textproduct: Portland

SYNOPSIS

Lingering showers through this afternoon as a shortwave trough exits the area. Dry weather returns Tuesday through Thursday, with fog/frost chances overnight across interior valleys. The next chances for precipitation return Thursday night to Friday. Conditions generally trend cooler and wetter weather pattern this weekend, but there remains some uncertainty with the exact set-up.

SHORT TERM...Today through Wednesday night

Radar imagery as of early Monday afternoon depicts scattered showers across northwest Oregon and southwest Washington as positive vorticity lingers aloft following the passage of a weak upper-level shortwave trough. Showers will gradually decrease tonight as this shortwave further exits the region and high pressure re- builds. High pressure will remain over the region Tuesday to Thursday, returning dry weather and mostly sunny skies with near-normal high temperatures. An exception would be a 20-40% chance of snow showers across the Linn and Lane County Cascades from Tuesday afternoon through Wednesday evening as a low pressure system moves into northern California.

Beginning tonight, clearing skies and light winds will favor fog and frost development across interior lowland valleys through midweek. Fog and/or frost would be more patchy tonight as there may still be some lingering cloud cover. However, widespread frost development is more likely than fog Tuesday night-Thursday night as efficient radiational cooling brings overnight low temperatures down to near or slightly below freezing for many interior valleys. Latest guidance suggests a 50-80% chance for morning lows of 32 degrees F or lower each night from Tuesday- Thursday, with the highest chances across the Tualatin Valley, southern Willamette Valley, and Upper Hood River Valley. -10

LONG TERM...Thursday through Sunday

At the end of the week, ensemble guidance suggests ridging will shift east of the Rockies and broad upper-level troughing will enter the western U.S. An initial frontal system will mark this pattern shift with rain returning as early as Thursday evening to the coast and spreading inland through Friday. Chances for precipitation linger through the weekend as the LREF ensemble mean keeps general troughing over the west, however, individual ensemble members show that there is still uncertainty with the exact magnitude and placement of the troughing. In general, it appears conditions will trend wetter and cooler heading into the weekend.

Cooler temperatures at the end of the week will bring snow levels back down to 3000-4000 ft, returning snow through the Cascade passes. There is still uncertainty with exact snow amounts, but current guidance suggests a 35-55% chance for 12+ inches of snow or greater through the passes in a 72 hour period from 4 AM Saturday to 4 AM Tuesday, with the highest chances through Santiam and Willamette Passes. There is also a 60-80% chance that snow levels fall below 2000 ft, which would bring snow chances down to the Cascade foothills and Coast Range this weekend.

Overnight low temperatures will also fall near or slightly below freezing across lowland valleys during this time if the trough is deep enough. Since this would coincide with moisture, we could see some wet snow or a rain/snow mix down to the Valley floor Saturday night-Sunday morning and Sunday night-Monday morning. Current guidance shows a 1-5% chance for 0.1 inch of snow or more across the Willamette Valley and southwest Washington lowlands Saturday night-Sunday morning and Sunday night-Monday morning when the temperatures are coldest. Fortunately, it appears that temperatures will be marginal enough due to onshore flow and precipitation will be light enough to suggest that if we did see some snow or a rain/snow mix, it would most likely melt upon impact with the ground and struggle to accumulate. If there were to be any accumulations in the lowlands, it would be short-lived as temperatures will still warm-up well above freezing during the day. This also does not appear to be a freezing rain pattern, so any frozen precipitation would fall as snow. -10/36

AVIATION

Scattered post-frontal rain showers will continue across the area, mainly to the north of KEUG, through 00-03z Tuesday. Until then, expect a mix of VFR and MVFR cigs, albeit with VFR cigs occurring the majority of the time at most terminals.

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 late 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 light rain showers through 00-03z Tuesday. Showers should quickly diminish thereafter. There is a 20-40% chance of fog between 06-11z Tuesday. Light offshore flow develops thereafter along with increasing high clouds, which should help mitigate fog later Tuesday morning. -23

MARINE

Small Craft Advisories are now in effect for all marine zones, including the Columbia River Bar, through 1 AM PST Tuesday. This is for continued seas of 9 to 11 ft at 13 seconds. Southwest winds around 5 to 15 kt with gusts as high as 20 kt will continue the rest of today before eventually becoming northerly late tonight into Tuesday morning as a thermal trough builds northward up the Oregon coast. Expect this to bring wind gusts up to 20 kt over the inner waters and up to 25 kt over the outer waters Tuesday afternoon through Wednesday morning, which is a slight increase from previous forecast updates. Therefore, have decided to issue a second Small Craft Advisory over the outer waters to cover the threat of increasing winds. It appears winds will stay just below small craft advisory criteria over the inner waters, however conditions will be close and future hazard updates may warrant an expansion to include the inner waters. Despite the increasing northerly winds Tuesday into Wednesday, seas should be below 10 ft during that time.

Long-range guidance continues to favor a return to more active and impactful weather late in the week and through the weekend. Multiple frontal systems may bring repeated chances of rain and elevated seas. While the threat for abnormally strong winds appears low (chances for gale force wind gusts only peak around 5-15% Friday through the weekend), confidence is high seas will be on the increase. The current forecast suggests seas will peak around 14-16 ft Friday morning with the arrival of a long period westerly swell. There is a 1-10% chances seas will peak as high as 16-19 ft. -23

BEACH HAZARDS

A high threat for sneaker waves is likely Friday through Sunday evening due to a long period westerly swell. Waves can run up significantly farther on a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning. Caution should be used when in or near the water, especially those who will be razor clamming. Keep children and pets away from the surf zone. Keep off of jetties, rocks and logs near the surf zone. If you see someone swept into the sea do not swim in after them. Call 911 and keep an eye on them until help arrives. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 1 AM PST Tuesday for PZZ210-251>253- 271>273.

Small Craft Advisory from 4 PM Tuesday to 10 AM PST Wednesday for PZZ271.

Small Craft Advisory from 10 AM Tuesday to 4 PM PST Wednesday for PZZ272-273.