textproduct: Portland

SYNOPSIS

A low pressure system moving into northern California and southern Oregon will bring push rain showers northward into northwest Oregon today. Most of the rain is expected from Lincoln/Polk/Marion County southward. Expect lower chances for rain north of these counties. Brief high pressure returns relatively drier conditions return on Monday. The next system arrives Tuesday-Wednesday, bringing widespread rain and pass-level Cascade snow. Chances for precipitation continue through the end of the week.

DISCUSSION...Today through Saturday

Radar and satellite imagery as of early Sunday morning depicts high clouds with some passing virga as a cut-off low pressure system approaches the region from the Pacific. This low will swing through northern California and southern Oregon, bringing wrap-around showers into the southern-most portions of northwest Oregon today. Most of the showers are expected from Lincoln/Polk/Marion Counties southward. For locations north of these counties, there is about a 15-30% chance for rain at any given hour through this evening, with the lowest chances across southwest Washington since they'll be much further north from the system.

There is uncertainty with exact rain amounts given the showery nature of the precipitation, however there is high confidence based on HREF/REFS guidance that 24 hour rain amounts ending 4 AM Monday remain under 0.50 inch for the majority of northwest Oregon. An exception could be the Linn and Lane Counties, where chances for exceeding 0.50 inch are around 20-30% for the lowlands and 50-70% for the adjacent foothills and high Cascades due to orographic enhancement. Lane County is expected to get the most precipitation, with the wettest scenario (10% chance) depicted by HREF/REFS being around 0.70-1 inch in 24 hours. This would be most likely to come to fruition if heavier showers pass through. Snow levels will also remain above 6000 to 7000 feet through at least Monday, keeping snowfall well-above the Cascade passes.

Transient ridging returns to the Pacific Northwest on Monday, returning a brief period of relatively drier conditions and warmer temperatures in the upper 50s. Could see some cloud breaks, but the high pressure won't be strong enough or last long enough to completely rule out some light precipitation. 15-30% chances for precipitation continue on Monday, highest over the Coast Range and Cascades.

The next weather system approaches the region on Tuesday with the majority of ensemble members (>80%) suggesting widespread rain beginning along the coast by Tuesday morning and spreading inland by the afternoon. Meanwhile, 20-40% of ensemble members have rain starting earlier around late Monday evening into early Tuesday morning. In either scenario, dry weather will come to an end by Tuesday evening. More substantial rain arrives Wednesday, with the chances for liquid precipitation exceeding 1 inch ending between 4 AM Tuesday to 4 AM Thursday being around 15-30% along the I-5 corridor from Cowlitz to Lane County, 45-65% along the coast, and 70-90% for the Coast Range and Cascades. At this point, not expecting wind impacts with this system, though could see some gusts up to 25 mph, locally up to 35-40 mph over the terrain, as the front passes on Wednesday. Snow will also return to the Cascade passes as snow levels drop to around 3500-4500 feet on Wednesday. Chances for 6+ inches of snow in a 48 hour period from 4 PM Tuesday through 4 PM Thursday are around 80-95% along the Santiam and Willamette Passes and 50% for Highway 26 at Government Camp, with the most snow falling late Tuesday into Wednesday. Those planning to travel through the Cascades mid-week should check road conditions before leaving and carry an emergency supply kit.

Thursday to Saturday, there is still some uncertainty with the exact upper level pattern over our area, but the majority of ensemble members are suggesting ridging far offshore over the northeast Pacific. The offshore placement of this ridge won't be favorable for a return to dry and sunny weather. Most ensemble guidance suggests that moisture will ride the northern/northeastern periphery of the ridge and track towards the Pacific Northwest, maintain chances for precipitation through the end of the week. -10

AVIATION

Satellite imagery and terminal observations as of early Sunday morning depicts VFR conditions with high clouds across northwest Oregon ahead of the next low pressure system. This system will move into northern California and southern Oregon today, spreading rain showers from south to north beginning 18-21z Sun and lasting through 06-09z Mon. Most of the showers will be from KSLE/KONP southward. CIGs will also gradually lower throughout the day, with chances for MVFR CIGs increasing to 50-70% across the Willamette Valley and the central Oregon coast at any given hour after 00z Mon. The highest chances will be at KONP/KEUG given that these terminals are further south and closer to the incoming system. Variable winds under 5 kt across the region today except for light east winds through the Columbia River Gorge and far east Portland Metro.

KPDX AND APPROACHES..Predominately VFR conditions through the TAF period with a 20-40% chance for MVFR CIGs at any given hour after 06z Mon as the system from the south spreads lower CIGs. 15-20% chance for rain after 00z Mon. Variable winds around 5 kt or less. -10

MARINE

Benign conditions expected through Monday night. Surface high pressure offshore and a weak thermal trough along the coast will maintain northeasterly winds 5-10 kt across the waters today. Seas fall to 3-4 ft at 9-10 sec tonight into Monday. Winds turn more southerly on Monday ahead of the next system, but remain light. The next frontal system arrives on Tuesday, likely returning small craft conditions. There is a 60-70% chance for frequent and widespread southerly wind gusts of 22 kt or greater across the waters on Tuesday. Seas will also gradually build on Wednesday and Thursday as a westerly swell moves in. Chances for seas exceeding 10 ft are 30-50% during this time, with the highest chances beyond 10-20 NM offshore. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.