textproduct: Portland

SYNOPSIS

An active weather pattern will continue into next week and 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

Our latest upper-level shortwave trough and accompanying frontal boundary are pushing eastward tonight with lingering showers rapidly decreasing and tapering off. This is followed by increasing upper-level heights Saturday morning as the overall longwave pattern amplifies due in part to a strengthening low over the eastern Pacific - thisll come in play late in the day. In the mean time, much of Saturday ends up fairly pleasant for early November, especially across the interior valleys. Thanks to the warm southerly flow aloft, daytime temperatures should jump into the mid 60s along the I-5 corridor as winds help to mix out fog and low clouds present early this morning.

Our attention shifts to another weather system that will push a stronger front eastward into the region late this afternoon and evening. As far as these impacts are concerned, the latest NBM guidance still has a 40-50% chance for wind gusts of 58 mph or greater along the coast, and the REFS guidance has probabilities of 60-70% along coastal beaches and headlands. The overall set-up appears to support these probabilities as well, 925mb winds (~1800ft) right along the coast maxing out around 55-65 knots along the frontal boundary. Without a notable temperature inversion to keep these winds pinned above the surface, heavier precipitation and more convective elements along the the front will definitely have an opportunity to mix towards the surface. Winds along the coast likely ramp up Saturday afternoon with the highest gusts along the coast likely occurring between 4-8PM.

Winds also increase across the interior lowlands Saturday evening and overnight, albeit they will not be nearly as strong as the coast. Both the latest NBM and REFS guidance indicate there is a 20-30% chance along the I-5 corridor for peak wind gusts above 40 mph Saturday evening, which would produce at least isolated down tree limbs and power outages. The majority of wind gusts will likely be in the 20-35 mph range, though.

Rainfall totals with this late Saturday storm system do not appear concerning for river flooding given the quick moving nature of the front. Confidence is high the region breaks to showers the second half of Saturday night into Sunday morning, followed by showers ending near sunset Sunday evening. That said, there is a signal among the high resolution model guidance for a narrow cold frontal rain band to materialize Saturday evening, but at this point, it still only appears there is a 10-20% chance for hourly rainfall totals exceeding a 0.25" per hour Saturday evening at any given location along the Interstate 5 corridor. With that said, REFS guidance suggests the probabilities are above 50% for the Salem area specifically. Either way, this hourly rainfall rate is typically what is needed to produce nuisance urban flooding in the October- December timeframe when deciduous trees are dropping their leaves and storm drains clog easily. Once showers end Sunday evening dry weather returns overnight with patchy fog across the inland valleys.

Looking at next week, ensembles are in agreement that the progressive pattern will continue, but there is relatively high uncertainty in the details. Global ensembles are in strong agreement with over 90% of the membership suggesting the next storm system arrives late Monday or Tuesday. The majority of the global ensemble membership also suggest rainfall amounts that will be beneficial. The probability we see rainfall amounts exceed 2.5-3 inches in a 24-hour period across even the higher terrain appears less than 5-10%.

While there is general agreement shortwave ridging will temporarily build across the region Wednesday into Thursday, there is a still a sizable chunk of the global ensembles that suggest a shortwave trough offshore will place an elongated SSW->NNE oriented atmospheric river close enough to the coast that it will continue rain there during this period. Nonetheless, the majority of the global ensemble guidance suggests this atmospheric river will remain offshore and be directed farther north towards the Olympic Peninsula and Vancouver Island with the region ultimately getting a reprieve from the rain.

All three ensemble systems are in broad agreement another storm system will bring rain and wind to the region late next week or next weekend. Fortunately, any signs of an overly impactful wind or rain event in the global ensembles is still relatively low (10-20% chance or less) at this point, though will need to keep an eye on flooding concerns depending on the strength of these weather systems as soils become more saturated from multiple rounds of rain.

AVIATION...Through 12Z Sunday

Satellite imagery and observations indicate a mixture of LIFR, IFR, MVFR and VFR conditions across the region early this morning. Low clouds continue to shift northwestward resulting in skies clearing from southeast to northwest across the region. This is allowing fog to develop in the southern and central Willamette Valley early this morning and expect fog and/or IFR conditions to develop at KSLE and KEUG shortly. Farther north, light winds and low level moisture have created a mix of conditions, but suspect many sites will trend into LIFR to IFR conditions over the next several hours.

Increasing winds ahead of the next storm system should result in conditions trending into VFR thresholds at all sites between 18-22z Saturday. A sharp cold front will bring lowering ceilings as it moves eastward across the area this evening. At this point it appears most likely to arrive on the coast between 00-03z Sunday and through the Willamette Valley taf sites between 03-06z Sunday.

KPDX AND APPROACHES...A mix of LIFR, IFR and MVFR conditions will continue to plague the terminal through about 18z Saturday. Increasing low level easterly winds should then result in conditions trending VFR before a sharp cold front pushes over the terminal between 03-06z Sunday. Expect gusty south-southwest winds of 25-35 kt along and behind the frontal passage.

MARINE

An active weather pattern will continue across the waters into next week. A persistent westerly swell will maintain steep and hazardous seas of 14 to 17 ft through Saturday morning. Therefore, a Hazardous Seas Warning remains in effect through 10 AM Sat across all waters including the Columbia River Bar.

The next stronger front moves eastward across the waters Saturday afternoon and evening. There is high confidence (>90% chance) for high-end gale force wind gusts of 40-45 kt across the waters south of Cape Foulweather with an 80-90% chance for at least a brief period of Storm Force wind gusts of 50-55 kt across the waters north of Cape Foulweather. The strongest winds will most likely occur across the waters off the Columbia River mouth. Winds appear most likely to peak across the northern outer waters between 1500-1800 and 1700-2000 for our inner waters. Gusty west winds will persist across our northern outer waters well into Saturday night.

A large westerly swell will then push into the waters early Sunday morning with high confidence for seas climbing above 20 ft for a 12-18 hour stretch. At this point, it appears seas will likely reach 20 ft between 0200-0500 on Sunday and not drop below 20 ft until 1800-2100 on Sunday. Latest guidance continues to suggest seas peak around 24-25 ft across the northern waters and 21-22 ft across the southern waters. Hedged towards seas peaking a couple feet higher than this in the official forecast given the pattern.

Seas gradually subside Sunday night into Monday. Additional storm systems will bring additional rounds of Gale Force winds to the waters with seas generally hovering between 10-20 ft for much of next week. The next round of Gale Force winds appears most likely (80% chance) to arrive late Monday into Tuesday.

PQR WATCHES/WARNINGS/ADVISORIES

OR...High Wind Warning from 3 PM this afternoon to 11 PM PST this evening for ORZ101-102. High Surf Warning from 4 AM to 10 PM PST Sunday for ORZ101-102. High Surf Advisory from 4 AM to 10 PM PST Sunday for ORZ103. WA...High Wind Warning from 3 PM this afternoon to 11 PM PST this evening for WAZ201. High Surf Warning from 4 AM to 10 PM PST Sunday for WAZ201. PZ...Hazardous Seas Warning until 10 AM PST this morning for PZZ210- 251>253-271>273. Storm Warning from 10 AM this morning to 4 AM PST Sunday for PZZ210-251-252-271-272. Gale Warning from 10 AM this morning to 4 AM PST Sunday for PZZ253-273.