textproduct: Portland

SYNOPSIS

Cool and showery conditions continue through Friday as an upper-level low sits offshore. Winter Weather Advisories remain in effect across the Cascades through this evening. 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 late Wednesday morning depicts scattered showers across northwest Oregon and southwest Washington as an upper-level low spins off the coast of the Vancouver Island. For elevations above 4500 feet, webcams and surface observations currently show temperatures around 32F with some accumulating snow. Winter Weather Advisories remain in effect for the north Oregon and south Washington Cascades through 11 PM this afternoon, and through 5 PM for the Marion, Linn, and Lane County Cascades. High resolution guidance shows some moderate to locally heavy snow showers continuing over the north Oregon and south Washington Cascades through this evening, hence the later end time for the advisory. Showers gradually taper off this evening for the Cascades south of Clackamas County. Expect an additional trace to 3 inches of snow for elevations above 4500 feet, except 3 to 6 inches above 6000 feet. Despite advisory-level snow amounts not being met, the advisory was issued to consider any hikers or backpackers along the PCT or forest roads that may still be open above the main passes.

Otherwise, expect a typically fall day west of the Cascades with chances for showers continuing through this evening and temperatures in the 50s for most locations. There is a slight chance (15-20%) for thunderstorms this afternoon across interior southwest Washington, the Portland-Vancouver Metro Area, and Columbia River Gorge as we see some cloud breaks that allows for some surface heating and increased instability. However, chances for thunderstorms decrease once the sun starts going down after 4-5 PM. Any passing thunderstorms may produce lightning, heavy rain, gusty winds, and/or small hail.

Tonight into Thursday, the aforementioned upper trough moves 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. Chances for 24 hour liquid precipitation exceeding 0.10 inch are around 50-90% along the coast, 40-60% around the Portland- Vancouver Metro Area to the Cowlitz Valley, and 10-25% from the Salem southward and 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 Rockies and Central U.S. Friday night into early Saturday morning, 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 60-90% across the southern Willamette Valley, northern Clark County (Battle Ground area), and the Upper Hood River Valley. -10

LONG TERM...Saturday through Tuesday

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 light precipitation amounts. However, there is still a low chance (10-20%) for higher amounts of 0.10-0.20 inch between 5 AM Saturday to 5 AM Sunday across northwest Oregon and southwest Washington. 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 Tuesday, the majority of ensemble members (75%) are showing an upper-level ridging amplifying over the northeast Pacific and shifting over the Pacific Northwest. If this pans out, will see a trending trend across the region with a temperatures increase of a few degrees. However, 25% of ensemble members (mainly from the GFS and Canadian models) are showing troughing moving into the Pacific Northwest by Monday-Tuesday. If this pans out, then will maintain potential for cooler and wetter weather. For now, chances for precipitation on Monday and Tuesday are only around 10-25%. -10

AVIATION...Through 18Z Thursday

At 1730Z Wednesday, a mix of VFR/MVFR ceilings were being observed across the area with scattered rain showers and breezy south to southwest winds. VFR ceilings are most predominant. Expect this trend to continue through this evening, albeit with showers decreasing in coverage after approximately 00Z Thursday.

Winds become relatively lighter tonight with cloud cover becoming scattered to broken. However, chances for rain showers after 10Z Thursday have increased, except from KSLE to KEUG. Probabilities for fog and surface visibilities below 1 SM are generally less than 10% through tonight and tomorrow morning, except 20-30% at KHIO and KEUG. Overall coverage of fog should be fairly patchy, forming in sheltered valleys where winds turn calm and cloud cover clears for a long enough duration of time. Confidence for at least a brief period of fog is highest at KEUG.

KPDX AND APPROACHES...Occasional rain showers will persist through 00-02Z Thursday, bringing a mix of VFR/MVFR flight conditions, albeit predominately VFR. Trending VFR most of tonight, before chances for showers increase again towards 10Z Thursday. Suspect a mix of VFR/MVFR ceilings will occur tomorrow morning, but predominately VFR as chances for ceilings below 3000 ft at any given tomorrow are only around 5-15% between 12-17Z Thursday and should be mainly tied to passing showers. -23

MARINE

Buoy observations from early Wednesday morning showed westerly winds around 10 to 20 kt with seas ranging between 7 and 10 ft at 8 to 10 seconds, with the highest winds and seas occurring over the northern and central outer waters. Therefore, these two marine zones have a Small Craft Advisory in effect through 5 PM PDT Wednesday. Meanwhile, the inner waters and southern waters have no headlines in effect as winds and seas have fallen below small craft advisory criteria.

Winds and seas will slowly subside this evening into tonight in the wake of a low pressure system. Seas decrease further Thursday through the weekend. 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 will temporarily push seas up. Seas on the bar are expected to peak near 10 ft with west winds of 10 to 15 kt during the very strong ebb current Wednesday evening. Seas should peak near 8 ft with south winds of 10 to 15 kt during the very strong ebb current Thursday evening.

Seas will build across the coastal waters on Monday in response to the arrival of a long-period west-northwest swell with a dominant wave period near 15-17 seconds. Significant wave heights will most likely peak near 10 ft with the arrival of the long- period swell, however there is a 20-40% chance wave heights will peak over 10 ft. The highest probabilities for this to occur are over the northern and central outer waters. Light offshore winds are expected on Monday, suggesting seas will be primarily driven by the long-period swell rather than wind waves. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 11 PM PDT this evening for ORZ126.

Winter Weather Advisory until 5 PM PDT this afternoon for ORZ127- 128.

WA...Winter Weather Advisory until 11 PM PDT this evening for WAZ211.

PZ...Small Craft Advisory from 4 PM this afternoon to 9 PM PDT this evening for PZZ210.

Small Craft Advisory from 5 PM to 10 PM PDT Thursday for PZZ210.

Small Craft Advisory until 5 PM PDT this afternoon for PZZ271- 272.