textproduct: Portland

SYNOPSIS

Largely dry weather expected through midweek with temperatures remaining near to above normal for mid-October. Chances for wetter and cooler weather return late in the week.

DISCUSSION...Now through Sunday

Tranquil early fall weather will continue into the workweek despite an upper-level trough approaching the coast through tomorrow. There remains good consensus that the trough will elongate and pinch off into a closed low well offshore which will ultimately drift south and away from the Pacific Northwest. Locally, cloud cover will increase this afternoon through tonight as the system makes its closest approach, which will stymie radiational cooling and mitigate frost concerns for most. The typical coldest spots in the far upper Hood River Valley near Dee and Parkdale, north Oregon Coast Range valleys around Vernonia and Grand Ronde, and peripheral valleys around the southern Willamette Valley (along the Middle and Coast Fork Willamette River and the McKenzie River) may see some areas at risk of developing frost, and those with agricultural interests or sensitive outdoor plants should take precautionary actions to prevent damage. Chances for rain from this system tonight through Monday are minimal, with the Cascade foothills and Coast Range south of US-20 having a 15-25% chance of light rain or drizzle accumulating to 0.01 inches. Otherwise, dry weather is expected with near-normal afternoon high temperatures in the 50s along the coast and in terrain, and 60s in inland valleys.

As the closed low drifts south into California, light offshore flow on its northern flank will help clear skies and push temperatures some 10 degrees above normal Tuesday and Wednesday, when afternoon highs should reach the 60s along the coast and in terrain, and the low to mid 70s in inland valleys. Despite warm afternoons, clear skies overnight may allow for nightly areas of frost within inland valleys; those with agricultural interests or sensitive outdoor plants should continue to monitor the forecast for potential Frost Advisories.

By the latter half of the workweek, the pattern looks to become more active, favoring a return to cooler and wetter weather. An upper-level trough looks track inland in conjunction with the aforementioned closed low in California, bringing chances for light rain from Thursday through early Friday, although potential rainfall amounts appear very low. There is increasing consensus that a more potent system may affect the Pacific Northwest sometime this weekend, possibly the first atmospheric river of the season. There are elevated chances for more substantial rainfall amounts, but uncertainty remains high at this longer lead time. -36

AVIATION...Through 06z Tuesday

A low pressure system dropping southward off the coast will spread mid and high level clouds across the region through 06z Tuesday. Expect light winds and predominantly VFR conditions across the area through this time period. It should be noted that around 40-50% of our high resolution weather models suggest a mixture of IFR and MVFR stratus will develop along the central coast of Oregon between 06-12z Monday before spreading eastward into the southern Willamette Valley towards 12-18z Monday. Confidence in this scenario is not high, but it does seem modestly plausible. Patches of stratus are just beginning to develop along the Cascade foothills, but these seem to be in the 4-7kft ASL range per ASOS observations. Will keep TAFS VFR for now for later tonight into Monday morning, but will be monitoring trends and additional guidance and may need to make amendments given the uncertainty. Also, dewpoint depressions are currently lowest at KAST, KONP and KEUG so fog concerns are highest at these airports if these sites continue to radiate out overnight.

KPDX AND APPROACHES...A low pressure system dropping southward off the coast will spread plenty of mid and high level clouds across the region through 06z Tuesday. Expect predominantly VFR conditions and light winds through this time period. There is a 20% chance for MVFR or high end IFR conditions to develop in the 12-18z Monday timeframe, but suspect a large band of mid and high clouds just beginning to move overhead will minimize fog and low cloud chances in the vicinity of KPDX later tonight.

MARINE

A continued west-northwesterly swell and weakening northerly winds will see seas of 8-9 ft today ease to 6-7 ft by early Monday morning. A system dropping southward well offshore will generate a stronger westerly swell arriving late Monday which will push seas to 8-11 ft, highest beyond 10 NM and south of Cape Falcon, where Small Craft Advisories have been hoisted from 5 PM PDT Monday through 5 AM PDT Tuesday before seas ease below 9-10 ft. Strengthening northerly flow in the wake of this passing system may support gusts peaking at 20-25 kt on Tuesday, potentially requiring an extension or expansion of the advisory if confidence in the coverage and duration of stronger winds increases. Conditions otherwise improve midweek as winds and seas ease, but a more active pattern looks to return late in the week. Increasing northwesterly winds and a northwesterly swell could push seas close to 10 ft again from Thursday into Friday, and a more potent weather system next weekend will also see increased chances for seas above 10 ft and gusty, potentially gale-force winds. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory from 5 PM Monday to 5 AM PDT Tuesday for PZZ272-273.