textproduct: Portland

SYNOPSIS

A series of weather systems are expected to bring widespread wet conditions to much the region on Tuesday through Thursday with rising snow-levels helping to mitigate any significant travel concerns over the Oregon Cascades passes leading into Thanksgiving. That said, guidance continues to show a trend towards a cooler and somewhat drier weather pattern late weekend into early next week. Due to high model uncertainty beyond Friday, the exact impact of this pattern change is rather nebulous, however, the potential for our coldest temperatures of the season are likely within reach by Monday.

DISCUSSION...Tuesday through Wednesday

Fog and low stratus are being observed across much of the NW Oregon and SW Washington interior lowlands early Tuesday morning with satellite imagery indicating mid to high level clouds streaming over the area ahead of an incoming frontal system. Surface temperature observations show the central and southern Willamette Valley have fallen into the mid 30s with mid to low 30s being observed across the foothills. Expect some locally frosty conditions early this morning as temperatures could fall another couple of degrees early this morning. A weather system is slated to move inland from the Pacific later this morning through tonight, ushering in warmer moist air and widespread precipitation. As temperatures will still be on the colder side when the front initially moves in, snow-levels will initially begin around pass- level (3,000-4,000ft) before rapidly jumping to 6,000-8,000ft this evening. Model soundings continue to show a brief period of warm-air overrunning during this transition coupled with a weak easterly wind component near the surface in the Cascades, so after some snowfall, a few hours of a wintry mix can't be ruled near the passes before temperatures fully warm to above freezing and transition precipitation to solely rain.

Otherwise, an elongated band of rainfall is expected to sit over over Western Oregon and Southwest Washington tonight through Wednesday. Given the extended duration of this moisture streaming overhead and a high probability (80%+) for IVT values greater than 250kg/m/s in both the EPS and GEFS, this set-up would technically be classified as an "atmospheric river", albeit a weak one. There remains some uncertainty as to how the axis of this moisture and the heaviest relative rainfall fluctuates longitudinally, particularly on Wednesday, with most models showing a shift southward into central Oregon before swinging northward again later Wednesday in response to a developing low over the eastern Pacific. Heaviest rainfall amounts are likely to be north of Lane County with 0.5-1.25 inches across the interior lowlands and 1.25-2.5 inches across the coast, Coast Range, and Cascades with localized amounts up to 3-3.5 inches. It's difficult to pinpoint where those localized higher rainfall amounts will be and will depend on where the front stalls. Some of the more responsive rivers in the coast range and Willapa Hills may be more impacted from the rain depending on where the front and moisture stall, but the chances for minor river flooding appear low (<15%) - still worth keeping an eye on.

For the developing low off the coast bringing the next weather system, the 00z and 06z ensemble runs for the EC, GEFS, and GEPS have both slowed down the low's progression inland and shifted the trajectory farther south into the Washington/Oregon coast. There still remains uncertainty on exactly where along the coast the low will move inland, but the majority of the ensemble members now suggest this will be somewhere between central Washington and northern Oregon instead of British Columbia as previous guidance suggested. Additionally, guidance has pushed back the onset of precipitation to be closer to sometime on Thursday morning instead of Wednesday night with the low pushing inland later on Thursday. The 01z NBM, which the current forecast is based on, hasn't fully picked up on this transition, but if this trend continues with the 12z and beyond ensemble runs, expect the NBM to begin shifting to this scenario. Ultimately, there's not much change in the overall impacts with this system except for potentially slightly increased winds depending on exactly where the low sets up. Current probability for 30+ mph winds are 20-40% for interior lowlands and 70-80% along the coast. Probability for 40+ mph winds drop to less than 10% across the interior lowlands and 30-45% along the coast. Rain amount forecast could fluctuate over the next few model runs, but overall not expecting a significant amount of rain, generally 0.15-0.25 inch for the central and southern Willamette Valley, 0.3-0.75 inch for the northern Willamette Valley into the SW Washington lowlands, 0.5-1.5 inches along the coast and Coast Range, and 0.5-2 inches along the Cascades. Snow levels are expected to remain above pass level until early Friday, but by then conditions are expected to become showery with limited precipitation accumulation.

The rest of Friday into the weekend and beyond, model uncertainty increases significantly as both deterministic and ensemble systems struggle to resolve the placement of the upper- level longwave pattern. Guidance is split as to whether another shortwave trough off the coast of British Columbia dives southward into the Pacific Northwest for Saturday, or if a high pressure ridge over the eastern Pacific strengthens into the Pacific Northwest and shunts this feature well to our east. At the very least all models show the development of broad northerly flow over the weekend which favors a cooling trend by Sunday and Monday. The scenario in which a trough digs southward just east of the Cascades would allow for a colder airmass to seep into the region through the Columbia Gorge and Cascade Gaps punching temperatures well below normal (overnight lows in the mid 20s to near 30 across the lowlands). On the flip side, scenario would keep the amplifying trough near the Rockies with a more predominate ridge feature overhead, resulting in less of an easterly wind influence. In any case, the NBM probability for subfreezing low temperatures across the lowlands is decent (generally 45-75%) by Monday morning which will give us a fighting shot for the lowest temperatures of the season thus far. For reference our lowest overnight minimum temperatures from Oct through Nov are as follows: Portland 36F, Salem 32F, Hillsboro 32F, and Eugene 30F. All in all, the overall model spread is large during this period with any impacts beyond the decrease in temperatures tough to nail down - forecast confidence is low Friday onward.

Definitely keep an eye on the forecast as we go through this week, especially if you have holiday travel plans. -99/03

AVIATION

As of 12z Tue, observations indicate fog/low stratus formation across much of the Willamette Valley with either vsbys below 1 SM and/or IFR/LIFR cigs observed, except for at KTTD due to the development of light east winds from the Columbia River Gorge. Fog and LIFR/IFR cigs likely begin gradually lifting towards 13-16z Tuesday as widespread stratiform rain begins pushing into the region from the west ahead of a frontal system. Once rain begins Tuesday morning, expect rain to persist through the day at all terminals. Cigs will trend towards a mix of MVFR/VFR thresholds, except IFR/MVFR at the coast and in southwest Washington, especially after 20-22z Tuesday as the front moves through the region, bringing heavier rain.

KPDX AND APPROACHES...Fog and/or low stratus is pushing into the terminal from the north with LIFR to IFR vis and/or cigs. By 14-15z Tuesday, fog will likely lift with cigs gradually trending towards MVFR or even low- end VFR thresholds as persistent rain develops. Once rain begins Tuesday morning, expect rain to persist through the day. Cigs and surface visibilities trend lower again around 00z Wednesday onward with a 60-80% chance for cigs below 2000-3000 ft and a 30-40% chance for cigs below 1000 ft. -02/03

MARINE

A frontal system is approaching the waters Tuesday morning and will move over the waters this afternoon through early Wednesday morning. Southerly wind gusts will increase ahead of and along the front with widespread gusts up to 30-34 kt expected. There is a 10-15% chance for occasional gale force wind gusts over 34 kt over the northern and central waters, though not frequent or widespread enough to justify the issuance of a Gale Warning. Therefore, a Small Craft Advisory remains in effect. Seas will rise to 9-11 ft at 12 seconds by Tuesday night, continuing through early Wednesday morning, so the Small Craft Advisory continues into Wednesday morning for the seas.

Another frontal system approaches the waters sometime Wednesday night into Thursday, though there are still uncertainties in exact timing. Another round of increased southerly wind gusts up to at least 30 kt is expected. There's a 45-60% chance for maximum wind gusts to peak at gale force wind gusts over 34 kt for all marine zones, though 3 hourly probabilities drop to 10-20%, indicating widespread gale force gusts are unlikely. Seas are expected to rise again, peak near at least 14-15 ft on Thursday. There is a 15-25% chance seas will peak around 18-20 ft, with a 5-15% chance seas will reach 20-22 ft. This is due to the uncertainty regarding the exact strength and track of the closed surface low and the frontal system accompanying the low. Regardless of the outcome for this system, it appears winds and seas will lower significantly on Friday, especially late in the day. Winds and seas over the upcoming weekend are currently forecast to stay below small craft advisory thresholds with a period of offshore flow. -23/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 10 AM this morning to 1 AM PST Wednesday for PZZ210.

Small Craft Advisory from 10 AM this morning to 10 AM PST Wednesday for PZZ251-252-273.

Small Craft Advisory from 4 PM this afternoon to 10 AM PST Wednesday for PZZ253.

Small Craft Advisory until 10 AM PST Wednesday for PZZ271-272.