textproduct: Portland

SYNOPSIS

An active weather pattern into next week will produce periods of rain and windy conditions, particularly along the coast. A large westerly swell will bring high surf Sunday and an elevated threat of sneaker waves through the weekend.

DISCUSSION

Water vapor satellite imagery early this morning reveals a shortwave trough over the northeast Pacific that is actively sliding eastward towards the region. A surface low pressure is currently undergoing cyclogenesis in the northeast Pacific and is modeled to move towards Haida Gwaii/Vancouver Island. This will result in a front and attendant band of rain pushing southeastward across the area today. 24 hour rainfall amounts appear non-impactful for flooding larger river systems and the probability that 1-hour rainfall rates exceed 0.25" per hour appear to be well less than 10% along the I-5 corridor so few concerns exist for even nuisance urban flooding later today.

Our attention then turns to a shortwave trough currently located well south of the Aleutian Islands in the northern Pacific. Another surface low pressure tied to this storm system will undergo cyclogenesis and move northeastward towards Haida Gwaii late Saturday. This will push a stronger front eastward into the region Saturday evening and Saturday night. NBM guidance has a 30-40% chance for gusts to 58 mph at KAST. High resolution model guidance suggests our windiest sites like Clatsop Spit and Cape Disappointment stand a 80-90% chance for gusts to reach 58 mph. As a result, confidence is increasing that a High Wind product will be needed for at least the Clatsop and Pacific County coastal zones for Saturday evening, but the focus of it will likely be for exposed beaches and headlands such as Fort Stevens SP and the Long Beach Peninsula with fewer impacts in more wind protected areas like Astoria and Raymond.

Farther inland southerly winds will increase in the Willamette Valley. At this point, there is a 25% chance at any given ASOS location of observing a wind gust to 40 mph Saturday evening per the NBM so we will be in the running to have a few down tree limbs and isolated power outages, but confidence is still rather low in this scenario. With that said, REFS guidance has that probability more in the 50-60% range for many areas in the central and northern Willamette Valley.

Rainfall totals with the late Saturday storm system do not appear concerning for river flooding. There is a better signal for an intense enough narrow cold frontal rain band to materialize, but at this point, it still only appears there is a 20-30% chance for hourly rainfall totals exceeding a 0.25" per hour Saturday evening. This is typically the hourly rainfall amounts that are needed to produce nuisance urban flooding in October-December timeframe.

Aside from there being general agreement among the global ensembles in another storm system impacting the region late Monday and/or Tuesday, details on the exact timing and magnitude of wind and rainfall next week is fairly uncertain at this point. Fortunately, any signs of an overly impactful wind or rain event in the global ensembles is still relatively low (10% chance or less) at this point.

AVIATION...Through 12Z Saturday

Some breaks in the high clouds overnight have allowed some patchy fog and low clouds to develop across the area this morning. The fog and low clouds should remain rather isolated and short lived this morning as cloud cover overhead thickens and winds increase ahead of an advancing cold front.

A southwest-northeast oriented band of rain will drop southeastward across the area today. As the lower atmosphere gradually saturates, our coastal and northern terminals should trend into MVFR thresholds this evening. As the front dies and begins to lift northward overnight, clearing across the southern portions of our area will open the door to some fog and low cloud development in the 06-12z Saturday timeframe. As a result, our hourly statistical guidance suggests a 20-30% chance for IFR conditions developing at any given hour at KONP and KEUG during that time period with lower probabilities for terminals farther northward.

KPDX AND APPROACHES...Low level easterly winds and thickening mid and high level clouds will keep conditions VFR through 00z Saturday. An approaching front and attendant band of rain will drop southeastward into the area this evening and result in a gradual moistening of the lower level atmosphere. There is a 30-40% chance at any given hour for MVFR conditions to occur between 03-12z Saturday. The probabilities for IFR conditions remain less than 10% through at least 09z Saturday after which point clearing conditions and weakening winds could open the door to fog and low cloud development.

MARINE

An active weather pattern will continue across the waters into next week. Expect south winds to increase early this morning as our next frontal system pushes towards the waters. Confidence is high wind gusts will exceed 34 knots Cape Falcon northward today. Steep and hazardous seas of 14 to 18 ft will also build across our remaining waters later today.

Winds and seas ease briefly early Saturday before the next stronger front moves eastward across the waters late Saturday afternoon into Saturday evening. There is high confidence (>85% chance) that at least Gale Force wind gusts of 40-45 kt will materialize across the waters Saturday evening. There is even a 50% chance for Storm Force wind gusts across the northern waters and a 30% chance for Storm Force wind gusts across the waters between Cape Falcon and Cape Foulweather.

A large westerly swell will then push into the waters on Sunday with a high probability (>80% chance) of seas eclipsing 20 ft. The active weather pattern continues into next week as additional weather systems impact the waters off the Oregon and Washington coast.

BEACH HAZARDS

A westerly swell of 14-16 ft with a dominant period of 15-16 seconds will push onto the coast today and bring a high threat for sneaker waves. This threat will persist through Saturday. More importantly, confidence is increasing an even larger westerly swell of 20-25 ft with a dominant period of 15-16 seconds will impact the coast on Sunday. This appears increasingly likely to necessitate at least a High Surf Advisory. NBM guidance suggests a 10% chance at any given hour of seas reaching 25-26 ft between 7 AM and 1 PM Sunday, at least along the far north Oregon and south Washington coast, which given those probabilities are at specific points in time and not necessarily the peak magnitude in that entire time period, it would suggest we are probably pushing a 50:50 shot of seeing an observation like Buoy 29 reach 25-26 ft at 15-16 seconds at some point during that time period. This would be supportive of a low end High Surf Warning for at least Clatsop, Tillamook and Pacific County coastline Sunday morning. The sneaker wave threat will be even more elevated on Sunday and it is advised to either avoid or use a lot of caution if you plan to recreate on area beaches and jetties on Sunday. High surf will stand the highest chance of producing damaging along the coast around high tide on Sunday, which will occur around 11AM-Noon for the north coast.

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Gale Warning from 7 AM this morning to 7 PM PST this evening for PZZ210-251. Hazardous Seas Warning from 7 PM this evening to 10 AM PST Saturday for PZZ210-251. Storm Watch from Saturday morning through late Saturday night for PZZ210-251-271. Hazardous Seas Warning from 7 AM this morning to 10 PM PST this evening for PZZ252-253-272-273. Small Craft Advisory from 10 PM this evening to 10 AM PST Saturday for PZZ252-253-272-273. Gale Watch from Saturday morning through late Saturday night for PZZ252-253-272-273. Gale Warning until 4 PM PST this afternoon for PZZ271. Hazardous Seas Warning from 4 PM this afternoon to 10 AM PST Saturday for PZZ271.