textproduct: Portland

SYNOPSIS

A weak system currently impacting California brings high cloud cover to the region, with chances of light precipitation over areas south of Salem and Lincoln City. Today into Sunday, chances of light precipitation will gradually expand north towards Portland and Astoria. Dry and warmer conditions return Monday as brief ridging forms overhead. A stronger system arrives Tuesday into Wednesday, bringing widespread rain and pass-level Cascade snow. The pattern then transitions to showers late in the week and into the weekend.

DISCUSSION...Now through Saturday

As a cut off low pressure system tracks through northern California, wrap-around moisture reaches areas south of Salem and Lincoln City. Heading into Sunday, the wrap-around moisture will push further north and potentially reach Portland and Astoria. Current chances of precipitation remains around 20-40% for Corvallis southward, which will then expand/increase to a 30-60% chance for Salem southward by tonight (highest chances to the south). Sunday morning, chances of precipitation will expand to Aurora and Tillamook, then to Portland and Astoria by Sunday afternoon/evening. As a whole, chances for precipitation on Sunday will be around 20-50% north of Salem and 50-90% Salem southward. The Portland/Vancouver metro will have a 15-30% chance of precipitation on Sunday.

Precipitation chances will dwindle through Monday morning as a brief ridge builds. From now until Monday morning, expect total rainfall amounts under 0.05 inch north of Salem and Tillamook and 0.1 to 0.2 inch Salem and Tillamook southward. The Cascades and Cascade foothills of Lane, Linn, and Marion counties will receive greater total rainfall amounts due to orographic lift, ranging between 0.4 to 1.0 inch through Monday morning. Snow levels will remain between 6000 to 7000 ft through at least Monday, limiting snowfall to the high Cascades.

Monday, enjoy dry conditions and afternoon temperatures in the upper 50s, as Tuesday marks the beginning of wet conditions through the rest of the week. Tuesday morning, widespread precipitation moves in, bringing more substantial rain amounts to the region. The arrival time of this system still varies, with timing ranging from Tuesday morning to Tuesday afternoon. Forecasted rainfall totals for Tuesday and Wednesday are rather similar; around 0.1 to 0.3 inch in the valley, 0.5 to 0.7 inch along the coast, 0.75 to 1.25 inch in the Coast Range, and 0.5 to 1.0 inch in the Cascades. Current guidance suggests chances of 1+ inch of total rainfall over 48 hours around 10-20% in the valley and 30-40% along the coast.

One difference on Wednesday is the forecasted precipitation amounts in the Cascades, as snow levels around 3500 to 4500 ft will cause precipitation to fall in the form of snow. Current guidance suggests a 40-70% chance of 6+ inches of total snow at Santiam and Willamette passes early Wednesday through early Thursday. For Government Camp, there is a 20-30% chance of 6+ inches of total snow Wednesday through Thursday. Guidance continues to suggest peak snowfall rates to occur late Tuesday into early Wednesday. Specifically, between 4 AM and 10 AM Wednesday, expect 1.5 to 2.0 inches of total snow at both passes, with a 60-70% chance of 2+ inches during the aforementioned times.

Winds will have little to no impact during this Tuesday into Wednesday weather system, with typical gusts up to 20 to 25 mph in the lowlands and gusts up to 35 to 40 mph in the higher terrain during the frontal passage.

Late in the week, widespread stratiform rain weakens and transitions towards a showery pattern. Snow showers in the Cascades will continue as snow levels are expected to remain around 3000 to 4500 ft through at least Friday morning. Heading into Saturday, ridging over the northeast Pacific tries to build into the region, but another trough approaches from the northwest and squashes the ridge. This attempt of a building ridge will cause snow levels to rise and shut off the snow showers in the Cascades by Saturday. However, there is moderate agreement from ensembles that rain showers will continue in the lowlands heading into the weekend. ~12

AVIATION

Mid and upper level clouds continue to increase over the area as an upper level low to the southwest supports moist southwest flow. Conditions are expected to remain largely VFR through the TAF period with only a 10-20% chance of MVFR conditions in the southern and central Willamette Valley tonight. Chances for light rain showers increase this afternoon as a weak disturbance within the upper level flow moves in from the south. Rain chances will be highest in the southern and central Willamette Valley. Inland winds will be northerly at around 5 kt or less with easterly offshore winds along the coast up to 10 kt.

KPDX AND APPROACHES..VFR conditions expected to prevail through the TAF period with increasing high clouds. There is a low 10-20% chance for rain after 12z on Sunday but conditions are expected to remain VFR. -19

MARINE

Benign conditions expected through early next week. Winds and seas will continue to ease through the rest of the weekend. Winds 5-10 kt from the NE-E through Sunday then turning more southerly on Monday but remaining light. Seas less than 10 feet and generally around 4-7 feet at 8-10 seconds through Tuesday. A weak disturbance approaches the waters on Tuesday, increasing southerly winds with a 50-70% chance of Small Craft winds. Seas increase Wednesday and Thursday but only have a 30-50% chance of reaching 10 feet or greater. -19

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ271>273.