textproduct: Portland

SYNOPSIS

The bulk of our impacts the next several days center around an atmospheric river event which is currently underway across the region. Expect periods of moderate to heavy rain, breezy conditions, and mountain snow (mainly WA Cascades). While the probability for mainstem river flooding is low, rises on rivers and streams across much of northwestern Oregon and southwest Washington will be a feature to watch due to the potential for anomalous rainfall rates - a Flood Watch is in effect through Friday evening. From there, another weather system clipping the area to the north late in the weekend may bring additional precipitation in addition to ushering in much warmer temperatures for early next week.

LONG TERM...Saturday through Tuesday night

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 MVFR and VFR conditions inland with IFR and LIFR conditions along the coast as steady rain continues over much of the area. Sub-VFR conditions are expected through much of the TAF period as steady rain continues. Southerly to southwesterly wind gusts of 20-30 kt are expected to continue over the next 6 to 10 hours, decreasing after a cold front drops through the area. Winds after 18z will be lighter at 5 to 10 kt.

KPDX AND APPROACHES...Steady rain continues through the TAF period, with MVFR CIGs and VIS expected to persist. There is a 50/50 chance that VFR conditions develop around 12-15z for a short time. Winds south to southwesterly 10-15 kt with gusts to 25-30 kt through 12z, decreasing to less than 10 kt thereafter. -19

MARINE

A strong frontal boundary moves over the waters, bringing varying conditions over the next 24 to 48 hours. Southwest winds will continue to increase through this afternoon, bringing Gales Force winds (34+ kt) over all of the waters through this evening. The strongest gusts will be located within 10 NM of the coast due to nearshore enhancement, allowing Gale Force winds to persist a bit longer compared to the waters beyond 10 NM. Gale Warnings are in effect until 10 PM PDT this evening for the outer waters (beyond 10 NM) and until 2 AM PDT Thursday for the inner waters (within 10 NM), including the Columbia River Bar. During this time, seas will also become steep and hazardous, continuing beyond the Gale Warning time. With seas expected to build to around 13 to 15 ft at 10 to 11 seconds, a Hazardous Seas Warning has been issued for the outer waters from 10 PM this evening to 5 AM PDT Thursday. Small Craft Advisories will follow as seas remain elevated, with a Small Craft Advisory in effect for the inner waters by 2 AM Thursday and outer waters by 5 AM Thursday. The Small Craft Advisories will continue until late Thursday.

As the front finally exits southward on Friday, winds and seas will finally ease to below hazard criteria. 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. ~12

HYDROLOGY

Hydrologic concerns increase today through Friday as a atmospheric river brings a prolonged period of moderate to heavy rainfall to portions of western Oregon and southwest Washington. While confidence is high that a moisture plume will impact the region, confidence remains moderate regarding where the heaviest rainfall will set up in conjunction with the two primary impulses of rainfall - tonight into Thursday morning and again on Friday. While most main-stem rivers have less than a 5% chance of minor flooding, a few of our "flashier" rivers have greater chances. The Pudding River at Aurora, Johnson Creek near Sycamore, and the Wilson/Trask near Tillamook are expected to reach minor Flood Stage with a 5-20% to reach moderate. 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 11 PM PDT Thursday for ORZ126.

WA...Winter Storm Warning until 5 AM PDT Friday for WAZ211.

PZ...Gale Warning until 2 AM PDT Thursday for PZZ210-251>253.

Small Craft Advisory until 5 PM PDT Thursday for PZZ210- 251>253.

Hazardous Seas Warning until 5 AM PDT Thursday for PZZ271>273.

Small Craft Advisory from 5 AM to 5 PM PDT Thursday for PZZ271>273.