textproduct: Portland

SYNOPSIS

Our extended dry stretch likely continues the remainder of the week as high pressure stubbornly holds overhead. 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. Gusty/breezy winds persist through the Columbia Gorge as well. 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...Now through Monday Night

This afternoon satellite imagery shows a few high clouds streaming across the region with otherwise mostly clear skies sitting overhead. Going through Wednesday (really through the end of the week) expect largely a persistence forecast with similar conditions day to day thanks to a robust ridge of high pressure holding overhead, although it is beginning to temporarily weaken. The pressure gradient between KPDX to KDLS has been strong today peaking around -10 mb with peak gusts of 60-75mph recorded near Corbett, and generally 25-45 mph gusts across the eastern Portland Metro, strongest out towards Troutdale and Gresham. The EPS indicates as the ridge weakens into Thursday so will this gradient but if you live in a spot prone to easterly winds they likely won't let up for some time, only slowly decrease.

Away from the immediate influence of the Columbia River Gorge these clear/mostly clear skies and light winds will facilitate rather efficient radiational cooling each night through the end of the week, continuing to drop overnight lows below freezing across the Willamette Valley, Tualatin Valley, southwest Washington lowlands, coast range valleys, and Upper Hood River Valley - frost is likely too. This set-up in combination with our subsidence inversion aloft often leads to boundary layer decoupling which can push temperatures even lower for our climatologically cooler spots like Hillsboro, Battleground, Eugene, etc. We will likely see apparent temperatures dip below 25F for some of these areas approaching sunrise each morning, but the confidence in the duration and areal coverage of these sub-25F temperatures are not high enough to issue a Cold Weather Advisory. If temperatures trend a touch lower, we may need to explore issuing this headline. Freezing fog also develops 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 the subsidence inversion, leading to air quality concerns through Thursday into Friday. Mixing heights will remain low and transport winds will be weak through the week. Therefore, the Air Stagnation Advisory was extended through midday 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.

Headed into Thursday, the majority of GFS, Euro, and Canadian ensemble members and their deterministic counterparts 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. This eventually leaves us in a dry Rex Block pattern later on. The subsidence inversion and very tight pressure gradients over the Cascades should degrade with a moderating of overnight temperatures and poor RH recoveries in the foothills and higher terrain. This shift in the upper-level pattern will also allow for a weak shortwave to move southward along the flow over the PacNW, however precipitation chances locally are now rather low, only 5-15%, so the dry weather likely persist.

Then, a significant number of ensemble members suggest a low moving through the NE Pacific will finally flatten and break down the ridging sometime late Sunday and Monday, with increasing chances of precipitation to around 30-50%. 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. -99/03

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 fog development. Pockets of IFR/LIFR conditions within low stratus/fog may redevelop but likely wont be as widespread as previous night in sheltered portions of the southern Willamette and Cowlitz Valleys late this evening into tonight, as well as MVFR cigs in valleys east of the Cascade crest including at K4S2. 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 30 kt will continue at KTTD into tonight, with gusts up to 45-60 kt east of KTTD at the western end of the Columbia River Gorge. Winds will ease for a time overnight but gusts to 20-25 kt will remain possible at KTTD.

KPDX AND APPROACHES...VFR with increasing high clouds. Southeast winds around 10 kts will ease after midnight before increasing again Wednesday morning. East winds remain breezy for east approaches with gusts to around 30 kt at KTTD. -19/DH

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 currently at around 5 to 6 ft with a period of 15 seconds slowly subside to around 4 to 5 ft at 12-13 seconds on Wednesday. Not much change to the sea state as a persistent westerly swell ebbs and flows through the week. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until noon PST Friday for ORZ109- 114>118.

WA...None.

PZ...None.