textproduct: Portland

SYNOPSIS

A weak atmospheric river event will keep conditions soggy into Tuesday although weather related impacts appear rather minor during this period. Wednesday into the weekend, the weather pattern appears fairly benign while trending drier, at least for the inland valleys. Conditions likely remain within our typical climatological norms late week into the, weekend but forecast uncertainty begins to noticeably increase late Saturday/Sunday onward.

EVENING UPDATE

A Winter Weather Advisory has been issued for elevations above 1000 feet in the Upper Hood River Valley through 10 AM Tuesday. Surface observations as of late Monday evening show temperatures ranging between 31 and 34 degrees, coldest above 1000 feet elevation. Webcams and public reports indicate that there is at least a rain/snow mix or light snow in the Parkdale area. Tripcheck observations also indicate potential traffic slow-down on Highway 34 between Odell and Parkdale. Considering that cold air commonly gets trapped here for extended periods of time and a stalled front will maintain precipitation through Tuesday morning, went ahead and issued the advisory. Low confidence with exact snow amounts, but could see anywhere between a trace and 1 inch for the highest elevations of the Upper Hood River Valley. Those traveling in this area should use extra caution through Tuesday morning. -10

LONG TERM...Thursday through Sunday

Colder air is expected to be funneled into the PacNW behind Wednesday's shortwave. This along with clearing skies will lead to colder overnight temperatures Wednesday night with a 60-90% chance of low temperatures Thursday morning falling below freezing in the central and southern Willamette Valley and a 30-50% chance for the SW Washington lowlands. For low elevation locations that still have precipitation chances during this time, there is the potential for some flurries to mix with rain or rain to turn a little slushy. However, there is a less than 5% chance of any accumulating snow.

Ensemble guidance indicates zonal flow returns for Thursday through the weekend. Low precipitation chances continue across the area each day, mainly over the terrain, with limited accumulation expected. The closed low remnant from the current longwave trough is slated to move back east towards the West Coast late weekend to early next week. Most ensemble members suggest it will move inland along the CA coast, but exactly how far north or south is uncertain. This uncertainty is part of the reason for the low precipitation chances.

However, the jet stream along the zonal flow is expected to dip south into central OR, which would allow for colder air to impact the area once again Saturday night and Sunday night. There's a 30-60% chance overnight temperatures fall below freezing again each night for the lowlands outside of the Portland metro area. Similar to Thursday morning, a rain/snow mix may show up in the forecast for lower elevations that have a chance of precipitation, but there is less than 5% chance of accumulating snow. -03

AVIATION

Radar imagery as of late Monday evening depicts widespread rain as a stalled front remains over the region. High confidence (>90% chance) for at least MVFR CIGs across the majority of terminals tonight and through the remainder of the TAF period. An exception is KAST, where CIGs will trend VFR as the heaviest rain is further south. There is also a 40-70% chance everywhere else for IFR CIGs through the TAF period, with the highest chances (60-70%) across the central and southern Willamette Valley. Moderate rain may result in VIS reductions to 3-5 SM at any given terminal. After 18z Tue, rain will decrease and transition to showers. Drier conditions return in the late evening. Easing pressure gradients keep winds under 10 kt across the area. Easterly winds along the coast and northerly winds inland turn more westerly/variable Tuesday afternoon.

KPDX AND APPROACHES...Predominately low-end MVFR CIGs (1-2 kft) through the TAF period with rain. 30-50% chance for IFR CIGs through 15z Tue. Rain turns more showery in the afternoon. Variable winds around 5 kt or less. -10

MARINE

Seas and winds continue to trend down today and through the middle of the week. Seas remain steep through tonight with a Hazardous Seas Warning remaining in effect until 7 PM today before transition to a Small Craft Advisory which is in effect until 10 AM Tuesday. The pattern is expected to remain benign through Thursday before a WNW swell entering the waters Friday morning pushes seas to around 10 feet with a period in the mid teens. -19

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 10 AM PST Tuesday for ORZ121.

WA...None.

PZ...Small Craft Advisory until 10 AM PST Tuesday for PZZ210-251>253- 271>273.