textproduct: Portland

SYNOPSIS

We'll embark on a noticeable warming trend the next several days with high temperatures peaking across the region on Monday and Tuesday. Widespread Moderate HeatRisk and localized Major HeatRisk for the Portland-Vancouver Metro Area is anticipated. Fairly warm overnight temperatures for interior valleys will provide limited overnight relief as well, especially in urban areas. With these things in mind, a Heat Advisory is in effect for many of the interior valleys and Columbia River Gorge Monday through Tuesday. At least confidence is increasing for cooler temperatures and even rainfall chances (30-60%) by late Thursday/Friday.

DISCUSSION...Today through Friday

Strong onshore flow remains in place this afternoon as morning clouds have taken their time dissipating. Forecast high temperatures today were lowered generally into low to mid 70s across the inland valleys to compensate for the more persistent cloud cover. All in all, these conditions are right around normal for mid to late June and it's only up from here.

On Sunday temperatures begin to rebound thanks to an upper-level ridge of high pressure amplifying over the eastern Pacific with building heights overhead. Still, we'll maintain some degree of onshore flow which keeps temperatures across western Oregon and southwest Washington more moderated than they otherwise would be. Expect highs in the low to mid 80s across interior valleys Sunday afternoon. The hottest days over the next week are still expected Monday and Tuesday during which the vast majority of ensemble members show the ridge sliding right over-top the Pacific Northwest. There is high confidence (>90% chance) for high temperatures exceeding 90 degrees F across the Willamette Valley and southwest Washington lowlands on Monday and Tuesday with a 10-30% chance for temperatures exceeding 100 degrees F from Salem to Portland-Vancouver on Monday, increasing to 30-50% on Tuesday. Overnight lows will also be warm (near 60 to mid 60s), especially for urban centers away from the coast. Overnight lows near or above 65 degrees would provide limited overnight relief from the heat. As a result, there will be widespread Moderate HeatRisk across much of northwest Oregon and southwest Washington during this period, with localized Major HeatRisk across the Portland-Vancvouer Metro Area through the Columbia River Gorge due to the warm overnight temperatures. A Heat Advisory is now in effect for much of the the southwestern Washington lowlands through interior valleys of western Oregon. Make sure to practice heat safety, including staying hydrated, wearing light-colored and loose-fitted clothing, wearing sun protection, and taking frequent breaks from the heat. If possible, schedule outdoor plans during the early morning or late evening when temperatures will be less intense.

All that said, there are a couple forecasting pitfalls worth highlighting that may keep temperatures for spots like the Portland metro closer to the mid 90s Monday and Tuesday compared to NBM 5.0 (and the current forecast) which projects highs in the mid to upper 90s to near 100. The first of these pitfalls is slight high bias occasionally observed in mid to long term temperature projections with the latest 5.0 version of the NBM and a feature we continue to assess here locally since this is only our second heatwave leaning on 5.0 - we'd like to have a more statistically sound reason to modify the NBM 5.0 output beyond day ~3 and we're not quite were yet. We're also monitoring the placement and strength of a near surface thermal trough during this Mon/Tue period which if centered along or just west of the coastline helps to shield the I-5 corridor from onshore influence and enhances easterly flow - typically a mesoscale feature observed when we have a shot at hitting the 100 degree mark. Exploring the latest deterministic guidance from the GFS, NAM, ECMWF, etc. shows the rough axis of the thermal trough oscillating from the coast range into the I-5 corridor (even as far east as the Cascades foothills) which would allow just enough NNW flow into the interior valleys to keep both high and low temperatures 2-4 degrees cooler than currently forecast. The lack of modest easterly flow near the boundary layer of the atmosphere (850mb) also backs this assertion, especially on Tuesday. With all this in mind, would only give moderate confidence to the high temperatures forecast on Monday and Tuesday, and should the trend with the thermal trough placement hold, highs may need to be decreased a few degrees along the I-5 corridor Mon/Tue compared to what the NBM would otherwise suggest. But even if temperatures are only in the mid 90s, it's still going to be abnormally warm early next week.

The ridge of high pressure overhead begins to drift to our east on Wednesday although there still remains forecast uncertainty as to just how quickly it'll progress. It would seem the majority of ensemble members 70% kick it well east of the Cascade crest-line which would allow for temperatures to decrease into the upper 80s to low 90s across the interior lowlands, if not further. The latest NBM still gives a 20-40% chance for highs to exceed 95 degrees Wednesday afternoon. Late week there's now a growing number of ensemble members (60-70%) depicting a trough of low pressure moving into the Pacific Northwest which would increase chances for light rainfall and below normal temperatures. -99

AVIATION

As of 22Z Sat, widespread VFR conditions throughout the airspace. Marine stratus will push back into the coast around 01-03Z Sun, bringing MVFR CIGs and a 10-30% chance of IFR CIGs to coastal terminals through at least 18Z Sun. There is a 60-80% chance for coastal terminals to improve to VFR after 18Z Sun. Inland terminals will likely stay VFR through the TAF period, with just a 10-20% chance of MVFR CIGs from 12-18Z Sun as marine stratus attempts to push through the Coast Range gaps. Otherwise, expect north to northwest winds around 5-10 kt, strongest during the afternoon.

KPDX AND APPROACHES...VFR conditions through the TAF period, with just a 10-20% chance of MVFR CIGs from 12-18Z Sun. West- northwesterly winds 5-10 kt, strongest during the afternoon. ~12

MARINE

High pressure offshore has weakened, resulting in weaker northwesterly winds tonight through Sunday morning. Steep and choppy seas around 7 to 9 ft continue to subside toward 5 to 7 ft through Sunday. A Small Craft Advisory remains in effect across the outer waters (beyond 10 NM offshore) until 11 PM tonight. Late Sunday into early next week, wind gusts around 20 to 25 kt return, while seas remain around 5 to 7 ft. There is only a 10 to 20 percent chance for seas to exceed 7 ft from Monday to Wednesday. ~12

PQR WATCHES/WARNINGS/ADVISORIES

OR...Heat Advisory from 11 AM Monday to 11 PM PDT Tuesday for ORZ108>123. WA...Heat Advisory from 11 AM Monday to 11 PM PDT Tuesday for WAZ204>210. PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ271>273.