Portland, OR 97258 Weather

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 maintan 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 preciptation 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 12Z Saturday: Moist, southwest flow aloft will persist through this evening as low pressure remains offshore. As of 10z Friday, IFR to LIFR CIGs persist across the southern and central Willamette Valley, while radar imagery is showing light rain showers beginning to redevelop. This will likely bring more widespread MVFR conditions through the Willamette Valley, around a 40-80% chance for CIGs below 3 kft, highest near KSLE where there is around a 50-60% chance for IFR conditions to persist through 18z. Showers are then expected to decrease in coverage later this afternoon and evening, with conditions favoring VFR by 18-21z Friday. Winds generally remain south to southeast 10 kt or less.

KPDX AND APPROACHES: VFR conditions are favored through the period. But rain showers increasing this morning will bring around a 30% chance for MVFR CIGs at any given hour between 14-20z Friday. Dry conditions expected by 02z Saturday. Light winds are expected to become southeasterly around 4-7 kt. /DH

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>103-107-108-111>113-119-124-125.

Freeze Watch from late tonight through Saturday morning for ORZ105-109-114-115-123.

WA: Frost Advisory from midnight tonight to 9 AM PDT Saturday for WAZ203-204-206-207.

Freeze Watch from late tonight through Saturday morning for WAZ205.

PZ: None.