textproduct: Portland

SYNOPSIS

Mainly dry conditions persist this afternoon in a brief lull between systems. A deep upper-level trough will drop south Monday into Tuesday, ushering in much cooler air and lowering snow levels to between 500 and 1500 feet. Unsettled and showery weather will likely continue through much of next week. While widespread lowland snow is not the most likely outcome, probabilities for measurable snowfall increase gradually through mid to late week. Snow levels slowly rise again next weekend.

DISCUSSION...Now through Saturday

As of this afternoon, the region remains between systems with largely dry conditions in place. This quiet period will be brief as a significant pattern change develops Monday. A shortwave trough dropping south from the Gulf of Alaska will push a cold front into the Pacific Northwest Monday morning. Snow levels will begin relatively high, around 4000 to 4500 feet, limiting early accumulations to the higher Cascades and pass levels, but colder air will steadily filter into the region through the day.

By Monday evening, snow levels are expected to fall to around 750 to 1500 feet as instability increases and showers become more widespread. Confidence has increased sufficiently to issue a Winter Weather Advisory for the Cascades, Cascade foothills, and Coast Range from 4 AM Monday to 4 AM Thursday. Advisory- level snowfall in the Cascades appears likely during this period, with ensemble guidance indicating a 60 to 70% probability of exceeding 6 inches at pass level Monday into Monday night.

By Tuesday, ensemble agreement remains strong that an anomalously deep upper-level trough will be positioned over the Pacific Northwest, with 850 mb temperatures falling to around -4 to -6 Celsius. This will support a cold, unstable, and showery regime through midweek. While overall precipitation amounts are not expected to be particularly high, locally heavier showers could briefly enhance snowfall rates where temperatures are supportive.

The air mass through the work week will be relatively convective in nature. Along the coast, precipitation will most often fall as snow given the colder profile; however, there is roughly a 5% chance that stronger isolated showers could briefly intensify and produce graupel at times. Inland, stronger isolated showers during morning hours could also produce brief moments of graupel. During daytime hours, inland precipitation will likely remain a rain/snow mix.

Across the Coast Range, including Highway 26, probabilities of at least 2 inches of snow accumulation are around 70-80% on Monday, decreasing to 50-70% on Tuesday and Wednesday, 40-50% on Thursday, and 20-30% by Friday as forcing gradually weakens. Snowfall is expected to continue beyond 4 AM Thursday; however, confidence in advisory-level impacts beyond that time frame remains low to moderate. The advisory may need to be extended in future forecast updates as confidence improves.

For the lowlands, measurable snowfall potential increases incrementally through the week as the cold trough settles overhead. The probability of at least 0.1 inch of snow is around 5-10% on Monday, increasing to 10-20% on Tuesday, 20-30% on Wednesday, and 30-40% on Thursday. While most guidance still favors little to no accumulation at valley floor, there remains a 5-15% chance of at least 1 inch of snow on Thursday should an organized mesoscale band form. Confidence in that higher- scenario remains low, and details will continue to evolve over the coming days. To add, we will be keeping a close eye on on the outer southeast Portland metro, West Hills, Chehalem Mountains, Eola Hills, Corbett, and the Upper Hood River Valley for later in the week as models look to trend towards possible accumulating snow.

Later in the week, troughing persists but gradually moderates. Snow levels may begin to edge upward next weekend, though cool and unsettled conditions may linger and maintain light snow for the mountains. ~12

AVIATION

Late this evening conditions are generally MVFR across the region with patches of IFR/LIFR CIGs in the southern Willamette Valley near KEUG as of 05-06z. Expect CIGs/VIS to remain largely a persistence forecast the remainder of the night through the morning hours as a frontal systems pivots from the south during this time period before a secondary system spreads showers west to east midday into the afternoon hours. Guidance does decrease probabilities for MVFR conditions during the afternoon hours once this transition to the showery airmass occurs, however, confidence is low regarding the timing of any defined periods of VFR CIGs/VIS as they'll most likely found between shower activity. It's worth noting there is also a 10-15% chance of thunderstorms at the coast later this morning and afternoon. Small hail or small graupel will be possible with stronger showers or thunderstorms, as well as gusty and erratic outflow wind gusts up to 25-30 kt.

KPDX AND APPROACHES...MVFR conditions likely(65-70%) persist the remainder of the overnight hours. Starting 12-14z light rain is expected to build over the terminal from the south before transitioning to showers during the afternoon hours (20-22z). Winds generally stay less than 10 knots through the TAF period. -99

MARINE

Low pressure off the northern California coast will drift south today as a cold front drops out of the Gulf of Alaska. Northerly to northeasterly winds have fallen below 21 kt a bit sooner than expected. Therefore, the Small Craft Advisory that was previously in effect through Sunday evening for the central and southern outer waters has been cancelled. Buoy reports from early Sunday morning showed seas ranging between 8 and 11 ft at 15 seconds. Seas gradually subside to around 7 to 8 ft later tonight into Monday morning.

Winds become westerly 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. However, stronger showers that develop will have the potential to produce brief outflow wind gusts 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, highest over the central and southern waters beyond 10-20 NM offshore. Seas gradually subside Wednesday into Thursday with wind gusts most likely ranging between 15-25 kt and seas falling to around 10 ft or less. Winds and seas will likely increase next weekend as a stronger system moves towards the waters. There is currently a 60-80% chance for gale force wind gusts over 34 kt on Feb. 21-22 with a 50% chance for seas of 12 to 15 ft and a 5-10% chance for seas as high as 17-20 ft. -23/DH

BEACH HAZARDS

A high threat for sneaker persists with another long period westerly swell arriving for Tuesday. Sneaker waves can run up significantly farther on a beach than normal, including over rocks and jetties. These 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. Instead , call 911 and keep an eye on them until help arrives.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 4 AM Monday to 4 AM PST Thursday for ORZ106-107-123>128. WA...Winter Weather Advisory from 4 AM Monday to 4 AM PST Thursday for WAZ203-208-211. PZ...Small Craft Advisory from 4 PM Monday to 4 AM PST Tuesday for PZZ251>253. Small Craft Advisory from 10 AM Monday to 4 AM PST Tuesday for PZZ271>273.