textproduct: Portland

DISCUSSION...Now through Thursday

A more robust frontal system is moving inland early this morning, bringing widespread rain to the region. This feature carries a stronger plume of moisture than yesterdays system, with integrated vapor transport values generally running in the 500-700 kg/ms range. This is enough to support persistent rainfall through the day, including areas farther south that were largely missed during the previous event. Rainfall from now through 5 AM Saturday is expected to total about 0.25 to 0.50 inches across interior lowlands. Totals will be notably higher along the coast, generally in the 1.0 to 1.5 inch range, with 2 to 3 inches common across the Coast Range and Cascades. Winds today will turn breezy from the southwest, with gusts most frequently reaching 25 to 35 mph through most of the day. Expect the frontal passage to arrive around 7-10 AM today.

Showers linger into Saturday as a westerly flow pattern persists. Another frontal wave will arrive Sunday and bring yet another round of steady rainfall to the region. Ensemble guidance suggests IVT values with Sundays system in the 300-500 kg/ms range, producing a widespread soaking. Rainfall totals from Saturday through sunday are currently projected to fall in the 0.5 to 1.0 inch range across the interior lowlands, with 0.75 to 1.0 inches along the coast and 1 to 2 inches in the Coast Range. The Cascades are expected to receive roughly 1.5 to 3.0 inches, except for the Lane County Cascades where amounts are closer to 0.5 to 1.5 inches. Forecast confidence in the exact high and low ends of these ranges remains moderate. Widespread impacts are not anticipated through Sunday; however, if rainfall rates sustain around 0.2 to 0.3 inches per hour for several hours, a few faster-reponding basins, such as the Grays at Rosburg and the Willapa River, could rise quickly.

Attention then turns to the Monday-Wednesday period, where models continue to suggest a multi-day atmospheric river. Over the past several cycles, ensembles have become increasingly consistent in highlighting two distinct surges of moisture: one centered roughly on Monday, and a second sometime Tuesday or Wednesday. Between these peaks, the moisture feed is unlikely to shut down completely, leading to a long-duration event rather than two isolated episodes.

Ensemble IVT guidance continues to show a broad range but retains a strong signal for an impactful event. For the first plume on Monday, GEFS and ECMWF are similar, with members clustering around 600-650 kg/ms and higher-end solutions near 750-800 kg/ms. The second surge maintains mean values in the 500-600 kg/ms range with high-end members closer to 750-850 kg/ms. The precise timing and latitude of these plumes remain uncertain, but confidence continues to increase that the region will experience a prolonged period of moderate to heavy rainfall. Winds early next week may also trend breezy, but ensemble spread remains wide. Even moderate gusts in a 30-40 mph range could be sufficient to bring down isolated trees due to increasingly saturated soils. This remains a detail to refine as the event draws closer.

Overall, the Monday-Wednesday period remains the primary focus for potential high-impact hydrologic (see hydrology section below for more details) and wind concerns. Confidence in a multi-day atmospheric river is rising, but exact rainfall totals and timing details still require several more forecast cycles to resolve. While rain is looking to continue through at least Thursday, rain amounts look much less Thursday and onward. ~12

HYDROLOGY

A series of frontal systems will keep conditions wet through the weekend and much of next week. The most notable period will be late Monday through late Wednesday, where a prolonged atmospheric river may bring significant rainfall and rising rivers across southwest Washington and northwest Oregon. Rivers draining the Coast Range and Willapa Hills have a 10-25% chance of reaching moderate flood stage and a 8-15% chance of reaching major flood stage. There is also the potential for flooding along small creeks and streams, as well as urban flooding. Flooding of the Willamette mainstem rivers is unlikely to occur (less than 10%). Probabilities for specific river points can be found at the National Water Prediction Service website. Periods of heavy rain will also increase the risk for landslides in areas of steep terrain, and debris flows over recently burned areas.

AVIATION

Radar imagery and terminal observations as of late Friday morning depict a mix of IFR/low-end MVFR CIGs and VIS across all terminals as a warm front lifts over the region and brings rain and mist. Moderate to heavy rain at times are bringing brief reductions in VIS to 2-4 SM. Expect this trend to continue through this morning and early afternoon. After 21z Fri-00z Sat, rain will decrease and transition to post-frontal showers. Visibility should trend back up to VFR thresholds and CIGs should begin to lift toward high-end MVFR/low-end VFR. Expect west-southwesterly winds today across all terminals, around 10-15 kt with gusts up to 25 kt along the coast and 20 kt across the Willamette Valley. Winds will gradually weaken overnight as the system exits the area.

KPDX AND APPROACHES...Low-end MVFR CIGs continue through the early afternoon with the warm front passage. Moderate to heavy rain at times may bring brief VIS reductions to 3-5 SM. After 21z Fri, rain becomes light and transitions into showers. CIGs will then trend toward high-end MVFR (above 2000 ft). Southwesterly winds 10-13 kt with gusts up to 20 kt today, weakening below 10 kt tonight. -10

MARINE

A frontal system will move over the waters today, with westerly winds increase to 15 to 25 kt with gusts up to 30 kt, strongest over the northern waters. Small Craft Advisories remain in effect across all waters including the Columbia River Bar beginning 4 AM Friday, lasting through Saturday evening. Seas of 4-5 ft early this morning will build to around 7-9 ft at 10-11 seconds by Friday evening as a northwesterly swell moves in with the next system. Will note that a very strong ebb may lead to higher seas of 10-11 ft in the Main Channel of the Columbia River Bar around 4-5 PM Friday. For the rest of the waters, seas likely (80% chance) build above 10 ft by late Saturday morning. There is also a 25% chance that seas build above 13 ft, with the highest chances north of Cape Lookout and beyond 10 NM. There is a 1-5% chance seas peak as high as 15 ft.

The parade of fronts continue through the weekend and into early next week. Chances for widespread and frequent Gale force wind gusts of 34 kt or greater remain under 15% through early next week; however, chances for brief and isolated Gale force wind gusts are around 40-50% on Sunday and 50-70% on Monday. Seas are forecast to hover close to 15 ft from late Monday through late Wednesday, with a 10% chance seas peak around 17-18 ft. -10/23

BEACH HAZARDS

Coastal Flood Advisories remain in effect from for minor tidal overflow during high tide from 9 AM to 3 PM Friday and Saturday for the south Washington coast and Clatsop County coast in Oregon. Minor flooding up to 1 foot above ground level is possible during high tide in low lying areas near bays, sloughs, and the low reaches of the coastal rivers. Coastal residents in the warned area should remain alert for rising water and take appropriate action to protect life and property. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Coastal Flood Advisory from 9 AM this morning to 3 PM PST this afternoon for ORZ101.

Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for ORZ101.

WA...Coastal Flood Advisory from 9 AM this morning to 3 PM PST this afternoon for WAZ201.

Coastal Flood Advisory from 9 AM to 3 PM PST Saturday for WAZ201.

PZ...Small Craft Advisory until 10 PM PST Saturday for PZZ210-251>253- 271>273.