textproduct: Portland

SYNOPSIS

Dry weather is expected to continue through most of the weekend as high pressure stubbornly holds over the region. There is a slight chanced of showers tonight over the coast and Cascades as a weak shortwave trough moves over the forecast area. Gusty winds persist through the Columbia Gorge. Expect cold overnight lows in the in the low 30s to low 20s each night through morning hours into Monday morning. 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 by the middle of next week, but details on the exact timing and impacts remain rather uncertain at this time.

DISCUSSION...Tonight through Wednesday night

An upper level ridge of high pressure remains entrenched over the Pac NW and NE Pacific through the end of the week. While current satellite and radar observations are showing a shortwave trough and light precipitation impacting areas around the Astoria, OR and the Long Beach Peninsula, am not expecting any significant precipitation along the coast or inland. So, given that there has been minimal change in the overall synoptic pattern, expect another night of very cold temperatures across the CWA. Overnight lows are expected to be generally be in the low to mid 20s and while winds are expected to decrease into this evening, expect apparent/wind chill/skin feel temperatures to make it feel like temperatures are in the low 20s. It should be noted that the incoming shortwave will bring clouds into the region and they could provide some insulation, thus keeping areas slightly warmer. However, model ensembles are pointing towards clearing skies overnight. Therefore, have issued a Cold Weather Advisory for the majority of the CWA, with the exception of Coastal locations and elevations above 2000 ft within the Central Oregon Cascades.

Stagnant air remains a concern with the persistent ridge and associated subsidence inversion. This is leading to air quality concerns into Friday as mixing heights will remain low and transport winds will be weak. Therefore, an Air Stagnation Advisory is in effect through at least Friday, after which there is moderate confidence on overall improvement. At this time, the Portland Metro area from the West Hills eastward were not included in this advisory due to increased winds from the Gorge maintaining a more mixed atmosphere.

As the weekend approaches, ensembles are in very good agreement that the ridge rebuilds over the CWA late Friday into Sunday. This will maintain the already dry conditions and bring a return of breezy east winds through the western Gorge into the eastern Portland metro with peak gusts of 40 mph. Skies are also expected to clear by Friday evening, and with the surface inversion strengthening again, strong radiational cooling, and a colder temperatures pushed into the region behind the shortwave, Friday night into Saturday morning is looking to see the coldest conditions of the winter so far. There's an 85-95% chance of apparent temperatures/wind chills falling towards 25 degrees or lower across the majority of the Willamette Valley, Coast Range, and SW Washington lowlands including the outskirts of the Portland/Vancouver metro area. Probabilities are a touch lower for the inner Portland/Vancouver metro area, around 60-80%. The higher elevations of the Upper Hood River Valley could also meet their Cold Weather Advisory criteria of wind chills at or below 15 degrees for at least 4 hours with a 65-90% chance. Cold overnight to morning temperatures will continue into at least Monday morning as high pressure persists, though the potential for Cold Weather Advisory criteria lowers each night as temperatures warm very slightly each night.

A pattern change remains on the horizon for Sunday/Monday as the GFS, ECMWF, and Canadian ensembles along with their deterministic counterparts still suggest a low pressure system moving through the NE Pacific and into the Pac NW. This system looks to briefly breaking down the strong ridge which has been anchored over the region. It should be noted that subsequent runs as well as 500mb cluster analysis are showing this system getting weaker with every new run as it gets closer to land, and around 50% of ensemble members suggest this low might not even be able to break down the ridge at all as this system could be the "sacrificial" system that could help bolster the following system next Tuesday/Wednesday. There's now only a 10-20% chance of precipitation, mainly over the coast and terrain, with limited accumulation.

As the middle of next week approaches, ensemble guidance remains in agreement that a moderate, Pacific low will finally break down the strong ridging over the region, though details with this weather system are frankly all over the place. Overall, there is a lot of uncertainty for this system. At this time, model ensemble guidance is pointing towards a moderate PoP (35-60% chance) with relatively low/moderate QPF (0.10-0.50 inches total) event. At this point, there are no major impacts expected with this weather system, and if precipitation does occur, it would be more beneficial than impactful, especially after the recent dry spell. /42/03

AVIATION

High clouds linger overnight as the frontal system that brought light precipitation to the region exits. Will see dry conditions prevail over the next 24-hours as high pressure rebuilds. VFR conditions with light winds becoming easterly. Temperatures will drop considerably overnight well below freezing for inland areas. Frost is likely in areas with light winds which may accumulate on surfaces. Frost will dissipate with the sun before redeveloping overnight.

KPDX AND APPROACHES...VFR over the next 24-hours. Minimal change as easterly winds dominate. Gusts will begin to increase in the afternoon with brief daytime heating. 10% chance of gusts as high as 30kt at KPDX. Aloft at 2000 ft, easterly gusts as high as 40 kt can be expected after 14Z Friday. -27

MARINE

Rather benign conditions continue across the coastal waters. High pressure offshore will maintain northerly breezes over the waters today. A very weak front drops across the waters tonight, turning winds back offshore by Friday morning. East to northeasterly winds increase through Friday evening as the thermal trough along the coast expands northward and high pressure rebuilds inland. Winds generally increase to around 10-15 kt with gusts up to 20 kt, but isolated gusts to 25 kt will be possible extending from gaps in the coast range and across the far outer coastal waters. Winds ease again through the weekend as pressure gradients decrease. Weak low pressure drifting offshore will cause the winds to turn southerly by Monday. The next frontal system is likely to approach the waters by the middle of next week returning increasing chances for Small Craft Advisory conditions on Wednesday. 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 10 PM this evening to noon PST Friday for ORZ104>125.

WA...Air Stagnation Advisory until noon PST Friday for WAZ204>206.

Cold Weather Advisory from 10 PM this evening to noon PST Friday for WAZ202>210.

PZ...None.