textproduct: Portland

SYNOPSIS

Enjoy one final day of below normal temperatures before conditions rapidly warm/dry Monday into the middle of the week. Compared to prior forecasts, heat related impacts have embarked on a rather stark upward trajectory as highs are now forecast to reach into the mid to upper 90s across the inland valleys and 70s/80s at the coast Tuesday/Wednesday. This will result in Moderate to locally Major HeatRisk across the region. Offshore easterly winds help to increase wildfire smoke west of the Cascades on Monday and Tuesday as well. Conditions slowly moderate late week into the weekend.

LONG TERM...Thursday through Saturday

Model uncertainty has increased late week, particularly on Thursday, thanks in-part to deterministic and ensemble guidance deviating on just how quickly the ridge of high pressure moves eastward. A faster progression would lead to more noticeable moderation on Thursday, however, guidance has been trending towards a slower and thus warmer scenario which leaves the influence of the ridge overhead a touch longer. Of particular note is the difference in projected high temperatures on Thursday between longstanding numerical ensemble modeling systems and their newer AI based counterparts. The latter AI ensembles (ECMWF AIFS, AIGEFS) lean towards a slowly (warmer) solution compared to the former which has increasing onshore influence resulting in a roughly 3-6 degree discrepancy (low 90s vs mid to upper 90s inland). It'll be interesting to see which of these systems end up winning out in the end but the forecast reflects the more "optimistic" numerical modeling at the moment. Should the current trend of model run to run temperatures increases continue, high temperatures on Thursday may need to be boosted further, thus extending heat impacts. Fortunately, there appears to be more consensus for temperatures to moderate back towards normal Friday into the weekend as weak onshore influence returns. The increasing onshore flow will also help clear out any smoke and haze lingering in the area as well, a nice added bonus. -99

AVIATION...Through 12Z Mon

Generally VFR conditions remain over the airspace this morning with CIGs SCT to BKN at 4-6kft. The latest guidance suggests chances for MVFR CIGs along the coast through 18z with hourly probabilities of 20-40%. VFR conditions are likely to persist inland but there is a low (10-20%) chance of MVFR CIGs between 12-19z but likely not persistent over one particular area. High confidence in VFR conditions this afternoon inland terminals. Overall light northwest winds becoming more northerly after 15-18z with wind gusts of 15-25 kt possible into the evening hours.

KPDX AND APPROACHES...VFR conditions are heavily favored through the TAF period, with a ~15% chance of MVFR CIGs developing between 12-18Z Sun. Based on current observations, this low probability seems reasonable. Expect north to northwest winds to peak in the late afternoon with gusts around 20 knots possible between 23z-04z. -99

MARINE

Behind the frontal boundary which passed overhead yesterday (Saturday) northerly flow begins to increase in earnest across the coast waters today, especially this afternoon. Chances for maximum wind gusts of at least 21 knots remain around 85-95%+ from this late afternoon and evening through the middle of the week with the highest relative probabilities across the outer waters. Thus, a Small Craft Advisory remains in effect starting this afternoon across the inner and outer waters including the Columbia River Bar. It still appears the Small Craft wind concerns shift moreso to the outer waters on Wednesday through the end of the week. Expect wave heights through the week to hold around 4-8 ft at 7-9 seconds. -99/12

FIRE WEATHER...Monday through Wednesday

There remain elevated fire weather concerns in portions of the Willamette Valley on Monday and Tuesday during the afternoon and evening hours due to breezy north-northeast winds and low humidity. However, confidence levels in Red Flag Warning criteria being met are still lower than 50% on both days, ranging between 20-40%. Due to developing easterly flow the western Columbia River Gorge is also an area to watch Tuesday into Wednesday as well. Fire Weather Watches have not been issued at this time, but this time period bears a close watch.

The outcome remains highly dependent on the evolution of a surface thermal trough that will begin lifting northward into western Oregon Monday into Tuesday. Typically it builds overhead from southwestern Oregon (dry northerly winds when this occurs) then it modulates diurnally back-and-forth from the coast/coast range (AM) towards I-5 and/or the Cascades (PM) helping to influence the magnitude and placement of offshore flow. Models indicate it'll develop along the south Oregon coast Monday morning before moving inland and further to the north in the afternoon. This progression would cause northerly winds to amplify in the Willamette Valley, and increase easterly winds aloft around 5000 ft, including in the high Cascades. These easterly winds will bring in drier air from east of the Cascades and the downsloping action will increase warming and drying in the lower elevations and in the Willamette Valley. Combine this with daytime heating and a deep layer of dry air, we will see relative humidity drop within the interior lowlands on Monday to near 25%. As the thermal trough builds into the area, could see gusts up to 20-25 mph within the Willamette Valley, Clark County lowlands, and along the coast. In the southern Willamette Valley around Eugene there is a higher probability of seeing these wind speeds than in the north. This is due to more of a channeling/fetch component and a tighter north-south pressure gradient. These dry and breezy conditions will lead to elevated fire weather concerns as they overlap mid/late afternoon into the early evening on Monday, however winds may be just shy of meeting criteria for a Red Flag Warning in most locations. Tuesday will be even drier with humidity values well below critical thresholds, however northerly winds will be trending weaker and are not expected to meet criteria for a Red Flag Warning at this time.

Our attention also turns to the western Columbia River Gorge Tuesday where offshore easterly winds appear increasingly likely to start the morning, continuing through the day while relative humidity values degrade. Localized gusts for places like Corbett and Three Corner Rock generally stay in the 25-35 mph range, closer to 15-25 mph far east Portland Metro. At least concerns are partially tempered by the preceding cooler/moister conditions today. While the near surface thermal trough driving these winds weakens by Wednesday morning, persistent offshore winds remain present. For Tuesday, if forecast winds increase any further, and/or forecast RHs drop, this is another region which may need a Red Flag Watch/Warning assuming fuels are ready to burn. Lastly, the pattern favors poor relative humidity recoveries for mid to upper slope locations in the Cascades, Cascade foothills, and portions of the coast range starting Monday night through the middle of the week. -99/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Monday for PZZ210. Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Tuesday for PZZ251-252-271-272. Small Craft Advisory from 2 PM this afternoon to 5 AM PDT Tuesday for PZZ253-273.