textproduct: Portland

SYNOPSIS

Wetter and more active weather begins as the first in a series of frontal systems spreads rain inland through this afternoon. Additional chances for rain Thursday night into Friday, Saturday afternoon into Sunday, and Monday through the middle of next week.

DISCUSSION...Now through Tuesday

A progressive pattern through the next week will bring repeated frontal systems into the Pacific Northwest, although light precipitation amounts are not expected to yield widespread flooding concerns. An initial cold front moving onshore through this afternoon will spread rain inland tonight with drying conditions then expected through Thursday morning. Rainfall amounts are likely to remain less than a quarter inch and minimally impactful. Brief upper ridging in the wake of this first system will see dry conditions continue into Thursday night before the next cold front arrives early Friday morning. This weak system will result in less widespread rain, although the areas which do see more persistent rain are likely to end up with higher totals than the first system. Expected rainfall amounts are a half inch or more along the Clatsop and Pacific County coasts, between a tenth and a quarter inch elsewhere on the northern and central Oregon coast, near Portland, and across inland southwest Washington, less than a tenth of an inch elsewhere at low elevations inland, and minimal amounts in the Oregon Cascades.

Another brief period of upper-level ridging with southwest flow aloft will see temperatures warm into the 60s on Saturday with largely dry weather across the region through the morning. Yet another frontal system will bring widespread higher rainfall totals from Saturday afternoon into Sunday. While remaining relatively progressive, vapor transport aloft is likely to peak in the "moderate" range for atmospheric rivers. While this may produce heavier rainfall rates, the short duration will limit total accumulations from becoming too impactful. Rainfall amounts on the west slopes of area terrain may reach a half inch from Saturday afternoon through Sunday evening (65-85% chance), those along the Willamette Valley are much less likely (15-30% chance). Given the warm nature of this system, snow levels are likely to remain well above pass level.

Long-range guidance continues to support a fourth system affecting the region during the midweek period, but uncertainty in the evolution of this system, in terms of both location and intensity, remains high. There is potential for both higher vapor transport aloft as well as a longer duration of resulting precipitation over the region, which could combine to support more substantial rainfall accumulations. Given antecedent rainfall from earlier systems, some rise in background river levels is expected which could prime area rivers for potential flooding if rainfall amounts are high enough. A period of strong south-southwest winds may also occur at the frontal passage, with chances for peak wind gusts of 40 mph or higher reaching 30-40% along the coast and 5-15% along the I-5 corridor. -36

AVIATION...Through 06Z Friday

Radar, satellite, and surface weather observations from 05Z Thursday depicted a mix of IFR and MVFR ceilings at the coast, and a mix of MVFR and VFR ceilings and visibilities inland. Scattered light rain showers lingered at KHIO, KPDX, KTTD with showers in the vicinity of KUAO, KSLE, and KEUG. Expect ongoing showers to dissipate by approximately 09Z Thursday in the wake of a frontal passage, with cloud cover temporarily becoming more scattered inland. Given increasing cloud breaks tonight with light winds and wet ground conditions, areas of fog and low stratus are expected to develop, resulting in fluctuating ceilings and visibilities with a complicated mix of LIFR/IFR/MVFR flight conditions. The lowest ceilings and visibilities are expected to occur between 10-18Z Thursday before conditions improve to MVFR/VFR thresholds thereafter.

KPDX AND APPROACHES...Expect occasional light rain showers through approximately 08-09Z Thursday with MVFR ceilings and visibilities. Cloud cover should become more scattered thereafter, which will help set the stage for potential fog and/or low stratus development. The most likely outcome at this time is for IFR ceilings to develop as chances for ceilings below 1000 ft increase to 50% by 14-16Z Thursday. Chances for surface visibilities around 1 SM or lower peak near 45% during that time. Conditions should improve after 18Z Thursday, however the exact timing is uncertain. -23

MARINE

The pattern remains very active as a series of fronts moves through the waters through the weekend. The first front recently finished moving over the coastal waters and pushing inland late Wednesday evening. Buoy observations from 9 PM PST showed southwest winds weakening in the wake of the frontal passage, sustained around 5-10 kt with gusts up to 15 kt. Although winds have eased, a westerly swell will begin moving into the waters late tonight, building seas to around 9 to 11 ft at 14 seconds by Thursday morning.

South winds then increase again late Thursday into Friday as another frontal system approaches the waters. There is a 60-80% chance for Gales for the waters north of Cape Falcon, including the Columbia River Bar. Steep and hazardous seas will also be possible later Friday as significant wave heights likely build to at least 14-17 ft. There is a 1-10% chance seas will peak as high as 20 ft.

A potentially stronger front is expected to quickly approach the area late Saturday, bringing more widespread chances for Gales (40-80%, highest to the north) across the coastal waters. Another westerly swell will likely build seas into the mid to upper teens, with around a 15-30% chance of wave heights exceeding 20 ft. The active weather pattern likely continues into early next week. -19/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 10 PM PST this evening for PZZ210.

Small Craft Advisory from 7 AM to 10 PM PST Thursday for PZZ210- 253-273.

Gale Watch from Thursday evening through Friday afternoon for PZZ210-251-271.

Small Craft Advisory until 10 PM PST Thursday for PZZ251-252-271-272.