textproduct: Portland

SYNOPSIS

Temperatures re-bound today as high pressure re- builds offshore. The hottest days of the week are expected on Monday and Tuesday with widespread Moderate HeatRisk and a 30-50% chance for Major HeatRisk across the Portland-Vancouver Metro Area and Columbia River Gorge. A Heat Advisory is in effect during these days. Fairly warm overnight for interior valleys will provide limited overnight relief, especially in urban areas. Temperatures cool down slightly Wednesday with increasing confidence for a cooler and wetter pattern by the end of the week.

DISCUSSION...Today through Saturday

Satellite imagery as of early Sunday morning depicts widespread marine stratus along the coast with some isolated areas of stratus across interior valleys. As daytime heating progresses today, we'll see increased mixing and stratus breaking out by late morning.

Temperatures begin to rebound today thanks to an upper-level ridge of high pressure amplifying over the eastern Pacific with building 500 mb 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 this 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 (70-95% chance) for high temperatures exceeding 90 degrees F across the Willamette Valley and southwest Washington lowlands on Monday and Tuesday with a 5-15% 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 a 30-50% chance for Major HeatRisk across the Portland-Vancvouer Metro Area and Columbia River Gorge due to the warm overnight temperatures. A Heat Advisory remains in effect from 11 AM Monday to 11 PM Tuesday for much of 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 caveats worth highlighting that may keep temperatures for spots like the Portland metro closer to the low to 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 for Tuesday. First is a 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 of the year leaning on NBM 5.0. We're also monitoring the placement and strength of a thermal trough during this Mon/Tue period. Current deterministic guidance is trending the thermal trough further east now, anywhere between the I-5 corridor and the Cascade crest. This should help bring northerly-northwesterly flow into interior valleys and keep temperatures a few degrees cooler. If the thermal trough were to lie along the coast (which it doesn't appear to be for this heat event), we would see increased easterly winds across the region which would make conditions much warmer. Given the more eastward placement of the trough now, the lack of strong offshore flow is supporting a temperature forecast slightly cooler on Monday, with highs peaking in the low to mid 90s across interior valleys. As we get closer to the start of this heat event, we'll see if Tuesday ends up taking a downward trend as well. Either way, it's going to be abnormally hot and people should make sure to practice heat safety.

The ridge of high pressure overhead begins to gradually drift to our east on Wednesday although there still remains forecast uncertainty as to just how quickly it'll progress. The vast majority of ensemble members kick the ridge axis well east of the Cascade crest-line by Wednesday 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 exceeding 95 degrees Wednesday afternoon from Salem to the Portland-Vancouver Metro. Thursday through the start of the weekend, the majority of ensemble members are now showing a trough of low pressure moving into the Pacific Northwest, returning cooler and wetter pattern. There still remains uncertainty with the exact timing, placement, and magnitude of the trough, and thus there is uncertainty with exact precipitation amounts. However, precipitation at the end of the week generally looks light and non-impactful at this point. -10/99

AVIATION

Northwest flow aloft continues with weak upper level troughing helping to deepen the marine layer along the coast. Marine stratus with MVFR CIGs is expected to persist at coastal terminals through at least 17-18z Sunday. CIGs are expected to lift with improvement to VFR becoming likely (80-90% chance) after 18z Sunday. Stratus may move across the Willamette Valley this morning, as chances for MVFR CIGs across inland terminals are around 30-50% at any given hour between 12-17z Sunday. Otherwise, expect predominately VFR conditions for inland terminals through the TAF period has high pressure builds. North to northwest winds continue, strongest during the afternoon/early evening with gusts up to 20 kt along the central Oregon coast.

KPDX AND APPROACHES...VFR with a few high clouds through the TAF period with a 30-50% chance for MVFR CIGs at any given hour between 11-16z Sunday. Light northwesterly winds this morning, increasing near 10 kt in the afternoon as pressure gradients tighten. -10/DH

MARINE

High pressure offshore has weakened, resulting in weaker northwesterly winds under 20 kt today through Monday morning with steep and choppy seas around 5 to 7 ft. High pressure re-builds Monday afternoon into Tuesday, tightening pressure gradients and increasing north-northwesterly winds with gusts up to 25 kt. Seas generally remain around 6 to 7 ft Mon- Tue. Small Craft Advisories are in effect for the outer waters (beyond 10 NM offshore) from late this afternoon to 5 AM Tuesday for the waters south of Cape Falcon. For the outer waters north of Cape Falcon, the Small Craft Advisory is in effect through 5 AM Monday. There is only a 10-20% chance for seas exceeding 7 feet at any given hour from Monday to Wednesday. -10

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 from 5 PM this afternoon to 5 AM PDT Monday for PZZ271.

Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Tuesday for PZZ272.

Small Craft Advisory from 2 PM this afternoon to 5 AM PDT Tuesday for PZZ273.