textproduct: Portland

SYNOPSIS

A passing feature will bring scattered showers, diminishing in coverage through today as building high pressure supports a drying trend. Snow levels around 3500 feet will see accumulating snow at and above the Cascade passes while these showers persist. Dry weather is then expected Tuesday through Thursday before more active weather returns Thursday night into the weekend.

LONG TERM...Thursday through Sunday

There is strong consensus among long-range ensemble guidance that dry weather will persist through Thursday before the pattern becomes more active. Ridging aloft will shift east of the Rockies while upper-level troughing takes its place over the West Coast late Thursday and continuing through the weekend. An initial frontal system will mark this pattern shift with rain returning as early as Thursday evening to the coast and spreading inland through Thursday night. With ensemble mean troughing then in place overhead, chances for precipitation persist through the weekend, although details of the flow remain very uncertain at this point. Cooler air within the upper trough favors lowering snow levels, allowing for more accumulating snowfall at pass-level in the Cascades, as well as potentially to elevations below 3000 ft in the foothills. -36

AVIATION

Scattered post-frontal rain showers continue across the area this morning, with mainly VFR cigs and visibilities aside from brief drops to MVFR cigs at the coast with passing showers. At this point, inland TAF sites are generally staying VFR even with passing showers, however brief MVFR cigs cannot be ruled out with heavier showers. Expect this trend to continue through this afternoon with showers gradually decreasing in coverage towards 00z Tuesday. By 06z Tuesday, dry conditions will return to all of northwest OR and southwest WA as weak low-level offshore flow develops. This will bring easterly winds around 10-15 kt at KTTD. Expect light northerly winds in the Willamette Valley later tonight, strongest at KEUG where sustained winds should approach 10 kt.

While offshore flow develops, high clouds will increase from south to north through the night. This should help mitigate widespread fog and low stratus development. That said, at least some patchy fog is possible (20-40% chance). Probabilities for fog are highest between 06-11z Tuesday when high clouds will be less prevalent and offshore flow will be weakest. Confidence in fog development at the terminals is not high enough to reflect in the 18z TAFs, except at KHIO where fog appears most likely to occur.

KPDX AND APPROACHES...Expect mainly VFR flight conditions through 18z Tuesday with occasional rain showers through 00-01z Tuesday. Showers should diminish thereafter. There is a 20-40% chance for fog between 06-11z Tuesday. Light offshore flow develops thereafter, which will help mitigate fog later Tuesday morning. -23

MARINE

Buoy observations early this morning of seas around 9-10 ft at 12-14 seconds support a continued hazard to small craft as the westerly swell peaks around 10 ft today before easing through midweek. Small Craft Advisories therefore remain in effect through 4 AM PST Tuesday across the coastal waters, however nearshore zones (those within 10 NM of the coast) are more likely to see early hazard cancellations as conditions improve through this evening. Relatively weak ebb currents at the mouth of the Columbia River of 1-4 kt at peak tide coincident with these marginally hazardous seas also merit continuation of the Small Craft Advisory for the Columbia River Bar through 1 AM PST Tuesday.

Modest high pressure building midweek will return tranquil weather and benign marine conditions to the region from Tuesday through Thursday as seas fall to 5-8 ft with northerly winds of 10-15 kt. Long-range guidance continues to favor a return to more active and impactful weather late in the week and through the weekend, with seas of 10-15 ft returning as early as Thursday night. Multiple frontal systems may bring repeated chances of rain, gusty winds, and elevated seas, however at this point, the chances of hazardous gale-force wind gusts are only 10-30% through the weekend. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 1 AM PST Tuesday for PZZ210.

Small Craft Advisory until 4 AM PST Tuesday for PZZ251>253- 271>273.