textproduct: Portland

SYNOPSIS

Light rain continues this afternoon as a frontal boundary lifts north across northwest Oregon and southwest Washington. Conditions trend mainly dry on Sunday as the parent low stalls well off the northern California coast. A strong upper-level trough will drop south early next week, bringing cooler, more unstable conditions and lowering snow levels to around 1000 to 1500 feet. Unsettled and showery weather is likely to persist through much of next week, with increasing potential for higher- elevation snowfall later in the week.

DISCUSSION...Now through Friday

As of this afternoon, radar and satellite imagery show widespread light rain associated with a lifting frontal boundary. The most persistent rainfall remains focused across the southern portions of northwest Oregon (generally south of Salem and Tillamook), where totals through tonight may reach around one quarter inch. Elsewhere, rainfall amounts will generally remain light, with most locations receiving around a tenth of an inch or less.

Snow levels currently remain elevated in the Cascades and southern Willamette Valley, generally around 3000 to 5000 feet, while snow levels elsewhere are around 2500 feet. This will limit meaningful snow impacts to the higher Cascades, where an additional 1 to 3 inches of accumulation is possible through tonight. Travel impacts at the major Cascade passes should remain limited under these relatively high snow levels.

By Sunday, the region transitions into a relative lull as the upper-level pattern briefly becomes less organized. The cutoff low drifts southward off the California coast while northwest Oregon and southwest Washington sit beneath weak flow aloft. This should allow precipitation to taper, with much of Sunday expected to remain dry or only spotty light showers at most.

Attention then turns to Monday as a reinforcing trough drops south from the Gulf of Alaska. This feature will push a cold front through the region and introduce noticeably cooler air aloft. Increasing instability will support more widespread showers beginning Monday morning and continuing into Monday night. Snow levels are expected to lower to around 1000 to 2000 feet across the Coast Range and south Washington Cascade foothills. While advisory-level snowfall is not currently favored, probabilities of impactful accumulations in the Coast Range and Cascades remain in the 20 to 40% range through Monday night.

Ensemble guidance remains in strong agreement that a deep upper-level trough will settle over the Pacific Northwest by Tuesday. 850 mb temperatures falling into the -4 to -6 Celsius range will support a cool and showery pattern. Low pressure is likely to become vertically stacked off the north Oregon coast, prolonging unsettled conditions. 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. The highest potential for more substantial Cascade snowfall currently appears to be Thursday into Friday, when ensemble guidance indicates a 20 to 30% 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 around 20 to 30%.

Ensemble guidance also continues to indicate a low-probability but non-zero winter weather scenario late next week. Current solutions suggest roughly a 5 to 10% chance that a more organized disturbance 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 inch of snow, with a worst-case outcome approaching 2 inches if mesoscale banding develops. Confidence in this outcome remains low and forecast details will likely evolve as we approach next week. ~12

AVIATION

A frontal boundary is currently set-up across the region with intermittent light rain ongoing as of 05-06z. Models are adamant this feature will weaken the remainder of the night into Sunday morning allowing for prevailing dry weather to return. However, expect flight conditions to be a mixed bag with CIGs/VIS deteriorating at most sites by 12-14z - the inland valleys have a 40-80% chance for MVFR or lower conditions. We'll also have to keep an eye out for patchy fog development over the interior lowlands too, which could result in localized LIFR conditions. While the best changes for patchy fog will be in the Tualatin Valley and central/southern Willamette valley, confidence is low regarding exact visibilities any ceilings at any given terminal Sunday morning. Probabilities trend back towards VFR for Sunday afternoon although there is still a 20-35% chance inland terminals, including PDX, hold onto MVFR CIGs through 00z Mon.

KPDX AND APPROACHES...Rain likely decreases by 07-08z with drier weather returning the latter half of the night as the front overhead weakens. By 10-15z, probabilities for MVFR conditions peak near 40-45% before decreasing the remainder of the morning. It's worth mentioning there is a 15% chance for patchy fog and low stratus development during this 10-15z period as well but the latest guidance keeps it outside of the terminal area thanks to persistent high clouds overhead. Winds generally remain night, below 5-10 knots. -99

MARINE

Low pressure develops off the northern California coast today while high pressure builds well offshore. This has resulted in a northeasterly flow regime over the northern, central, and southern waters. Winds around 15-20 kt this afternoon will increase through this evening with gusts up to 25-30 kt across the outer coastal waters, strongest south of Cascade Head. Northeasterly winds continue on Sunday, but will gradually ease in strength Sunday night. A west-northwesterly swell will persist through the weekend with seas remaining within the 8 to 10 foot range, lowest Sunday afternoon and evening. This is when conditions are most likely to fall below small craft advisory criteria.

However, this break will be short-lived as a cold front will be approaching the waters by early Monday morning. This front will bring a wind shift to westerly winds by late Monday morning with wind gusts increasing to around 20 kt. Weak low pressure drops south toward the north Oregon coast on Tuesday, bringing another wind shift with winds becoming more southerly. Wind speeds will most likely remain below small craft advisory thresholds, however another northwesterly swell is expected to build seas again to around 13 to 15 ft on Tuesday. Seas gradually decrease Wednesday into Thursday with wind gusts most likely ranging between 15-25 kt. Chances for gale force wind gusts during that time are only around 10%. -23/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. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 AM PST Sunday for PZZ210-251>253. Small Craft Advisory until 10 AM PST Sunday for PZZ271>273.