Wilsonville, OR 97070 Weather

Synopsis

Cool and showery conditions continue through this evening as an upper-level low sits offshore. Clear skies and calm winds may bring frost and/or fog development across much of NW OR and SW WA in the mornings Saturday through Monday. Mostly dry conditions return this weekend. More uncertainty for the return of precipitation early next week with limited weather impacts expected.

Discussion

through Thursday: Radar imagery early Friday afternoon shows scattered showers continuing across much of NW OR and SW WA as a shortwave along the closed low pressure system spinning off of the PacNW coast moves through the region. Showers will dissipate by late evening with mostly dry conditions expected overnight.

A northerly jet streak on the back side of the upper low will deepen overnight causing the upper level pattern to stretch into an elongated trough near the coast while the surface low drifts south and weakens offshore on Saturday. This will lead to a mostly dry day on Saturday, though as the trough finally moves inland Saturday, some wrap around moisture will bring 20-60% chance of rain showers over the OR Cascades and foothills late afternoon and evening with a 10-15% chance for Valley locations long and east of Interstate 5 south of Salem after 5 PM. Showers are expected to be light though on the chilly side as valley temperatures will quickly fall into the 40s around sunset. Completely dry conditions return Sunday as upper level ridging moves inland as the upper trough slides east of the Cascades on Sunday.

Temperatures tonight into Saturday morning are forecast to become much cooler than previous nights with widespread low temperatures in the 30s, though cloud cover will be an important factor in whether temperatures will fall to near or below freezing. Latest model guidance indicates skies will become mostly clear late this evening into the overnight hours as showers dissipate. Clear skies combined with calm winds and ground moisture from recent rain will allow for radiational cooling but also for fog formation. If temperatures fall quicker than forecast under clear skies before fog forms, then low temperatures near or below freezing are more possible. However, if fog ends up forming, this could limit how low temperatures can fall in areas with fog. Because of this uncertainty, have converted the Freeze Watch to a Frost Advisory, which is in effect for much of the area west of the Cascades, including the coast. Outside of the major metro areas and foggy locations, there are higher chances for minimum temperatures to fall below freezing (around 60-80%). Otherwise, probabilities are closer to 20-40%. Frosty conditions are likely again Sunday and Monday mornings. However, freeze and frost headlines are only issued during the growing season of April 1 through October 31. Therefore, no additional freeze or frost headlines will be issued after Saturday.

Ensemble guidance remains in relatively good agreement that upper level ridging and subsequent dry conditions remain over the region into early next week as another low pressure system develops in the eastern Pacific west of the OR/CA border and eventually moving inland over California midweek. However, there are a handful of ensemble members that bring the low pressure system inland over the PacNW sometime late Monday into Tuesday as a weak trough, allowing for a 10-20% chance of precipitation to return. If the drier solutions pan out, expect a slight warming trend with afternoon highs near 60 degrees for the interior lowlands through the middle of next week. Chances for rain increase slightly late week as some ensemble members indicate a frontal system approaching the area, but there is high uncertainty in specifics. At this point, no significant impacts are expected with the upcoming weather pattern. -03/DH

Aviation

Through 00Z Saturday: Radar imagery and terminal observations as of late Friday morning depicts scattered showers and a mix of MVFR/VFR CIGs across northwest Oregon and southwest Washington as a low pressure system sits offshore. This system will gradually weaken this afternoon/evening with showers tapering off. Conditions should improve to predominately VFR after 00z Sat with increasing clearing of low clouds.

A moist ground, clearing skies, and calm winds overnight will support fog or frost development across the Willamette Valley as temperatures drop into the low to mid 30s. The biggest uncertainty right now is whether or not terminals receive fog or frost. Guidance suggests a 20-40% chance for LIFR VIS/CIGs due to fog across the Willamette Valley at any given hour between 11-17z Sat. If fog doesn't develop, then expect frost development over metal surfaces. Eastern Portland Metro terminals such as KPDX and KTTD may be less likely for frost due to warmer temperatures, and light easterly winds at KTTD tonight would also prevent fog. Southerly to southwesterly winds under 10 kt across the Valley today. Along the coast, winds are more southeasterly but turn more easterly overnight.

KPDX AND APPROACHES: Predominately VFR with intermittent drops to MVFR during heavier showers. Clearing skies tonight with light winds will support potential fog development. Guidance suggests a 20-40% chance for LIFR VIS due to fog at any given hour between 11-17z Sat. Southeasterly winds 4-7 kt through this afternoon. -10

Marine

Weakening low pressure offshore will maintain southerly winds across the waters through this weekend, generally remaining under 20 kt. Winds weaken over the weekend. Seas around 5-6 ft today gradually subside to 3-4 ft on Saturday. Offshore winds develop late Sunday and persist into early next week. A long period west-northwest swell also arrives Sunday night, building seas to around 8 to 10 ft at 15-18 seconds by Monday. Chances for seas over 10 ft on Monday remain at around 40-60% across the outer waters. This long period swell will result in a moderate threat for sneaker waves Monday and Tuesday. Caution should be used when in or near the water. The next frontal system likely approaches the waters mid to late next week, returning southerly winds. -10/DH

Watches, warnings and advisories

OR: Frost Advisory from midnight tonight to 9 AM PDT Saturday for ORZ101>103-105-107>109-111>115-119-123>125.

WA: Frost Advisory from midnight tonight to 9 AM PDT Saturday for WAZ203>207.

PZ: None.