textproduct: Portland

SYNOPSIS

A strong frontal system with an associated atmospheric river continues impacting the region through tonight. High confidence in widespread Minor and Moderate river flooding with increasing confidence in Major river flooding for a few river points. Urban and small stream flooding concerns are increased through tonight, as well. Breezy winds continue through this evening. Cascade snow returns early Friday, continuing into Saturday morning. Rain persists through early next week with continued river flooding through Saturday.

LONG TERM...Saturday through Wednesday

On Saturday, the general flow becomes more zonal, allowing conditions to calm somewhat. The remnants from the atmospheric river will linger through Saturday with scattered showers persisting through the day. Accumulation will be light and more typical for this time of year. Areas that will receive the most precipitation will be along the coast and the south Washington Cascades where 0.5-0.75 inch is possible.

By Sunday, ensemble guidance indicates troughing develops in the eastern Pacific and will continue through the rest of the extended forecast period with multiple shortwaves likely to move along the flow. Details on exact impacts with these waves are uncertain at this point, but precipitation chances continue each day Sunday through Wednesday. There is more confidence in a surge of renewed widespread rain Sunday into Monday, though amounts will be significantly less than with previous weather systems as this is not an atmospheric river. Current forecast indicates around 0.5-0.8 inches for the interior lowlands, 0.9-2 inches along the coast, Coast Range, Willapa Hills, and Oregon Cascades, and up to 2.5 inches for the south Washington Cascades Sunday through Monday. NBM indicates around a 60-80% chance for areas outside of the interior lowlands reach or exceed 1 inch of rain in 48 hours ending 4 AM Tuesday. Beyond Tuesday, uncertainty increases. Keep an eye on the forecast, especially those who have travel plans around the holiday. -03/27

AVIATION

VFR to MVFR conditions continuing across the majority of the airspace. The exception at this time is KONP as LIFR conditions are expected to continue through around 08Z Friday. 20-30% probability for intermittent IFR conditions through the TAF period, but timing and location of said conditions are challenging. The warm front has generally moved out of the air space, while a cold front follows closely behind. After the cold frontal passage, widespread rain will become more showery in nature. Behind the cold front, expect winds to become more southwesterly as well as decrease towards 10-12 kt or less. Flight conditions are expected to trend more towards VFR with occasional MVFR behind the front as well.

A 15-30% probability for thunderstorms is also expected across the airspace starting around 15Z Friday and likely through 12Z Saturday. Any thunderstorms could produce locally gusty winds and small hail.

KPDX AND APPROACHES...MVFR/VFR conditions expected to continue through the TAF period. Rain will become more showery behind an approaching cold front around 08Z-10Z Friday. A 15-30% probability for thunderstorms also returns around 20Z Saturday through 03Z Saturday. Any thunderstorms could produce locally gusty winds and small hail. /42

MARINE

Wind gusts have decreased by Wednesday afternoon with gusts below 20 kts. Seas still remain elevated around 13 to 15 ft at 10 to 11 seconds but are expected to decrease to around 11 to 13 feet overnight. A Hazardous Seas Warning remains in effect through 4 AM Thursday. These conditions won't last long as the next weather system begins impacting the waters early Thursday morning. Winds will quickly ramp up after 4 to 6 AM with widespread south to southwest gale force wind gusts through the day, decreasing after 7 to 10 PM. A coastal jet is forecast to develop south of Rockaway Beach with gusts up to 55 kts possible up to 10-20 NM west of the coastline. A Gale Watch has been issued for all marine zones except for PZZ252 and 253, for which a Storm Watch has been issued. Both watches are from 4 AM through 10 PM on Thursday. Seas will also rebuild to around 15 to 16 ft at 10 to 11 seconds, except locally up to 17 to 19 feet in vicinity of the coastal jet. There's also a 15-25% chance of seas reaching or exceeding 20 feet in the vicinity of the coastal jet. -03

HYDROLOGY

Today through Friday will be heads-up days in regards to river flooding due to the abundant rainfall from a strong atmospheric river. Heaviest rainfall occurs for a relatively short period of time through tonight, mainly for locations in NW Oregon. Due to already saturated soils and swollen rivers from previous rain, rivers will be more prone to experience flooding. As is typical in atmospheric rivers, the highest rainfall totals are likely to be over areas of terrain including the Coast Range and Cascades, while lesser totals can be expected across inland valleys. High rain rates this evening will promote urban and small stream flooding. It's important to watch your surroundings as urban flooding can come upon quickly and without notice.

As we are now in the middle of the event, a number of river points have already reached flood stage with around 20-25 total river points forecast to reach at least minor flood stage. Three river points are forecast to reach Major flood stage as of the late Thursday afternoon update: Clackamas River near Estacada, Clackamas River near Oregon City, and Johnson Creek near Sycamore. Eight additional river points are now forecast to reach Moderate flood stage. River flooding will continue into at least Friday, with some slower responding locations continuing into the weekend. These river forecasts remain very sensitive to rainfall forecasts, and interested parties should expect changes as the event continues. The latest river forecasts across the region can be found at www.water.noaa.gov/wfo/pqr.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory from 10 AM to 4 PM PST Friday for ORZ101>103.

Winter Weather Advisory from 4 AM Friday to 10 AM PST Saturday for ORZ126-127.

Winter Weather Advisory from 4 AM Friday to 4 AM PST Saturday for ORZ128.

WA...Winter Weather Advisory from 4 AM Friday to 10 AM PST Saturday for WAZ211.

PZ...Gale Warning until 10 PM PST this evening for PZZ210-251-271.

Hazardous Seas Warning until 10 AM PST Friday for PZZ210-251>253- 271>273.

Small Craft Advisory from 10 AM to 9 PM PST Friday for PZZ210- 251>253-271>273.

Storm Warning until 10 PM PST this evening for PZZ252-253-272- 273.