textproduct: Portland

SYNOPSIS

High pressure maintains dry weather through Thursday with nightly chances for fog and frost in interior valleys and gusty east winds through the Columbia River Gorge. There is moderate to high confidence in a return to cooler and wetter weather Thursday night through the weekend.

SHORT TERM...Today through Thursday

An upper-level ridge of high pressure over the area will maintain dry weather across northwest Oregon and southwest Washington through Thursday. An exception would be the Linn and Lane County Cascades, where moisture from an low pressure system moving into northern California will result in a 10-25% chance for light snow showers.

Satellite imagery as of early Tuesday afternoon show most sunny conditions except for the southern Willamette Valley where low stratus is lingering. Stratus across the central Valley is now dissipating, so the southern Valley should begin to clear up by 2-4 PM. Winds generally remain light and northerly in the Willamette Valley except for the Columbia River Gorge, where KTTD-KDLS pressure gradients of -5 mb are resulting in breezy easterly winds with gusts up to 25-35 mph in the far eastern Portland Metro. A weak thermal trough is also leading to light offshore flow along the coast. Ensemble guidance suggests that offshore flow pressure gradients across the area will continue through Wednesday night.

Clear skies and calm winds each night beginning tonight will favor fog and frost development across interior valleys. Frost is more likely tonight and Wednesday night as temperatures are forecast to fall near or slightly below freezing. The highest chances for frost will be the southern Willamette Valley, west- central Willamette Valley, Upper Hood River Valley, and northern Clark County lowlands as these locations have a 60-80% chance for morning low temperatures falling below 32 degrees F each night. If fog ends up developing rather than frost, make sure to use extra caution if you'll be commuting in the morning. -10

LONG TERM...Thursday night through Monday

Late in the week, ensemble guidance remains confident that ridging will shift east of the Rockies and broad upper-level troughing will dip down from the Gulf of Alaska and set up over the western CONUS. An initial frontal system will mark this pattern shift with rain returning as early as Thursday night to the coast and spreading inland through Friday.

Chances for precipitation then linger through the weekend as the forecast area remains within the upper trough. There does remain some uncertainty in the details of the flow aloft, however, with a cluster of ensemble members are favoring a scenario where the upper trough pinches off into a closed low over California. While this scenario would leave the Pacific Northwest drier through the weekend, those members still suggest increased chances for precipitation early next week.

Cooler temperatures within the upper trough will bring snow levels back down to 3000-4000 ft, allowing for accumulating snow at the Cascade passes. Given uncertainty in the pattern aloft though, confidence remains low in forecast snow amounts. There is a 45-55% chance of 6 inches or more of snow from 4 AM Saturday through 4 AM Tuesday, and 10-25% chances of 1 foot or more snow over the same period.

There is additionally a 15-35% chance that snow levels fall below 1000 ft each night from Saturday night to Monday night, which could bring light snow down to the Cascade foothills or even the West Hills. The latest guidance has also brought chances for snow reaching the valley floor to 0-1% for Saturday night and Sunday night and 2-5% for the Monday night as forecast precipitation amounts have decreased. Even if snow were to fall down to the fall to the Valley floor during these nights, marginal temperatures would severely limit any accumulations, and thus impacts are unlikely. -10/36

AVIATION

At 21z Tuesday, skies were mostly clear across southwest WA and northwest OR aside from a few thin high clouds. One exception is the southern Willamette Valley, including KEUG, where low stratus persists due to persistent northerly upslope winds. Overall confidence is low regarding exactly when low stratus will clear out at KEUG, if at all. That being said, current satellite imagery does show this stratus deck beginning to erode from the north. If the current progression continues as is, expect clearing at KEUG around by 22-23z Tuesday. Even if clearing does occur, there is a now a 60-65% chance low stratus and/or fog will redevelop at KEUG by 06z Wednesday.

For all other terminals, expect VFR flight conditions to most likely continue through the TAF period with offshore winds. Wind speeds will generally stay around 5-10 kt or less, except at KTTD where offshore winds are strongest with easterly wind gusts up to 25 kt. Probabilities for fog tonight are below 10% at all TAF sites, except at KEUG as mentioned above.

KPDX AND APPROACHES...Expect mostly clear skies to continue through 00z Thursday with relatively light winds under 10 kt. There is a 5% chance for fog development tonight. Frost is much more likely to occur than fog as temperatures dip into the mid 30s. -23

MARINE

Northerly winds are beginning to increase over the coastal waters as a thermal trough builds northward up the coast. Still expecting wind gusts up to 25 kt from late Tuesday afternoon through Wednesday afternoon beyond 10-20 nm offshore, except up to 30 kt beyond 40-50 nm offshore. Confidence has increased wind gusts will remain below 20 kt over the inner waters out to 10 nm due to an offshore component to the wind direction. That being said, locally breezier winds are likely downwind of gaps in the coastal terrain. Small Craft Advisories remain in effect over the outer waters through 7 PM PST Wednesday, except through 10 AM PST Wednesday for the northern outer waters (PZZ271).

Seas are expected to range between 6 and 9 feet through Thursday while winds become westerly Thursday afternoon, albeit relatively weak in strength. Winds then become southerly Thursday night into Friday ahead of a frontal passage, but are not expected to exceed 15 kt. Despite winds remaining weak, a long period westerly swell is set to arrive Thursday night into Friday. This will rapidly push seas up to 13-16 ft by early Friday morning. There is a 10% chance seas will peak as high as 17-18 ft, which represents the worst case scenario. Seas gradually decrease late Friday into the weekend, hovering close to 9-10 ft Saturday through Sunday. -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 from 4 PM this afternoon to 10 AM PST Wednesday for PZZ271.

Small Craft Advisory until 7 PM PST Wednesday for PZZ272-273.