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 00z Friday

High and mid level clouds can be seen streaming in along westerly flow ahead of an approaching cold front. The front is currently ~100 mi offshore and will continue to progress toward the coast late this afternoon. Light returns are showing up on radar but this is likely virga which will transition to light rain as the atmosphere moistens. VFR conditions currently will trend toward lower end VFR to MVFR along the coast as light rain increases and CIGs drop to between 1000-3000 feet through 03-04z this evening. Rain expected to move inland this evening but CIGs not expected to drop as low, generally between 3000-5000 feet though isolated MVFR conditions are possible. South winds ahead of the front up to 10 kt with gusts 15-20 kt. Winds turn westerly along the coast but remain southerly elsewhere.

KPDX AND APPROACHES...Predominately VFR conditions are expected through 06z. Light rain is expected to begin this evening sometime by 02Z ahead of an incoming front. Chances for ceilings below 3000 ft increase between 05-08z Thu to around 50%, lower beyond this time at 20-30%. South winds around 4-7 kt through early tonight, becoming light and variable after 12z. -19

MARINE

The pattern remains very active as a series of fronts moves through the waters through the weekend. The first front is currently approaching the outer waters. Southerly winds are expected to increase ahead of the front with buoy 46089 reporting gusts between 25-30 kt which will continue to spread eastward. Small Craft Advisories are in place for winds gusting up to 25-30 kt through 10 PM. Winds shift southwest to westerly behind the front as it pushes onshore tonight. Winds ease, but 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. Therefore, have extended Small Craft Advisories through Thursday evening.

After a brief lull in winds, southerlies increase again late Thursday into Friday as another frontal system approaches the waters. There is a 60-80% chance for Gales for 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 into at least the mid- teens, though currently just a 10% or less chance for seas reaching 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/DH

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.

Small Craft Advisory until 7 PM PST this evening for PZZ253- 273.