textproduct: Portland

SYNOPSIS

Showery conditions will persist through tonight as a cold upper trough settles across the region. Snow levels will continue to lower this evening, bringing low chances for minor low-elevation accumulation in parts of southwest Washington and the Portland/Vancouver metro overnight into early Tuesday. A transition toward a warmer and wetter pattern begins midweek as snow levels rise and a moisture plume approaches. The heaviest rainfall potential late week will depend on the placement of the atmospheric river, with uncertainty increasing into the weekend.

LONG TERM...Thursday through Sunday

The pattern turns wetter on Thursday as the moisture plume becomes better established. Thursday into Friday currently appears to hold the highest probability for heavier rainfall. Mean 24-hour QPF ending Friday morning ranges from around 2.0 to 2.5 inches in the south Washington Cascades and 1.5 to 1.9 inches along the north Oregon Coast Range and Willapa Hills. Inland totals are lower, generally 1.0 to 1.25 inches near Portland, decreasing to around 0.3 to 0.7 inch Salem southward.

If the atmospheric river axis shifts farther south towards Oregon and less so towards Washington, rainfall totals could increase meaningfully across our forecast area, particularly along the Coast Range and Willapa Hills. This long duration moderate atmospheric river will also come directly out of the west, improving orographic lift efficiency which will result in greater rainfall amounts west of the Coast Range and lesser amounts on the east side of the Coast Range. Ensemble river guidance continues to show flooding probabilities mostly around 5-15%, and even lower for slower-responding rivers. Recent guidance has given one exception, with a 20% chance of minor flooding at the Wilson River near Tillamook on Thursday. Trends will need to be monitored closely over the next 48 hours as hydro concerns may grow as ensemble river guidance hones in on the upcoming precipitation.

Heading into the weekend, uncertainty increases substantially. Ensemble cluster analysis shows roughly equal support for wetter versus drier solutions and warmer versus cooler outcomes. The spread in 10th to 90th percentile temperatures highlights this uncertainty, with potential swings exceeding 10 degrees in many locations on Saturday and around 18 degrees on Sunday. The strength and placement of Pacific high pressure and the position of the jet stream relative to the region will influence the outcome. -12

AVIATION

Mainly VFR conditions persists over the area. Scattered showers have decreased in coverage a bit but still continue over the area with the highest coverage over the higher terrain of the coast range and Cascades. Expecting shower activity to decrease late tonight before the next round of showers moves into the area Tuesday morning. There are low chances for low- elevation snow from KKLS northward along the Cowlitz Valley, although light snowfall rates and minimal surface accumulations will limit surface impacts. Chances for MVFR or lower CIGs also increase during this time with a 20-50% chance throughout the Willamette Valley during any given hour. Light winds overnight, around 5 kt then S-SW winds increase Tuesday morning to 10-15 kts with gusts to 20 kt.

KPDX AND APPROACHES...VFR flying conditions likely with low chances for MVFR CIG/VIS to 20-50% any given hour. Scattered showers continuing to decrease in coverage tonight before increasing again Tuesday morning. Light winds overnight increasing from the southwest to around 10 kt with gusts to 20 kt Tuesday morning. -19

MARINE

Scattered post-frontal rain showers continue this afternoon while an isolated thundershower also cannot be ruled out, particularly north of Cape Falcon through this evening. Seas of 8-11 ft per buoy observations early this afternoon are expected to slowly increase to 10-12 ft, then maintaining those levels through Tuesday night, while west to northwest winds around 15 kt with gusts to 20-25 kt will ease tonight, then turn out of the southwest and build to around 20 kt through over the same period. Small Craft Advisories will therefore expand to cover all the coastal waters by 8 PM PDT this evening, and further to include the Columbia River Bar by 8 AM PDT Tuesday morning.

A stronger frontal system will traverse the waters Wednesday and Wednesday night. The locally tightened surface pressure gradient will see winds and seas increase through the day. There is high confidence (80% or higher chance) that at least isolated gale- force gusts will occur over the waters, however more persistent or widespread gale-force gusts remain less likely (25% or less chance in any given hour). The enhancement of southerly winds within 10 NM of the coast as southwest winds are blocked and deflected by coastal terrain render nearshore areas more likely to see gales, but locations where coastal enhancement will occur remains sensitive to the position of the frontal system and its parent area of low pressure. As such, a Gale Watch has been issued for areas north of Cape Foulweather from 10 AM PDT Wednesday through 7 AM PDT Thursday, during which time the strongest wind gusts are expected. Meanwhile, seas may build to 15 ft or higher, although again, the location of the highest seas is dependent upon the frontal position. Additional Hazardous Seas Watches may be needed if forecast swell height increases and the coverage of higher seas expands beyond the area of high winds, and additional Small Craft Advisories may be issued for areas likely to remain below Gale or Hazardous Seas conditions.

The pattern remains active late this week and into this weekend, but confidence in the occurrence of hazardous conditions is low at this time. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 5 PM PDT Tuesday for ORZ106.

Winter Weather Advisory until 5 PM PDT Wednesday for ORZ126- 127.

WA...Winter Weather Advisory until 5 PM PDT Tuesday for WAZ211.

Winter Storm Warning from 5 PM Tuesday to 5 PM PDT Wednesday for WAZ211.

PZ...Small Craft Advisory from 8 AM Tuesday to 8 AM PDT Wednesday for PZZ210.

Small Craft Advisory until 8 AM PDT Wednesday for PZZ251>253- 271>273.

Gale Watch from Wednesday morning through late Wednesday night for PZZ251-252-271-272.