textproduct: Portland

SYNOPSIS

The hottest days of the week are expected today and Tuesday as high pressure builds over the area, bringing widespread Moderate HeatRisk across interior valleys. Onshore flow will increase Tuesday night into Wednesday, bringing a slight cool down, but still remaining hot. A pattern change occurs Thursday through the weekend as an upper-level trough approaches the Pacific Northwest and returns cooler and wetter weather.

DISCUSSION...Today through Sunday

Satellite imagery and surface observations as of early Monday morning depicts marine stratus along the coast and mostly clear skies elsewhere. As daytime heating progresses today, we'll see increased mixing and stratus breaking out by late morning.

Upper-level ridging will move overhead today, warming temperatures into the upper 80s to low 90s across interior valleys which is about 15-20 degrees above normal. Diurnal northerly winds will increase this afternoon, with gusts between 15-20 mph along the coast and I-5 corridor. The warm-up we're experiencing today will result in widespread Moderate HeatRisk across all interior valleys. This level of heat will affect those who are sensitive to heat, especially those without adequate cooling/hydration. Tonight into early Tuesday morning, overnight lows in the mid to upper 50s for most areas will provide some relief from the heat. However, an exception is from Salem to the Portland/Vancouver Metro Area where Tuesday morning lows between 60 to 65 degrees will result in limited overnight relief. Chances for Tuesday morning lows exceeding 65 degrees across the Portland/Vancouver Metro are around 10-20%, possible due to urban heat island effects. Therefore, there is also a 20-40% chance for Major HeatRisk across the Portland/Vancouver Metro Area today.

The Tuesday high temperature forecast has trended a few degrees cooler, but still hot with interior valleys forecast to peak in the low to mid 90s. We'll have another day of widespread Moderate HeatRisk across interior valleys, with a 30-50% chance for Major HeatRisk across the Portland/Vancouver Metro Area. A Heat Advisory is in effect from 11 AM today to 11 PM Tuesday for the I-5 corridor from Salem to the Cowlitz Valley, along with the Columbia River Gorge and Upper Hood River Valley. Make sure to stay hydrated, wear light-colored and loose-fitted clothing, wear sun protection, and take frequent breaks from the heat. If possible, schedule outdoor plans during the early morning or late evening when temperatures will be less intense.

One note about Tuesday's forecast is that deterministic and ensemble guidance is beginning to show some weak troughing moving into the area as well. It's a dry trough, so rain is not expected but it will increase onshore flow. Winds on Tuesday shift more west-northwesterly, breeziest in the evening. Tuesday night into Wednesday morning, overnight lows cool down a few degrees compared to tonight due to the increased onshore flow. However, the Portland/Vancouver Metro Area will still have limited overnight relief as it remains warmer than most locations due to urban heat islands effects. Chances for Wednesday morning lows exceeding 65 degrees across the Portland/Vancouver Metro Area are around 15-30%.

On Wednesday, the vast majority of ensemble members kick the broader upper-level ridge axis east of the Cascade crest- line during the daytime hours. This should allow for high temperatures to drop a few degrees into the upper 80s to low 90s across interior valleys although the greatest day the day temperature change will likely be felt along the coast and in the some of the coast range valleys as westerly flow continues to increase. The latest NBM gives a 15-35% chance for high temperatures exceeding 95 degrees Wednesday afternoon from Salem to the Portland/Vancouver Metro. At least confidence is very high temperatures begin a noticeable descent back to normal, then below normal, the remainder of the week as the upper-level pattern undergoes a significant shift.

Thursday through the start of the weekend the majority of deterministic and ensemble guidance are depicting a upper-level trough of low pressure moving into the Pacific Northwest ushering in a cooler and wetter pattern. There still remains some uncertainty with the exact timing, placement, and magnitude of the trough, and thus there is uncertainty regarding exact precipitation amounts. The latest ensembles, including the NBM, are highlighting late Thursday night into Friday morning as best chance (70-95%) for rainfall during this late week period, mainly focused along the coast, Coast Range, Portland/Vancouver Metro through SW Washington, and in the Cascades. Should the core of the upper-level trough/low track overhead Friday and Saturday, post-frontal thunderstorms are another concern we'll have to watch. At the end of the day, any precipitation be can get this time of year is largely beneficial and we'll take what we can get during this late week/early weekend period. -10/99

AVIATION

Westerly flow aloft with weak upper level ridging will maintain high pressure over the region today. As of 17z, marine stratus is beginning to retreat from the Coast Range valleys and is expected to move offshore by 19z this afternoon. Expect widespread VFR conditions under mostly clear skies to persist inland through the TAF period. Conditions are likely to improve to VFR at coastal terminals around 18-20z, with chances for marine stratus returning to the coast increasing again at KONP after 01z Tuesday. CIGs are most likely to return IFR/LIFR conditions by 03z Tue (50-70% chance) along the central Oregon coast, while the northern OR coast sees lower chances (20-30%) after 04z Tue. North to northwest winds increase this afternoon with gusts up to 20 kt possible.

KPDX AND APPROACHES...VFR expected through the TAF period with mostly clear skies. Northwest winds around 5 kt expected to increase to around 10 kt with gusts up to 18 kt later this afternoon. /DH

MARINE

Northwesterly winds will generally stay under 20 kt through this morning. High pressure strengthens this afternoon into Tuesday, tightening pressure gradients and increasing north- northwesterly winds with gusts up to 25 kt. Small Craft Advisories remain in effect for the waters south of Cape Falcon and beyond 10 NM offshore through 5 AM Tuesday. Thursday to Friday, an approaching frontal system will shift winds more southwesterly, however, there is only a 5-10% chance for widespread and frequent small craft wind gusts of 21 kt or greater. High pressure rebuilds over the weekend, returning northwesterly winds. Seas remain steep and choppy around 5-7 ft through Wednesday, subsiding to 4-5 ft Thursday and Friday. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Heat Advisory until 11 PM PDT Tuesday for ORZ108>115-119>123.

WA...Heat Advisory until 11 PM PDT Tuesday for WAZ204>210.

PZ...Small Craft Advisory until 5 AM PDT Tuesday for PZZ272-273.