textproduct: Portland

SYNOPSIS

A slow-moving frontal boundary will bring periods of light rain to southwest Washington and along the far north Oregon coast now through Monday morning, while the rest of western Oregon stays dry with light winds and high clouds. A very strong high pressure system will strengthen and remain anchored over the Pacific Northwest Tuesday through next weekend. This will result in a prolonged stretch of dry weather with abnormally mild temperatures and light winds. Expect at least 6 to 8 consecutive dry days in a row.

DISCUSSION...Now through Saturday night

Aside from periods of light rain over portions of southwest Washington and the far north Oregon coast through Sunday night/Monday morning, the weather forecast over the next 6 to 8 days is dominated by a prolonged stretch of dry and calm conditions with above normal temperatures for this time of year. The primary cause of these conditions is an anomalously strong high pressure system that models and their ensembles continue to show a broad area of high pressure anchored over the Pac NW/ NE Pacific through this week and into next weekend. Models also show that the ridge will amplify over western OR and WA Monday through Wednesday. The overall synoptic patters has a well established, upper level ridge that transitions into a Rex Blocking pattern towards the latter part of this week. It should be noted that models are showing the ridge flattening as Alaskan lows ride the crest of the ridge, but these systems are not expected to break the overall blocking pattern. 500 mb heights look to peak near 590 dm over northern California and southern Oregon, and near 585 dm over northern Oregon. Note these values are around the 95th-99th percentile compared to climatology, which means high pressure of this magnitude is quite rare this time of year, especially given the extended duration.

At this time, it is not clear exactly when the ridge/blocking pattern will break down and precipitation will return, however some model guidance is suggesting that a pattern change could occur around Jan. 21-23 with precipitation coming back into the conversation. Still, confidence on this occurring is very low given that we are looking at model solutions around 10 days in the future. Also, blocking patterns (via forecaster pattern recognition) will typically last 7-8 days or 10-11 days. Until then, expect more of the same each day with varying degrees of cloud cover, light winds, and mild temperatures with afternoon highs running anywhere from 5-15 degrees above normal for this time of year as daytime highs are expected to be generally in the 50s to 60s. The NBM suggests highs will range in the low to upper 50s lowlands, mid 50s to mid 60s in the Cascade foothills and low to upper 50s along the Coast. Currently, the warmest days in the forecast are Thursday and Friday.

The coolest daytime temperatures will occur in portions of the central and southern Willamette Valley where chances for fog and low clouds each night/morning may limit daytime heating and keep highs closer to 50 degrees. The reason the Oregon Cascade foothills will likely be warmest is due to a combination of more hours of sunshine and the fact that these elevations are high enough to poke above a strong low-level temperature inversion that will remain in place through the week, thus tapping into relatively warmer air aloft. Locations such as Oakridge will feel more spring like rather than typical mid January daytime highs.

It should be noted that a high pressure aloft will result in a rather stable atmosphere IE...minimal air mixing and movement. This will lead to the potential for degraded air quality as this pattern is expected to persist. So, prolonged multi-day stretch of very low vertical mixing heights, which helps trap air pollutants near the surface, will bring about stagnant air.

As a result of these conditions, we have issued a Air Stagnation Advisory for the majority of the Willamette Valley and it could expand as the week progresses. State air quality agencies highly recommend that no outdoor burning occur and that residential wood burning devices be limited as much as possible. According to state air quality agencies, prolonged periods of stagnant air can hold pollutants close to the ground where people live and breathe. Check with your local burn agency for any current restrictions in your area.

Also, people with respiratory illness should follow their physician's advice for dealing with high levels of air pollution during periods of stagnant air, especially if an Air Quality Alert is in effect for your area. To see if an Air Quality Alert is in effect for your area, go to www.weather.gov/pqr and left- click your location on the homepage map display. Alternatively, you could also type your location into the search bar found on the top-left hand side of the homepage. From there, you will see an Air Quality Alert listed at the top if one is in effect for your location; you can click the alert to read more details if applicable. You can also find the current Air Quality Index (AQI) for your location at www.airnow.gov. /42-23

AVIATION

This afternoon VFR conditions currently prevail across the region outside of KAST where the southern edge of a frontal boundary has kept occasional light rain and MVFR CIGs in place. Headed through tonight the primary flight impacts will be associated with the potential for patchy fog within portions of the Willamette Valley and Tualatin valley. Confidence is still rather low regarding exactly where and when fog will develop - if it even develops at all. Similar to the past couple of nights, broken high clouds will largely work against widespread fog formation, but this setup has generally resulted in patchy fog and fluctuating visibilities at KHIO, KSLE, and KUAO. KHIO has the highest change for fog of any TAF site, with probabilities reaching 30-50% for surface visibilities below 1 SM between 09-18z Monday. Probabilities for fog range between 15-30% at KUAO, KSLE and KEUG. We'll also have to watch a push of low clouds from the west at KONP which would degrade flight conditions after 10-14z Monday. Otherwise, VFR conditions are favored to persist at KPDX and KTTD. It's worth noting just outside of the TAF period (Monday night into Tuesday morning) probabilities for fog and LIFR/IFR conditions across the inland valleys increase significantly.

KPDX AND APPROACHES...Expect VFR flight conditions to prevail through the TAF period as high clouds stream overhead. Light ESE winds continue as well helping to prevent any fog formation locally. -99

BEACH HAZARDS

There is a high threat of sneaker waves at the coast through Monday afternoon due to a continued long- period southwesterly to westerly swell. Sneaker waves can create potentially life-threatening conditions as waves may run farther up the beach than expected. These waves can easily catch people off guard and cause beachgoers to be knocked off their feet and pulled out into cold ocean waters. Never turn your back to the ocean. Caution should be used when in or near the water, and those with children should be especially watchful. Sneaker waves can lift or roll large, heavy logs or rocks which can lead to serious death or injury. Keep off rocks, logs, and jetties near the waterline on beaches. /42

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 4 AM PST Friday for ORZ109>111- 114>118.

WA...Air Stagnation Advisory until 4 AM PST Friday for WAZ205-206.

PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210.

Hazardous Seas Warning until 4 PM PST Monday for PZZ210.

Hazardous Seas Warning until 4 PM PST Monday for PZZ251-252-271- 272.

Small Craft Advisory from 4 PM Monday to 10 AM PST Tuesday for PZZ251-252-271-272.

Small Craft Advisory until 4 AM PST Tuesday for PZZ253.

Hazardous Seas Warning until 10 PM PST this evening for PZZ273.

Small Craft Advisory from 10 PM this evening to 4 AM PST Tuesday for PZZ273.