textproduct: Portland

SYNOPSIS

An atmospheric river will move over the area today bringing periods of moderate to heavy rain, and mountain snow. Expecting southerly winds to become quite gusty both over the marine waters and inland. The atmospheric river persists through Friday with potentially impactful accumulations, and continued snow over the south Washington Cascades. Another weather system late in the weekend may continue precipitation into early next week.

LONG TERM...Saturday through Tuesday

Rainfall tapers late Saturday as weak ridging develops. This ridge of high pressure will come into play on Sunday and Monday though as yet another atmospheric river is on deck. A broad low pressure system forms in the Gulf of Alaska on Saturday night. Ensemble 500 mb heights show good consistency between clusters in the ridge amplifying, a deep digging trough over the northern Plains and the low pressure system in the Gulf of Alaska staying to the north. About half of the clusters show a drier solution, while the other half favor rain. Vapor transport again shows a spike in values on Monday into Tuesday but uncertainty is quite high. If the ridge remains dominate we will see high temperatures well above climatology. For example in Eugene on Monday, the high temperature spread is around 67-73 degrees F (25th to 75th percentiles) with a 10% chance of seeing temperatures as high as 75 degrees. Further north in Kelso, WA the max temperature spread is closer to 55-65 degrees F with a 10% chance of highs of 67 degrees F.

There is high uncertainty overall in the long-term forecast so we will be continuing to evaluate as time passes. -27

AVIATION

A mix of VFR and MVFR conditions as rain increases over the area. Rain has begun to pick up along the coast causing some of the MVFR CIGs to fall to IFR. Inland sites are beginning to experience more persistent rain and dropping CIGs. Overall going to see low end MVFR conditions with periodic improvements between rain bands. The other concern is the southerly wind transition. We will see winds begin to amplify with gusts increasing through all north-south aligned terrain. This will especially be along coastal terminals and the Willamette Valley. For example, KONP could see winds gust as high as 40 kt after 20Z Wed. The Willamette Valley could see gusts as high as 30 kt after 22Z Wed. LLWS is a threat for many inland sites - especially those where the airport itself is not susceptible for southerly surface winds like KTTD. In heavier rain with gusty winds expect visibility to drop.

KPDX AND APPROACHES...Steady rain with MVFR CIGs. Southerly winds begin to increase around 15Z Wed. Southerly winds up to 50 kt at 3000 ft after 18Z Wed. Low confidence in LLWS threat due to the surface wind speeds. If surface wind gusts remain below 20 kt, then LLWS is likely up to 2000 ft. However, if they rise then LLWS is not as significant of a threat. Important to note though that winds aloft for east approaches will be strong with speeds in the 50-60 kt range between 3000-5000 ft. -27

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 this 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. There is a 10% chance of hazardous seas within PZZ253/273.

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. -27/36

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 20% chance at the Nehalem River near Foss and 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, and Tualatin River near Dilley. River responses will likely vary in time due to rainfall duration and location, with some likely occurring late tonight 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.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 5 PM PDT this afternoon for ORZ126-127. WA...Winter Storm Warning until 5 AM PDT Friday for WAZ211. PZ...Small Craft Advisory until 2 PM PDT this afternoon for PZZ210. Gale Warning from 2 PM this afternoon to 2 AM PDT Thursday for PZZ210. Small Craft Advisory until 11 AM PDT this morning for PZZ251- 252-271-272. Gale Warning from 11 AM this morning to 5 AM PDT Thursday for PZZ251-252. Small Craft Advisory until 5 PM PDT Thursday for PZZ253-273. Gale Warning from 11 AM this morning to 2 AM PDT Thursday for PZZ271-272.