textproduct: Portland

SYNOPSIS

A pair of quick moving frontal systems associated with subtropical moisture will bring additional rounds of heavier rainfall and river rises on Monday and Tuesday. The probability for exceeding minor flood stage for a handful of rivers has increased to around 30-50%, mainly focused toward later in the week. Breezy to gusty southerly winds are also expected both days with the strongest winds likely Tuesday night. Onshore flow continues with showers on Wednesday. Lowering snow levels will allow for snow to fall at passes in the Cascades on Wednesday and again by Friday. Precipitation continues through late next week, with potential for another strong atmospheric river system on Thursday.

LONG TERM...Wednesday through Saturday

Snow levels are expected to drop significantly by Wednesday morning with the passage of the upper trough. This will allow for snow to begin accumulating at pass level on Wednesday. The weather setup on Wednesday will feature onshore flow with zonal flow aloft, likely maintaining showers enhanced by orographic effects. There still remains some uncertainty regarding snowfall amounts through Wednesday night, as the higher end amounts have come down (10% or less chance of exceeding 12 inches of snow in 24 hours), except in the south Washington Cascades where snow levels are likely to drop to around 2500-3000 ft. For Oregon Cascade passes, there remains around a 30-50% chance for exceeding 6 inches of snowfall through Wednesday night. The NBM's 10th to 90th percentiles at both Santiam Pass and Government Camp gives a range now of around 2 to 11 inches of snowfall. So, for those preparing to travel over the Cascades, keep an eye on the updated forecasts.

As the synoptic pattern progresses into late next week there is a trend for yet another atmospheric river on Thursday into Friday. At this point, confidence remains low as there remains significant differences between ensembles, but these have gradually been coming into better agreement over the past 48-hours. The WPC (Weather Prediction Center) has maintained a slight risk of excessive rainfall across most of western Oregon, which means there is increasing probability of exceeding flash flooding guidance. With our soils very saturated and rivers full, adding the Thursday- Friday system, local area rivers could see potential flooding. There remains quite a bit of variability as to where the plume of moisture and highest IVT values set up, but indications are that these 24 to 48-hr rainfall amounts Thursday into early Friday are likely to be the highest of the week. Latest model runs have shifted the system farther north with more west-southwest flow. This would also push snow levels higher in the Cascades through Thursday night.

With the potential for heavier rainfall late in the week, probabilities for local tributary rivers reaching at least minor flood stage have increased to around 30-50%. Moderate flood stage remains low around 10-20%. Again, still quite a bit of uncertainty. But moving into the weekend, conditions likely ease with clusters showing low amplitude troughing likely over the Pacific NW. Showers will likely continue with lower snow levels. There is around a 10% chance that snow flakes or a snow/rain mix falls to around 1500 ft in the foothills. /DH

AVIATION

Satellite and radar observations show out next frontal system beginning to progress into the region ushering in widespread rain and an increase in winds. While conditions as of 06z are primarily VFR inland, MVFR cigs remain favored once the steadiest/heaviest rain builds overhead with generally 10-25% chances of IFR cigs after 12z at both inland and coastal terminals. Surface winds will also continue to increase as high- resolution guidance favors wind gusts reaching at least 30-35 kt along the coast and 20-25 kt inland. A 50-60 kt low-level jet out of the south-southwest at only a few thousand feet off the surface will additionally yield increasing low-level wind shear of 35-55 kt through Monday morning at all terminals. This shear will be unidirectional for most, however surface winds remaining southeasterly later into the period at Portland-area terminals will result in a directional shear component as well. It's worth noting once the front passes late morning into the afternoon showers will trend more showery in nature but may be fairly frequent, especially along the coast and around higher terrain features.

KPDX AND APPROACHES...Currently as of 06z VFR conditions are holding in place as intermittent stratiform rainfall begins to overtake the region. Cigs/vis is expected to trend to MVFR by 10-12z Mon while winds turn toward the south and build to around 15 kt with gusts to 25 kt tonight and through Monday morning. Some high-resolution guidance supports even stronger gusts exceeding 30 kt, most likely between ~14-20z Mon. A strong low-level jet out of the south-southwest will also support low-level wind shear of 30-45 kt. This concern rapidly dissipates ~19-20z behind the frontal passage. -99/36

MARINE

A strong cold-frontal system is slated to pass through the waters today (Monday) leading to fairly impactful conditions across the waters. Gale Warnings are in effect for the inner and outer waters, including the Columbia River Bar, through the daytime hours with high confidence in sustained winds of 25-35 kt with gusts of 40-50 kt. At this point isolated storm-force gusts associated with a coastal jet accompanying the frontal boundary can't be ruled out Monday morning with a 35-45% chance of occurring somewhere along the southwest Washington or northwest Oregon coasts, and only 10-20% chances across the outer waters. While winds peak Monday morning, seas continue to build into Monday afternoon before slowly easing on Tuesday. Seas will most likely reach into the 15-18 ft range Monday afternoon, with a 10-35% chance of surpassing 18 ft beyond 30 NM north of Cape Falcon - 10% or less elsewhere. By Tuesday afternoon, significant wave heights are likely closer to 11-14 ft with a dominate period of 11-12 seconds.

Going forward a rather active weather pattern, even by December standards, continues through the work week bringing additional winds and wave hazards to the coastal waters. Seas are very likely to remain elevated above 10 ft (greater than 90% confidence) from Wednesday through Friday as additional weather systems move onshore. At least models are beginning to hint as some larger breaks between weather disturbances next weekend into the following week, but at this 7-8 day time-scale, the confidence in this slowdown materializing is only low to moderate at this time. -99/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Wind Advisory until 4 PM PST Monday for ORZ106-107. WA...Wind Advisory until 4 PM PST Monday for WAZ203. PZ...Small Craft Advisory until 1 AM PST Monday for PZZ210-253-273. Gale Warning until 4 PM PST Monday for PZZ210-253-273. Hazardous Seas Warning from 4 PM Monday to 4 AM PST Tuesday for PZZ210-251>253-271>273. Gale Warning until 4 PM PST Monday for PZZ251-252-271-272.