Grand Ronde, OR 97347 Weather

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 interior valleys Saturday morning. Mostly dry conditions return this Halloween weekend. Slight chances for precipitation return early next week.

Short term

Today through Friday night: Radar imagery as of early Thursday morning show scattered showers across the CWA as an upper-level low spins off the coast of the Vancouver Island. Snow levels will fluctuate between 3500-4500 ft through Friday which will result in light snow showers continuing but additional snow accumulations are expected to be minimal as QPF values are minimal. For the remainder of the work week, the aforementioned upper level trough will leisurely move southward but remains offshore of the Pacific Northwest. This will maintain chances for light showers across the area, with the highest chances along the coast. Some locations along the I-5 corridor, particularly from Salem southward, may remain drier given that guidance isn't showing this low moving inland. It should also be noted that cold air aloft, along with clouds breaks through the day is also resulting in a slight increase in overall instability for the CWA. Various CAMs are showing CAPE values around 100 J/kg, which is resulting in a 5-15% chance for thunderstorms across the CWA through this evening. It should also be noted that there is a chance for cold core funnels to develop again today. Chances for 24 hour liquid precipitation, exceeding 0.10 inch from 5AM Thursday through 5AM Friday are around 60-80% along the coast, 30-50% for the I-5 corridor and 10-25% across the Cascades. Friday is a similar trend but slightly drier as the upper low is kept offshore due to upper- level ridging building over the Great Basin through Saturday morning. /42

Long term

Saturday through Wednesday..The conditions mentioned above will result in clearing skies and calm winds will favor temperatures dropping into the low to mid 30s across interior valleys, which may lead to fog or frost development. Chances for Saturday morning lows dropping below 32 degrees is 55-85% across the southern Willamette Valley, northern Clark County (Battle Ground area), and the Upper Hood River Valley.

Heading into the weekend/Halloween, the majority of ensemble members are showing the offshore upper-level low moving into the Pacific Northwest. It appears that there will be limited moisture with this low coming in, so most ensembles show either dry conditions or very light precipitation amounts. However, there is still a a 10-20% chance for new precipitation amounts around 0.10 inches between 5 AM Saturday to 5 AM Sunday across northwest Oregon and southwest Washington, with the highest probability along the coast and the Central Oregon Cascades. For those going out on Halloween, the weather is not looking too impactful as we are not expecting heavy rain or strong winds.

Sunday to Wednesday, looking at WPC 500 mb clusters around 75% of the ensemble member show a ridge amplifying over the northeast Pacific and shifting over the Pacific Northwest, with around 50% of those ensemble members developing into an Omega Blocking pattern. However, 25% of ensemble members are showing an upper level trough moving into the Pacific Northwest by Monday-Tuesday. If the ridge solution manifests, a warming and drying trend is on deck for the start of next week and if the trough solution manifests then a cooler and wetter trend would be on deck for the start of next week. /42

Aviation

Through 12Z Friday: Moist, south/southwesterly flow thanks to a nearly stationary low north of the Olympic Peninsula, will keep scattered showers, with predominately VFR conditions and a 10-20% chance of MVFR conditions through around 16Z-19Z Thursday. After 19Z Thursday the chance for MVFR conditions drops to 5-10%, but light, isolated showers (which might not impact any TAF locations) are still expected through 03Z Thursday. Colder air aloft and clearing skies through the day is also resulting in a 5-15% chance for thunderstorms across the airspace through around 03Z Friday. Probs for fog and reduced surface visby below 1 SM are generally less than 10% through 16Z Thursday, with the exception of locations around KEUG which will maintain a 15-25% chance through 16Z Thursday as well. Towards the end of the TAF period, expect a return of VFR to MVFR cloud cover along with isolated to scattered showers as the aforementioned low will send another pulse of moisture into the region.

KPDX AND APPROACHES: Scattered showers, with predominately VFR conditions and a 10-20% chance of MVFR conditions through around 19Z Thursday. After 19Z Thursday chances for MVFR conditions drops to 5-10%, but light, isolated showers (which might not impact any TAF locations) are still expected through 03Z Thursday. 5-15% chance for thunderstorms across the airspace through around 03Z Friday. Towards the end of the TAF period, expect a return of VFR to MVFR cloud cover along with isolated to scattered showers. /42

Marine

Winds and seas will continue to subside, with gusts less than 20 kt and seas 4-6 ft through the remainder of the week and into the weekend. Peak gusts approaching 20 kt are expected on Friday. This will result in relatively benign conditions across the coastal waters, with an exception within the main channel of the Columbia River Bar where very strong ebb currents Wednesday evening and Thursday evening that will temporarily result in seas building towards 8 to 10 ft with each strong ebb.

Towards the start of next week, seas will start to re-build towards 8-10 ft as a fresh, long period west-northwest swell with a dominant period of 15-17 seconds enters into the region. Light offshore winds are expected on Monday, suggesting seas will be primarily driven by the long-period swell rather than wind waves. /42-23

Watches, warnings and advisories

OR: None. WA: None. PZ: Small Craft Advisory from 5 PM this afternoon to 10 PM PDT this evening for PZZ210.