textproduct: Portland

SYNOPSIS

High pressure and dry weather continues through at least the end of the week with breezy easterly winds through the Columbia River Gorge and eastern Portland/Vancouver Metro. As long as high pressure prevails, clear skies and light winds away from the Columbia Gorge will result in frost and/or freezing fog formation across most interior lowland valleys. Near/below freezing temperatures in combination with any fog may lead to locally slick road conditions. Lower confidence for fog/frost for locations that remain windy and/or have persistent stratus. Chances for precipitation return this weekend, but details on the exact timing and impacts remain nebulous.

DISCUSSION...Tuesday morning through Monday

Satellite imagery as of 2 AM Tuesday shows mostly clear skies across NW Oregon and SW Washington except for a few patches of fog/low stratus in the southern Willamette Valley, northern Cowlitz valley, and the Hood River Valley and surrounding areas. Surface observations indicate temperatures are currently near to below 30 degrees across much of the area except for along the coast as well as for elevations between 1500 ft to 5000 ft due to a continued strong subsidence inversion under strong high pressure. The last few ensemble runs have indicated that this strong high pressure that has refused to budge over the West Coast/eastern Pacific will now likely stick around through at least Saturday, though subtle changes to the ridge over the PacNW will lead to subtle changes to the forecast beginning Thursday.

Between now and Wednesday, conditions will remain fairly similar to previous days. The current pattern has resulted in a tightening of pressure gradients over the Cascades, though these have eased a bit compared to previous days. CAMs indicate the pressure gradient will slacken slightly in the overnight and early morning hours each day before tightening again in the late morning through evening hours, allowing breezy east winds through the Columbia River Gorge into the eastern Portland metro to decrease then increase accordingly. Current KPDX to KDLS pressure gradient is around -6 to -7 mb, and wind gusts have decreased to around 20-30 mph through the eastern Portland metro, including Troutdale, and 40-55 mph for winder locations east of Troutdale. As pressure gradients increase to -7 to -9 mb later this morning and afternoon, peak gusts will likely increase to 25-35 mph in the eastern Portland metro and 45-60+ mph east of Troutdale. Rinse and repeat through at least Wednesday evening.

Clear skies and light winds away from the winds from the Columbia River Gorge will lead to efficient radiational cooling across the area each night through the end of the week, continuing to drop overnight lows to near or below freezing across the Willamette Valley, southwest Washington lowlands, and Upper Hood River Valley. These are favorable conditions for widespread frost development, especially over grasses and metal surfaces. Freezing fog may also develop in some locations, particularly across the southern Willamette Valley and lower elevations of Cowlitz County where surface conditions are more moist and protected from the dry air filtering through the Cascade gaps. Any freezing fog that develops could lead to locally slick road conditions.

Additionally, stagnant air will be a continued concern under this subsidence inversion as the winds decrease, leading to air quality concerns through at least mid-week. Mixing heights will remain low and transport winds will be weak. Therefore, an Air Stagnation Advisory remains in effect for most of the Willamette Valley due to this concern through 4 PM Wednesday. 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.

On Wednesday into Thursday, the majority of GFS, Euro, and Canadian ensemble members and their deterministic models indicate a trough moving through the central Pacific that will pinch off into a cutoff low and move towards California, undercutting the ridge of high pressure instead of breaking it down completely, thus leaving us in a drier Rex Block pattern. However, this change will cause a subtle shift of the ridge off of the West Coast from a slight positive tilt to a slight negative tilt by Friday. The subsidence inversion and very tight pressure gradients over the Cascades are likely to break down Thursday into Friday, leading to a moderating of overnight temperatures in the foothills and terrain as well as the breezy east winds through the Gorge, though weak offshore flow is expected to continue.

Under this ridging, dry weather is likely to continue through at least Saturday. However, around 15% of ensemble members suggest the slight negative tilt of the high pressure could allow for a weak shortwave to move south along the flow over the PacNW, bringing a 10-20% chance of light showers to the coast and terrain Friday and Saturday. Temperatures are likely to remain on the cold side, especially in the overnight hours due to a push of colder arctic air from the east.

Then, around 80% of ensemble members suggest a low moving through the NE Pacific will finally break down the ridging sometime Sunday into Monday, with increasing chances of precipitation of around 30-60%. This would be a pretty weak system by the time it moves into the PacNW with limited precipitation accumulation. However, there are still a couple of items to keep eyes on as this system moves inland. First, if there is precipitation late Saturday night into Sunday morning, a handful of ensemble members suggest that overnight/early morning temperatures will remain cold enough 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 and roads would be too warm to accumulate snow. Second is precipitation type in the central Columbia River Gorge and Upper Hood River Valley as precipitation begins. If easterly winds continue into the weekend, 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. -03/99

AVIATION

Dry northwesterly flow aloft today as an upper level ridge persists over the region. This will maintain widespread VFR conditions for most terminals through the period. The main exceptions will be the development of fog or low stratus overnight into the morning hours underneath the surface inversion. As of 17z, freezing fog around a half mile visby persists at KEUG. This should mix out by 20z this afternoon. By 08z Wednesday, probabilities for IFR to LIFR fog/stratus increase to around 50% for KEUG, and up to 20-30% elsewhere for locations such as KSLE and KHIO tonight. In the lower levels the offshore pressure gradient continues to support easterly winds through the Columbia River Gorge and across portions of the eastern Portland metro. Expect gusts to around 35 kt at KTTD and up to 65 kt at the western end of the Gorge. Model guidance suggests winds ease again somewhat by 08z Wednesday.

KPDX AND APPROACHES...VFR with a few high clouds in the area today. Otherwise, light winds this morning expected to become southeast by 20z with gusts up to 20 kt. East winds remain breezy for east approaches with gusts to around 35 kt at KTTD. /DH

MARINE

Strong ridging aloft continues to favor persistent offshore easterly flow through midweek. While winds are largely at 10 kt or less, areas downwind of gaps in the coastal terrain may see stronger flow with gusts up to 15-20 kt, mainly west of the mouth of the Columbia River. Seas remain below 10 ft at 10-13 seconds through the middle of the week. -03/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 4 PM PST Wednesday for ORZ109- 114>118.

Freezing Fog Advisory until 11 AM PST this morning for ORZ117- 118.

WA...None. PZ...None.