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SYNOPSIS

Weak low pressure over the region will maintain cool temperatures with light scattered showers today. High pressure re-builds Sunday to Monday, returning dry and sunny conditions. Cool nights will bring chances for morning frost in rural valleys away from urban cores this weekend into early next week. Next chances for light rain arrive Tuesday to Wednesday. Temperatures trend warmer late in the week as an upper-level ridge amplifies over the region.

MORNING UPDATE

The Frost Advisory for the Upper Hood River Valley has been cancelled for this morning. Low cloud cover did not clear out, maintaining warmer temperatures in the low to mid 40s. Clouds are not expected to clear out until later today, so temperatures are unlikely to fall into the low to mid 30s this morning. -10

SHORT TERM...Today through Monday night

Radar imagery as of early Saturday morning depicts light, scattered showers across northwest Oregon and southwest Washington as upper-level troughing remains over the region following a frontal passage yesterday. The upper trough will develop into a closed upper- level low today and progress southward, bringing wrap-around moisture on the relatively cooler backside of the low over western OR/WA. This will maintain chances for light showers throughout the day with rain totals less than what we saw yesterday. There is high confidence (80-90% chance) that rain totals today will be less than 0.10 inch across the region. Expect another cool day with temperatures peaking in the low 60s across interior valleys and 50s along the coast.

A Frost Advisory remains in effect this morning for the Upper Hood River Valley. However, low-level cloud cover is keeping conditions much warmer in the mid to upper 40s as of 3 AM Saturday. High-resolution REFS guidance suggests that low clouds will remain through the rest of the morning, which would prevent radiational cooling. Will continue to monitor temperatures of the next few hours, and if clouds don't clear out, frost development will be less likely and the Frost Advisory may be canceled early.

On the contrast, chances for more widespread and impactful frost increase tonight into Sunday morning as the aforementioned upper low drops southward and high pressure builds overhead, returning clear skies and calm winds. There is high confidence for frost and morning lows between 33-36 degrees over the Upper Hood River Valley, Cascade foothills, north Oregon Coast Range valleys, Battle Ground/Brush Prairie area, and portions of the central/southern Willamette Valley to the east of the I-5 corridor near the foothills (e.g. Lebanon, Scio, Albany, Halsey, Brownsville, Sweet Home, Marcola, Mohawk, Jasper). Sub-freezing temperatures less than 32 degrees are likely in Dee and Parkdale (70-90% chance). Therefore, Frost Advisories remain in effect for tonight where confidence in frost formation is highest. Anyone growing outdoor plants that are susceptible to frost damage should cover plants or move plants indoors if possible.

Expect dry and sunny conditions Sunday to Monday as upper-level ridging amplifies over the Pacific Northwest, warming temperatures into the mid to upper 60s. Will continue to see chances for frost development each night through Monday night/Tuesday morning as clear skies and calm winds support radiational cooling, especially in rural areas and sheltered valleys. These areas will likely (50-60%) experience temperatures into the mid 30s each morning. -10/23

LONG TERM...Tuesday through Friday

High pressure will maintain mostly dry conditions with seasonable temperatures Tuesday to Wednesday for the majority of the area. An exception would be southwest Washington and the north Oregon Coast Range and Cascades, where chances for light rain increase to 10-30% as a weak cold front over the coastal waters attempts pushing inland. Even if this front is able to move inland, it would be decaying, which suggests rain amounts would likely be very light. Chances for 48 hour rain totals exceeding 0.10 inch from 5 AM Tuesday to 5 AM Thursday are 20-40% across southwest Washington and the north Oregon Coast Range and Cascades, and 5-15% across the Willamette Valley. Will also see increased cloud cover by Wednesday from this weak front, which will keep overnight temperatures warmer and reduce the threat of widespread frost by Tuesday night/Wednesday morning.

By Thursday and Friday, the majority of ensemble members are in agreement that upper-level ridging will amplify again over the western U.S., bringing above-average 500 mb heights. This will result in temperatures rebounding to the low to mid 70s across interior valleys at the end of the week. -10

AVIATION...Through 12Z Sunday

Radar imagery as of early Saturday morning depicts light scattered showers across northwest Oregon and southwest Washington as weak low pressure remains over the region. Conditions remain predominately VFR, except for KONP where there will be intermittent drops to IFR/MVFR CIGs/VIS through this morning. Expect lingering showers across the area through 21z Sat. Afterwards, high pressure will re-build and return drier conditions with increasing cloud breaks. Northwesterly winds around 4-7 kt for all terminals today. Tonight, clearing skies and calm winds will support temperatures falling to 33-36 degrees for more rural areas and sheltered valleys. Areas that are able to cool down to these temperatures may see frost development over metal surfaces.

KPDX AND APPROACHES...VFR with light showers through 21z Sat. Increasing cloud breaks late this afternoon/evening with clear skies overnight. Northwesterly winds 3-4 kt. -10

MARINE

Buoy observations as of early Saturday morning show northwesterly winds around 10-20 kt with gusts up to 25 kt, along with seas around 8-10 ft at 10 seconds. The Hazardous Seas Warning for the waters north of Cape Falcon and beyond 10 NM as been cancelled given that seas have now fallen below 11-12 ft. However, Small Craft Advisories remain in effect for most of the waters through this afternoon for steep and choppy seas. Winds will gradually weaken today and seas subside to 7-9 ft by this evening.

Tonight into Sunday, will see a renewed round of small craft conditions as tightening pressure gradients from a thermal trough result in breezy north/northeasterly winds with gusts up to 25 kt for the waters south of Cape Falcon. Seas remain steep and choppy around 7-10 ft at 9 seconds during this time, steepest as you go beyond 10-20 NM. Winds weaken on Monday and remain northerly through early Tuesday morning, before turning more southwesterly mid-week ahead of an approaching weak front. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from midnight tonight to 9 AM PDT Sunday for ORZ104-106-113-117-121-123>125.

WA...Frost Advisory from midnight tonight to 9 AM PDT Sunday for WAZ205-208.

PZ...Small Craft Advisory until 8 PM PDT this evening for PZZ210.

Small Craft Advisory until 2 PM PDT this afternoon for PZZ251- 252-271>273.

Small Craft Advisory from 2 PM to 11 PM PDT Sunday for PZZ252- 253.

Small Craft Advisory from 2 AM to 11 PM PDT Sunday for PZZ272- 273.