Portland, OR 97209 Weather

: Updated aviation and watches/warnings/advisories sections:

Synopsis

Cool and showery conditions continue through Friday as an upper-level low sits offshore. Clear skies and calm winds may bring frost 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: Water vapor satellite imagery early Friday morning shows a vertically stacked low pressure system spinning off the Washington coast. Expect rain showers to increase in coverage across the area later this morning as a shortwave impulse lifts over NW Oregon into SW Washington. Accumulations at most locations today will likely see only a trace to 0.10 inch, but there is around a 20% chance that heavier showers bring over 0.25 inch to some areas. Chances for precipitation decrease to below 10% by Friday evening, except 10-30% along the north OR/south WA coast.

A northerly jet streak on the back side of the upper low will dig late Friday 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 largely reduce precipitation chances for much of Saturday. But, as the trough moves over the region, some wrap around moisture will bring around a 10- 30% chance of light rain or sprinkles across the Willamette Valley for Saturday evening, with higher chances over the Cascades. As the upper trough slides east of the Cascades on Sunday, an upper level ridge of high pressure will begin to build over the region leading. to dry conditions.

Sky cover Friday night into Saturday morning will be an important factor in overnight temperatures potentially falling to near or below freezing. Latest model guidance suggests clear to partly cloudy skies over inland valleys along with calm winds should allow for radiational cooling to occur. Outside of the major metro areas, there are high chances for minimum temperatures to fall below freezing. Will maintain the Freeze Watch and Frost Advisories in place for Fri night into Sat morning. Afternoon temperatures are expected to warm into the lower to mid-50s on Saturday before falling back into the 40s Sat evening after sunset. Similar overnight low temps are likely Sat and Sunday nights, 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 is in relatively good agreement that upper level ridging over the region maintains dry conditions into early next week. But, there remains a few solutions that shift the ridge far enough east allowing for low chances of precipitation to persist late Monday into Tuesday. If the drier solutions pan out, expect a slight warming trend with afternoon highs near 60 degrees through the middle of next week. Chances for rain increase again by Thursday as there is increasing confidence for a frontal system to approach the area. At this point, no significant impacts are expected with the upcoming weather pattern. /DH

Aviation

Through 18Z Saturday: Radar imagery and terminal observations as of late Friday morning depicts scattered showers and a mix of IFR/MVFR/VFR CIGs and VIS 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 21z Fri-00z Sat with 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: MVFR CIGs with intermittent IFR CIGs through 19-20z Fri, then improving to predominately VFR. 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 4-6 kt today. -10

Marine

Low pressure will continue to weaken as it drifts south off the Oregon coast through Saturday. Southerly winds increase again Friday afternoon, but should generally remain 20 kt or less. Winds then weaken over the weekend. Seas around 5 to 6 ft today gradually subside to around 4 ft on Saturday. Offshore winds develop late Sunday and persist into early next week. A long period west-northwest swell also arrives building seas to around 8 to 11 ft on Monday. Chances for seas over 10 ft on Monday remain at around 40-60% across the outer waters. The next frontal system likely approaches the waters mid to late next week. /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.