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. 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 may lead to locally slick road conditions. Chances for precipitation return late this weekend into next week, but details on the exact timing and impacts remain rather nebulous at this time.

DISCUSSION...Wednesday morning through Tuesday

Dry weather is here to stay through much of the weekend as ensemble guidance (GFS, Euro, and Canadian) are in good agreement that the ridge of high pressure will remain over the Eastern Pacific/Pac NW. However, ensembles indicate the heights will slowly lower over the PacNW each day, allowing a break down of the strong pressure gradients over the Cascades by late this week. Another day of gusty offshore/east winds is expected through the western Columbia River Gorge and into the eastern Portland metro with peak gusts up to 35-40 mph through the eastern Portland metro, including Troutdale, and up to 55-65 mph for windy locations east of Troutdale. As the pressure gradient slowly weakens each day, so will the extent and strength of gusty offshore winds through the Gorge. However, wind prone areas near the Gorge will continue to see breezy winds into the weekend.

Satellite imagery early Wednesday morning depict scattered high clouds moving over NW Oregon and SW Washington on the north side of a cutoff low pressure system moving into central California. Other than these high clouds, mostly sunny skies are in store 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 outside of the influence of offshore winds through the Gorge. Some high clouds through Friday morning will keep widespread apparent temperatures (often called wind chill or "feels like" temperature) mainly above 25 degrees, though known colder pockets could see parent temperatures fall below 25 degrees in the early mornings for a couple of hours. By Friday night, skies are expected to be mostly clear of high clouds, which will allow for peak radiational cooling, and parent temperatures could fall below 25 degrees over a wider area. This becomes a concern for a potential Cold Weather Advisory, which conditions are met when locations west of the Cascades see apparent temperatures at or below 25 degrees for at least 4 hours or more. At this point, there's around a 60-70% chance of these conditions for most locations west of the Cascades except for the coast for Friday night/Saturday morning and about a 40-50% chance for Saturday night/Sunday morning. Will continue to monitor whether this threshold will be met for a widespread enough area and/or timeframe for the hazard to be issued. With the cold temperatures, freezing fog and low stratus 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 that develops could lead to locally slick road 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.

Looking at early next week, ensemble guidance continues to push back the return of precipitation to NW Oregon and SW Washington. Most ensemble members are in good agreement that a low pressure system will move through the NE Pacific and finally break down the strong high pressure Sunday into Monday. However, latest guidance suggests the low will weaken significantly before it makes landfall along our coast with only a 20-40% chance of precipitation now beginning late Sunday night, continuing into Monday afternoon. Higher chances for precipitation are for the far northern Oregon and SW Washington coast, Coast Range, and Cascades and the SW Washington lowlands. However, there are still a couple of items to keep eyes on as this system moves inland. First, a handful of ensemble members suggest that overnight/early morning temperatures will remain cold enough Sunday night into Monday morning to result in wet, non- impactful snow or rain/snow mix falling down to the Valley floor when precipitation returns. If this occurs, impacts would be minimal given air temperatures will be marginally freezing due to onshore flow, roads would be too warm to accumulate snow, and precipitation amounts would be limited in general. Second is precipitation type in the central Columbia River Gorge and Upper Hood River Valley as precipitation begins. If easterly winds continue into early next week, then colder air may linger longer for these areas. This could result in wintery precipitation, especially if the incoming moisture overlaps at the same time as the freezing air near the surface. These areas can remain much colder than other interior valleys like the Willamette Valley and southwest WA lowlands, and model guidance is generally too quick in scouring out the cold air. Specific details will become more clear as we get closer to precipitation returning.

There is good ensemble agreement that weak transient ridging moves inland late Monday into Tuesday. Temperatures are expected to remain on the moderate side as onshore flow from the Sunday/Monday weather system will have scoured out the colder air and cloud cover is expected to remain over the region. However, this ridging is not expected to last long as there are higher chances for another round of precipitation to return sometime late Tuesday into Wednesday. -03/99

AVIATION

Persistent high pressure remains over the region, supporting mainly VFR conditions. An upper level low moving over California has sent some high levels clouds northwards and may inhibit widespread fog/stratus development, especially in sheltered areas of the southern Willamette Valley. Pockets of MVFR/IFR/LIFR conditions within low stratus/fog have redeveloped in the southern Willamette Valley and northern Cowlitz valleys as of 10z Wed, but they likely won't become as widespread as previous mornings. Expect conditions to fluctuate between all flight levels through 16-19z Wed at KEUG. An offshore pressure gradient continues to support easterly winds at Portland- area and coastal terminals, and light northerly winds along the Willamette Valley. Stronger easterly gusts of 25-30 kt will continue at KTTD, with gusts up to 45-60 kt east of KTTD at the western end of the Columbia River Gorge. Winds will begin easing after 21z Wed - 00z Thu as the pressure gradient begins to ease.

KPDX AND APPROACHES...VFR with increasing high clouds. Southeast winds around 10 kts. East winds remain breezy for east approaches with gusts to around 30 kt at KTTD.

MARINE

High pressure inland and lower pressure over the coastal waters will maintain offshore easterly flow through Wednesday. High pressure builds offshore by Thursday, returning more northerly winds across the waters later this week. Winds generally remain around 10 kt or less, except areas downwind of gaps in the coastal terrain, where gusts up to 20 kt will be possible. Winds turn offshore again this weekend.

Seas around 4 to 6 ft at 12-14 seconds continue through the week. Not much change to the sea state as a persistent weak westerly swell ebbs and flows through the week. -DH/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until noon PST Friday for ORZ109- 114>118. WA...None. PZ...None.