271-272.

Hazardous Seas Warning from 4 AM Sunday to 4 AM PST Monday for PZZ210-251>253-271>273.

Gale Warning until 4 AM PST Sunday for PZZ253-273.