textproduct: Portland

SYNOPSIS

Cold and showery conditions persist this afternoon as snow levels remain confined primarily to the higher terrain. Ongoing winter weather impacts continue in the Cascades, while a marginal threat of light accumulating snow continues in the Coast Range. The marginal threat for light accumulating snow for the interior lowlands has ended. A transition toward a much wetter and warmer pattern begins tonight as an atmospheric river develops midweek. Heavy mountain snow remains possible through Wednesday afternoon, then becomes light mountain snow as potentially impactful rainfall enters the region Wednesday through Friday. Another weather system late in the weekend may continue precipitation into early next week.

LONG TERM...Thursday through Monday

The atmospheric river signal continues through Wednesday and into Thursday and Friday. Climatological tools indicate anomalous QPF values for this time of year. Looking at 500 mb models, a strong jet structure located near Vancouver Island has speeds around 120 kt, with speeds around 85 kt over our area (Southwest Washington/Northwest Oregon). While the strongest part of the jet is not over our area, the location and increased speeds of the jet do bring favored precipitation enhancement and convection. The moisture plume will persist for 48 to 72 hours depending on placement, with ensemble guidance continuing to show substantial spread for 48 hour totals from Thursday morning to Saturday morning. Low-end solutions bring roughly 0.75 to 1.5 inches along the coast, 1.5 to 2.0 inches in the Coast Range, and around 1 inch inland near Portland. High- end outcomes suggest 2.5 to 4.0 inches along the coast, 4 to 6 inches in the Coast Range, 3.0 to 3.5 inches near Portland, and over 6 inches in the Cascades. The high-end scenario represents a direct and prolonged atmospheric river impact, while the low- end scenario reflects a northward displacement of the moisture plume.

Rainfall tapers late Saturday as weak ridging develops. Another atmospheric river signal emerges Sunday night into Monday, though current long-range guidance favors a more northwesterly trajectory initially, then gradually shifting westerly to southwesterly through early next week.

Overall, the latter half of the week remains active, with the primary forecast challenge centered on precise atmospheric river positioning and resulting rainfall amounts. ~12

HYDROLOGY

Hydrologic concerns increasing Wednesday through Friday as a developing atmospheric river brings a prolonged period of moderate to heavy rainfall. While confidence is high that a moisture plume will impact the region, confidence remains moderate regarding where the heaviest rainfall will set up. While most rivers have less than a 5% chance of minor flooding, a few have greater chances. These rivers with greater chances of minor flooding include a 35% chance for the Wilson River near Tillamook and a 10-20% chance for the Willapa River near Willapa, Johnson Creek at Sycamore, Trask River above Cedar Creek, Nehalem River near Foss, and Tualatin River near Dilley. River responses will likely vary in time due to rainfall duration and location, with some likely occurring late Wednesday through Saturday, and others occurring late Thursday through Saturday. The most likely outcome at this time is elevated river levels and within-bank rises, though trends will need to be monitored.

AVIATION

Largely VFR conditions continue during rain showers. Brief restrictions to cigs/vis may occur at area terminals, but the timing of these impacts is low confidence due to the scattered nature of showers. Rain showers will transition to a steadier rain by 00z Wed or later, supporting mixed VFR/MVFR conditions which will trend to MVFR across the region (50-70% chance) by 06-12z Wed as cigs lower. The exception to this may be for inland terminals south of US-20 including KEUG/KCVO where lighter rainfall rates and higher ceilings will instead favor persistent low-end VFR conditions. Southerly to southwesterly winds at 5-10 kt will continue inland through the period, with southerly gusts to 20 kt along the central Willamette Valley developing overnight after 09z Wed. Along the coast, gusts will increase above 20 kt after 12-15z Wed as the next stronger cold front approaches.

KPDX AND APPROACHES...VFR flying conditions with southerly winds near 10 kt favored to continue. Brief vis/cigs restrictions are possible within rain showers, but timing of these impacts is low confidence through this early afternoon. After 00z Wed, showers will transition to steadier rain, with cigs trending down to MVFR through this evening. -36

MARINE

A strong frontal boundary over the waters will bring varying conditions over the next 72 hours. Ahead of the boundary, southwest winds will increase through tonight, culminating in gales over the waters north of Cape Falcon by Wednesday afternoon. Nearshore enhancements of the wind speed will favor the strongest gusts within 10 NM of the coast while also allowing gale-force winds to persist later into Thursday morning. Gale Warnings have therefore been hoisted north of Cape Falcon beginning at 11 AM PDT Wednesday and lasting through 2 AM PDT Thursday beyond 10 NM, and through 5 AM PDT Thursday within 10 NM. Gale conditions are expected over the Columbia River Bar from 2 PM PDT Wednesday through 2 AM PDT Thursday. During this time, seas will also become steep and hazardous, building to 14-17 ft at 8-9 seconds by late Wednesday evening. Within the Columbia Bar, seas of peak seas of 12-14 are more likely. To the south of Cape Falcon, both lighter wind speeds and lower coverage of the highest winds do not support a Gale Warning, however intermittent or isolated gusts reaching up to 35-40 kt cannot be ruled out. With seas reaching 12-14 ft and wind gusts of 25-30 kt, the existing Small Craft Advisory will remain in effect through Thursday afternoon before seas fall below 10 ft and winds below 15-20 kt.

As the front finally exits southward on Friday, winds and seas will finally ease below levels hazardous to small craft. As high pressure builds over the Northeast Pacific, winds will shift northerly Friday into the weekend while seas continue to ease below 5-6 ft through this weekend. -36

PQR WATCHES/WARNINGS/ADVISORIES

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

WA...Winter Weather Advisory until 5 PM PDT this afternoon for WAZ211.

Winter Storm Warning until 5 PM PDT Wednesday for WAZ211.

PZ...Small Craft Advisory until 2 PM PDT Wednesday for PZZ210.

Gale Warning from 2 PM Wednesday to 2 AM PDT Thursday for PZZ210.

Small Craft Advisory until 11 AM PDT Wednesday for PZZ251-252- 271-272.

Gale Warning from 11 AM Wednesday to 5 AM PDT Thursday for PZZ251-252.

Small Craft Advisory until 5 PM PDT Thursday for PZZ253-273.

Gale Warning from 11 AM Wednesday to 2 AM PDT Thursday for PZZ271-272.