textproduct: Portland

SYNOPSIS

Light rain early this morning is expected to dissipate by sunrise as conditions trend mainly dry today. A deep upper-level trough will drop south Monday into Tuesday, bringing cooler, more unstable conditions and lowering snow levels to around 500 to 1500 feet. Unsettled and showery weather is likely to persist through much of next week. There is around a 10% chance for a scenario occurring that could bring an inch or two of snow down to the valley floor mid to late week. Snow levels slowly rise again next weekend.

DISCUSSION...Now through Saturday

Light rain early this morning is expected to give way to drier conditions today as we briefly transition in a relative lull between systems. Expect seasonable temperatures today.

A pattern change is in store for next week which will feature cold upper level troughing and modified Arctic air, allowing snow levels to drop to their lowest levels this winter:

An initial shortwave trough will drop south out of the Gulf of Alaska on Monday. This feature will push a cold front toward the region Monday morning. At the same time, model solutions are trending toward a deformation band setting up across the central and north Oregon Cascades. Snow levels over the Cascades will still be relatively high, at around 4000-4500 ft. This will likely limit the snowfall Monday morning to pass levels. By Monday afternoon, colder air aloft will be filtering across the entire area west of the Cascades. Increasing instability will support more widespread showers on Monday while snow levels lower to around 1000 to 1500 ft by Monday evening. There is increasing confidence for advisory level snowfall in the Cascades as guidance suggests there is around a 40-60% chance of exceeding 6 inches at pass level. Snowfall amounts in the Coast Range through Monday night remain low confidence, but there is potential for at least advisory level snow there as well. A lot will depend on the location of the surface low as it drops south into the coastal waters of Washington or north Oregon as another shortwave trough rotates around the parent trough, potentially intensifying showers near the coast and into the Coast Range overnight.

Ensemble guidance remains in very good agreement that an anomalously deep upper level trough settles over the Pacific Northwest on Tuesday, while 850 mb temperatures fall to around -5 to -6 C. Low pressure offshore will likely maintain an unstable and showery pattern. While overall precipitation amounts do not appear especially high at this time, light snow accumulation above roughly 1000 feet will remain possible through midweek.

Looking toward the latter half of the week, troughing persists across the region, keeping conditions cool and periodically showery. Potential for more substantial Cascade snowfall has generally decreased as guidance indicates a 10 to 20% probability of exceeding 12 inches in 24 hours in the higher Cascades. In the Coast Range, including passes along Highways 6 and 26, probabilities of exceeding 4 inches during that same period are similarly around 10 to 20%.

Ensemble guidance also continues to indicate a low-probability winter weather scenario for the lowlands next week. Current solutions suggest roughly a 10% chance that a more organized disturbance Wednesday night into Thursday could result in producing accumulating snow in portions of the lowland valleys. While the most likely outcome remains little to no accumulation at valley floor, a lower-end impact scenario could bring around 1 to 2 inches of snow, if mesoscale banding develops. Confidence in this outcome remains low and forecast details will likely evolve as we approach next week. DH/12

AVIATION

Moist, southwest flow aloft continues while a frontal boundary remains across the area with intermittent light rain ongoing as of 09-10z. Drier conditions are expected after 14z this morning and through much of the day. However, flight conditions will likely remain a mix of VFR and MVFR, with pockets of IFR as more settled conditions return. Guidance suggests CIGs and VIS deteriorating at most sites by 12-14z as inland valleys have a 40-80% chance for MVFR or lower conditions. Low chances also remain for patchy fog development within the interior lowlands, which could result in localized LIFR conditions. IFR CIGs are likely to continue in the southern Willamette Valley as northerly flow and moist conditions persist through this morning. Probs trend back towards VFR this afternoon although there is still a 20-35% chance inland terminals hold onto MVFR CIGs through 00z Monday.

KPDX AND APPROACHES...Conditions remain VFR as of 09z, even as a band of light rain moves across the area. Drier conditions are expected after 12z, but probs for MVFR CIGs increase to around 50-60% through 20-21z this afternoon. There is also around a 10-20% chance for patchy IFR fog this morning, but is unlikely to occur if the mid to high clouds persist. VFR becomes more likely after 21z, but there remains around a 30% chance for MVFR stratus through this evening. Generally northwest winds around 5 kt or less expected. /DH

MARINE

Low pressure off the northern California coast will drift south today as a cold front drops out of the Gulf of Alaska. Northeasterly winds will gradually ease across the waters through tonight, but still expect gusts up to 25 kt to persist for the outer coastal waters south of Cape Falcon through this evening. Buoy reports maintain seas at around 9 to 11 ft early this morning. Seas gradually subside to around 7 to 8 ft later tonight into Monday morning.

Winds begin to shift early Monday as the front approaches the waters and quickly pushes inland by Monday afternoon. Expect increasing westerly winds behind this cold front with wind gusts generally up to 20 kt. But, stronger showers that develop will also have the potential to produce gustier winds up to 30 kt. Surface low pressure drops south toward the south Washington or north Oregon coast late Monday into Tuesday, bringing more south to southwesterly winds across the waters. Another west- northwesterly swell is expected to push into the coastal waters Monday night, building seas to around 13 to 15 ft on Tuesday. Seas gradually subside Wednesday into Thursday with wind gusts most likely ranging between 15-25 kt. /DH

BEACH HAZARDS

A high threat for sneaker persists through Sunday evening due to a long period westerly swell. Waves can run 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. Caution should be used when in or near the water, especially those who will be razor clamming. Keep children and pets away from the surf zone. Keep off of jetties, rocks and logs near the surf zone. If you see someone swept into the sea do not swim in after them. Call 911 and keep an eye on them until help arrives.

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 AM PST early this morning for PZZ210-251>253.

Small Craft Advisory until 10 AM PST this morning for PZZ271.

Small Craft Advisory until 10 PM PST this evening for PZZ272-273.