textproduct: Portland

SYNOPSIS

A deep, upper level trough looks to impact the region overnight and into Thursday. This low looks to bring a rain or rain/snow mixture to elevations below 1000 ft, while maintaining steady snow accumulations for elevations above 1000 ft. As the end of the week and weekend approaches, conditions look to generally warm and dry. However, a more active pattern looks to return to the region by the start of next week.

DISCUSSION...Now through Tuesday

Model guidance continues to be in fairly good agreement that a weak low pressure system will this evening along the north Oregon coast. Models are favoring the low coming onshore somewhere between Astoria and Tillamook, OR this evening. This track will favor accumulating snow mainly over portions of Southwest WA and the North Oregon Coast. This will include the Portland West Hills, Chehalem Mountain, North Coast Range Mountains of Oregon, North Oregon Coast Range Lowlands and Willapa Hills. Models also continue to maintain 850 mb temperatures around -5 to -7 degrees C. Now, when it comes to the timing of the precipitation, simulated CAM (Convective Allowing Models) reflectivity has a series of frontal bands moving inland this evening through tomorrow. The first band looks to start cross over the Coast Range around 5-7 PM this evening with the second frontal system expected around 8-10 PM tonight. The 850 mb temperatures as well as the showery/convective nature of the precipitation will result in a mainly rain or rain/snow mixture (75-90% probability) for elevations below 500 ft as warmer, southerly winds will help to keep overnight lows a few degrees above freezing for the majority of the I-5 corridor to include Eugene, Salem and the Portland/Vancouver Metro area. Now, there is a caveat in that precipitation rates, at times, could pull some of the colder air down towards the surface (via the wet bulb effect) and isolated snow flurries could be observed below 500 ft. With the lower elevations covered, lets focus in on the Portland West Hills, Chehalem Mountain, North Coast Range Mountains of Oregon, North Oregon Coast Range Lowlands and Willapa Hills. Simulated CAM reflectivity, do show a weak frontal feature bringing a enhanced band of precipitation to the area, along with the potential for cold air damming. As a result, have now included the Portland West Hills, Chehalem Mountains and North Oregon Coast Range Lowlands as snow accumulations of 1 to 4 inches will be possible (65-85% probability). It should be noted, that some uncertainty remains in the forecast, but we will continue to monitor as conditions develop through Thursday morning. Expect snow to occur between 8 PM tonight through 9 AM Thursday morning for elevations between 500-1000 ft.

Showers are expected to continue through at least Thursday night. For the South Washington Cascades and the Northern and Central Cascades of Oregon, expect additional snow accumulations of 4 to 10 inches through tonight and another 2 to 4 inches of snow Thursday morning through Thursday night. For the Foothills of the Northern and Central Oregon Cascades, North and Central Coast Range Mountains of Oregon and South Washington Cascade Foothills, expect additional snow accumulations of 2 to 7 inches through tonight and another 1 to 3 inches of snow Thursday morning through Thursday night.

Overall, for elevations above 1000 ft expect snow with areas below 1000 ft experiencing rain or a rain/snow mixture with little to no snow accumulations through Thursday morning. As the day progresses, any rain/snow mix being observed for elevations below 1000 ft will transition to rain, resulting in any snow that does fall in the lowlands would quickly melt as afternoon temperatures are expected to warm back into the 40s. While there remains some uncertainty with accumulating snowfall, there is very high confidence (85-95% probability) that freezing rain will NOT occur. Also, we are NOT expecting strong winds with this system.

Showers are expected to gradually decrease through Thursday, with light snow accumulations continuing for the Cascades. Overnight temperatures Thursday night into Friday morning will likely cool again into the upper 20s to lower 30s. Thus, any remaining moisture would freeze, resulting in slick conditions on most surfaces. As we move into the end of this week, mostly dry and warm conditions return Friday as a weak, upper level ridge builds over the area. The exception to the mostly dry conditions could be along the Coast and in the Cascades as some lingering, light showers are expected.

Looking beyond Friday, into the weekend and the start of next week, WPC 500 mb cluster analysis continues to show decent agreement that the upper level ridge shifts eastward towards the Rockies on Sunday. As this ridge builds and moves eastward, 850 mb temps are expected to warm towards 0 to 2 degrees C by Saturday and 1 to 3 degrees C by Sunday. This will result in snow levels rising towards 3500-4500 ft on Sunday. In addition to the warming pattern, models and their ensembles also bring precipitation back into the forecast by the start of next week. However, there is still high uncertainty in regards to precipitation amounts and location of the heaviest rainfall. Models indicate the potential for a weak atmospheric river somewhere along the western US coast Sunday into Monday, with the highest probabilities of enhanced rainfall currently pointed at portions of southern Oregon and California. While this is still well into the future, depending on precipitation amounts as well as snow melt, there could be hydrological concerns as the start of next week approaches. Again, there is still a significant amount of uncertainty in the forecast in the extended. Nevertheless, we will continue to monitor conditions as they develop. /42

AVIATION

A cold upper level trough remains over the region while northwest flow aloft becomes more westerly today. Showers continue over the area with periods of heavier accumulations. Where these heavier precipitation bands are, some snow is mixing into the rain. However, accumulation in low elevation terminals is minimal at best. Conditions remain a mix of VFR to MVFR, with lower MVFR CIGs and pockets of IFR developing where showers have eased and where our more stratus prone areas are.

Conditions are expected to improve this afternoon as probabilities favor more widespread VFR by 22Z. A slightly stronger low pressure system nears the region, pushing an initial frontal band onshore after 01Z Thu, with showers ramping up ahead of the main frontal band. Precipitation type will be quite variable and complicated as the southerly wind will maintain warmer air in the area. Cannot rule out a rain/snow mix but there is a low probability it would accumulate significantly at area terminals.

KPDX AND APPROACHES...Mixture of MVFR/VFR CIGs today with passing showers. Low probability of snow but if it does fall, there is a trend towards it being a "wet snow". -27

MARINE

Weak surface low pressure lingers near the Washington coast today maintaining onshore flow with wind speeds generally 15 kt or less. Slightly stronger surface low pressure develops near the north Oregon coast later this evening and moves south across the coastal waters through Thursday morning. Stronger showers may produce brief wind gusts up to 20-25 kt through this period. Seas remain around 11-13 ft at 13 seconds.

Winds and seas are expected to increase this weekend as a stronger system nears the waters. An extended period of southerly gales is likely (at least 80% chance) across the outer coastal waters as a series of low pressure systems move north. With the stronger southerly winds, there is increasing chances for storm force winds. Current peak wind gusts show an 50-75% chance of gusts of 48 kt or greater. However, these may not be prolonged and appear to be isolated to the outer waters of PZZ273. Seas are expected to respond as wind waves and a fresh southerly swell will build combined seas into the teens (80% chance beyond 20 NM). -27/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 4 AM PST Friday for ORZ106-107- 123>128.

WA...Winter Weather Advisory until 4 AM PST Friday for WAZ203-208-211.

PZ...Small Craft Advisory until 10 AM PST this morning for PZZ251>253.

Small Craft Advisory until 4 PM PST this afternoon for PZZ271>273.