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 into early next week.

LONG TERM...Thursday Night through Tuesday

Upper-level ridging will continue to slide eastward away from the region while upper-level troughing shifts overhead from the Gulf of Alaska late in the week and persists into early next week. An initial frontal system will return rain to the region beginning late Thursday night at the coast and spreading inland through Friday. Ensemble guidance has come into better agreement that by the end of the weekend, the upper trough will pinch off into a closed low to the south while the Pacific Northwest instead remains in the col or saddle of the 500-hPa height field.

This pattern would place the best synoptic support for ascent to the south of the region, while disorganized hit-or-miss showers are more likely locally. As such, post-frontal precipitation chances will persist through the weekend, but will trend lower as the cut-off low develops. Weaker support for ascent and the scattered nature of showery precipitation both contribute to a lowering of potential precipitation totals through the period. Snow levels falling to 3000-4000 ft within the upper trough will allow for accumulating snow at the Cascade passes, with 25-45% chances of six inches or more falling from Friday through Sunday afternoons, and 10% or less chances for one foot of snow.

Valley locales will also see cooler temperatures within the upper trough, as morning lows fall to near or below freezing each of Sunday, Monday, and Tuesday mornings. Particularly on Monday and Tuesday, there is a 10-25% chance that snow levels will fall to 1000 ft or lower, which could see snow fall across the Coast Range, Cascade foothills, and possibly even to the valley floor in the early morning hours. Chances for even 0.1 inches of snowfall along the I-5 corridor remain only some 2-3%, and any snow that does accumulate would quickly melt as marginal morning temperatures warm above freezing each day, thereby limiting potential impacts. -36

AVIATION

Largely VFR flying conditions expected through the period beneath clear skies. Fog has again developed in the southern Willamette Valley including at KEUG yielding IFR vis/cigs which are expected to persist through 18-20z Wed before burning off. Elsewhere, colder temperatures early this morning favor frost formation instead of fog, however there remains a 10% or less chance of IFR conditions within isolated fog at other area terminals. An offshore pressure gradient will see light winds of 5 kt or less through 16-18z Wed will turn easterly to northeasterly along the coast and in the Portland area, and northerly along the Willamette Valley, generally at 5-10 kt. Stronger east flow is expected west of the Columbia River Gorge, with gusts up to 20-25 kt at KTTD.

KPDX AND APPROACHES...VFR conditions with clear skies and light north to northwest winds this morning, becoming northeasterly at 5-10 kt after 16-18z Wed. Chances for IFR fog early this morning remain less than 10%. -36

MARINE

Thermal troughing along the coast will maintain breezy north winds over the waters into today, with gusts as high as 25-30 kt across the outer waters, particularly to the south of Cape Falcon. Small Craft Advisories remain in effect until 10 AM PST beyond 10 NM from Cape Foulweather to Cape Falcon, and until 7 PM PST beyond 10 NM from Cape Falcon to Florence. Nearer to the shore, a slight offshore component to the wind (north- northeast flow) will minimize the risk of gusts hazardous to small craft, however locally higher winds can be expected downwind of gaps in the coastal terrain. As winds ease to 10 kt or less through tonight, seas continue at 5-8 ft into Thursday.

Ahead of an approaching frontal system, winds will shift out of the south Thursday night into Friday, but are not expected to increase above 15 kt. Seas, however, will rise dramatically late in the week as a westerly long-period swell arrives, yielding increasing chances for seas to reach 14-16 ft by early Friday morning, and low (less than 20%) chances for seas reaching into the upper teens. Seas look to ease to 7-10 ft by Saturday, continuing at those levels into early next week. -36

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 10 AM PST this morning for PZZ271.

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