textproduct: Portland

SYNOPSIS

Dry weather is expected to continue through most of the weekend as high pressure stubbornly holds overhead. Gusty winds persist through the Columbia Gorge. Expect cold overnight lows in the in the low 30s to mid 20s tonight and likely tomorrow night. Clear skies, and light winds away from the Columbia River Gorge will result in frost and/or freezing fog formation for a decent swath of the interior lowland valleys and coast range Valleys. Near/below freezing temperatures in combination with any fog/frost may lead to locally slick road conditions. Chances for precipitation return towards the end of the week as well as by the middle of next week, but details on the exact timing and impacts remain rather uncertain at this time.

DISCUSSION...Now through Wednesday

Dry weather is here to stay through much of the weekend and into the start of next week. Ensemble guidance (GFS, Euro, and Canadian) are in good agreement that the ridge of high pressure will remain anchored over the Eastern Pacific/Pac NW. However, ensembles indicate that an incoming, shortwave trough will result in heights slowly lowering over the PacNW through the latter part of the week. This will allow a slow break down of the strong pressure gradients over the Cascades for the latter part of this week. As the pressure gradient slowly weakens over the next 48-72 hours, the extent and strength of gusty offshore winds through the Gorge will follow suit. Wind prone areas near the Gorge will continue to see breezy winds into the weekend.

Even with the weak shortwave moving in from the north, expect mostly sunny skies for the region through the end of the week. Under mostly clear skies, overnight temperatures will continue to be on the cold side with morning low temperatures near to below freezing across most of the region, for those areas outside of the influence of offshore winds through the Gorge. Clearing skies tonight will result in overnight lows as well as apparent temperatures (often called wind chill or "feels like" temperatures) dropping into the low 30s to mid 20s for lowland areas. For higher elevations, the temperature inversion that has been keeping higher elevations 5 to 10 degrees warmer will persist.

Have issued a Cold Weather Advisory for the majority of lowland areas within the Willamette Valley to include: North and Central Coast Range Valleys of Oregon, Lower Columbia River and Cowlitz River Valleys, Clackamas County Cascade Foothills, North Clark County Lowlands, Inner Vancouver Metro, Central and Southern Willamette Valley, Tualatin Valley, North and Central Coast Range Mountains of Oregon, Portland Metro Hills, and South Washington Cascade Foothills. These areas are expected to experience at least 4 hours or more of apparent temperatures at or below 25 degrees. Because of the persistent pattern with minimal changes, there is a 50-70% chance of similar Cold Weather Advisory conditions returning Friday night into Saturday morning and even again on Sunday morning. With these cold temperatures freezing fog or frost concerns continue in the southern Willamette Valley, northern Cowlitz county valleys, and the Upper Hood River Valley each overnight through morning hours into the weekend. Any freezing fog or frost that develops could lead to locally slick road or sidewalk conditions.

Additionally, stagnant air will be a continued concern under the subsidence inversion, leading to air quality concerns into Friday. Mixing heights will remain low and transport winds will be weak through the week. Therefore, an Air Stagnation Advisory is in effect through Friday after which point there's moderate confidence in improvement. The Portland Metro area from the West Hills eastward were not included in this advisory due to stronger winds from the Gorge maintaining a more mixed atmosphere.

Ensemble guidance continues to push back the return of precipitation to NW Oregon and SW Washington. Initially, the pump appears to be primed by the shortwave arriving late Thursday into Friday. While there is around a 15-30% chance of precipitation, this system may not be enough to produce widespread rain but it will aid in weakening the ridge and making space for a potential wetter system early next week. Ensembles are now showing a signal for a more significant pattern change starting late Monday into Tuesday. Models are showing a broad area of low pressure, sliding into the region from the NE Pacific and that could bring relatively warm and moist air into the Pac NW. Still, it should be noted, that model runs keep pushing the pattern change further and further into the future. Nevertheless, we will continue to diligently monitor and update the forecast as needed. -42/03

AVIATION

Dry northwesterly flow will continue aloft today as high pressure remains over the region. An upper low off the California coast will maintain scattered high clouds over northwest Oregon today. Otherwise, expect predominately VFR conditions to persist through this evening across the area. Chances for IFR to LIFR fog/stratus returning to the southern Willamette Valley increase to 50% at KEUG by 10z Thursday. Given the persistent pattern, have included LIFR conditions in the forecast. East winds with gusts to around 30 kt will continue at KTTD through this evening.

KPDX AND APPROACHES...VFR with scattered high clouds decreasing through the day. Southeast winds around 10-14 kt expected through this evening. The offshore pressure gradient in the lower levels continues to support easterly winds through the Portland area. East winds remain breezy for east approaches with gusts to around 30 kt at KTTD. -27/DH

MARINE

Rather benign conditions continue across the coastal waters. High pressure builds slightly offshore and persists over the waters through the end of the week, while the high pressure inland weakens somewhat. This will allow for the offshore winds to diminish as the northerly winds become more dominant by Thursday. Offshore winds return Friday as the thermal trough along the coast expands northward, and higher pressure rebuilds inland through the weekend. Generally, winds are expected to remain 15 kt or less.

Seas are expected to persist at around 4 to 6 ft with wave periods varying between 11-15 seconds through the weekend. Not much change to the overall sea state into early next week. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until noon PST Friday for ORZ104-105- 108-109-114>118-123. Cold Weather Advisory from midnight tonight to 10 AM PST Thursday for ORZ104>110-113>118-123. WA...Air Stagnation Advisory until noon PST Friday for WAZ204>206. Cold Weather Advisory from midnight tonight to 10 AM PST Thursday for WAZ204>206-208. PZ...None.