Milwaukie, OR 97222 Weather

Updated Aviation Discussion:

Synopsis

Cool and showery conditions continue through Friday as an upper-level low sits offshore. Clear skies and calm winds may bring frost development across much of NW OR and SW WA in the mornings Saturday through Tuesday. Mostly dry conditions return this Halloween weekend except along the OR Cascades. High uncertainty for the return of precipitation early next week with limited weather impacts expected.

Discussion

through Wednesday: Scattered showers continue across NW OR and SW WA Thursday afternoon. Some stronger showers could produce heavy rain and small hail, and with SPC mesoanalysis indicating CAPE values are around 100-250 J/kg this afternoon, can't rule out a lightning strike or two along the far NW OR and SW WA coast through the early evening hours. Scattered showers continue tonight into Friday as the center of the upper level low pressure system causing the showers slowly deepens along the WA coast. By late Friday into Saturday, ensemble guidance is in good agreement that this low deepens and tilts negatively enough for widespread precipitation chances to lower below 10% except for along the OR Cascades and foothills where shower chances increase Saturday afternoon and evening to 20-60%. The majority of NW OR and SW WA is expected to see dry conditions for Halloween festivities, though daytime temperatures in the low 50s could fall into the 40s by the evening hours once the sun sets.

Skies are forecast to become more clear Friday night into early Saturday, and winds will be nearly calm. This will allow for temperatures to become quite chilly by Saturday morning. There's a 50-90% chance of frost across the majority of the forecast area Saturday morning as well as a 50-85% chance of temperatures below freezing for most inland valleys outside of the Portland metro area. A combination of a Freeze Watch and Frost Advisories have been issued for midnight to 9 AM on Saturday, including for coastal locations off of the beaches. There are similar probabilities for below freezing conditions Sunday and Monday mornings, though probabilities are slightly lower on Sunday due to uncertainty of cloud cover. 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 good agreement that the upper level low will finally push inland Saturday night, moving east of the Cascades Sunday morning with ridging building over the PacNW later Sunday into Monday as another closed low pressure system forms in the eastern Pacific. Dry weather returns Sunday into Monday with mostly sunny skies and the previously mentioned cold mornings. By late Monday, ensemble members diverge on solutions for the movement of the eastern Pacific low. Around half of the members think the low will remain over the ocean early next week with ridging remaining over the PacNW, keeping dry conditions. The others indicate the low moving onshore somewhere over central or northern California, though timing and north- south extent differ, leading to uncertainty in weather precipitation could return on the north side of the low. At this point, no significant impacts are expected with the upcoming weather pattern. -03

Aviation

Through 06Z Saturday: Currently, rain showers weaken as they drift northward through the Willamette Valley and along the coast, bringing MVFR to IFR conditions to some terminals. While a few terminals may remain MVFR to IFR through tonight, others may remain VFR through at least 12Z Fri. By 12-14Z Fri, more rain will enter the airspace, bringing nearly widespread MVFR conditions between 12-22Z Fri. Probabilistic guidance suggests a 20-40% chance of MVFR conditions along the coast and northern Willamette Valley between 12-22Z Fri, while terminals at KUAO southward have a 40-70% chance of MVFR, 30-40% chance of IFR, and 20-30% chance of LIFR from now until 22Z Fri. Highest chances for MVFR CIGs will coincide with passing rain showers in the northern Willamette Valley. In contrast, highest chances of IFR/LIFR conditions in the central/southern Willamette Valley coincide with less rain showers and more clearing, supporting low stratus development. After 22-24Z Fri, rain will taper off toward the coast, bringing VFR conditions to inland terminals and possibly at coastal terminals as well. Winds will remain generally south to southeasterly through the TAF period, 7-10 kt with gusts up to 18 kt possible along the coast and 5-8 kt at inland terminals.

KPDX AND APPROACHES: Currently VFR with light winds out of the southeast. Chances for MVFR CIGs increase to 20-30% between 08-19Z Fri, with the highest chances with passing rain showers. Expect predominantly VFR conditions after 19-21Z Fri. Expect generally southeasterly winds of 4-7 kt through the period. ~12

Marine

Winds and seas will continue to subside this evening through tonight, resulting in relatively benign conditions. One exception is over the main channel of the Columbia River Bar with a very strong ebb current Thursday evening, which will temporarily push seas up to around 8 ft. As such, a Small Craft Advisory remains in effect for the Columbia River Bar from 5-9 PM PDT Thursday.

South winds increase over the waters on Friday, but should generally stay under 20 kt, except over the northern inner waters and the main channel of the Columbia River Bar where gusts will likely top out near 20-21 kt. Winds weaken again over the weekend with seas hovering near 4 to 5 ft.

Monday and Tuesday are highlighted by light offshore winds with seas building to 8-11 ft with the arrival of a long period west- northwest swell with a dominant wave period of 15-17 seconds. Given winds will be relatively light, seas will be primarily driven by the long-period swell rather than wind waves. Chances for seas over 10 ft on Monday have increased to 40-60% over the outer waters and 15-30% over the inner waters. -23

Watches, warnings and advisories

OR: Frost Advisory from midnight Friday night to 9 AM PDT Saturday for ORZ101>103-107-108-111>113-119-124-125.

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

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

Freeze Watch from late Friday night through Saturday morning for WAZ205.

PZ: None.