textproduct: Portland

SYNOPSIS

High pressure nudges into the region later tonight with dry conditions expected on Wednesday. Widespread fog and frost return tonight through tomorrow morning. A series of fronts will bring a period of wet weather Thursday through the weekend across southwest Washington and northwest Oregon with potentially more impactful rain next week.

DISCUSSION...Tuesday afternoon through Monday

Satellite imagery Tuesday afternoon shows low to mid-level clouds across much of NW Oregon and SW Washington, except for some clearing occurring in the central and southern Willamette Valley as northwest flow aloft increases. The few light showers that linger over the high terrain this afternoon should dissipate by this evening as high pressure builds overhead and into the eastern Pacific, with dry weather continuing into Wednesday. Clearing is expected to continue through the evening hours across the interior lowlands, and CAMs (HREF and REFS) indicate this leads to 70-90% chance for low stratus and 50-70% chance for fog to form late tonight into Wednesday morning across this area. Additionally, temperatures will be on the colder side again tonight, falling into the mid to upper 30s across much of the region except for 40s right along the coast line and 20s over the Cascades.

Our extended cool and mostly dry weather pattern comes to an end Thursday as a series of upper level shortwaves and attendant frontal systems are slated to impact the PacNW into next week. The parent low- pressure system associated with the frontal systems impacting the region Thursday into Friday is currently centered around the Aleutian Islands. This low is funneling tropical moisture north along the west side of the upper level ridge, which will then travel over the ridge then south into the PacNW by Thursday. As the first front moves through the region Thursday from north to south, IVT values are expected to increase above 250 kg/m/s. However, the interior valleys will likely be rainshadowed with not as much rain accumulating, though the higher elevations and the coast north of Lane County will likely see steady rain. However, the ridging will begin to flatten Thursday night into Friday, allowing for more zonal, westerly flow into the region. Friday is also expected to be when IVT values peak in the area around 500-750 kg/m/s. These components together will allow for less rainshadowing in the interior lowlands with continued steady rain over the terrain and coast through Friday. As the IVT stream continues into Saturday, the ridge continues to become zonal, with similar conditions to Friday. Latest forecast indicates rain totals from Thursday to Sunday will be around 1-1.5 inches in the interior lowlands and 1.5-3.5 inches along the coast, Coast Range, and Cascades. The locations with highest probabilities of exceeding 3 inches of rain are localized to the Coast Range in Tillamook County (40-60%) and far north Oregon and SW Washington Cascades (50-60%). By Sunday, upper level flow is expected to be completely zonal over the region with another front slated to move through. This will bring another round of widespread rain. Overall, widespread flooding impacts are not expected with this round of rain, though flashier rivers in the Coast Range and Willapa Hills such as the Grays River near Rosburg and Willapa River at Willapa could see significant rises if hourly rainfall rates exceed 0.25-0.3 inches for multiple hours in a row. Will have more details about potential rainfall rates as we get closer to this event.

By Monday into the middle of next week, more uncertainty is introduced in the forecast as ensemble members struggle to resolve specifics in the pattern. Ultimately, the GEFS and Euro ensembles both suggest the potential for another atmospheric river late Monday into Wednesday. However, there is significant spread in the members between where the strongest band of moisture will funnel between WA and OR along with the maximum strength of the IVT values. One thing to note is soils will already be saturated and river levels elevated from the days of rain leading up to this event. Depending on the strength of this this event, flooding could definitely be a concern. HREF guidance for many rivers indicate a 10-25% chance of certain rivers reaching action stage and a 5-10% chance for minor flood stage, mainly focused on the Dec 9-10 time period. Additionally, there are signals for breezy winds of some sort this the frontal system associated with this event, although the spread is also very wide, and if winds do end up even around 30-40 mph, saturated soils could cause some localized impacts due to downed trees. Keep in mind there's still the potential that this event could be more similar to the lower impact event of this weekend. It is worth keeping an eye on the forecast as we get closer to this event. -03

AVIATION

A mix of VFR and MVFR conditions as the cold front exits the region and low cloud cover tries to break up. Northerly winds 5-10 kts become light and variable overnight. Cloud cover continues to scatter out through the evening and will lead to widespread fog and low stratus development tonight into Wednesday morning throughout the Willamette Valley. Probabilities for 1/2SM or lower visibility are 40-80% by 10-13z Wednesday throughout the Willamette Valley. Guidance suggests fog and stratus start to dissipate after 16-19z and VFR conditions should return by late morning or early afternoon.

KPDX AND APPROACHES...Cloud cover around 1000 feet remains in place but should break up and lift to low end VFR for a period this evening. Fog and low stratus become increasingly likely tonight into Wednesday morning. Probabilities for IFR conditions increase to around 40-60% by 12z Wednesday. Northwesterly winds around 5 kt expected through the period. -19

MARINE

Northerly winds will weaken through the evening, dropping below 10 kt overnight. A long period westerly swell moving into the waters will keep wave heights around 8-10 feet with a period of 14-16 seconds through tonight. Waves drop to wound 5 feet by late Wednesday and should remain close to this level through at least Thursday night.

A more active pattern develops later in the week as a series of frontal systems is expected to move across the waters beginning Thursday. Winds turn southerly on Thursday with a 30-50% chance for wind gusts to reach 21 kt, highest probabilities across the northern waters. Seas expected to remain below 10 feet through there is a 10-15% chance of greater than 10 feet Saturday morning. Expect this parade of fronts to continue through the weekend, with the strongest of the fronts potentially on Sunday. -19

BEACH HAZARDS

A long-period westerly swell is expected along the northwestern Oregon and southwestern Washington coasts through Wednesday. 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 remains in effect through 10 PM Wednesday. Sneaker waves can create potentially life- threatening conditions in the surf zone when 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 2 PM on Thursday 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. -19

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory from 9 AM to 2 PM PST Thursday for ORZ101.

WA...Coastal Flood Advisory from 9 AM to 2 PM PST Thursday for WAZ201.

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

Small Craft Advisory until 10 PM PST this evening for PZZ271-272.